Mixtures of cations or halides with FAPbI3 (where FA is formamidinium) lead to high efficiency in perovskite solar cells (PSCs) but also to blue-shifted absorption and long-term stability issues caused by loss of volatile methylammonium (MA) and phase segregation. We report a deposition method using MA thiocyanate (MASCN) or FASCN vapor treatment to convert yellow δ-FAPbI3 perovskite films to the desired pure α-phase. NMR quantifies MA incorporation into the framework. Molecular dynamics simulations show that SCN– anions promote the formation and stabilization of α-FAPbI3 below the thermodynamic phase-transition temperature. We used these low-defect-density α-FAPbI3 films to make PSCs with >23% power-conversion efficiency and long-term operational and thermal stability, as well as a low (330 millivolts) open-circuit voltage loss and a low (0.75 volt) turn-on voltage of electroluminescence.

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The material class of hybrid organic-inorganic perovskites has risen rapidly from a virtually unknown material in photovoltaic applications a short 7 years ago into a ∼ 20% efficient thin-film solar cell material. As promising as this class of materials is, however, there are limitations associated with its poor long-term stability, non-optimal band gap, presence of environmentally-toxic Pb element, etc. We herein apply a functionality-directed theoretical materials selection approach as a filter for initial screening of the compounds that satisfy the desired intrinsic photovoltaic functionalities and might overcome the above limitations. First-principles calculations are employed to systemically study thermodynamic stability and photovoltaic-related properties of hundred of candidate hybrid perovskites. We have identified in this materials selection process fourteen Ge and Sn-based materials with potential superior bulk-material-intrinsic photovoltaic performance. A distinct class of compounds containing NH3COH+ with the organic molecule derived states intriguingly emerging at band-edges is found. Comparison of various candidate materials offers insights on how composition variation and microscopic structural changes affect key photovoltaic relevant properties in this family of materials.

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The steady-state spectra and fluorescence lifetimes are investigated under vacuum for methylammonium lead bromide and iodide (CH3NH3PbBr3 or MAPbBr3, and CH3NH3PbI3 or MAPbI3) thin films by stably controlling the sample temperature in the range of 78 K to 320 K. The transformation of spectrum features and lifetime components are proved to be quite sensitive to the temperatures in accordance with the phase transition of structures. Our work demonstrates that the halide anions I- and Br- lead to remarkable differences on optical characteristics. Due to the distinct behaviors of excitons, electron-hole pairs and free carriers in decay channels, MAPbI3 has much longer lifetime and higher low-temperature fluorescence efficiency than those of MAPbBr3. The findings provide possible choices for certain perovskites under various ambient temperature conditions to display photovoltaic or luminescent advantage.