Perovskites at the nanoscale: from fundamentals to applications.

“Perovskites” are materials with ABX3 stoichiometry (for example, calcium titanate, CaTiO3) that form crystal structures wherein “B” cations reside in the centers of corner-shared octahedra of “X” anions, with the “A” cations filling the resulting interstices. Perovskites have captivated researchers with properties such as superconductivity, ferroelectricity, piezoelectricity, ferromagnetism and antiferromagnetism, to name just a few. Even though most of the known perovskites are metal oxides, metal halide perovskites (MHP) have recently been taking the field by storm with their record shattering photovoltaic performance combined with low-cost processing methods and earth-abundant elemental compositions. Since the first report on MHP-based solar cells in 2009, the record MHP solar cell efficiency (certified) has now reached 21%. This represents the highest solar cell efficiency among all solution-processed materials and the fastest rate of efficiency improvement in the history of all photovoltaic materials (Fig. 1a). As evident in this themed issue and recent reviews, currently there is an explosive growth in research efforts on MHPs. Interestingly, the best solar cell performance has been obtained from hybrid organic–inorganic MHPs such as methylammonium lead iodide (Fig. 1b). The origin of the high performance is far from being understood and a vigorous international research effort is currently directed at this. The presence of organic cations and the structural complexity of these materials both raise interesting research questions. The organic cations have aspherical shape, electric dipole, rapid rotation at room temperature and hydrogen bonding to the halides in the inorganic network that cause complex structural dynamics and distortions. Previous studies have shown that the rotation and alignment of electric dipoles in organic cations may play a role in the long carrier lifetime responsible for high solar cell efficiency, ferroelectricity-like behavior, and low exciton binding energy. The precise mechanisms of these phenomena are still actively being studied but establishing the role of the organic cations in the properties of MHPs will be crucially important (Even et al., DOI: 10.1039/c5nr06386h; Leguy et al., DOI: 10.1039/c5nr05435d). Another characteristic of these materials is the presence of nanoscale structural disorder and a tendency to amorphization, and it will be critical to understand the role this plays in the properties. The presence of local structural distortions in the inorganic framework of these materials was noted early on, well before the materials became ‘plat du jour’. Later structural work indicated that solution-processed material had a significant amorphous and nanocrystalline component. How can such defective and structurally imperfect materials have such good electronic properties? The charge diffusion length in MHPs has been shown to vary widely depending on the sample preparation method, ranging from hundreds of nanometers in polycrystalline films to 175 microns in single crystals. These results seem to indicate that higher crystallinity will be beneficial for solar cell performance. However, MHPs turn on its head the conventional notion that high degree of crystallinity is a requirement for achieving high solar cell efficiency (>20%). A majority of MHP solar cells reported to date, including one of the record >20% solar cell results, employ solution processing at low temperature that results in polycrystalline thin films with grain sizes typically smaller than 1 micron. Spatially resolved photoluminescence mapping shows wide variation among different MHP grains, which suggests differences in structures at the surfaces as well as crystallinity among the grains. Together with the earlier reports of structural disorder, these results suggest that the origin of high MHP solar cell performance may not be high crystallinity but that the photocarriers are either protected from the disorder, or are somehow using it to their own ends. Answering these questions will be critical to accelerate the progress in MHP solar cell efficiencies as well as rationally Department of Chemical Engineering, University of Virginia, Charlottesville, VA 22904, USA. E-mail: [email protected] Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA. E-mail: [email protected]