Photovoltaic Devices

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?

The lifetime of the CsPbI3-based cell is shown to be longer in every respect

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.

Schematic of a perovskite photovoltaic cell
with a mesoporous hole blocking layer

 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.

Explanation of hysteresis observed in pin and nip cells
 

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.  

SEM image of a single PbI2 crystal before and after
reaction with MAI in solution and vapor phase

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.

Fermi level position of MAPbI3 film on HOPG after exposure
to vacuum (left) and O2 (right). The n-type doping of the film
in vacuum decreases after O2/air exposure.

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
 

The ability to grow perovskite single crystals opens up many new avenues of research.


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.