Goldschmidt tolerance factor (t) is an empirical index for predicting stable crystal structures of perovskite materials. A t value between 0.8 and 1.0 is favorable for cubic perovskite structure, and larger (>1) or smaller (<0.8) values of tolerance factor usually result in nonperovskite structures. CH­(NH2)2PbI3 (FAPbI3) can exist in the perovskite α-phase (black phase) with good photovoltaic properties. However, it has a large tolerance factor and is more stable in the hexagonal δH-phase (yellow phase), with δH-to-α phase-transition temperature higher than room temperature. On the other hand, CsPbI3 is stabilized to an orthorhombic structure (δO-phase) at room temperature due to its small tolerance factor. We find that, by alloying FAPbI3 with CsPbI3, the effective tolerance factor can be tuned, and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA1–x Cs x PbI3 is enhanced, which is in agreement with our first-principles calculations. Thin films of the FA0.85Cs0.15PbI3 perovskite alloy demonstrate much improved stability in a high-humidity environment; this contrasts significantly with the pure FAPbI3 film for which the α-to-δH phase transition (associated with yellowing appearance) is accelerated by humidity environment. Due to phase stabilization, the FA0.85Cs0.15PbI3 solid-state alloy showed better solar cell performance and device stability than its FAPbI3 counterparts. Our studies suggest that tuning the tolerance factor through solid-state alloying can be a general strategy to stabilize the desired perovskite structure for solar cell applications.

No abstract

High-performance perovskite solar cells (PSCs) based on organometal halide perovskite have emerged in the past five years as excellent devices for harvesting solar energy. Some remaining challenges should be resolved to continue the momentum in their development. The photocurrent density-voltage (J-V) responses of the PSCs demonstrate anomalous dependence on the voltage scan direction/rate/range, voltage conditioning history, and device configuration. The hysteretic J-V behavior presents a challenge for determining the accurate power conversion efficiency of the PSCs. Here, we review the recent progress on the investigation of the origin(s) of J-V hysteresis behavior in PSCs. We discuss the impact of slow transient capacitive current, trapping and detrapping process, ion migrations, and ferroelectric polarization on the hysteresis behavior. The remaining issues and future research required toward the understanding of J-V hysteresis in PSCs will also be discussed.

Photovoltaic applications of perovskite semiconductor material systems have generated considerable interest in part because of predictions that primary defect energy levels reside outside the bandgap. We present experimental evidence that this enabling material property is present in the halide-lead perovskite, CH 3 NH 3 PbI 3 (MAPbI 3 ), consistent with theoretical predictions. By performing X-ray photoemission spectroscopy, we induce and track dynamic chemical and electronic transformations in the perovskite. These data show compositional changes that begin immediately with exposure to X-ray irradiation, whereas the predominant electronic structure of the thin film on compact TiO 2 appears tolerant to the formation of compensating defect pairs of V I and V MA and for a large range of I/Pb ratios. Changing film composition is correlated with a shift of the valence-band maximum only as the halide−lead ratio drops below 2.5. This delay is attributed to the invariance of MAPbI 3 electronic structure to distributed defects that can significantly transform the electronic density of states only when in high concentrations.

The Shockley-Queisser limit for solar cell efficiency can be overcome if hot carriers can be harvested before they thermalize. Recently, carrier cooling time up to 100 picoseconds was observed in hybrid perovskites, but it is unclear whether these long-lived hot carriers can migrate long distance for efficient collection. We report direct visualization of hot-carrier migration in methylammonium lead iodide (CHNHPbI) thin films by ultrafast transient absorption microscopy, demonstrating three distinct transport regimes. Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers transport distance that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot-carrier devices based on hybrid perovskites.