Publications

Self-healing materials can become game changers for developing sustainable (opto)electronics. APbX₃ halide (=X^-) perovskites, HaPs, have shown a remarkable ability to self-heal damage. While we demonstrated self-healing in pure HaP compounds, in single crystals, and in polycrystalline thin films (as used in most devices), HaP compositions with multiple A⁺ (and X^-) constituents are preferred for solar cells. We now show self-healing in mixed A⁺ HaPs. Specifically, if at least 15 atom % of the methylammonium (MA⁺) A cation is substituted for by guanidinium (Gua⁺) or acetamidinium (AA⁺), then the self-healing rate after damage is enhanced. In contrast, replacing MA⁺ with dimethylammonium (DMA⁺), comparable in size to Gua⁺ or AA⁺, does not alter this rate. Based on the times for self-healing, we infer that the rate-determining step involves short-range diffusion of A⁺ and/or Pb²⁺ cations and that the self-healing rate correlates with the strain in the material, the A⁺ cation dipole moment, and H-bonding between A⁺ and I^-. These insights may offer clues for developing a detailed self-healing mechanism and understanding the kinetics to guide the design of self-healing materials. Fast recovery kinetics are important from the device perspective, as they allow complete recovery in devices during operation or when switched off (LEDs)/in the dark (photovoltaics).

The remarkable ability of natural proteins to conduct electricity in the dry state over long distances remains largely inexplicable despite intensive research. In some cases, a (weakly) exponential length-attenuation, as in off-resonant tunneling transport, extends to thicknesses even beyond 10 nm. This report deals with such charge transport characteristics observed in self-assembled multilayers of the protein bacteriorhodopsin (bR). ≈7.5 to 15.5 nm thick bR layers are prepared on conductive titanium nitride (TiN) substrates using aminohexylphosphonic acid and poly-diallyl-dimethylammonium electrostatic linkers. Using conical eutectic gallium-indium top contacts, an intriguing, mono-exponential conductance attenuation as a function of the bR layer thickness with a small attenuation coefficient β ≈ 0.8 nm⁻¹ is measured at zero bias. Variable-temperature measurements using evaporated Ti/Au top contacts yield effective energy barriers of ≈100 meV from fitting the data to tunneling, hopping, and carrier cascade transport models. The observed temperature-dependence is assigned to the protein-electrode interfaces. The transport length and temperature dependence of the current densities are consistent with tunneling through the proteinprotein, and protein-electrode interfaces, respectively. Importantly, the results call for new theoretical approaches to find the microscopic mechanism behind the remarkably efficient, long-range electron transport within bR.

We report continuous wave laser-assisted evaporation (CLE), a thin film deposition technique that yields phase-pure and stoichiometric thin films of halide perovskites (HaPs) from stoichiometric HaP targets. We use methylammonium lead bromide (MAPbBr₃) to demonstrate the ability to grow with CLE well-oriented and smooth thin films on various substrates. Further, we show the broader applicability of CLE by preparing films of several other 3D HaP compounds, viz., methylammonium lead iodide, formamidinium lead bromide, and a 2D one, butylammonium lead iodide. CLE is a single-source, solvent-free, room-temperature process that needs only roughing pump vacuum; it allows the deposition of hybrid organic-inorganic compound films without needing post-thermal treatment or an additional organic precursor source to yield the intended product. The resulting films are polycrystalline and highly oriented. All these features, and the fact that one stoichiometric source serves as the target, make for an attractive, potentially scalable dry deposition approach.

Self-healing (SH) of (opto)electronic material damage can have a huge impact on resource sustainability. The rising interest in halide perovskite (HaP) compounds over the past decade is due to their excellent semiconducting properties for crystals and films, even if made by low-temperature solution-based processing. Direct proof of self-healing in Pb-based HaPs is demonstrated through photoluminescence recovery from photodamage, fracture healing and their use as high-energy radiation and particle detectors. Here, the question of how to find additional semiconducting materials exhibiting SH, in particular lead-free ones is addressed. Applying a data-mining approach to identify semiconductors with favorable mechanical and thermal properties, for which Pb HaPs are clear outliers, it is found that the Cs₂Au^IAu^IIIX₆, (X = I, Br, Cl) family, which is synthesized and tested for SH. This is the first demonstration of self-healing of Pb-free inorganic HaP thin films, by photoluminescence recovery.

The class of materials termed halide perovskites has experienced a meteoric rise in popularity due to their potential for photovoltaic and related applications, rivaling the well-established silicon devices within a few short years of development. These materials are characterized by several intriguing properties, among them their mechanical behavior. The study of their response to stress is essential for proper device development, while being of fundamental scientific interest in its own right. In this perspective, we highlight the key concerns surrounding this topic, critically analyzing the measurement techniques and considering the challenges in the current level of understanding.

Two-dimensional (2D) halide perovskites, HaPs, can provide chemical stability to three-dimensional (3D) HaP surfaces, protecting them from exposure to ambient species and from reacting with contacting layers. Both actions occur with 2D HaPs, with the general stoichiometry R2PbI4 (R: long or bulky organic amine) covering the 3D ones. Adding such covering films can also boost power conversion efficiencies of photovoltaic cells by passivating surface/interface trap states. For maximum benefit, we need conformal ultrathin and phase-pure (n = 1) 2D layers to enable efficient tunneling of photogenerated charge carriers through the 2D film barrier. Conformal coverage of ultrathin (

The finding that electronic conductance across ultrathin protein films between metallic electrodes remains nearly constant from room temperature to just a few degrees Kelvin has posed a challenge. We show that a model based on a generalized Landauer formula explains the nearly constant conductance and predicts an Arrhenius-like dependence for low temperatures. A critical aspect of the model is that the relevant activation energy for conductance is either the difference between the HOMO and HOMO1 or the LUMO+1 and LUMO energies instead of the HOMOLUMO gap of the proteins. Analysis of experimental data confirms the Arrhenius-like law and allows us to extract the activation energies. We then calculate the energy differences with advanced DFT methods for proteins used in the experiments. Our main result is that the experimental and theoretical activation energies for these three different proteins and three differently prepared solid-state junctions match nearly perfectly, implying the mechanisms validity.

The development of vacuum-deposited perovskite materials and devices is partially slowed down by the minor research effort in this direction, due to the high cost of the required research tools. But there is also another factor, thermal co-deposition in high vacuum involves the simultaneous sublimation of several precursors with an overall deposition rate in the range of few Å s⁻¹. This leads to a deposition time of hours with only a single set of process parameters per batch, hence to a long timeframe to optimize even a single perovskite composition. Here we report the combinatorial vacuum deposition of wide bandgap perovskites using 4 sources and a non-rotating sample holder. By using small pixel substrates, more than 100 solar cells can be produced with different perovskite absorbers in a single deposition run. The materials are characterized by spatially resolved methods, including optical, morphological, and structural techniques. By fine-tuning of the deposition rates, the gradient can be altered and the best-performing formulations in standard depositions with rotation can be reproduced. This is viewed as an approach that can serve as a basis to prototype other compositions, overcoming the current limitations of vacuum deposition as a research tool for perovskite films.

Coupling electromagnetic radiation with matter, e.g., by resonant light fields in external optical cavities, is highly promising for tailoring the optoelectronic properties of functional materials on the nanoscale. Here, we demonstrate that even internal fields induced by coherent lattice motions can be used to control the transient excitonic optical response in CsPbBr3 halide perovskite crystals. Upon resonant photoexcitation, two-dimensional electronic spectroscopy reveals an excitonic peak structure oscillating persistently with a 100-fs period for up to ~2ps which does not match the frequency of any phonon modes of the crystals. Only at later times, beyond 2ps, two low-frequency phonons of the lead-bromide lattice dominate the dynamics. We rationalize these findings by an unusual exciton-phonon coupling inducing off-resonant 100-fs Rabi oscillations between 1s and 2p excitons driven by the low-frequency phonons. As such, prevailing models for the electron-phonon coupling in halide perovskites are insufficient to explain these results. We propose the coupling of characteristic low-frequency phonon fields to intra-excitonic transitions in halide perovskites as the key to control the anharmonic response of these materials in order to establish new routes for enhancing their optoelectronic properties.

Halide perovskites (HaPs) are functional semiconductors that can be prepared in a simple, near-room-temperature process. With thin polycrystalline HaP films, excellent solar cells, light-emitting diodes (LEDs), and (also as single crystals) highenergy radiation detectors have been demonstrated. The very low single-crystal defect densities make HaP thin single crystals (TSCs), instead of polycrystalline HaP films an attractive option, to boost device performances and for fundamental research. However, growing TSCs is challenging primarily because of random multiple nucleations, which, in the often-used space-confined geometry, is favored at the substrate boundaries, where loss of organo-amines and solvents occurs. We show that fewer and better quality thin crystals nucleate and grow reproducibly away from the substrate edges in the substrate center, if we localize the heating (needed for inverse-temperature crystallization, the preferred crystal growth method) there. Using a further finding of ours that lowers the crystallization temperature, TSCs of methylammonium lead bromide (MAPbBr3), the HaP we focus on here, grow also directly on flexible substrates. 1H NMR measurements show how the observed lower crystallization temperature results from slow humidity-mediated chemical changes in the HaP precursor solution during its storage.

We present a multi-head spray pyrolysis system and its application in high-throughput combinatorial synthesis for research of solid Li-ion conductors. Each spraying nozzle is fed with a separate precursor solution. The overlap of areas that are sprayed leads to unprecedented composition flexibility of the films obtained after pyrolysis. Thus, a library with a continuous composition spread of a Li-La-P-O model system is formed. The Li-ion conduction was determined on 169 cells of the library, using high throughput impedance measurements in a controlled environment. While the activation energies that were found were relatively small, Li-ion conduction was still low. This low mobility is hypothesized to originate from the sub-optimal occupation of Li sites in the non-stoichiometric materials' lattices, and/or porosity and tortuosity issues, which in turn, reduces their effective concentration and contribution to ion transport. In addition, porosity and tortuosity in sprayed electrolyte causes random orientation of the grains and grain boundaries in the solid followed by the random diffusion scattering of Li-ions and low Li-ions conductivity, despite low apparent activation energy of conduction.

Humidity is often reported to compromise the stability of lead halide perovskites or of devices based on them. Here we measure the humidity dependence of the elastic modulus and hardness for two series of lead halide perovskite single crystals, varying either by cation or by anion type. The results reveal a dependence on bond length between, hydrogen bonding with, and polarizability/polarization of these ions. The results show an intriguing inverse relation between modulus and hardness, in contrast to their positive correlation for most other materials. This anomaly persists and is strengthened by the effect of humidity. This, and our overall findings are ascribed to the materials unique atomic-scale structure and properties, viz nano-polar domains and strong dynamic disorder, yet high-quality average order. Our conclusions are based on comparing results obtained from several different nano-indentation techniques, which separate surface from bulk elastic modulus, and probe different manifestations of the hardness.

The interfaces between inorganic selective contacts and halide perovskites (HaPs) are possibly the greatest challenge for making stable and reproducible solar cells with these materials. NiO x , an attractive hole-transport layer as it fits the electronic structure of HaPs, is highly stable and can be produced at a low cost. Furthermore, NiO x can be fabricated via scalable and controlled physical deposition methods such as RF sputtering to facilitate the quest for scalable, solvent-free, vacuum-deposited HaP-based solar cells (PSCs). However, the interface between NiO x and HaPs is still not well-controlled, which leads at times to a lack of stability and V oc losses. Here, we use RF sputtering to fabricate NiO x and then cover it with a Ni y N layer without breaking vacuum. The Ni y N layer protects NiO x doubly during PSC production. Firstly, the Ni y N layer protects NiO x from Ni3+ species being reduced to Ni2+ by Ar plasma, thus maintaining NiO x conductivity. Secondly, it passivates the interface between NiO x and the HaPs, retaining PSC stability over time. This double effect improves PSC efficiency from an average of 16.5% with a 17.4% record cell to a 19% average with a 19.8% record cell and increases the device stability.

The origin of the low densities of electrically active defects in Pb halide perovskite (HaP), a crucial factor for their use in photovoltaics, light emission, and radiation detection, remains a matter of discussion, in part because of the difficulty in determining these densities. Here, we present a powerful approach to assess the defect densities, based on electric field mapping in working HaP-based solar cells. The minority carrier diffusion lengths were deduced from the electric field profile, measured by electron beam-induced current (EBIC). The EBIC method was used earlier to get the first direct evidence for the n-i-p junction structure, at the heart of efficient HaP-based PV cells, and later by us and others for further HaP studies. This manuscript includes EBIC results on illuminated cell cross sections (in operando) at several light intensities to compare optoelectronic characteristics of different cells made by different groups in several laboratories. We then apply a simple, effective single-level defect model that allows deriving the densities (Nr) of the defect acting as recombination center. We find Nr ≈ 1 × 1013 cm3 for mixed A cation lead bromide-based HaP films and ∼1 × 1014 cm3 for MAPbBr3(Cl). As EBIC photocurrents are similar at the grain bulk and boundaries, we suggest that the defects are at the interfaces with selective contacts rather than in the HaP film. These results are relevant for photovoltaic devices as the EBIC responses distinguish clearly between high- and low-efficiency devices. The most efficient devices have n-i-p structures with a close-to-intrinsic HaP film, and the selective contacts then dictate the electric field strength throughout the HaP absorber.

Attempts to dope halide perovskites (HaPs) extrinsically have been mostly unsuccessful. Still, oxygen (O2) is an efficient pdopant for polycrystalline HaP films. To an extent, this doping is reversible, i.e., the films can be dedoped by decreasing the O2 partial pressure. Here results are reported, aimed at understanding the mechanism of such reversible doping, as it has been argued that doping involves interaction of oxygen with defects inside bulk HaP. These experimental results clearly point out that O2surface interactions suffice to dope the bulk of the films. Such behavior fits what is known for other polycrystalline semiconductors, where surface charge transferadducts can form and be removed. Thus, controlling the O2 partial pressure to which the HaP film is exposed, can, after proper encapsulation, achieve the desired bulk doping of the film.

Ion migration and subsequent accumulation at interfaces, driven by the built-in potential (V_bi), are intrinsic properties of halide perovskite solar cells (PVSCs), which mostly decrease the device performance. To address this issue, we constructed favorable ion accumulation in perovskite solar cells via illumination to improve the performance of the quasi-2D PVSCs. This design dramatically improves the photo-carrier collection and enables significant device performance improvement from 14.6% to 19.05%, one of the best results for quasi-2D PVSCs. We argue that the light-triggered favorable ion accumulation originates from (1) the photo-induced quasi-Fermi level splitting that compensates the V_bi, so as to avoid the ion accumulation that decreases V_bi, and (2) the light-intensity-distribution-induced uneven ion potential further drives the segregation of mobile ions towards favorable ion accumulation, decreasing any I⁻ gradient between the anode and cathode. Our work provides insight into the fundamental understanding of ion accumulation in perovskite-based optoelectronic devices and paves the way to more stable, high-performance PVSCs.

