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
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
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
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 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.
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.