The certified power conversion efficiency (PCE) of state-of-the-art organic-inorganic metal halide perovskite solar cells (PSCs) has surpassed 25%, coming after single crystalline silicon solar cells. The most widely studied methylammonium (MA) lead iodide undergoes a structural phase transition and degradation due to the volatile MA cation, detrimental to the long-term stability of the devices. Formamidinium (FA), used as the majority cation in most PCE &gt;20% PSCs, is thermally more stable and has an optimal red-shifted bandgap. However, FA with the relatively large radius induces titled PbI6 octahedra in FAPbI3, leading to the photoinactive yellow δ-phase at room temperature. Herein, iodide and FA in FAPbI3 were gradually replaced by bromide and cesium (Cs) with smaller radius to suppress the yellow δ-phase and systematically explore the compositional space via regulating the tolerance factor, revealing the crucial link between the crystal structure and optoelectronic properties of the mixed-cation (FA/Cs) lead mixed-halide (I/Br) perovskite. An empirical expression for FA/Cs-Pb-I/Br bandgap as a function of composition was determined, which contributes to tailor bandgap for tandem solar cells. In addition, the FA/Cs-Pb-I/Br films were probed by advanced synchrotron X-ray characterization to understand the variation of crystal symmetry with composition.

Methylammonium-free compositional engineering for the design of high-performance perovskite solar cells

ACS Fall 2021

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