Considering the toxicity of lead ions, substituting Pb with nontoxic elements in halide perovskites, HaPs, has become one of the most significant challenges associated with these materials. Here, we report on replacing Pb with Sn and Ge, focusing on an all-inorganic HaP, CsSn x Ge1x Br3, and using a multihead spray deposition setup for thin-film formation to overcome the low solubility of the precursors and improve film coverage. We find that, in this way, we can form CsSn x Ge1x Br3 films up to high x values as homogeneous solid solutions; i.e., we obtain a range of compositions with one crystal structure (rather than clusters of two phases). The cubic structure of pure CsSnBr3 is maintained up to 77 atom % Ge, with the lattice spacing decreasing with increasing Ge concentration. The optical band gap is tunable between 1.8 and 2.5 eV, from pure Sn to pure Ge HaP. Most importantly, the perovskite structural stability increases with increasing concentration of Ge, with less oxidation of both Ge and Sn to the +4 state, which can be ascribed to less octahedral tilting and stronger bonding. Electrical and electronic transport measurements show the potential of these materials as Pb-free absorbers for solar cells, particularly, given their band gap range as the top cell of a tandem photovoltaic device.

Among many riddles posed by halide perovskites, the surprising apparent near-absence of harmful defects stands out. This is commonly explained by invoking defect tolerance (DT), but the term is used loosely, sometimes interchangeably with self-healing (SH). Also, the relation between underlying physical and chemical mechanisms and device behavior is often murky. Here, we offer our views as to what DT and SH constitute, the evidence for and against them, and what research challenges remain.

A central issue in protein electronics is how far the structural stability of the protein is preserved under the very high electrical field that it will experience once a bias voltage is applied. This question is studied on the redox protein Azurin in the solid-state Au/protein/Au junction by monitoring protein vibrations during current transport under applied bias, up to ≈1 GV m
⁻¹, by electrical detection of inelastic electron transport effects. Characteristic vibrational modes, such as C-H stretching, amide (N-H) bending, and A-S (of the bonds that connect the protein to an Au electrode), are not found to change noticeably up to 1.0 V. At >1.0 V, the N-H bending and C-H stretching inelastic features have disappeared, while the Au-S features persist till ≈2 V, i.e., the proteins remain Au bound. Three possible causes for the disappearance of the N-H and C-H inelastic features at high bias, namely, i) resonance transport, ii) metallic filament formation, and iii) bond rupture leading to structural changes in the protein are proposed and tested. The results support the last option and indicate that spectrally resolved inelastic features can serve to monitor in operando structural stability of biological macromolecules while they serve as electronic current conduit.

Direct detection of intrinsic defects in halide perovskites (HaPs) by standard methods utilizing optical excitation is quite challenging, due to the low density of defects in most samples of this family of materials (≤1015 cm-3 in polycrystalline thin films and ≤1011 cm-3 in single crystals, except melt-grown ones). While several electrical methods can detect defect densities 2 eV) HaPs. By measuring HaP layers on both hole- and electron-contact layers, as well as single crystals without contacts, we conclude that the observed deep defects are intrinsic to the Br-based HaP, and we propose a passivation route via the incorporation of a 2Dforming ligand into the precursor solution.

Ion diffusion affects the optoelectronic properties of halide-perovskites (HaPs). Until now, the fastest diffusion has been attributed to the movement of the halides, largely neglecting the contribution of protons, on the basis of computed density estimates. Here, the process of proton diffusion inside HaPs, following deuteriumhydrogen exchange and migration in MAPbI₃, MAPbBr₃, and FAPbBr₃ single crystals, is proven through D/H NMR quantification, Raman spectroscopy, and elastic recoil detection analysis, challenging the original assumption of halide-dominated diffusion. The results are confirmed by impedance spectroscopy, where MAPbBr₃- and CsPbBr₃-based solar cells respond at very different frequencies. Water plays a key role in allowing the migration of protons as deuteration is not detected in its absence. The water contribution is modeled to explain and forecast its effect as a function of its concentration in the perovskite structure. These findings are of great importance as they evidence how unexpected, water-dependent proton diffusion can be at the basis of the ≈7 orders of magnitude spread of diffusion (attributed to I⁻ and Br⁻) coefficient values, reported in the literature. The reported enhancement of the optoelectronic properties of HaP when exposed to small amounts of water may be related to the finding.

We track ultrafast charge carrier relaxation and the concurrent build-up of dynamical exciton screening on a 30-fs timescale, and probe strong electron-phonon couplings in halide perovskite crystals by temperature-dependent two-dimensional electronic spectroscopy.

We describe the growth of single crystals of the halide perovskites APbBr3, with varying ratios of Cs to methylammonium (MA) on the A site, by antisolvent vapor-assisted crystallization (AVC) and characterize the structural and compositional homogeneity of the grown crystals. We find improved thermostability for the Cs-rich mixed crystals and suggest that this is caused by a combination of locking-in of the relatively large MA in the smaller lattice of the Cs-rich material as well as by stronger hydrogen bonding between the nitrogen of MA and Br due to reduced lattice size and/or octahedral tilting with increased Cs. We also show that it is possible to grow either compositionally homogeneous crystals or core-shell, compositionally graded crystals by changing the ratio of antisolvent to solvent in the AVC method.

We show a simple modification of spin-coating to obtainpinhole-free, well-coveringanduniformfilms of an all-inorganic Pb-halide perovskite (Pb-HaP),on a flat substrate(i.e., not mesoporous)at standard temperature and pressure. In Pb-HaP-based devices the active film is generally fabricated by spin-coating. This is a challenging task for purely inorganic Pb-HaPs, as, except if nano-crystallites can be used, mostly rough and inhomogeneous thin films with poor optoelectronic quality are obtained. We describe agas flow-assistedmethod to fabricate thinconformalfilms of phase-pure CsPbBr3. First cells made with the resulting CsPbBr3-films, yield solar conversion efficiencies up to 2.5%. Electron beam-induced current measurements of device cross-sections show uniform charge generation profiles, implying >= similar to 0.2 mu m diffusion lengths. Non-encapsulated devices generate stable photocurrent for > 1 hr under continuous illumination at maximum power in ambient.

We study halide exchange in the prototypical halide perovskite, methylammonium lead trihalide, MAPbX(3) (X = halide), to test and possibly experimentally use halide diffusion in these materials. We use macroscopic single crystals to study the fundamental exchange process(es) so as to minimize possible grain boundary and surface diffusion effects. Initially, halide exchange creates normal concentration gradients of the outgoing and incoming halides, on a scale of a few microns to a few hundred microns. The depth (from the surface) of the substituted volume depends on the halide pair and on which one is exchanged and which is exchanging. The concentration gradient of the incoming halides decreases from the crystal surface toward its inner core and vice versa for the out-going halides; the profiles roughly fit diffusion in a semi-infinite specimen. This concentration gradient changes slowly with time, with the crystal becoming more homogeneous with storage time. Using the Boltzmann-Matano method and diffusion profiles from electron-dispersive spectroscopy, we evaluate the halide diffusion coefficients; these are not constant and depend on the halide couple. Although these gradients cause a lattice parameter change and may cause a symmetry change, X-ray diffraction shows that if the exchanging halides are of similar size (e.g., Br- and Cl-, Br- and I-, but not Cl- and I-), the resulting material remains single crystalline, prima facie evidence for bulk halide diffusion. These findings are valid, irrespective of which is the exchanged halide. These results suggest that for the similar-sized halide pairs the solid-state chemical exchange is topotactic such that the resulting crystal orientation is determined by that of the initial crystal. I-Cl exchange leads to loss of single crystallinity, suggesting lack of miscibility, a finding that might bear on the difficulty in finding Cl in MAPbI(3) samples grown from Cl-containing solutions.

We report on the chemical and electronic structure of cesium tin bromide (CsSiiBr(3)) and how it is impacted by the addition of 20 mol % tin fluoride (SnF2) to the precursor solution, using both surface-sensitive lab-based soft X-ray photoelectron spectroscopy (XPS) and near-surface bulk-sensitive synchrotron-based hard XPS (HAXPES). To determine the reproducibility and reliability of conclusions, several (nominally identically prepared) sample sets were investigated. The effects of deposition reproducibility, handling, and transport are found to cause significant changes in the measured properties of the films. Variations in the HAXPES-derived compositions between individual sample sets were observed, but in general, they confirm that the addition of 20 mol % SnF2 improves coverage of the titanium dioxide substrate by CsSnBr3 and decreases the oxidation of Sn-II to Sn-IV while also suppressing formation of secondary Br and Cs species. Furthermore, the (surface) composition is found to be Cs-deficient and Sn-rich compared to the nominal stoichiometry. The valence band (VB) shows a SnF2-induced redistribution of Sn 5s-derived density of states, reflecting the changing Sn-II/Sn-IV ratio. Notwithstanding some variability in the data, we conclude that SnF2 addition decreases the energy difference between the VB maximum of CsSnBr3 and the Fermi level, which we explain by defect chemistry considerations.

Extended x-ray absorption fine structure spectroscopy of the light-harvesting formamidinium lead bromide (FAPbBr(3)) perovskite, a system with attractive optoelectronic performance, shows anomalously large variance in Pb-Br bond length, some 50% larger than in its inorganic CsPbBr3 counterpart. Using first-principles molecular dynamics simulations, we find a significant contribution to this variance coming from the FA cation, and show that the FA does not just tumble in its cuboctahedral Br-12 cage, but instead stochastically sticks to, and detaches from one of the 12 nearest Br atoms after another, leading to the large variance in Pb-Br bond length. Our results demonstrate dynamic coupling between the FA-Br moiety and perovskite cage vibrations, and that tunability in dynamics can be achieved by changing the cation type and perovskite lattice parameter. Thus, our results provide information that needs to be considered in any of the intensely debated models of electron-phonon coupling in lead halide perovskites.

Solid-state electronic transport (ETp) via the electron-transfer copper protein azurin (Az) was measured in Au/Az/Au junction configurations down to 4 K, the lowest temperature for solid-state protein-based junctions. Not only does lowering the temperature help when observing fine features of electronic transport, but it also limits possible electron transport mechanisms. Practically, wire-bonded devices-on-chip, carrying Az-based microscopic junctions, were measured in liquid He, minimizing temperature gradients across the samples. Much smaller junctions, in conducting-probe atomic force microscopy measurements, served, between room temperature and the protein's denaturation temperature (similar to 323 K), to check that conductance behavior is independent of device configuration or contact nature and thus is a property of the protein itself. Temperature-independent currents were observed from similar to 320 to 4 K. The experimental results were fitted to a single-level Landauer model to extract effective energy barrier and electrode-molecule coupling strength values and to compare data sets. Our results strongly support that quantum tunneling, rather than hopping, dominates ETp via Az.

The primary sequence and secondary structure of a peptide are crucial to charge migration, not only in solution (electron transfer, ET), but also in the solid-state (electron transport, ETp). Hence, understanding the charge migration mechanisms is fundamental to the development of biomolecular devices and sensors. We report studies on four Aib-containing helical peptide analogues: two acyclic linear peptides with one and two electron-rich alkene-based side chains, respectively, and two peptides that are further rigidified into a macrocycle by a side bridge constraint, containing one or no alkene. ETp was investigated across Au/peptide/Au junctions, between 80 and 340 K in combination with the molecular dynamic (MD) simulations. The results reveal that the helical structure of the peptide and electron-rich side chain both facilitate the ETp. As temperature increases, the loss of helical structure, change of monolayer tilt angle, and increase of thermally activated fluctuations affect the conductance of peptides. Specifically, room temperature conductance across the peptide monolayers correlates well with previously observed ET rate constants, where an interplay between backbone rigidity and electron-rich side chains was revealed. Our findings provide new means to manipulate electronic transport across solid-state peptide junctions.

Halide perovskites are promising optoelectronic materials. Despite impressive device performance, especially in photovoltaics, the femtosecond dynamics of elementary optical excitations and their interactions are still debated. Here we combine ultrafast two-dimensional electronic spectroscopy (2DES) and semiconductor Bloch equations (SBEs) to probe the room-temperature dynamics of nonequilibrium excitations in CsPbBr3 crystals. Experimentally, we distinguish between excitonic and free-carrier transitions, extracting a similar to 30 meV exciton binding energy, in agreement with our SBE calculations and with recent experimental studies. The 2DES dynamics indicate remarkably short,

Although Pb Halide perovskites (HaPs) can be prepared as organic electronic materials, they resemble top-quality inorganic semiconductors, especially with respect to their low defect densities, as derived from optical and electronic transport studies. Among causes for such low defect densities were 'defecttolerance' (proposed) and 'self-healing' (experimentally identified). We show that HaPs are likely an example of a class of materials that cannot support static bulk defect densities significantly above thermodynamically-dictated densities. The reasons are (a) the free energy to form HaPs (from binary halides) is less than the formation energies of (static) defects in them and (b) the small kinetic stabilization of such defects. We summarize the evidence for such a situation and conclude that higher defect densities in polycrystalline films likely result from the (expected) smaller defect formation energy at surfaces and grain boundaries than in the bulk. This situation directly limits the options for doping such materials, and leads to the counter-intuitive conclusion that a low free energy of formation (from the binaries) can lead to self-healing and, consequently, to low densities of static defects, to be distinguished from dynamic ones. The latter can be benign in terms of (opto)electronic performance, because of their relatively short lifetimes. We propose that the conditions that we formulated can serve as search criteria for other low defect density materials, which can be of interest and beneficial, also for applications beyond optoelectronics.

Halide perovskites (HaPs) have rapidly grown as highly promising photovoltaic materials. Despite the impressive device efficiencies, however, the fundamental mechanisms governing their unusual optoelectronic properties are still unclear [1]. In particular, the nature of their elementary optical excitations, their dynamics and interactions on ultrafast timescales are currently debated. Due to the comparatively low exciton binding energy, typically

Lead bromide-based halide perovskites are of interest for wide-band-gap (>1.75 eV) absorbers for low-cost solar spectrum splitting to boost solar-to-electrical energy conversion efficiency/area by adding them to c-Si or Cu(In,Ga)Se-2 PV cells and for photoelectrochemical solar fuel synthesis. Deep in-gap electronic states in PV absorbers serve as recombination centers and are detrimental for the cell's photovoltaic performance, especially for the open-circuit voltage (V-oc). We find four different deep defect states in polycrystalline layers of mixed-cation lead tribromide from high-sensitivity modulated surface photovoltage (SPV) spectroscopy. Measurements were performed with different contact configurations, on complete solar cells and on samples before and after aging or stressing at 85 degrees C under illumination. Three of the four states, with energies of similar to 0.63, 0.73, and 1.35 eV below the conduction band edge, are assigned to intrinsic defects, whereas defect states in the middle of the band gap could be associated with (uncontrolled) impurities.

Design and modification of interfaces, always a critical issue for semiconductor devices, has become a primary tool to harness the full potential of halide perovskite (HaP)-based optoelectronics, including photovoltaics and light-emitting diodes. In particular, the outstanding improvements in HaP solar cell performance and stability can be primarily ascribed to a careful choice of the interfacial layout in the layer stack. In this review, we describe the unique challenges and opportunities of these approaches (section 1). For this purpose, we first elucidate the basic physical and chemical properties of the exposed HaP thin film and crystal surfaces, including topics such as surface termination, surface reactivity, and electronic structure (section 2). This is followed by discussing experimental results on the energetic alignment processes at the interfaces between the HaP and transport and buffer layers. This section includes understandings reached as well as commonly proposed and applied models, especially the often-questionable validity of vacuum level alignment, the importance of interface dipoles, and band bending as the result of interface formation (section 3). We follow this by elaborating on the impact of the interface formation on device performance, considering effects such as chemical reactions and surface passivation on interface energetics and stability. On the basis of these concepts, we propose a roadmap for the next steps in interfacial design for HaP semiconductors (section 4), emphasizing the importance of achieving control over the interface energetics and chemistry (i.e., reactivity) to allow predictive power for tailored interface optimization.

