Optoelectronic Materials

Halide Exchange in Single Crystal Halide Perovskites

Halide Perovskites (HaPs) with general the formula ABX₃, where A =MA: CH₃NH₃; FA: NH₂(CH)NH₂ or Cs, B =Pb or Sn and X = halide, have remarkable electronic and optical characteristics, but much is still unknown regarding the connection between their physical and chemical properties. Cation or anion substitution can change the optical absorption edge, with or without a change in structure. Successful halide exchange was reported in thin-films and in micron-sized single crystals (SCs). In this work we explored the halide exchange reaction in methylammonium lead tri-halide SCs in order to understand the process of exchange and the stability of the product(s). We demonstrate halide exchange in mm-sized MAPbX₃ SCs, achieved by diffusion. In these macro-sized crystals, the effect of surface and defects is significantly smaller than in the microcrystals and polycrystalline thin films studied previously. Therefore, they are better suited to examine the fundamental exchange process(es), unencumbered by possible grain boundary and surface diffusion effects. Initially, the halide exchange creates normal concentration gradients of the out-going and in-coming halides, on a scale of a few microns to a few hundred microns. The depth (from the surface) of the substituted area depends on the halide pair and the role of each halide (which one is being exchanged and which one is exchanging). As expected, the concentration gradient of the in-coming halides decreases from the surface of the crystal towards its inner core and vice versa for the out-going halides and seems to follow the solution of a semi-infinite specimen. Using the Boltzmann-Matano method and diffusion profiles obtained by electron dispersive spectroscopy, it is possible to evaluate the halide diffusion coefficients, which are not constant, for a given halide mixture. For all permutations, composition change strongly affects the optical and electrical properties such as the band gap of the semiconductor as seen in cathodoluminescence measurements in the scanning electron microscope. While these gradients do cause a lattice parameter change and may cause a symmetry change, x-ray diffraction measurements show that if the interchanged halide pair is such that their sizes are not very different (e.g., Br^- and Cl^-, Br^- and I^-, not Cl^-and I^-) the product crystal remains surprisingly single crystalline. These findings are valid, no matter which one of the two halides is being exchanged. These results suggest that for these relatively similar-sized halide pairs, this exchange occurs through a solid-state chemical reaction such that the resulting crystal orientation is determined by that of the initial crystal.

Self-Healing and Bulk Chemical Evolution of Photostimulated Reactions in Halide Perovskites

In this project we study the consequences of photo-stimulation of halide perovskites (HaPs) in the bulk. We investigate the chemical environment, kinetics and the influence that the environment can have on the bulk chemical properties of HaPs. Results of this project are of fundamental importance in defining the intrinsic stability and optical properties of HaPs.

On Ferroelectricity in Halide Perovskites

Halide peroskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with esp. tetragonal methylammonium lead tri-iodide (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 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 hallmakr 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, that is stable at room temperature). By chemical etching we also can image teh 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 non-centrosymmetry. We note that the material's ferroelectric nature, can, but need not be important in a PV cell at room temperature.

- Rakita, Y., Meirzadeh, E., Bendikov, T., Kalchenko, V., Lubomirsky, I., Hodes, G., Ehre, D. and Cahen, D., 2016. CH3NH3PbBr3 is not pyroelectric, excluding ferroelectric-enhanced photovoltaic performance. APL Materials, 4(5), p.051101.

Control Over Self-Doping in High Bandgap HaP's

While new information on high efficiency perovskite solar cells appears on a weekly basis, there are major gaps in our understanding of high bandgap perovskite devices; the selective contacts, doping, gap states and interface-related properties have not been thoroughly investigated in these materials. One of the questions is: what limits the open circuit voltages in high band gap (> 2eV) perovskite devices?

