Grain engineering through combined MACl and MAH 2 PO 2 additives in perovskite precursors improves the photovoltaic performance of perovskite/silicon tandem cells. MACl increases the grain size of wide-bandgap perovskite films and also produces smooth films. MAH 2 PO 2 suppresses non-radiative recombination sites at grain boundaries. The synergetic effects of MACl and MAH 2 PO 2 further promote grain growth and prolong the carrier recombination lifetime. This enables a power conversion efficiency of 25.4% for a perovskite/silicon tandem device. SUMMARYOrganic-inorganic halide perovskites are promising semiconductors to mate with silicon in tandem photovoltaic cells due to their solution processability and tunable complementary bandgaps. Herein, we show that a combination of two additives, MACl and MAH 2 PO 2 , in the perovskite precursor can significantly improve the grain morphology of wide-bandgap (1.64-1.70 eV) perovskite films, resulting in solar cells with increased photocurrent while reducing the open-circuit voltage deficit to 0.49-0.51 V. The addition of MACl enlarges the grain size, while MAH 2 PO 2 reduces non-radiative recombination through passivation of the perovskite grain boundaries, with good synergy of functions from MACl and MAH 2 PO 2 . Matching the photocurrent between the two subcells in a perovskite/silicon monolithic tandem solar cell by using a bandgap of 1.64 eV for the top cell results in a high tandem V oc of 1.80 V and improved power conversion efficiency of 25.4%.

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Single-crystal perovskites with excellent photophysical properties are considered to be ideal materials for optoelectronic devices, such as lasers, light-emitting diodes and photodetectors. However, the growth of large-scale perovskite single-crystal films (SCFs) with high optical gain by vapor-phase epitaxy remains challenging. Herein, we demonstrated a facile method to fabricate large-scale thin CsPbBr3 SCFs (∼300 nm) on the c-plane sapphire substrate. High temperature is found to be the key parameter to control low reactant concentration and sufficient surface diffusion length for the growth of continuous CsPbBr3 SCFs. Through the comprehensive study of the carrier dynamics, we clarify that the trapped-related exciton recombination has the main effect under low carrier density, while the recombination of excitons and free carriers coexist until free carriers plays the dominate role with increasing carrier density. Furthermore, an extremely low-threshold (∼8 μJ cm–2) amplified spontaneous emission was achieved at room temperature due to the high optical gain up to 1255 cm–1 at a pump power of 20 times threshold (∼20 P th). A microdisk array was prepared using a focused ion beam etching method, and a single-mode laser was achieved on a 3 μm diameter disk with the threshold of 1.6 μJ cm–2. Our experimental results not only present a versatile method to fabricate large-scale SCFs of CsPbBr3 but also supply an arena to boost the optoelectronic applications of CsPbBr3 with high performance.

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