The presence of excess lead iodide in halide perovskites has been key for surpassing 20% photon-to-power conversion efficiency. To achieve even higher power conversion efficiencies, it is important to understand the role of remnant lead iodide in these perovskites. To that end, we explored the mechanism facilitating this effect by identifying the impact of excess lead iodide within the perovskite film on charge diffusion length, using electron-beam-induced current measurements, and on film formation properties, from grazing-incidence wide-angle X-ray scattering and high-resolution transmission electron microscopy. Based on our results, we propose that excess lead iodide in the perovskite precursors can reduce the halide vacancy concentration and lead to formation of azimuthal angle-oriented cubic α-perovskite crystals in-between 0° and 90°. We further identify a higher perovskite carrier concentration inside the nanostructured titanium dioxide layer than in the capping layer. These effects are consistent with enhanced lead iodide-rich perovskite solar cell performance and illustrate the role of lead iodide.

Junctions built from metallic nanoparticles (NPs) can circumvent the diffraction limit and combine molecular/nanoelectronics with plasmonics. However, experimental advances in plasmon-assisted electron transport at the nanoscale have been limited. We construct junctions of a robust, molecule-free, suspended film, built solely from AuNPs, capped by SiO2 shells (Au@SiO2), which give insulating tunneling gaps up to 3.6 nm between the NPs. Current measured across monolayers of such AuNPs shows ultra-long-range, plasmon-enabled electron transport (P-transport), beyond the range of normal electron tunneling across insulators. This finding challenges the present understanding of electron transport in such systems and opens possibilities for future combinations of plasmonics and nanoelectronics.

Multi-heme cytochrome c (Cytc) proteins are key for transferring electrons out of cells, to enable intracellular oxidation to proceed in the absence of O-2. In these proteins most of the hemes are arranged in a linear array suggesting a facile path for electronic conduction. To test this, we studied solvent-free electron transport across two multi-heme Cytc-type proteins: MtrF (deca-heme Cytc) and STC (tetra-heme Cytc). Transport is measured across monolayers of these proteins in a solid state configuration between Au electrodes. Both proteins showed 1000x higher conductance than single heme, or heme-free proteins, but similar conductance to monolayers of conjugated organics. Conductance is found to be temperature-independent (320-80 K), suggesting tunneling as the transport mechanism. This mechanism is consistent with I-V curves modelling, results of which could be interpreted by having protein-electrode coupling as rate limiting, rather than transport within the proteins.

Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of semiconductors, including now also Halide Perovskites (HaPs), especially transport properties. However, as yet, no evaluation of their amenability and justification for the use of the results for the above-noted purposes has been reported. To check if we can learn from pulsed measurement results about steady-state phototransport properties, we show here that, although pulsed measurements can be useful to extract information on the recombination kinetics of HaPs, great care should be taken. One issue is that no changes in the material are induced during or as a result of the excitation, and another one concerns in how far pulsed excitation-derived data can be used to find relevant steady-state parameters. To answer the latter question, we revisited pulsed excitation and propose a novel way to compare between pulsed and steady state measurements at different excitation intensities. We performed steady-state photoconductivity and ambipolar diffusion length measurements, as well as pulsed time-resolved microwave conductivity and time-resolved photoluminescence measurements as a function of excitation intensity on the same samples of different MAPbI(3) thin films, and found good quasi-quantitative agreement between the results, explaining them with a generalized single level recombination model that describes the basic physics of phototransport of HaP absorbers. Moreover, we find the first experimental manifestation of the boundaries between several effective recombination regimes that exist in HaPs, by analyzing their phototransport behavior as a function of excitation intensity. Published by AlP Publishing.

Electrochemical CO2 reduction on Cu electrode has attracted the attention of many researchers in the last decades, because of its potential to generate significant amounts of hydrocarbons at high reaction rates over sustained periods of time. As a result, substantial effort has been devoted to determining the unique catalytic performance of Cu and to elucidate the mechanism through which hydrocarbons are formed. Here we report new insights into CO2 reduction on Cu by electrochemical impedance spectroscopy (EIS) in terms of adsorption/desorption of the reduction intermediates. The potential dependence of charge transfer kinetics is discussed on the basis of EIS results. We revisit the mechanism of the formation of hydrocarbons, taking into account the pH adjacent to the electrode surface, adsorption of HCO3- and CO32-, and the role of active hydrogen. In addition to the enol-like intermediate, proposed previously, we proposed that *COOH center dot radicals, originating from the active involvement of HCO3- and/or CO32- upon reduction are key intermediates for the formation of a variety of C2 and C3 products. Thus, our results provide an additional crucial guideline for the design of future catalysts that can efficiently and selectively reduce CO2 into value-added chemicals.

Inorganic and organic lead halide perovskite materials attract great interest in the scientific community because of their potential for low-cost, high efficiency solar cells. In this report we add a new property of these materials, namely their photochemical activity in the visible light range. Both inorganic (CsPbBr3) and organic (CH3NH3PbBr3-MAPbBr(3)) perovskite thin films were demonstrated to promote photo-dissociation of adsorbed ethyl chloride (EC), employing 532 nm pulsed laser irradiation under ultra-high vacuum (UHV) conditions. From the post-irradiation temperature programmed desorption (TPD) analysis, the yield of photoproduct formation was found to be up to two orders of magnitude higher than for UV light-excited EC molecules on metallic and oxide surfaces. Photo-reactivity on top of the CsPbBr3 surface is almost an order of magnitude more efficient than on the CH3NH3PbBr3 surface, apparently due to the lower density of defect and surface states. A direct correlation was found between electron-induced luminescence and photoluminescence intensities and the photoreactivity cross-sections. We conclude that both the intense luminescence and the well-known photovoltaic properties associated with these halide perovskite materials are consistent with the efficiency of photo-reactivity in the visible range, reported here for the first time.

Metalloproteins, proteins containing a transition metal ion cofactor, are electron transfer agents that perform key functions in cells. Inspired by this fact, electron transport across these proteins has been widely studied in solid-state settings, triggering the interest in examining potential use of proteins as building blocks in bioelectronic devices. Here, we report results of low-temperature (10 K) electron transport measurements via monolayer junctions based on the blue copper protein azurin (Az), which strongly suggest quantum tunneling of electrons as the dominant charge transport mechanism. Specifically, we show that, weakening the protein-electrode coupling by introducing a spacer, one can switch the electron transport from off-resonant to resonant tunneling. This is a consequence of reducing the electrode's perturbation of the Cu(II)localized electronic state, a pattern that has not been observed before in protein-based junctions. Moreover, we identify vibronic features of the Cu(II) coordination sphere in transport characteristics that show directly the active role of the metal ion in resonance tunneling. Our results illustrate how quantum mechanical effects may dominate electron transport via protein-based junctions.

Peptide-based molecular electronic devices are promising due to the large diversity and unique electronic properties of biomolecules. These electronic properties can change considerably with peptide structure, allowing diverse design possibilities. In this work, we explore the effect of the side-chain of the peptide on its electronic properties, by using both experimental and computational tools to detect the electronic energy levels of two model peptides. The peptides include 2Ala and 2Trp as well as their 3-mercaptopropionic acid linker which is used to form monolayers on an Au surface. Specifically, we compare experimental ultraviolet photoemission spectroscopy measurements with density functional theory based computational results. By analyzing differences in frontier energy levels and molecular orbitals between peptides in gas-phase and in a monolayer on gold, we find that the electronic properties of the peptide side-chain are maintained during binding of the peptide to the gold substrate. This indicates that the energy barrier for the peptide electron transport can be tuned by the amino acid compositions, which suggests a route for structural design of peptide-based electronic devices.

Molecular monolayers at metal/semiconductor heterointerfaces affect electronic energy level alignment at the interface by modifying the interface's electrical dipole. On a free surface, the molecular dipole is usually manipulated by means of substitution at its external end. However, at an interface such outer substituents are in close proximity to the top contact, making the distinction between molecular and interfacial effects difficult. To examine how the interface dipole would be influenced by a single atom, internal to the molecule, we used a series of three molecules of identical binding and tail groups, differing only in the inner atom: aryl vinyl ether (PhO), aryl vinyl sulfide (PhS), and the corresponding molecule with a CH2 group allyl benzene (PhC). Molecular monolayers based on all three molecules have been adsorbed on a flat, oxide-free Si surface. Extensive surface characterization, supported by density functional theory calculations, revealed high-quality, well-aligned monolayers exhibiting excellent chemical and electrical passivation of the silicon substrate, in all three cases. Current voltage and capacitance voltage analysis of Hg/PhX (X = C, 0, S)/Si interfaces established that the type of internal atom has a significant effect on the Schottky barrier height at the interface, i.e., on the energy level alignment. Surprisingly, despite the formal chemical separation of the internal atom and the metallic electrode, Schottky barrier heights were not correlated to changes in the semiconductor's effective work function, deduced from Kelvin probe and ultraviolet photoemission spectroscopy on the monolayer-adsorbed Si surface. Rather, these changes correlated well with the ionization potential of the surface-adsorbed molecules. This is interpreted in terms of additional polarization at the molecule/metal interface, driven by potential equilibration considerations even in the absence of a formal chemical bond to the top Hg contact.

Halide perovskite film-based devices (e.g., solar cells and LEDs) have shown unique device performance. These films are commonly prepared from toxic solutions of metal salts (e.g., Pb²⁺ in DMF or DMSO). We describe a method to form halide perovskite films by simply reacting metal (Pb or Sn) films with alcoholic solutions of monovalent alkali metal or alkyl ammonium halides, which avoids the use of toxic Pb²⁺ solutions in the manufacturing step. We show how the morphology of the films can be controlled by variation in reaction parameters and also how mixed halide perovskite films can be prepared. A mechanism for the metal-to-perovskite conversion is suggested. We further show how electrochemically assisted conversion can allow control over the oxidation state of the metal and increase the reaction rate greatly.

Negative capacitance in photovoltaic devices has been observed and reported in several cases, but its origin, at low or intermediate frequencies, is under debate. Here we unambiguously demonstrate a direct correlation between the observation of this capacitance and a corresponding decrease in performance of a halide perovskite (HaP; CsPbBr3)-based device, expressed as reduction of open-circuit voltage and fill factor. We have prepared highly stable CsPbBr3 HaPs that do not exhibit any degradation over the duration of the impedance spectroscopy measurements, ruling out degradation as the origin of the observed phenomena. Reconstruction of current voltage curves from the impedance spectroscopy provided further evidence of the deleterious role of negative capacitance on photoconversion performance.

Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI₃). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI₃ is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI₃ (unlike MAPbBr₃) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material's relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity's hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI₃, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material's noncentrosymmetry. We note that thematerial's ferroelectric nature, can, but need not be important in a PV cell at room temperature.

We review charge transport across molecular monolayers, which is central to molecular electronics (MolEl), using large-area junctions (NmJ). We strive to provide a wide conceptual overview of three main subtopics. First, a broad introduction places NmJ in perspective to related fields of research and to single-molecule junctions (1mJ) in addition to a brief historical account. As charge transport presents an ultrasensitive probe for the electronic perfection of interfaces, in the second part ways to form both the monolayer and the contacts are described to construct reliable, defect-free interfaces. The last part is dedicated to understanding and analyses of current-voltage (I-V) traces across molecular junctions. Notwithstanding the original motivation of MolEl, I-V traces are often not very sensitive to molecular details and then provide a poor probe for chemical information. Instead, we focus on how to analyze the net electrical performance of molecular junctions, from a functional device perspective. Finally, we point to creation of a built-in electric field as a key to achieve functionality, including nonlinear current-voltage characteristics that originate in the molecules or their contacts to the electrodes. This review is complemented by a another review that covers metal-molecule-semiconductor junctions and their unique hybrid effects.

Recent studies showed that positron annihilation methods can provide key insights into the nanostructure and electronic structure of thin film solar cells. In this study, positron annihilation lifetime spectroscopy (PALS) is applied to investigate CdSe quantum dot (QD) light absorbing layers, providing evidence of positron trapping at the surfaces of the QDs. This enables one to monitor their surface composition and electronic structure. Further, 2D-Angular Correlation of Annihilation Radiation (2D-ACAR) is used to investigate the nanostructure of divacancies in photovoltaic-high-quality a-Si:H films. The collected momentum distributions were converted by Fourier transformation to the direct space representation of the electron-positron autocorrelation function. The evolution of the size of the divacancies as a function of hydrogen dilution during deposition of a-Si:H thin films was examined. Finally, we present a first positron Doppler Broadening of Annihilation Radiation (DBAR) study of the emerging class of highly efficient thin film solar cells based on perovskites.

Simple organic salts are used as a cheap alternative for hole-conducting materials in methylammonium lead bromide perovskite solar cells and obtaining power conversion efficiency of 4.4%. The findings suggest that the polar organic salts interact with the perovskite surface, leading to formation of a surface dipole or change of an existing one on the perovskites that changes its effective work function.

Solar cells based on "halide perovskites" (HaPs) have demonstrated unprecedented high power conversion efficiencies in recent years. However, the well-known toxicity of lead (Pb), which is used in the most studied cells, may affect its widespread use. We explored an all-inorganic lead-free perovskite option, cesium tin bromide (CsSnBr₃), for optoelectronic applications. CsSnBr₃-based solar cells exhibited photoconversion efficiencies (PCEs) of 2.1%, with a short-circuit current (J_SC) of ∼9 mA cm^-2, an open circuit potential (V_OC) of 0.41 V, and a fill factor (FF) of 58% under 1 sun (100 mW cm^-2) illumination, which, even though meager compared to the Pb analogue-based cells, are among the best reported until now. As reported earlier, addition of tin fluoride (SnF₂) was found to be beneficial for obtaining good device performance, possibly due to reduction of the background carrier density by neutralizing traps, possibly via filling of cation vacancies. The roles of SnF₂ on the properties of the CsSnBr₃ were investigated using ultraviolet photoemission spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) analysis.