To date, the highest reported V_OC for perovskite solar cells is 1.65V using MAPbBr₃ as the absorber, a perovskite semiconductor with a band gap of 2.3eV. Considering a thermodynamic limitation resulting in loss of ca. 0.3V between E_g and V_OC, thre are gaps in our understanding as to 'where' and why do we lose the rest of the voltage. The aim is to study materials, as well as interface-related properties, to understand what limites the open circuit voltage in devices, based on high band gap (> 2eV) HaP's and related materials. Recently, we have shown that carefully selecting the combination of precursors in Br-based HaP absorbers allows some control over doping and charge transport properties in these materials. Four different bromide-based HaP's were used to show that, depending on the material composition, the optoelectronic properties can be dramatically changed. By introducing additional A cations to the precursor solution, the initial free hole concentration decreased by 1-4 orders of magnitude compared to the single cation equivalents, and two carriers charge transport properties were observed in the mixed A cation HaP, with electron and hole diffusion lengths of 640 and 990 nm, respectively. The multi A cation HaP-based solar cells are the first example of high band gap HaP-based solar cell devices with resemblance to p-i-n junction-like behavior.

What Limits the Open-Circuit Voltage of Bromide Perovskite-Based Solar Cells?

High bandgap Pb bromide perovskite, APbBr₃–based solar cells, where A is a mixture of formamidinium, methylammonium and Cs, show significantly higher, relative, V_OC losses than their iodide analogs. Using photoluminescence-, external quantum efficiency- and surface photovoltage- spectroscopy measurements, we show the absence of any significant electronically-active tail states within the bulk of the (FA_0.85MA_0.1Cs_0.05)PbBr₃absorber. All methods confirm that E_G = 2.28 eV for this Halide Perovskite, HaP. Contact potential difference measurements for this HaP, on different substrates, reveal a Z-shape dependence between the substrate work functions and that of the HaP, deposited on it, indicating that the HaP is relatively low doped, and that its Fermi level is affected by the substrate onto which it is deposited. We confirm results from electron beam-induced current (EBIC) and other measurements that most voltage loss of cells, made with these HaP films is at the HaP/selective-contact interface, specifically the TiO₂/HaP one and provide a complete account of these cells’ V_OC losses. Capacitance measurements indicate that 350 mV V_OC could be gained by eliminating (fast) interfacial states, emphasizing the importance of interface passivation. Still, even passivating the TiO₂/HaP interface cannot eliminate the band misalignment with Br-based HaPs.

Charge Carrier Dynamics and Recombination in HaP thin Films

Photovoltaic solar cells operate under steady state conditions that are established during the charge carrier excitation and recombination.  However, much of the research in the optoelectronic properties on these materials and cells relies on using illumination pulses of very short duration.  Among these studies time-resolved photoluminescence (TRPL) has become one of the standard characterization tools of these systems.  However, unlike the qualitative understanding and quantitative models that connect the cell parameters and quantities that are measured by steady state phototransport methods, the relation between these parameters and quantities derived by the transient parameters is not as clear.  In particular, the steady state minority carrier diffusion length, is a critical quantity in the determination of the solar cell efficiency.

The basic question that arises, in particular in the context of the HaPs, is in how far the conclusions derived from the transient type of measurements (including photocurrent decay) are relevant to the steady state photoelectronic properties that determine the solar cell operation.  In this research we focus on direct measurement of the electron and hole diffusion length (L) of HaPs, under steady state illumination conditions, as a function of the generation rate (G) and temperature, and show how a single type of recombination center model can explain our experimental results and resolve the dominant recombination mechanism as well as the trap density (N_r) in different types of MAPI films (solution processed and evaporated (e-MAPI)) in the given example shown in the Figure).

Levine, I., Gupta, S., Bera, A., Ceratti, D., Hodes, G., Cahen, D., Guo, D., Savenije, T.J., Avila, J., Bolink, H.J. and Millo, O., Can we use time-resolved measurements to get Steady-State Transport data for Halide perovskites? Journal of Applied Physics 124, 103103 (2018)

High bandgap perovskite devices

While new information on high efficiency perovskite solar cells appears on a weekly basis, there are major gaps in our understanding of high bandgap perovskite devices: the selective contacts, doping, gap states and interface-related properties have not been thoroughly investigated in these materials. One of the questions is: what limits the open circuit voltages in high band gap (> 2 eV) perovskite devices?

