Optoelectronic properties and photo-physics of large grain hybrid perovskites

Organic-Inorganic or (Hybrid) perovskites are a special class of perovskites with a general chemical formula of AMX3 formed by using low-temperature synthesis approaches. They exhibit exceptional fundamental properties that have been translated into proof-of-principle demonstrations of photovoltaics (1), light emitting diodes(2), photodetectors(3), thermoelectric devices, lasers(4), photo-catalysts(5) and gamma ray detectors(6, 7) among which energy harvesting in photovoltaics has been the most studied(1, 8-15). While tremendous progress is being made, a fundamental bottleneck in that has existed in field is the high degree of variability in crystalline quality, grain-size, and microstructure of hybrid perovskites reported by the community. This has resulted in multiple interpretations of experimental data and thus key fundamental mechanisms remain largely unresolved. An ideal solution to this issue of non reliable properties is the ability to reproducibly grow hybrid perovskite thin-films with high degree of crystallinity, which allow access to the intrinsic physical properties, which are otherwise masked by non-reliable processing dependent microstructure. Here we present our recently developed fast thin-film crystal growth technique termed as hot-casting, which allows us to grow high crystalline quality, uniform, pinhole free films of hybrid perovskites with hundreds of microns to mm-scale grain-size. Investigation of photo-physical properties reveals that the resulting large grains behave as classical III-V direct band-gap semiconductors. However, as the grain-size decreases, the properties can no longer be described using models described for direct gap semiconductors. When incorporated into a simple "inverted" photovoltaic bilayer architecture with ITO/PEDOT:PSS/Perovskite/PCBM/Al, with no minimal optimization a hysteresis free device with a current-voltage curve of 15.37%. Electrical characterization using capacitance-voltage measurements and light-intensity dependence of the open circuit voltage suggest that perovskite films are intrinsic and the photogenerated charge carrier recombine through a bimolecular process, only observed in high quality semiconducting materials.