The realization of high-quality optoelectronic properties in halide perovskite semiconductors through low-temperature, low energy processing is unprecedented. Understanding the unique aspects of the formation chemistry of these semiconductors is a critical step toward understanding the genesis of high quality material via simple preparation procedures. The toolbox of preparation procedures for halide perovskites grows rapidly. The prototypical reaction is that between lead iodide (PbI₂) and methylammonium iodide (CH₃NH₃I, abbr. MAI) to form the perovskite CH₃NH₃PbI₃ (MAPbI₃), which we discuss in this work. We investigate the conversion of small, single-crystalline PbI₂ crystallites to MAPbI₃ by two commonly used synthesis processes: reaction with MAI in solution or as a vapor. The single crystal nature of the PbI₂ precursor allows definitive conclusions to be made about the relationship between the precursors and the final product, illuminating previously unobserved aspects of the reaction process. From in situ photoluminescence microscopy, we find that the reaction in solution begins via isolated nucleation events followed by growth from the nuclei. We observe via X-ray diffraction and morphological characterization that there is a strong orientational and structural relationship between the final stage of the solution-reacted MAPbI₃ product and the initial PbI₂ crystallite. In all these measurements, we find that the reaction does not proceed below a certain MAI threshold concentration, which allows the first experimental determination of a free energy of formation for a widely used synthetic procedure of ∼0.1 eV. From these conclusions, we present a more detailed hypothesis about the reaction pathway than has yet been proposed: Our results suggest that the reaction in solution begins with a topotactic nucleation event followed by grain growth by dissolution-reconstruction. By similar techniques, we find the reaction via vapor phase produces material lacking a preferred orientation, suggesting the transformation is dominated by a deconstruction-reconstruction process due to the higher thermal energy involved. We also find that the crystal lattice structure of the vapor-reacted material is clearly different from that of the solution-phase reaction due to the temperature conditions of the synthesis.

Hysteresis in the current-voltage characteristics of hybrid organic-inorganic perovskite-based solar cells is one of the fundamental aspects of these cells that we do not understand well. One possible cause, suggested for the hysteresis, is polarization of the perovskite layer under applied voltage and illumination bias, due to ion migration within the perovskite. To study this problem systemically, current-voltage characteristics of both regular (light incident through the electron conducting contact) and so-called inverted (light incident through the hole conducting contact) perovskite cells were studied at different temperatures and scan rates. We explain our results by assuming that the effects of scan rate and temperature on hysteresis are strongly correlated to ion migration within the device, with the rate-determining step being ion migration at/across the interfaces of the perovskite layer with the contact materials. By correlating between the scan rate with the measurement temperature, we show that the inverted and regular cells operate in different hysteresis regimes, with different activation energies of 0.28 ± 0.04 eV and 0.59 ± 0.09 eV, respectively. We suggest that the differences observed between the two architectures are due to different rates of ion migration close to the interfaces, and conclude that the diffusion coefficient of migrating ions in the inverted cells is 3 orders of magnitude higher than in the regular cells, leading to different accumulation rates of ions near the interfaces. Analysis of V_OC as a function of temperature shows that the main recombination mechanism is trap-assisted (Shockley-Read Hall, SRH) in the space charge region, similar to what is the case for other thin film inorganic solar cells.

To experimentally (dis)prove ferroelectric effects on the properties of lead-halide perovskites and of solar cells, based on them, we used second-harmonic-generation spectroscopy and the periodic temperature change (Chynoweth) technique to detect the polar nature of methylammonium lead bromide (MAPbBr₃). We find that MAPbBr₃ is probably centrosymmetric and definitely non-polar; thus, it cannot be ferroelectric. Whenever pyroelectric-like signals were detected, they could be shown to be due to trapped charges, likely at the interface between the metal electrode and the MAPbBr₃ semiconductor. These results indicate that the ferroelectric effects do not affect steady-state performance of MAPbBr₃ solar cells.

ConspectusHybrid alkylammonium lead halide perovskite solar cells have, in a very few years of research, exceeded a light-to-electricity conversion efficiency of 20%, not far behind crystalline silicon cells. These perovskites do not contain any rare element, the amount of toxic lead used is very small, and the cells can be made with a low energy input. They therefore already conform to two of the three requirements for viable, commercial solar cells-efficient and cheap. The potential deal-breaker is their long-term stability. While reasonable short-term (hours) and even medium term (months) stability has been demonstrated, there is concern whether they will be stable for the two decades or more expected from commercial cells in view of the intrinsically unstable nature of these materials. In particular, they have a tendency to be sensitive to various types of irradiation, including sunlight, under certain conditions.This Account focuses on the effect of irradiation on the hybrid (and to a small degree, all-inorganic) lead halide perovskites and their solar cells. It is split up into two main sections. First, we look at the effect of electron beams on the materials. This is important, since such beams are used for characterization of both the perovskites themselves and cells made from them (electron microscopy for morphological and compositional characterization; electron beam-induced current to study cell operation mechanism; cathodoluminescence for charge carrier recombination studies). Since the perovskites are sensitive to electron beam irradiation, it is important to minimize beam damage to draw valid conclusions from such measurements. The second section treats the effect of visible and solar UV irradiation on the perovskites and their cells. As we show, there are many such effects. However, those affecting the perovskite directly need not necessarily always be detrimental to the cells, while those affecting the solar cells, which are composed of several other phases as well as the perovskite light absorber, are not always due to the perovskite itself. While we cannot yet say whether perovskite solar cells will or will not be stable over the long-term, the information in this Account should be a useful source to help achieve this goal.

Electron transport properties via a photochromic biological photoreceptor have been studied in junctions of monolayer assemblies in solid-state configurations. The photoreceptor studied was a member of the LOV domain protein family with a bound flavin chromophore, and its photochemically inactive mutant due to change of a crucial cysteine residue by a serine. The photochemical properties of the protein were maintained in dry, solid state conditions, indicating that the proteins in the junctions were assembled in native state-like conditions. Significant current magnitudes (>20 μA at 1.0 V applied bias) were observed with a mechanically deposited gold pad (area ∼0.002 cm²) as top electrode. The current magnitudes are ascribed to electrode-cofactor coupling originating from the apparent perpendicular orientation of the protein's cofactor embedded between the electrodes, and its proximity to the electrodes. Temperature independent electron transport across the protein monolayers demonstrated that solid-state electron transport is dominated by tunneling. Modulation of the observed current by illumination of the wildtype protein suggested conformation-dependent electron conduction efficiency across the solid-state protein junctions.

Halide perovskite-based solar cells still have limited reproducibility, stability, and incomplete understanding of how they work. We track electronic processes in [CH₃NH₃]PbI₃(Cl) ("perovskite") films in vacuo, and in N₂, air, and O₂, using impedance spectroscopy (IS), contact potential difference, and surface photovoltage measurements, providing direct evidence for perovskite sensitivity to the ambient environment. Two major characteristics of the perovskite IS response change with ambient environment, viz. -1- appearance of negative capacitance in vacuo or post-vacuo N₂ exposure, indicating for the first time an electrochemical process in the perovskite, and -2- orders of magnitude decrease in the film resistance upon transferring the film from O₂-rich ambient atmosphere to vacuum. The same change in ambient conditions also results in a 0.5 V decrease in the material work function. We suggest that facile adsorption of oxygen onto the film dedopes it from n-type toward intrinsic. These effects influence any material characterization, i.e., results may be ambient-dependent due to changes in the material's electrical properties and electrochemical reactivity, which can also affect material stability.

We report a combined ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) study of the electronic structure of aromatic self-assembled monolayers covalently bound to Si, using several different aromatic groups (phenyl, biphenyl, and fluorene) and binding groups (O, NH, and CH₂). We obtain excellent agreement between theory and experiment, which allows for a detailed interpretation of the experimental results. Our analysis reveals a significant effect of the binding group on state hybridization at the organic/inorganic interface. Specifically, it highlights that lone-pair electrons in the binding atom facilitate hybridization between the aromatic system and the Si substrate, resulting in a significant induced density of interface states (IDIS). These interface states are manifested as a broadened HOMO peak in the experimental UPS data and are clearly observed in a theoretical spatially-resolved density of states map. This provides means to control the degree of coupling between substrate and molecule, which may prove useful in the design of transport across organic/inorganic interfaces.

Surprisingly efficient solid-state electron transport has recently been demonstrated through "dry" proteins (with only structural, tightly bound H₂O left), suggesting proteins as promising candidates for molecular (bio)electronics. Using inelastic electron tunneling spectroscopy (IETS), we explored electron-phonon interaction in metal/protein/metal junctions, to help understand solid-state electronic transport across the redox protein azurin. To that end an oriented azurin monolayer on Au is contacted by soft Au electrodes. Characteristic vibrational modes of amide and amino acid side groups as well as of the azurin-electrode contact were observed, revealing the azurin native conformation in the junction and the critical role of side groups in the charge transport. The lack of abrupt changes in the conductance and the line shape of IETS point to far off-resonance tunneling as the dominant transport mechanism across azurin, in line with previously reported (and herein confirmed) azurin junctions. The inelastic current and hence electron-phonon interaction appear to be rather weak and comparable in magnitude with the inelastic fraction of tunneling current via alkyl chains, which may reflect the known structural rigidity of azurin.

The soft character of organic materials leads to strong coupling between molecular, nuclear and electronic dynamics. This coupling opens the way to influence charge transport in organic electronic devices by exciting molecular vibrational motions. However, despite encouraging theoretical predictions, experimental realization of such approach has remained elusive. Here we demonstrate experimentally that photoconductivity in a model organic optoelectronic device can be modulated by the selective excitation of molecular vibrations. Using an ultrafast infrared laser source to create a coherent superposition of vibrational motions in a pentacene/C₆₀ photoresistor, we observe that excitation of certain modes in the 1,500-1,700 cm^-1 region leads to photocurrent enhancement. Excited vibrations affect predominantly trapped carriers. The effect depends on the nature of the vibration and its mode-specific character can be well described by the vibrational modulation of intermolecular electronic couplings. This presents a new tool for studying electron-phonon coupling and charge dynamics in (bio)molecular materials.

Hybrid organic-inorganic lead halide perovskite photovoltaic cells have already surpassed 20% conversion efficiency in the few years that they have been seriously studied. However, many fundamental questions still remain unanswered as to why they are so good. One of these is "Is the organic cation really necessary to obtain high quality cells?" In this study, we show that an all-inorganic version of the lead bromide perovskite material works equally well as the organic one, in particular generating the high open circuit voltages that are an important feature of these cells.

The great promise of hybrid organic-inorganic lead halide perovskite (HOIP)-based solar cells is being challenged by its Pb content and its sensitivity to water. Here, the impact of rain on methylammonium lead iodide perovskite films was investigated by exposing such films to water of varying pH values, simulating exposure of the films to rain. The amount of Pb loss was determined using both gravimetric and inductively coupled plasma mass spectrometry measurements. Using our results, the extent of Pb loss to the environment, in the case of catastrophic module failure, was evaluated. Although very dependent on module siting, even total destruction of a large solar electrical power generating plant, based on HOIPs, while obviously highly undesirable, is estimated to be far from catastrophic for the environment.

A distinct odd-even effect on the electrical properties, induced by monolayers of alkyl-phenyl molecules directly bound to Si(111), is reported. Monomers of H₂C=CH-(CH₂)_n-phenyl, with n = 2-5, were adsorbed onto Si-H and formed high-quality monolayers with a binding density of 50-60% Si(111) surface atoms. Molecular dynamics simulations suggest that the binding proximity is close enough to allow efficient π-π interactions and therefore distinctly different packing and ring orientations for monomers with odd or even numbers of methylenes in their alkyl spacers. The odd-even alternation in molecular tilt was experimentally confirmed by contact angle, ellipsometry, FT-IR, and XPS with a close quantitative match to the simulation results. The orientations of both the ring plane and the long axis of the alkyl spacer are more perpendicular to the substrate plane for molecules with an even number of methylenes than for those with an odd number of methylenes. Interestingly, those with an even number conduct better than the effectively thinner monolayers of the molecules with the odd number of methylenes. We attribute this to a change in the orientation of the electron density on the aromatic rings with respect to the shortest tunneling path, which increases the barrier for electron transport through the odd monolayers. The high sensitivity of molecular charge transport to the orientation of an aromatic moiety might be relevant to better control over the electronic properties of interfaces in organic electronics.

Hybrid organic/lead halide perovskites are promising materials for solar cell fabrication, resulting in efficiencies up to 18%. The most commonly studied perovskites are CH₃NH₃PbI₃ and CH₃NH₃PbI_3-x Cl_x where x is small. Importantly, in the latter system, the presence of chloride ion source in the starting solutions used for the perovskite deposition results in a strong increase in the overall charge diff usion length. In this work we investigate the crystallization parameters relevant to fabrication of perovskite materials based on CH₃NH₃PbI₃ and CH₃NH₃PbBr₃. We find that the addition of PbCl₂ to the solutions used in the perovskite synthesis has a remarkable eff ect on the end product, because PbCl₂ nanocrystals are present during the fabrication process, acting as heterogeneous nucleation sites for the formation of perovskite crystals in solution. We base this conclusion on SEM studies, synthesis of perovskite single crystals, and on cryo-TEM imaging of the frozen mother liquid. Our studies also included the effect of different substrates and substrate temperatures on the perovskite nucleation efficiency. In view of our findings, we optimized the procedures for solar cells based on lead bromide perovskite, resulting in 5.4% efficiency and V_oc of 1.24 V, improving the performance in this class of devices. Insights gained from understanding the hybrid perovskite crystallization process can aid in rational design of the polycrystalline absorber films, leading to their enhanced performance.

Organometallic lead-halide perovskite-based solar cells now approach 18% efficiency. Introducing a mixture of bromide and iodide in the halide composition allows tuning of the optical bandgap. We prepare mixed bromide-iodide lead perovskite films CH₃NH₃Pb(I _1-xBr_x)₃ (0 ≤ x ≤ 1) by spin-coating from solution and obtain films with monotonically varying bandgaps across the full composition range. Photothermal deflection spectroscopy, photoluminescence, and X-ray diffraction show that following suitable fabrication protocols these mixed lead-halide perovskite films form a single phase. The optical absorption edge of the pure triiodide and tribromide perovskites is sharp with Urbach energies of 15 and 23 meV, respectively, and reaches a maximum of 90 meV for CH ₃NH₃PbI_1.2Br_1.8. We demonstrate a bromide-iodide lead perovskite film (CH₃NH₃PbI _1.2Br_1.8) with an optical bandgap of 1.94 eV, which is optimal for tandem cells of these materials with crystalline silicon devices.

The built-in voltage of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)-nSi hybrid solar cells is demonstrated to indicate strong inversion over most substrate donor concentrations. This implies a p-n homojunction, induced in the Si surface by the high work function of the top contact. This induced homojunction is then used to form the source and drain electrodes in a field-effect transistor.

Direct and inverse photoemission spectroscopies are used to determine materials electronic structure and energy level alignment in hybrid organic-inorganic perovskite layers grown on TiO₂. The results provide a quantitative basis for the analysis of perovskite-based solar cell performance and choice of an optimal hole-extraction layer. This journal is

Developments in organic-inorganic lead halide-based perovskite solar cells have been meteoric over the last 2 years, with small-area efficiencies surpassing 15%. We address the fundamental issue of how these cells work by applying a scanning electron microscopy-based technique to cell cross-sections. By mapping the variation in efficiency of charge separation and collection in the cross-sections, we show the presence of two prime high efficiency locations, one at/near the absorber/hole-blocking-layer, and the second at/near the absorber/electron-blocking-layer interfaces, with the former more pronounced. This 'twin-peaks' profile is characteristic of a p-i-n solar cell, with a layer of low-doped, high electronic quality semiconductor, between a p- and an n-layer. If the electron blocker is replaced by a gold contact, only a heterojunction at the absorber/hole-blocking interface remains.