To date, the highest reported VOC for perovskite solar cells is 1.5 V using MAPbBr3 as the absorber, a perovskite semiconductor with a band gap of 2.3 eV. Considering a thermodynamic limitation resulting in loss of ca. 0.3 V between Eg and VOC, there are gaps in our understanding as to “where” and why do we lose the rest of the voltage. The aim is to study materials, as well as interface-related properties, to understand what limits the open circuit voltage in devices, based on high band gap (> 2 eV) halide perovskites and related materials. Recently, direct comparison between perovskite-structured hybrid organic-inorganic - methyl ammonium lead bromide (MAPbBr3) and all-inorganic cesium lead bromide (CsPbBr3) have been done by our group for identifying possible fundamental differences in their structural, thermal and electronic characteristics. Both materials possess a similar direct optical band-gap, but CsPbBr3 demonstrates a higher thermal stability than MAPbBr3. In order to compare device properties we fabricated solar cells, with similarly synthesized MAPbBr3 or CsPbBr3, over mesoporous titania scaffolds. Both cell types demonstrated comparable photovoltaic performances under AM1.5 illumination, reaching power conversion efficiencies of ~6 %. Further analysis shows that Cs-based devices are as efficient as, and more stable than methyl ammonium-based ones after aging (storing the cells for 2 weeks in a dry (relative humidity 15-20%) air atmosphere in the dark) for 2 weeks, under constant illumination (at maximum power), and under electron beam irradiation.

 

Pb-free Inorganic Halide Perovskites as Opto-Electronic Materials

 

Over the past years there is tremendous growing interest in Hybrid Organic Inorganic metal halide Perovskite—HOIP (primarily methyl ammonium lead trihalides CH3NH3PbX3)-based solar cells.  These cells demonstrated power conversion efficiencies of over 20% for very small cells, up from a few % within a brief period of research (~5 years).  Recently, we showed that use of the all-inorganic halide perovskite CsPbBr3 can yield as good a PV performance, and a more stable one than the organic-inorganic hybrid one MAPbBr3.

 However, the metal, lead, used in the most studied, successful HOIP cell is toxic, impeding its widespread use and commercialization. The goal is to replace lead by much less toxic and chemically similar element such as tin (also earth-abundant) in an all-inorganic halide perovskite.  We are currently working on tin-based perovskite devices and measuring their device characteristics and efficiencies.  Since tin-based systems suffer from oxidation (Sn2+→Sn4+) and are extremely sensitive to moisture and oxygen, we are trying to stabilize them for long-term use.
 

The role of the electrical contacts in organic- and metal-organic-based optoelectronic devices.

 

In this research we investigate the role of the electrical contacts on the electronic properties of the hybrid organic-inorganic perovskite (HOIP) via two strategies:
(1) using a solar cell device structure, in which the HOIP absorber layer is sandwiched between different n- and p-type contact layers as selective contacts, and
(2) in a simple Metal-Semiconductor configuration using different metals (or TiO2) as substrates.

The first strategy is used to study the effect of the electrical contacts on the observed hysteresis on the current-voltage characteristics of hybrid organic-inorganic perovskite-based solar cells, which is one of the fundamental aspects of these cells that is not yet fully understood.  We perform I-V measurements as a function of temperature and scan rate. The second strategy is employed to study the effect of environmental conditions as well as the electronic properties of the substrate, on the surface electronic properties and doping level of the HOIP, using the Kelvin probe technique and photoelectron spectroscopy.
 

Understanding the unique formation chemistry of halide perovskites

 

The achievement of high-quality optoelectronic properties in halide perovskite semiconductors through low-temperature, low energy processing is unprecedented. Understanding the formation process of these semiconductors is a critical step toward understanding the origins of high-quality via these simple preparation procedures.  