Low-cost solar cells with high V_OC, relatively small (E _G - qV_OC), and high qV_OC/E_G ratio, where E_G is the absorber band gap, are long sought after, especially for use in tandem cells or other systems with spectral splitting. We report a significant improvement in CH₃NH₃PbBr₃-based cells, using CH₃NH₃PbBr_3-xCl_x, with E_G = 2.3 eV, as the absorber in a mesoporous p-i-n device configuration. By p-doping an organic hole transport material with a deep HOMO level and wide band gap to reduce recombination, the cell's V_OC increased to 1.5 V, a 0.2 V increase from our earlier results with the pristine Br analogue with an identical band gap. At the same time, in the most efficient devices, the current density increased from ∼1 to ∼4 mA/cm².

The alignment between the energy levels of the constituents of an organic solar cell plays a central role in determining the open-circuit voltage. However, tuning the energy levels of electrodes and/or active components via molecular modifiers placed at interfaces is not straightforward. The morphology of organic materials is commonly controlled by the substrate onto which they are deposited, and differences in morphology often lead to differences in energetics. Such a change in morphology may reduce the effect of surface modifications, as the modified surface is part of an interface with the organic material. Here we show, in an experimental model system, that by using binary molecular monolayers, in which dipolar molecules are buried in a protective nonpolar matrix, we can transform changes in the electrode surface dipole into interface dipole changes without significantly affecting the growth of pentacene onto the molecular layer, thus enabling the use of the full range of dipolar-induced open-circuit-voltage tuning.

We report on the passivation properties of molecularly modified, oxide-free Si(111) surfaces. The reaction of 1-alcohol with the H-passivated Si(111) surface can follow two possible paths, nucleophilic substitution (S_N) and radical chain reaction (RCR), depending on adsorption conditions. Moderate heating leads to the S_N reaction, whereas with UV irradiation RCR dominates, with S_N as a secondary path. We show that the site-sensitive S_N reaction leads to better electrical passivation, as indicated by smaller surface band bending and a longer lifetime of minority carriers. However, the surface-insensitive RCR reaction leads to more dense monolayers and, therefore, to much better chemical stability, with lasting protection of the Si surface against oxidation. Thus, our study reveals an inherent dissonance between electrical and chemical passivation. Alkoxy monolayers, formed under UV irradiation, benefit, though, from both chemical and electronic passivation because under these conditions both S_N and RCR occur. This is reflected in longer minority carrier lifetimes, lower reverse currents in the dark, and improved photovoltaic performance, over what is obtained if only one of the mechanisms operates. These results show how chemical kinetics and reaction paths impact electronic properties at the device level. It further suggests an approach for effective passivation of other semiconductors.

Integrating proteins in molecular electronic devices requires control over their solid-state electronic transport behavior. Unlike "traditional" electron transfer (ET) measurements of proteins that involve liquid environments and a redox cycle, no redox cofactor is needed for solid-state electron transport (ETp) across the protein. Here we show the fundamental difference between these two approaches by macroscopic area measurements, which allow measuring ETp temperature dependence down to cryogenic temperatures, via cytochrome C (Cyt C), an ET protein with a heme (Fe-porphyrin) prosthetic group as a redox centre. We compare the ETp to electrochemical ET measurements, and do so also for the protein without the Fe (with metal-free porphyrin) and without porphyrin. As removing the porphyrin irreversibly alters the protein's conformation, we repeat these measurements with human serum albumin (HSA), 'doped' (by non-covalent binding) with a single hemin equivalent, i.e., these natural and artificial proteins share a common prosthetic group. ETp via Cyt C and HSA-hemin are very similar in terms of current magnitude and temperature dependence, which suggests similar ETp mechanisms via these two systems, thermally activated hopping (with ∼0.1 eV activation energy) >190 K and tunneling by superexchange

Monolayers of the redox protein Cytochrome C (CytC) can be electrostatically formed on an H-terminated Si substrate, if the protein- and Si-surface are prepared so as to carry opposite charges. With such monolayers we study electron transport (ETp) via CytC, using a solid-state approach with macroscopic electrodes. We have revealed that currents via holo-CytC are almost 3 orders of magnitude higher than via the heme-depleted protein (→ apo-CytC). This large difference in currents is attributed to loss of the proteins' secondary structure upon heme removal. While removal of only the Fe ion (→ porphyrin-CytC) does not significantly change the currents via this protein at room temperature, the 30-335 K temperature dependence suggests opening of a new ETp pathway, which dominates at high temperatures (>285 K). These results suggest that the cofactor plays a major role in determining the ETp pathway(s) within CytC.

Mesoscopic solar cells, based on solution-processed organic-inorganic perovskite absorbers, are a promising avenue for converting solar to electrical energy. We used solution-processed organic-inorganic lead halide perovskite absorbers, in conjunction with organic hole conductors, to form high voltage solar cells. There is a dire need for low-cost cells of this type, to drive electrochemical reactions or as the high photon energy cell in a system with spectral splitting. These perovskite materials, although spin-coated from solution, form highly crystalline materials. Their simple synthesis, along with high chemical versatility, allows tuning their electronic and optical properties. By judicious selection of the perovskite lead halide-based absorber, matching organic hole conductor, and contacts, a cell with a ∼ 1.3 V open circuit voltage was made. While further study is needed, this achievement provides a general guideline for additional improvement of cell performance.

An eight-orders of magnitude enhancement in current across Hg/X-styrene-Si junctions is caused by merely altering a substituent, X. Interface states are passivated and, depending on X, the Si Schottky junction encompasses the full range from Ohmic to strongly rectifying. This powerful electrostatic molecular effect has immediate implications for interface band alignment and sensing.

Transition voltage spectroscopy (TVS) has become an accepted quantification tool for molecular transport characteristics, due to its simplicity and reproducibility. Alternatively, the Taylor expansion view, TyEx, of transport by tunneling suggests that conductance-voltage curves have approximately a generic parabolic shape, regardless of whether the tunneling model is derived from an average medium view (e.g., WKB) or from a scattering view (e.g., Landauer). Comparing TVS and TyEx approaches reveals that TVS is closely related to a bias-scaling factor, V₀, which is directly derived from the third coefficient of TyEx, namely, the second derivative of the conductance with respect to bias at 0 V. This interpretation of TVS leads to simple expressions that can be compared easily across primarily different tunneling models. Because the basic curve shape is mostly generic, the quality of model fitting is not informative on the actual tunneling model. However internal correlation between the conductance near 0 V and V₀ (TVS) provides genuine indication on fundamental tunneling features. Furthermore, we show that the prevailing concept that V₀ is proportional to the barrier height holds only in the case of resonant tunneling, while for off-resonant or deep tunneling, V₀ is proportional to the ratio of barrier height to barrier width. Finally, considering TVS as a measure of conductance nonlinearity, rather than as an indicator for energy level spectroscopy, explains the very low TVS values observed with a semiconducting (instead of metal) electrode, where transport is highly nonlinear due to the relatively small, bias-dependent density of states of the semiconducting electrode.

Organic monolayers derived from ω-fluoro-1-alkynes of varying carbon chain lengths (C₁₀-C₁₈) were prepared on Si(111) surfaces, resulting in changes of the physical and electronic properties of the surface. Analysis of the monolayers using XPS, Infrared Reflection Absorption Spectroscopy, ellipsometry and static water contact angle measurements provided information regarding the monolayer thickness, the tilt angle, and the surface coverage. Additionally, PCFF molecular mechanics studies were used to obtain information on the optimal packing density and the layer thickness, which were compared to the experimentally found data. From the results, it can be concluded that the monolayers derived from longer chain lengths are more ordered, possess a lower tilt angle, and have a higher surface coverage than monolayers derived from shorter chains. We also demonstrate that by substitution of an H by F atom in the terminal group, it is possible to controllably modify the surface potential and energy barrier for charge transport in a full metal/monolayer- semiconductor (MOMS) junction.

Keywords: Physics, Applied; Physics, Condensed Matter

Solid-state electron transport (ETp) via a monolayer of immobilized azurin (Az) was examined by conducting probe atomic force microscopy (CP-AFM), as a function of both temperature (248-373K) and applied tip force (6-15 nN). At low forces, ETp via holo-Az (with Cu²⁺) is temperature-independent, but thermally activated via the Cu-depleted form of Az, apo-Az. While this observation agrees with those of macroscopic-scale measurements, we find that for holo-Az the mechanism of ETp at high temperatures changes upon an increase in the force applied by the tip to the proteins; namely, above 310 K and forces >6 nN ETp becomes thermally activated. This is in contrast to apo-Az, where increasing applied force causes only small monotonic increases in currents due to decreased electrode separation. The distinct ETp temperature dependence of holo- and apo-Az is assigned to a difference in structural response to pressure between the two protein forms. An important implication of these CP-AFM results (of measurements over a significant temperature range) is that for reliable ETp measurements on flexible macromolecules, such as proteins, the pressure applied during the measurements should be controlled or at least monitored.

Despite the rapid increase in solar cell manufacturing capacity (∼50 GW _p in 2011), maintaining this continued expansion will require resolving some major fabrication issues. Crystalline Si, the most common type of cell, requires a large energy input in the manufacturing process, which results in an energy payback time of years. CdTe/CdS thin film cells, which have captured around 10% of the global market, may not be sustainable for very large-scale use because of limited Te availability. Thus, research in this field is emphasizing cells that are energy efficient and inexpensive and use readily available materials. The extremely thin absorber (ETA) cell, the subject of this Account, is one of these new generation cells. Since the active light absorber in an ETA cell is no more than tens of nanometers thick, the direct recombination of photogenerated electrons and holes in the absorber should not compete as much with charge removal in the form of photocurrent as in thicker absorber materials. As a result, researchers expect that poorer quality semiconductors can be used in an ETA cell, which would expand the choice of semiconductors over those currently in use.We first describe the ETA cell, comparing and contrasting it to the dye-sensitized cell (DSC) from which it developed and describing its potential advantages and disadvantages. We then explain the mechanism(s) of operation of the ETA cell, which remain controversial: different ETA cells most likely operate by different mechanisms, particularly in their photovoltage generation. We then present a general description of how we prepare ETA cells in our laboratory, emphasizing solution methods to form the various layers and solution treatments of these layers to minimize manufacturing costs. This is followed by a more specific discussion of the various layers and treatments used to make and complete a cell with emphasis on solution treatments that are important in optimizing cell performance and explaining the possible modes of action of each of these treatments. Finally, we show how ETA cells have improved over the years, their present efficiencies, our expectations for the future, and the challenges that we foresee to fulfill these expectations.

Electrical transport studies across nm-thick dielectric films can be complicated, and datasets compromised, by local electrical breakdown enhanced by nm-sized features. To avoid this problem we need to know the minimal voltage that causes the enhanced electrical breakdown, a task that usually requires numerous measurements and simulation of which is not trivial. Here we describe and use a model system, using a "floating" gold pad to contact Au nanoparticles, NPs, to simultaneously measure numerous junctions with high aspect ratio NP contacts, with a dielectric film, thus revealing the lowest electrical breakdown voltage of a specific dielectric-nanocontact combination. For a 48 ± 1.5 Å SiO ₂ layer and a ∼7 Å monolayer of organic molecules (to link the Au NPs) we show how the breakdown voltage decreases from 4.5 ± 0.4 V for a flat contact, to 2.4 ± 0.4 V if 5 nm Au NPs are introduced on the surface. The fact that larger Au NPs on the surface do not necessarily result in significantly higher breakdown voltages illustrates the need for combining experiments with model calculations. This combination shows two opposite effects of increasing the particle size, i.e., increase in defect density in the insulator and decrease in electric field strength. Understanding the process then explains why these systems are vulnerable to electrical breakdown as a result of spikes in regular electrical grids. Finally we use XPS-based chemically resolved electrical measurements to confirm that breakdown occurs indeed right below the nm-sized features.

We report near-perfect transfer of the electrical properties of oxide-free Si surface, modified by a molecular monolayer, to the interface of a junction made with that modified Si surface. Such behavior is highly unusual for a covalent, narrow bandgap semiconductor, such as Si. Short, ambient atmosphere, room temperature treatment of oxide-free Si(100) in hydroquinone (HQ)/alkyl alcohol solutions, fully passivates the Si surface, while allowing controlled change of the resulting surface potential. The junctions formed, upon contacting such surfaces with Hg, a metal that does not chemically interact with Si, follow the Schottky-Mott model for metal-semiconductor junctions closer than ever for Si-based junctions. Two examples of such ideal behavior are demonstrated: a) Tuning the molecular surface dipole over 400 mV, with only negligible band bending, by changing the alkyl chain length. Because of the excellent passivation this yields junctions with Hg with barrier heights that follow the change in the Si effective electron affinity nearly ideally. b) HQ/methanol passivation of Si is accompanied by a large surface dipole, which suffices, as interface dipole, to drive the Si into strong inversion as shown experimentally via its photovoltaic effect. With only ∼0.3 nm molecular interlayer between the metal and the Si, our results proves that it is passivation and prevention of metal-semiconductor interactions that allow ideal metal-semiconductor junction behavior, rather than an insulating transport barrier.

Solar cells based on crystalline semiconductors such as Si and GaAs provide nowadays the highest performance, but photovoltaic (PV) cells based on less pure materials, such as poly- or nano-crystalline or amorphous inorganic or organic materials, or a combination of these, should relax production requirements and lower the cost towards reliable, sustainable and economic electrical power from sunlight. So as to be able to compare the operation of different classes of solar cells we first summarize general photovoltaic principles and then consider implications of using less than ideal materials. In general, lower material purity means more disorder, which introduces a broad distribution of energy states of the electronic carriers that affects all the aspects of PV performance, from light absorption to the generation of voltage and current. Specifically, disorder penalizes energy output by enhanced recombination, with respect to the radiative limit, and also imposes a lowering of quasi-Fermi levels into the gap, which decreases their separation, i.e., reduces the photovoltage. In solar cells based on organic absorbers, such as dye-sensitized or bulk heterojunction solar cells, vibronic effects cause relaxation of carriers in the absorber, which implies an energy price in terms of obtainable output.

Freezing out of molecular motion and increased molecular tilt enhance the efficiency of electron transport through alkyl chain monolayers that are directly chemically bound to oxide-free Si. As a result, the current across such monolayers increases as the temperature decreases from room temperature to ∼80 K, i.e., opposite to thermally activated transport such as hopping or semiconductor transport. The 30-fold change for transport through an 18-carbon long alkyl monolayer is several times the resistance change for actual metals over this range. FTIR vibrational spectroscopic measurements indicate that cooling increases the packing density and reduces the motional freedom of the alkyl chains by first stretching the chains and then gradually tilting the adsorbed molecules away from the surface normal. Ultraviolet photoelectron spectroscopy shows drastic sharpening of the valence band structure as the temperature decreases, which we ascribe to decreased electron-phonon coupling. Although conformational changes are typical in soft molecular systems, in molecular electronics they are rarely observed experimentally or considered theoretically. Our findings, though, indicate that the molecular conformational changes are a prominent feature, which imply behavior that differs qualitatively from that described by models of electronic transport through inorganic mesoscopic solids.

We compare the charge transport characteristics of heavy-doped p ⁺⁺- and n ⁺⁺-Si-alkyl chain/Hg junctions. Based on negative differential resistance in an analogous semiconductor-inorganic insulator/metal junction we suggest that for both p ⁺⁺- and n ⁺⁺-type junctions, the energy difference between the Fermi level and lowest unoccupied molecular orbital (LUMO), i.e., electron tunneling, controls charge transport. This conclusion is supported by results from photoelectron spectroscopy (ultraviolet photoemission spectroscopy, inverse photoelectron spectroscopy, and x-ray photoemission spectroscopy) for the molecule-Si band alignment at equilibrium, which clearly indicate that the energy difference between the Fermi level and the LUMO is much smaller than that between the Fermi level and the highest occupied molecular orbital (HOMO). Furthermore, the experimentally determined Fermi level - LUMO energy difference, agrees with the non-resonant tunneling barrier height, deduced from the exponential length attenuation of the current.

Despite many recent research efforts, the influence of grain boundaries (GBs) on device properties of CuIn_1-xGa_xSe₂ solar cells is still not fully understood Here, we present a microscopic approach to characterizing GBs in polycrystalline CuIn_1-xGa _xSe₂ films with x = 0.33. On samples from the same deposition process we applied methods giving complementary information, i.e., electron backscatter diffraction (EBSD), electron-beam induced current measurements (EBIC), conductive atomic force microscopy (c-AFM), variable-temperature Kelvin probe force microscopy (KPFM), and scanning capacitance microscopy (SCM). By combining EBIC with EBSD, we find a decrease in charge-carrier collection for non-σ3 GBs, while σ 3 GBs exhibit no variation with respect to grain interiors. In contrast, a higher conductance of GBs compared to grain interiors was found by c-AFM at low bias and under illumination. By KPFM, we directly measured the band bending at GBs, finding a variation from - 80 up to + 115 mV. Depletion and even inversion at GBs was confirmed by SCM. We comparatively discuss the apparent differences between the results obtained by various microscopic techniques.

Understanding how quantum dot (QD)-sensitized solar cells operate requires accurate determination of the offset between the lowest-unoccupied molecular orbital (LUMO) of the sensitizer quantum dot and the conduction band of the metal oxide electrode. We present detailed optical spectroscopy, low-energy photoelectron spectroscopy, and two-photon photoemission studies of the energetics of size-selected CdSe colloidal QDs deposited on TiO₂ electrodes. Our experimental findings show that in contrast to the prediction of simplified models based on bulk band offsets and effective mass considerations, band alignment in this system is strongly modified by the interaction between the QDs and the electrode. In particular, we find relatively small conduction band- LUMO offsets, and near "pinning" of the QD LUMO relative to the conduction band of the TiO₂ electrode, which is explained by the strongQD-electrode interaction. That interaction is the origin for the highly efficientQDto electrode charge transfer, and it also bears on the possibility of hot carrier injection in these types of cells.

Focus During the last decade the direct conversion of solar energy to electricity by photovoltaic cells has emerged from a pilot technology to one that produced 11 GW_p of electricity generating capacity in 2009. With production growing at 50%70% a year (at least until 2009) photovoltaics (PV) is becoming an important contributor to the next generation of renewable green power production. The question is that of how we can move to the terawatt (TW) scale [1]. Synopsis The rapid evolution of PV as an alternative means of energy generation is bringing it closer to the point where it can make a significant contribution to challenges posed by the rapid growth of worldwide energy demand and the associated environmental issues. Together with the main existing technology, which is based on silicon (Si), the growth of the field is intertwined with the development of new materials and fabrication approaches. The PV industry, which was, until recently, based primarily on crystalline, polycrystalline, and amorphous Si, grew at an average annual rate of 50% during 20002010. This rate was increasing, at least until the 2008 economic crisis, with production of ~11 gigawatts (GW_p) per year in 2009 [2]. While this may seem a very large number, PV installations in total are still supplying only

Protein structures can facilitate long-range electron transfer in solution. But a fundamental question remains: can these structures also serve as solid-state electronic conductors? Answering this question requires methods for studying conductivity of the "dry" protein (which only contains tightly bound structured water molecules) sandwiched between two electronic conductors in a solid-state type configuration. If successful, such systems could serve as the basis for future, bioinspired electronic device technology. In this Account, we survey, analyze, and compare macroscopic and nanoscopic (scanning probe) solid-state conductivities of proteins, noting the inherent constraints of each of these, and provide the first status report on this research area. This analysis shows convincing evidence that "dry" proteins pass orders of magnitude higher currents than saturated molecules with comparable thickness and that proteins with known electrical activity show electronic conductivity, nearly comparable to that of conjugated molecules ("wires"). These findings suggest that the structural features of proteins must have elements that facilitate electronic conductivity, even if they do not have a known electron transfer function. As a result, proteins could serve not only as sensing, polar,or photoactive elements in devices (such as field-effect transistor configurations) but also as electronic conductors. Current knowledge of peptide synthesis and protein modification paves the way toward a greater understanding of how changes in a protein's structure affect its conductivity. Such an approach could minimize the need for biochemical cascades in systems such as enzyme-based circuits, which transduce the protein's response to electronic current. In addition, as precision and sensitivity of solid-state measurements increase, and as knowledge of the structure and function of "dry" proteins grows, electronic conductivity may become an additional approach to study electron transfer in proteins and solvent effects without the introduction of donor or acceptor moieties. We are particularly interested in whether evolution might have prompted the electronic carrier transport capabilities of proteins for which no electrically active function is known in their native biological environment and anticipate that further research may help address this fascinating question.

One of the major problems in molecular electronics is how to make electronically conducting contact to the "soft" organic and biomolecules without altering the molecules. As a result, only a small number of metals can be applied, mostly by special deposition methods with severe limitations. Transferring a predefined thin metal leaf onto a molecular layer provides a nondestructive, noninvasive contacting method that is, in principle, applicable to many types of metal and a variety of metal/molecules combinations. Here we report a modification of our earlier lift-off, float-on (LOFO) method, using as a basis its offspring, the polymer-assisted lift-off (PALO) method, where a backing polymer enables simultaneous deposition of multiple contacts as well as reduces wrinkles in the thin metal leaf. The modified PALO (MoPALO) method, reported here, adds lithography steps to obviate the need to punch through the polymer, as is done to complete PALO contacts. Morphological characterization of the electrodes indicates highly uniform, wrinkle-free contacts of negligible roughness. The good electrical performance of the MoPALO contacts was proven with metal/organic-monolayer/semiconductor (MOMS) junctions, which are known to be very sensitive to molecular degradation and metal penetration. We also show how MoPALO contacts enabled us to compare the effect of varying the metal work function and contact area on the current-voltage characteristics of MOMS devices.

Electron transfer (ET) through proteins, a fundamental element of many biochemical reactions, is studied Intensively in aqueous solutions. Over the past decade, attempts were made to Integrate proteins into solid-state Junctions in order to study their electronic conductance properties. Most such studies to date were conducted with one or very few molecules In the Junction, using scanning probe techniques. Here we present the high-yield, reproducible preparation of large-area monolayer Junctions, assembled on a Si platform, of proteins of three different families: azurln (Az), a blue-copper ET protein, bacterlorhodopsln (bR), a membrane proteln-chromophore complex with a proton pumping function, and bovine serum albumin (BSA). We achieve highly reproducible electrical current measurements with these three types of monolayers using appropriate top electrodes. Notably, the current-voltage (i-V) measurements on such Junctions show relatively minor differences between Az and bR, even though the latter lacks any known ET function. Electron Transport (ETp) across both Az and bR is much more efficient than across BSA, but even for the latter the measured currents are higher than those through a monolayer of organic, C18 alkyl chains that is about half as wide, therefore suggesting transport mechanlsm(s) different from the often considered coherent mechanism. Our results show that the employed proteins maintain their conformation under these conditions. The relatively efficient ETp through these proteins opens up possibilities for using such blomolecules as current-carrying elements in solid-state electronic devices.

Basic scientific interest in using a semiconducting electrode in moleculebased electronics arises from the rich electrostatic landscape presented by semiconductor interfaces. Technological interest rests on the promise that combining existing semiconductor (primarily Si) electronics with (mostly organic) molecules will result in a whole that is larger than the sum of its parts. Such a hybrid approach appears presently particularly relevant for sensors and photovoltaics. Semiconductors, especially Si, present an important experimental test-bed for assessing electronic transport behavior of molecules, because they allow varying the critical interface energetics without, to a first approximation, altering the interfacial chemistry. To investigate semiconductor-molecule electronics we need reproducible, high-yield preparations of samples that allow reliable and reproducible data collection. Only in that way can we explore how the molecule/electrode interfaces affect or even dictate charge transport, which may then provide a basis for models with predictive power. To consider these issues and questions we will, in this Progress Report, o review junctions based on direct bonding of molecules to oxide-fřee Si. o describe the possible charge transport mechanisms across such interfaces and evaluate in how far they can be quantified, o investigate to what extent imperfections in the monolayer are important for transport across the monolayer, o revisit the concept of energy levels in such hybrid systems.

Bromine-terminated alkyl-chain monolayers, bound to oxide-free Si substrates, were prepared by self-assembly. Infrared spectroscopy and atomic force microscopy imply that monolayer packing density improves after hydrolysis, despite an increase in the presence of oxide. The probable reason is that OH-mediated intermolecular H-bonding along the monolayer emerges after hydrolysis and rearranges the molecular components of the insulating layer. Current-voltage and differential capacitance measurements show that also the interfacial electronic properties of these junctions are changed by hydrolysis of the Br groups. This is expressed in an increased effective Schottky barrier height and a decreased junction ideality factor. We correlate the proposed structural changes of the monolayer with the change in the interfacial electronic properties, with the help of the inhomogeneous Schottky barrier height model. The role of oxide in the charge transport through the monolayer is discussed, as well.

We report on electronic transport measurements through dense monolayers of CH ₃(CH ₂) _nPO ₃H ₂ molecules of varying chain lengths, with a strong and stable bond through the phosphonic acid end group to a GaAs surface and a Hg top contact. The monolayers maintain their high quality during and after the electrical measurements. Analyses of the electronic transport measurements of junctions, and of UV and inverse photoemission spectroscopy data on band alignments of free surfaces, yield insight about the electrical transport mechanism. Transport characteristics for n-GaAs junctions at low forward bias are identical for different chain lengths, a strong indication of high-quality monolayers. Tunneling barrier and carrier effective mass values for n- and p-GaAs samples were deduced from the transport data. In this way we find a tunneling barrier for n-GaAs of 1.3 eV, while UPS data for the lowest unoccupied system orbital (LUSO) point to a 2.4 eV barrier. This discrepancy can be understood by invoking states, closer to the Fermi level than the LUSO state, that contribute to charge transport. Such states lead to a manifold of transitions, each having a different probability, both because of differences in the tunnel barrier and because of differences in density of these interface-induced states; i.e., the single barrier, deduced from J-V measurements, is an effective value only.

A low-cost dichroic mirror can be used successfully for solar spectrum splitting to enhance solar to electrical energy conversion. The mirror is optimized for use with a polycrystalline silicon photovoltaic cell (pc-Si). With the dichroic mirror simultaneous excitation of a medium-efficient (11.1%) commercial pc-Si and a custom-made high band gap GaInP cell (12.3%), yields 16.8% efficiency, with both cells operating at maximum power. Our results clearly show that what is missing for this simple low-cost enhancement of Si solar cell efficiency are low-cost high band gap cells. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3081510]

After some definitions to establish common ground and illustrate the issues in terms of orders of magnitude, we note that meeting the Energy challenge will require suitable materials. Luckily, we can count on the availability of natural resources for most materials. We briefly illustrate the connection between materials and energy and review the past and the present situations, to focus on the future. We wrap up by arguing that more than bare economics is required to use the fruits of science and technology towards a world order, built on sustainable energy (and materials) resources.

Because conventional photovoltaic (PV) cells are threshold systems in terms of optical absorption, "photon management "is an obvious way to improve their performance. Calculations to optimize photon utilization in a single-junction PV cell show -1.4 eV to be the optimal bandgap for terrestrial solar to electrical power conversion. For Si, with a slightly sub-optimal gap, continuous efforts have yielded single-junction laboratory cells, quite close to the theoretical limit. One of the repeatedly proposed directions to improve photon management is that of up-and down-conversion of photon energy. In up-conversion two photons with energy hv G (the band gap) create one photon with hv > E_G, while in down-conversion one photon with energy hv > 2E_G, yields two photons with energy hv > E_G. Multi-exciton generation (MEG), although not a "photon management" process, can achieve effects like down-conversion, which, though, is more limited than MEG. In MEG one photon with energy hv > HEG yields n electron-hole pairs with energy E_G. Because MEG has clear advantages over down-conversion, in the following we will, instead of considering both, consider MEG. We find that a straightforward analysis of this approach to "photon management" for a single junction cell under the detailed balance limit shows clearly that, even if we assume (highly unrealistic) 100% efficient up-conversion and MEG, a new theoretical PV conversion limit of 49 %, instead of 31% is arrived at, a maximum possible gain of =60%. The main attractive feature of the combination of up-conversion and MEG is a significant broadening of the optimal band-gap range. Rough estimates for the very highest possibly feasible efficiencies for up-conversion and MEG (25% and 70% respectively), yield at most slightly less than 40% PV conversion efficiency, i.e., only a -25% gain over conventional single band gap semiconductor.

The preparation of alkoxy monolayers on oxide-free Si (100) and on electronic current transport measurements through junctions that are made up of these monolayers, sandwiched between Si as one electrode and a Hg drop as the other electrode, was reported. Based on the polarized IR spectra, the average tilt angles of the alkoxy and alkyl chains with respect to the surface normal are found to be very similar at 28° and 27° respectively. The monolayer thickness value for the OC12 layer, calculated using an inelastic mean free path of 3.3 nm, is 15∓2Å. The atomic force microscopy (AFM) and contact angle measurements indicate that neither binding chemistry nor molecular length alter the monlayer density. The X-ray photoelectron spectroscopy (XPS) binding energy of the C atom closets to the Si is lower than that of the C atom of the alkyl backbone.

The interfacing of functional proteins with solid supports and the study of related protein-adsorption behavior are promising and important for potential device applications. In this study, we describe the preparation of bacteriorhodopsin (bR) monolayers on Br-terminated solid supports through covalent attachment. The bonding, by chemical reaction of the exposed free amine groups of bR with the pendant Br group of the chemically modified solid surface, was confirmed both by negative AFM results obtained when acetylated bR (instead of native bR) was used as a control and by weak bands observed at around 1610 cm^-1 in the FTIR spectrum. The coverage of the resultant bR monolayer was significantly increased by changing the pH of the purple-membrane suspension from 9.2 to 6.8. Although bR, which is an exceptionally stable protein, showed a pronounced loss of its photoactivity in these bR monolayers, it retained full photoactivity after covalent binding to Br-terminated alkyls in solution. Several characterization methods, including atomic force microscopy (AFM), contact potential difference (CPD) measurements, and UV/Vis and Fourier transform infrared (FTIR) spectroscopy, verified that these bR monolayers behaved significantly different from native bR. Current-voltage (I-V) measurements (and optical absorption spectroscopy) suggest that the retinal chromophore is probably still present in the protein, whereas the UV/Vis spectrum suggests that it lacks the characteristic covalent protonated Schiff base linkage. This finding sheds light on the unique interactions of biomolecules with solid surfaces and may be significant for the design of protein-containing device structures.