The toolbox of preparation procedures grows by the day, however the fundamental reaction remains the spontaneous re-organization of a metal halide (PbI2, for instance) and a halide salt (CH3NH3I, abbr. MAI, for instance) to form the perovskite (CH3NH3PbI3 or MAPbI3 in this case). The reverse of this process is also a major decomposition pathway of the perovskite meaning that this reaction is also critical to understanding the stability of the material. A major component of understanding the formation process is to determine the steps of the reaction pathway. Our group has focused on the simplest reaction, between PbI2 and MAI. This reaction is especially interesting because PbI2 has a layered structure and it could be that the reaction proceeds by intercalation of MAI and topotactic conversion to the perovskite. In order to learn about the reaction process, we have chosen to study the conversion of single-crystalline PbI2 crystallites to the perovskite MAPbI3 by vapor and liquid phase reaction methods. The well-defined starting material allows us to accurately determine dimensional, morphological, and orientational changes that the crystallites undergo during reaction. 
 

The effect of composition on the thermodynamics of charge carrier mobilities in perovskites 

 

In those hybrid organic-inorganic perovskites which were previously used to form efficient solar cells, the A cation is organic; however, recent work from our group utilized the perovskite CsPbBr3 to produce a solar cell with an efficiency which is comparable to the efficiencies our lab has achieved for MAPbBr3-based devices. Despite the efficiency of the CsPbBr3 cell, as of September 2015 no one has yet attempted to quantify, with a direct comparison, how the replacement of the organic cation with cesium affects the transport properties of the perovskite material itself.   Considering that the organic MA and FA ions have dipoles, whereas Cs does not, some have surmised that this dipole gives rise to ferroelectricity or other effects which may be responsible for the characteristically long carrier lifetimes observed in perovskites.  Therefore, an improved understanding of the differences in transport properties between the hybrid organic-inorganic perovskites and the all-inorganic cesium perovskites could enhance our knowledge of those physical processes responsible for the high efficiencies of perovskite solar cells. We seek to understand how the substitution of an organic cation such as MA with an inorganic one such as Cs affects the electronic transport properties of the material.  More specifically, another group has recently published measurements of mobility in MAPbI3 by terahertz spectroscopy which strongly suggest that the mobilities of the charge carriers are limited only by vibrations in the material.  We seek to verify whether this finding holds on a larger scale for both MA and Cs lead halide perovskites.  To that end, we conduct current-voltage and capacitance-voltage measurements at various temperatures across the materials’ phase transitions.  The observation of a similar dependence of mobility upon temperature for all of the lead halide perovskites, together with previous work from our group showing similar mechanical properties for all members of the family, would demonstrate a general principle that the mobilities of lead halide perovskites are determined only by mechanical vibrations.
 

Interface Modifications of Semiconductors to Control Charge Transport in Photovoltaics

 

Halide perovskite (HaP)-based photovoltaic (PV) devices have reached ~20% power conversion efficiency within a few years of research. However, HaP-based PV cells still suffer from limited reproducibility, stability and incomplete understanding of how they work. Understanding the electronic processes involved in the PV charge transport, esp. those concerning the HaP interfaces in the cells, should allow to improve cell preparation and enhance device performance.

We study electronic processes in films of methyl ammonium (MA)-PbI3, MA-PbBr3 or CsPbBr3 HaPs and their interfaces with hole conductors such as Spiro- OMeTAD (C81H68N4O8) as function of environment to understand what limits charge collection from the HaP. The hole conduction layer can influence the interface quality and affect charge recombination. We provide direct evidence for the electronic sensitivity of a MAPbI3(Cl) layer to the measurement ambient, indicating an electrochemical process by using Impedance spectroscopy (IS) and surface potential measurements. A decrease in the film resistance upon transferring the film from O2-rich to vacuum ambient suggest that facile adsorption of oxygen onto the film de-dopes it from n-type towards intrinsic. 

 

Understanding the Bulk Properties of Halide Perovskites for Photovoltaic Applications
 

To understand what stands behind the success of halide perovskites, we address our research towards the main core of this new technology – the material. Since we are dealing with a complex system of an integrated organic and inorganic framework, we want to understand whether it possesses unique properties, how these influence photovoltaic performance and, more importantly, whether we can direct the field toward new materials with desired properties.
To achieve that, we start with growing single-crystals of the halide perovskites in their pure form – having the ability to grow them from several microns up to centimeters. Having that, we wish to answer questions that their answers should lead us towards a better understanding of the fundamental properties of these materials, to improve their integration in future devices and draw guidelines towards the next generation of materials for photovoltaics.