Molecular electronics is very much about contacts, and thus understanding of any generic contact effect is essential to its advance. For example, it is still not obvious in how far variations in electrode roughness of macroscopic contacts can lead to rectification. Here we report an investigation of this contact effect on electronic transport properties using metal-insulator-metal planar junctions with a 5 nm thick bacteriorhodopsin-based insulator as model system. We demonstrate that the experimentally observed rectifying behavior is not an intrinsic property of the molecules used, but rather of the local contact quality. Even a slight increase in surface roughness of the bottom electrode gives rise to distinct rectifying behavior in these and, by extrapolation, possibly other molecular junctions.

Temperature-dependent transport measurements through alkyl chain monolayers that are directly chemically bound to Si, show that the currents decrease as the temperature increases. We relate this temperature dependence primarily to a gradual un-tilting of the adsorbed molecules, which leads to increasing of the film thickness, resulting in a wider tunnel barrier. Following that, we conclude that a significant part of transport through these alkyl chain monolayers occurs "through space". The experimental finding and its interpretation result from the high reproducibility and accuracy of the transport results for the semiconductor/alkyl chain/metal junctions that we study.

The deficit of top scientists pursuing alternative energy (AE)-oriented research is a major problem in meeting the global energy challenge. This paucity has the potential to self-proliferate, as a limited core of experienced researchers will encourage a limited group of talented students and post-docs to seek research opportunities in AE research. To discontinue this pattern, a selected group of 24 of the nation's top, final-year PhD students from all areas of physical, life, and engineering sciences, assembled in a Galilee mountain location, along with 20 senior Israeli and foreign scientists, who are all experts in various aspects of AE. The meeting also included seminars that made students aware of, and knowledgeable about, the energy challenges and related worldwide scientific research. The seminar also allowed them to form their own opinions about directions for fruitful avenues of research for possible new approaches to AE.

A reliable and reproducible method for preparing bacteriorhodopsin (bR)-containing metal-biomolecule-monolayer-metal planar junctions via vesicle fusion tactics and soft deposition of Au top electrodes is reported. Optimum monolayer and junction preparations, including contact effects, are discussed. The electron-transport characteristics of bR-containing membranes are studied systematically by incorporating native bR or artificial bR pigments derived from synthetic retinal analogues, into single solid-supported lipid bilayers. Current-voltage (I-V) measurements at ambient conditions show that a single layer of such bR-containing artificial lipid bilayers pass current in solid electrode/bilayer/solid electrode structures. The current is passed only if retinal or its analogue is present in the protein. Furthermore, the preparations show photoconductivity as long as the retinal can isomerize following light absorption. Optical characterization suggests that the junction photocurrents might be associated with a photochemically induced M-like intermediate of bR. I-V measurements along with theoretical estimates reveal that electron transfer through the protein is over four orders of magnitude more efficient than what would be estimated for direct tunneling through 5 nm of water-free peptides. Our results furthermore suggest that the light-driven proton-pumping activity of the sandwiched solid-state bR monolayer contributes negligibly to the steady-state light currents that are observed, and that the orientation of bR does not significantly affect the observed I-V characteristics.

We show that electron beam evaporation of metal onto a monolayer of organic molecules can yield reproducible electrical contacts, if evaporation is indirect and the sample is on a cooled substrate. The metal contact forms without damaging even the molecules' outermost groups. In contrast, direct evaporation seriously damages the molecules. By comparing molecular effects on metal/molecular layer/GaAs junctions, prepared by indirect evaporation and by other soft contacting methods, we confirm experimentally that Au is not an optimal choice as an evaporated contact metal. We ascribe this to the ease by which Au can diffuse between molecules, something that can, apart from direct contact-substrate connections, lead to undesired and uncontrollable interfacial interactions. Such phenomena are largely absent with Pd as evaporated contact.

Local electrical transport measurements with scanning probe microscopy on polycrystalline (PX) p-CuInSe2 and p-Cu(In,Ga)Se-2 films show that the photovoltaic and dark currents for bias voltages smaller than 1 V flow mainly through grain boundaries (GBs), indicating inversion at the GBs. Photocurrent for higher bias flows mainly via the grains. Based on these results and our finding of similar to 100 meV GB band bending we deduce the potential landscape around the GBs. We suggest that high grain material quality, leading to large carrier mobilities, and electron-hole separation at the GBs, by chemical and electrical potential gradients, result in the high performance of these PX solar cells. (c) 2006 Elsevier B.V. All rights reserved.

A dipole-layer approach is adapted to describe the electrostatic potential and electronic transport through metal/semiconductor junctions with a discontinuous monolayer of polar molecules at the metal/semiconductor interface. The effective barrier height of those junctions, which have small pinholes, embedded in a molecular layer, which introduces a negative {positive} dipole (i.e., a dipole whose negative {positive} pole is the one that is closest to the semiconductor surface) on an n-type {p-type} semiconductor, is often "tunable" by the magnitude and density of the dipoles. If the lateral dimensions of a molecule-free pinhole at the interface exceed the semiconductor depletion width, carrier transport is not influenced by the molecular layer and the "effective" barrier height is the nominal metal/semiconductor barrier height. If the molecular layer introduces a positive {negative} dipole on an n-type {p-type} semiconductor, enhanced field emission at edges of small pinholes might lead to a leakage- and/or an edge-current component resulting in an effective barrier height lower than the nominal one. We support these conclusions by direct measurements of the nm-scale electronic behaviour of a Au/n-GaAs diode with a discontinuous monolayer of dicarboxylic acids at the interface, using Ballistic Electron Emission Microscopy (BEEM).

Coupling of noble-metal nanoparticles (NPs) into electronics might lead to interesting new avenues in nanoelectronics. It was found that currents through metal//organic (or inorganic) insulator/Au NP monolayer//metal planar junctions (2) are orders of magnitude higher than what is obtained without the NP monolayer (1). (Graph Presented).

CdSe is homogeneously deposited into nanoporous TiO₂ films and used in liquid junction photoelectrochemical solar cells. The effect of the deposition parameters on the cell are studied, in particular differences between ion-by-ion and cluster deposition mechanisms. CdSe deposition on a Cd-rich CdS film that was deposited first into the TiO₂ film, or selenization of the Cd-rich CdS layer with selenosulphate solution improves the cell parameters. Photocurrent spectral response measurements indicate photocurrent losses due to poor collection efficiencies, as shown by the strong spectral dependence on illumination intensity. Cell efficiencies up to 2.8% under solar conditions have been obtained.

Studying electron transport (ET) through proteins is hampered by achieving reproducible experimental configurations, particularly electronic contacts to the proteins. The transmembrane protein bacteriorhodopsin (bR), a natural light-activated proton pump in purple membranes of Halobacterium salinarum, is well studied for biomolecular electronics because of its sturdiness over a wide range of conditions. To date, related studies of dry bR systems focused on photovoltage generation and photoconduction with multilayers, rather than on the ET ability of bR, which is understandable because ET across 5-nm-thick, apparently insulating membranes is not obvious. Here we show that electronic current passes through bR-containing artificial lipid bilayers in solid "electrode-bilayer-electrode" structures and that the current through the protein is more than four orders of magnitude higher than would be estimated for direct tunneling through 5-nm, water-free peptides. We find that ET occurs only if retinal or a close analogue is present in the protein. As long as the retinal can isomerize after light absorption, there is a photo-ET effect. The contribution of light-driven proton pumping to the steady-state photocurrents is negligible. Possible implications in view of the suggested early evolutionary origin of halobacteria are noted.

The superior performance of certain polycrystalline (PX) solar cells compared to that of corresponding single-crystal ones has been an enigma until recently. Conventional knowledge predicted that grain boundaries serve as traps and recombination centers for the photogenerated carriers, which should decrease cell performance. To understand if cell performance is limited by grain bulk, grain surface, and/or grain boundaries (GBs), we performed high-resolution mapping of electronic properties of single GBs and grain surfaces in PX p-CdTe/n-CdS solar cells. Combining results from scanning electron and scanning probe microscopies, viz., capacitance, Kelvin probe, and conductive probe atomic force microscopies, and comparing images taken under varying conditions, allowed elimination of topography-related artifacts and verification of the measured properties. Our experimental results led to several interesting conclusions: 1) current is depleted near GBs, while photocurrents are enhanced along the GB cores; 2) GB cores are inverted, which explains GB core conduction. Conclusions (1) and (2) imply that the regions around the GBs function as an extension of the carrier-collection volume, i.e., they participate actively in the photovoltaic conversion process, while conclusion (2) implies minimal recombination at the GB cores; 3) the surface potential is diminished near the GBs; and 4) the photovoltaic and metallurgical junction in the n-CdS/p-CdTe devices coincide. These conclusions, taken together with gettering of defects and impurities from the bulk into the GBs, explain the good photovoltaic performance of these PX cells (at the expense of some voltage loss, as is indeed observed). We show that these CdTe GB features are induced by the CdCl ₂ heat treatment used to optimize these cells in the production process.

Molecular modification of dye-sensitized, mesoporous TiO2 electrodes changes their electronic properties. We show that the open-circuit voltage (V-oc) of dye-sensitized solar cells varies linearly with the dipole moment of coadsorbed phosphonic, benzoic, and dicarboxylic acid derivatives. A similar dependence is observed for the short-circuit current density (I-sc). Photovoltage spectroscopy measurements show a shift of the signal onset as a function of dipole moment. We explain the dipole dependence of the V-oc in terms of a TiO2 conduction band shift with respect to the redox potential of the electrolyte, which is partially followed by the energy level of the dye. The I-sc shift is explained by a dipole-dependent driving force for the electron current and a dipole-dependent recombination current.

Diodes made by (indirectly) evaporating Au on a monolayer of molecules that are adsorbed chemically onto GaAs, via either disulfide or dicarboxylate groups, show roughly linear but opposite dependence of their effective barrier height on the dipole moment of the molecules. We explain this by Au-molecule (electrical) interactions not only with the exposed end groups of the molecule but also with its binding groups. We arrive at this conclusion by characterizing the interface by in situ UPS-XPS, ex situ XPS, TOF-SIMS, and Kelvin probe measurements, by scanning microscopy of the surfaces, and by current-voltage measurements of the devices. While there is a very limited interaction of Au with the dicarboxylic binding groups, there is a much stronger interaction with the disulfide groups. We suggest that these very different interactions lead to different (growth) morphologies of the evaporated gold layer, resulting in opposite effects of the molecular dipole on the junction barrier height.

The effect of surface treatments on p-CdTe/n-CdS solar cell performance was examined. Adsorption of organic molecules with various magnitudes and directions of the dipole moment on p-CdTe resulted in controlled changes in electron affinity and surface band bending. Similar adsorption on CdTe in state-of-the-art p-CdTe/n-CdS solar cells changes the cell performance, and we explain this by a combination of increased series resistance and changes in light absorption and in cell photovoltage. While at this stage no improvement in performance has been found with these cell structures, which are the result of years of empirical optimization, the molecular effect on the photovoltage shows that it is possible in this way to control the photovoltaic effect at this junction. Separate optimization may well lead to improvement by inserting a dipole layer near the photovoltaic interface. Our results also show that this is even possible when dipole adsorption is performed on the complete polycrystalline thin-film cell.

The electric potential distribution in dye-sensitized solar cells plays a major role in the operation of such cells. Models based on a built-in electric field which sets the upper limit for the open circuit voltage (V _∝) and/or the possibility of a Schottky barrier at the interface between the mesoporous wide band gap semiconductor and the transparent conducting substrate have been presented. We show that I-V characteristics in the dark and upon illumination are very well explained by electron tunneling, rather than transport over a Schottky barrier, at this interface. Our calculations, based on tunnel currents, show that a discontinuity of the conduction band at the TiO₂/FTO interface, rather than a built-in electric field, suffices for efficient electron transfer through this interface, and, thus, for efficient operation of this type of solar cell. Clearly, this will hold only if the photoinduced electrostatic potential barrier between the transparent conducting substrate and the mesoporous wide band gap semiconductor drops over a region that is sufficiently narrow to allow efficient tunneling through it.

We show reproducible, stable negative differential resistance (NDR) at room temperature in molecule-controlled, solvent-free devices, based on reversible changes in molecule-electrode interface properties. The active component is the cyclic disulfide end of a series of molecules adsorbed onto mercury. As this active component is reduced, the Hg-molecule contact is broken, and an insulating barrier at the molecule-electrode interface is formed. Therefore, the alignment of the molecular energy levels, relative to the Fermi levels of the electrodes, is changed. This effect results in a decrease in the current with voltage increase as the reduction process progresses, leading to the so-called NDR behavior. The effect is reproducible and repeatable over more than 50 scans without any reduction in the current. The stability of the system, which is in the "solid state" except for the Hg, is due to the molecular design where long alkyl chains keep the molecules aligned with respect to the Hg electrode, even when they are not bound to it any longer.

The single-crystal outperformance of thin film polycrystalline (PX) CdTe/CdS solar cells was analyzed. The defect density of grains was reduced by gettering of defects at the grain boundaries (GB). Scanning capacitance microscopy (SCM) alongwith scanning Kelvin probe microscopy (SKPM) characterize the grain surface electrically. The results show that the separation and collection of photogenerated charge carriers was enhanced by CdTe GB.

We compile, compare, and discuss experimental results on low-bias, room-temperature currents through organic molecules obtained in different electrode-molecule-electrode test-beds. Currents are normalized to single-molecule values for comparison and are quoted at 0.2 and 0.5 V function bias. Emphasis is on currents through saturated alkane chains where many comparable measurements have been reported, but comparison to conjugated molecules is also made. We discuss factors that affect the magnitude of the measured current, such as tunneling attenuation factor, molecular energy gap and conformation, molecule/electrode contacts, and electrode material.

Epitaxial films of CuInSe₂ on Si(1 1 1) were modified by the application of an electric field through a movable tip. The electric field induces stable junction regions which are identified by separation and collection of electron beam-induced charge carriers. The movable tip allows scribing of these junction regions and one-sided connection to a contact pad. The junction formation is mainly due to electric field application and, in contrast to what was found to be the case for bulk samples, is not accompanied by a significant temperature rise. The junctions can be explained by symmetrical p/p⁺/n/p⁺/p regions formed within the CuInSe₂ epilayers. The reported method presents a new way for junction patterning in two dimensions.

We show that, for molecules with particularly strong dipoles, their organization into a monomolecular layer can lead to depolarization, something that limits the range over which the substrate's work function can be changed. It appears that, with molecules, depolarization is achieved by changes in orientation and conformation, rather than by charge transfer to the substrate as is common for atomic layers.

A study was performed on molecular modification of an ionic semiconductor-metal interface. It was found that with ZnO the interface behavior was 0.55, five times that with GaAs. It agreed well with the results for junctions of these materials with different metals, prepared in ultrahigh vacuum.

The effect of the presence or absence of chemical bonds between alkyl chain monolayers and the contacts in metal/molecule/semiconductor junctions on the current-voltage characteristics was studied. Three types of junctions were used: Hg/alkylthiols/SiO₂/p-Si, Hg/alkylthiols/p-Si-H, and Hg/alkylsilanes/SiO₂/p-Si. While in the first two junctions current is attenuated exponentially as a function of the length of the alkyl chain, a characteristic behavior of tunneling, the current through the third junction does not reveal such behavior, suggesting that current transport is different in this case. We postulate that this is because in the first two junctions the monolayers are covalently bound to the Hg, while in the third junction, the alkysilanes are anchored to the Si surface only at a few points and are best viewed as not bonded to either side of the junction. The mechanism of current flow through the first two junctions is thought to be through-bond tunneling, and our results indicate that a chemical bond to at least one of the electrode surfaces is essential for this mechanism to operate. Electrostriction causes changes in the current-voltage characteristics of the first two junctions. Evidence is presented suggesting that electrostriction tilts short chains (≤C₁₂), resulting in an additional route to charge transport by tunneling through space. In contrast, long chains (≥C₁₄) do not tilt under pressure; instead, gauche defects are formed in their initial all-trans configuration decreasing the efficiency of electronic coupling through them. The use of p-type Si in this study ensures that at low bias voltages holes are the dominant charge carriers. Holes are found to tunnel more efficiently than electrons in agreement with theoretical predictions.

The mechanism by which dicarboxylic acid molecules adsorb onto ambient-exposed GaAs (1 0 0) surfaces is studied by combining infrared spectroscopy and measurements that are sensitive to changes of the electric potential on the surface. For the latter we used the recently developed molecular controlled semiconductor resistor. By comparing the time dependences of the two measurements we conclude that adsorption proceeds sequentially, with virtually all of the rearrangement of electrical charge in the adsorbates taking place when the first carboxylic group, in each molecule, binds to the surface. This can be understood if charge rearrangement is necessary for forming a close-packed adsorbed layer. Since creation of such a layer, that is made up of molecules with significant molecular dipoles, requires some degree of depolarization of the molecules.

Junction function: Molecule-controlled devices based on metal-semiconductor junctions can exhibit negative differential resistance (NDR) at room temperature. The active component is a self-assembled monolayer of molecular dipoles (see schematic representation; X = CF₃, CN, H, OMe). A systematic change in the dipole moment of these molecules results in a corresponding systematic change in the NDR effect, thus establishing for the first time molecularly tunable NDR.

This article examines a somewhat counter-intuitive approach to molecular-based electronic devices. Control over the electronic energy levels at the surfaces of conventional semiconductors and metals is achieved by assembling on the solid surfaces, poorly organized, partial monolayers (MLs) of molecules instead of the more commonly used ideal ones. Once those surfaces become interfaces, these layers exert electrostatic rather than electrodynamic control over the resulting devices, based on both electrical monopole and dipole effects of the molecules. Thus electronic transport devices, incorporating molecules, can be constructed without current flow through the molecules. This is illustrated for a gallium arsenide (GaAs) sensor as well as for gold-silicon (Au-Si) and Au-GaAs diodes. Incorporating molecules into solid interfaces becomes possible, using a 'soft' electrical contacting procedure, so as not to damage the molecules. Because there are only a few molecular restrictions, this approach opens up possibilities for the use of more complex (including biologically active) molecules as it circumvents requirements for ideal MLs and for molecules that can tolerate actual electron transport through them.

High efficiency CdTe/CdS thin-film solar cells require low resistance contacts to p-CdTe, which is frequently achieved by addition of Cu. Decreases in cell efficiency over time. however. have been associated with Cu from the contact. The question that is considered here is if Cu is really detrimental to cell performance? By performing a series of thermal stress tests the authors reach a far more optimistic conclusion than what has hitherto been assumed. The Figure shows the proposed model for action of Cu (and Cl) in the CdTe/CdS cell.

The onset wavelengths of the surface photovoltage (SPV) in dye-sensitized solar cells (DSSCs) with different mesoporous, wide-band gap electron conductor anode materials, viz., TiO₂ (anatase), Nb₂O₅ (amorphous and crystalline), and SrTiO₃, using the same Ru bis-bipyridyl dye for all experiments, are different. We find a clear dependence of these onset wavelengths on the conduction band edge energies (E_CB) of these oxides. This is manifested in a blue-shift for cells with Nb₂O₅ and SrTiO₃ compared to those with TiO₂. The E_CB levels of Nb₂O₅ and SrTiO₃ are known to be some 200-250 meV closer to the Vacuum level than that of our anatase films, while there is no significant difference between the optical absorption spectra of the dye on the various films. We, therefore, suggest that the blue shift is due to electron injection from excited-state dye levels above the LUMO into Nb₂O₅ and SrTiO₃. Such injection comes about because, in contrast to what is the case for anatase, the LUMO of the adsorbed dye in the solution is below the E_CB of these semiconductors, necessitating the involvement of higher vibrational and/or electronic levels of the dye, with the former being more likely than the latter. While for Nb₂O₅ hot electron injection has been proposed earlier, on the basis of flash photolysis experiments, this is the first evidence for such ballistic electron-transfer involving SrTiO₃, a material very similar to anatase but with a significantly smaller electron affinity. Additional features in the SPV spectra of SrTiO₃ and amorphous Nb₂O₅ (but not in those of crystalline Nb₂O₅) can be understood in terms of hole injection from the dye into the oxide via intraband gap surface states.

We suggest an alternative technique for electroluminescent device fabrication, based on our earlier findings of electric field (E-field)-induced bipolar transistor creation in Si, doped with Li. An external electric field served to induce μm sized electroluminescent device structures in Si, that had been doped prior to E-field application, with Li, and Er via thermal in-diffusion. Such devices exhibit low temperature, near infrared (IR) electroluminescence at ∼ 1.16 and 1.55 μm, corresponding to transitions associated with Li and Er levels, respectively, in the forbidden gap. While Li also creates radiative recombination centers in Si, the Er-based IR radiation is the most desirable one. At the same time Li-doping is what makes E-field-induced p-n junction fabrication possible.

We explore chemical and physical limits to semiconductor device miniaturization. Minimal sizes for space charge-based devices can be estimated from Debye screening lengths of the materials used. Because a doped semiconductor can be viewed as a mixed electronic-ionic conductor, with the dopants as mobile ions, dopant intermixing across a p/n junction presents a chemical limit. Given a desired lifetime, simple relations can be derived between size and dopant intermixing for reverse- or forward-biased devices. Mostly, conditions for significant dopant mobility are far from those where the material is used. Thus, it is generally held that elemental and III-V-based p-n junctions are immune to this problem and persist because of kinetic stability. Indeed, we find this to be so for Si in the foreseeable future, but not for III-V- and II-VI-based ones. The limitation is more severe in structures with very thin undoped layers sandwiched between doped ones or vice versa, where even 1% intermixing can be critical. This decreases lifetime nearly 100 times. For example, for structures containing a 10 nm critical dimension, none of the components can have an average diffusion coefficient higher than 10^-24 cm²/s for a 3 year lifetime. Ways to overcome or mitigate this limitation are indicated.

Stability aspects of the Mo/Cu(In,Ga)Se₂/CdS/ZnO solar cell are reviewed and assessed. These include (i) the chemical stability of the various interfaces present in the device, (ii) the long-term behavior of metastable defects found in the Cu(In,Ga)Se₂ (CIGS) compound, and (iii) the impact of Cu migration on device performance and lifetime. We find that (i) all interfaces within the structure are chemically stable, (ii) metastable defects have a beneficial effect on performance, and (iii) Cu migration effects are reversible and their possible detrimental effects are eclipsed by the beneficial effect of the metastable states. Moreover, Cu out-diffusion from the CIGS layer is absent in photovoltaic-quality CIGS. Finally, we propose a model that explains the exceptional radiation hardness and impurity tolerance of CIGS-based devices, based on the synergetic effect of copper migration and point defect reactions.

The use of molecules to control electron transport is an interesting possibility, not least because of the anticipated role of molecules in future electronic devices(1). But physical implementations using discrete molecules are neither conceptually(2,3) simple nor technically straightforward (difficulties arise in connecting the molecules to the macroscopic environment). But the use of molecules in electronic devices is not limited to single molecules, molecular wires or bulk material. Here we demonstrate that molecules can control the electrical characteristics of conventional metal-semiconductor junctions, apparently without the need for electrons to be transferred onto and through the molecules. We modify diodes by adsorbing small molecules onto single crystals of n-type GaAs semiconductor. Gold contacts were deposited onto the modified surface, using a 'soft' method to avoid damaging the molecules(4). By using a series of multifunctional molecules whose dipole is varied systematically, we produce diodes with an effective barrier height that is tuned by the molecule's dipole moment. These barrier heights correlate well with the change in work function of the GaAs surface after molecular modification. This behaviour is consistent with that of unmodified metal-semiconductor diodes, in which the barrier height can depend on the metal's work function.

Sequential self-assembly of a two-component system on a solid support is described with respect to structure and function. Two ligands, which bind to the semiconductor surface through one end and axially ligate a heme analogue at the other end, are described. Monolayer assemblies of complexes formed by these ligands and iron-porphyrin perform reversible binding of molecular oxygen. In the monolayer, a metalloporphyrin (the sensing unit) is held by the intervening ligand that serves as a 'hinge', away from the solid surface. Sensing events based on porphyrin chemistry are communicated via the ligand to the solid support. The transduction manifests itself as a change in the solid's surface electronic properties. Synthesis of the ligands and analysis of its complex formation with Fen(III)-porphyrin are described. The anisotropic orientation of the porphyrin ring within the ligand cavity, due to restricted rotation around the Fe(III)-N imidazole bonds, was probed by ¹H NMR measurements in solution. We show that the porphyrin substituents stand as barriers for the free rotation even at room temperature. Molecular modeling supports the NMR evidence and reveals the stable conformations for the porphyrin's orientation relative to the solid support. The complexes were assembled as films on the (0001) surface of etched n-CdSe single crystals, and the films were characterized using transmission Fourier transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopies. Contact potential difference (CPD) and steady-state photoluminescence (PL) measurements of the derivatized CdSe show that the intervening ligands yield better conjugation and stronger binding of the sensing unit to the semiconductor surface, relative to direct adsorption of metalloporphyrins. Furthermore, the PL changes in the CdSe can be used to follow the interaction of the surface-bound Fe(III)-porphyrin-ligand complexes with molecular oxygen. A model is proposed to explain the electronic changes resulting from binding of O₂ to the monolayer.

The chemical effects of oxygenation of Cu(In,Ga)Se₂ (CIGS) interfaces are analyzed and are shown to involve passivation of Se deficiencies and Cu removal. The former effect is beneficial at grain boundaries, but detrimental at the CdS/CIGS interface. The latter effect is purely detrimental. Na and chemical bath deposition (CBD) treatments are shown to isolate the `good' oxygenation effect from the `bad' ones. Na is shown to promote oxygenation already before the deposition of the buffer and window layers, which allows a maximization of the benefits of Se deficiency passivation and a minimization of Cu removal. Next, the CBD of the CdS buffer layer restores the interface charge, due to creation of Cd_Cu interface donors and possibly a removal of O_Se interface acceptors. This highlights the crucial role that interface redox engineering plays in optimizing the performance of CIGS-based solar cells.

Surface passivation due to the interaction of a given molecule with n- and p-GaAs surfaces is explained well by a highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) interaction between the frontier orbitals of the molecules and the semiconductor surface states. The observed electronic changes depend on the nature of the molecules, on the one hand, and on that of the surface states, on the other. Considering semiconductor surface passivation as a frontier orbital interaction mechanism is expected to lead to its quantitative understanding, and use of such a model for designing molecular treatments of electronic materials provides a new tool for fine-tuning semiconductor device structures.

We present a simple, compact, and robust arrangement for surface photovoltage measurements of free semiconductor surfaces immersed in liquids. It is based on the classical Kelvin probe arrangement, where the semiconductor sample is put in a liquid-containing, electrically insulating vessel, with an optically transparent window, situated between the sample and the Kelvin probe. At the price of permitting relative, rather than absolute, contact potential difference values, this modification enables easy, routine surface photovoltage measurements of semiconductors in any kind of liquid ambient. The validity and efficiency of this approach are demonstrated by surface photovoltage spectra obtained from the p-InP(100) surface in various liquid etchants.

Communication: Is "self-healing" the source of the stability of Cu(In, Ga)Se-2-based solar modules? The proven remarkable stability and radiation hardness of Cu(In,Ga)Se-2 (CIGS) solar cells stand in apparent contradiction to the fact that CIGS shows both short-range (metastable defect centers) and long-range (significant Cu migration) instabilities. The authors suggest that these instabilities may in fact be a prerequisite for CIGS's stability as they allow a degree of flexibility or "smartness" in accommodating externally imposed changes. Two self-healing cycles are proposed, in which copper species play a particularly important role.

Photoelectron spectroscopy and admittance spectroscopy were used to analyze Cu(In,Ga)Se₂ (CIGS) based thin films and heterojunction solar cells based on chemical oxygenation and post-deposition air-annealing effects. The effect of CIGS surface chemistry on the electronic structure of the heterojunction solar cells and the influence on the CIGS layer of oxygenation-induced Cu redistribution were analyzed. Results showed that charge redistribution and compensation of the effective acceptor density can be achieved in the bulk of the absorber.

Dopant flux in a semiconductor junction due to chemical diffusion and drift (electromigration) was analyzed as a possible determining factor for device life expectancy at room temperature. Simple relations are derived and/or recalled to allow estimates of lifetimes. They are shown to be appropriate for III-V heterojunction bipolar transistors. We suggest that this chemical factor must be considered for compound semiconductor devices, as their dimensions shrink.

We show how phase separation, in the form of a redistribution of impurities (dopants in a semiconductor), can occur at impurity concentrations that are more than one order of magnitude lower than hitherto observed. This phenomenon results from the balance between long-range electrostatic repulsion and the elastic attraction of the dopants, which deforms the anisotropic host lattice. We observed such a phase separation for Ag in (Cd, Hg)Te at Ag concentrations

We have developed a new, efficient method to dope bulk single crystals of CdTe by In, via gas phase diffusion, using In₄Te₃ as the source. Doping was carried out on crystals of very high resistivity (>5 MΩ cm), following annealing in the temperature range of 350-1000°C. Resulting crystals showed n-type conductivity with a free carrier concentration in the range of 10¹⁵-10¹⁸ cm^-3 and carrier mobility of 100-750 cm²/(V s), depending on the annealing temperature and time, and on the cooling conditions. Incorporation of In was found to be a function of annealing time and temperature only. Up to 650°C, the In and the free electron concentrations are roughly the same.

Polymer electrochemical cells have been reported to emit light. The current and light output increase rapidly with voltage, apparently beyond 2 V. The polymer is an ionic conductor as well as an electronic (electron/hole) conductor, i.e. a mixed ionic-electronic conductor (MIEC). The I-V relations are explained here to be those of an MIEC of a particular defect model placed between two ion blocking electrodes. This MIEC defect model assumes a large concentration of mobile ions and small concentrations of electrons and holes. A p and an n region are formed in the MIEC. However, there is no space charge within the MIEC and it stays neutral. The resulting I-V relations are exponential. A good fit to the experimental data is obtained when electrode over-potential and heating of the polymer under current are taken into consideration.