Rational Design on Chemical Regulation of Interfacial Microstress Engineering by Matching Young's Modulus in a CsPbBr Perovskite Film with Mechanical Compatibility toward Enhanced Photoelectric Conversion Efficiency.

Thermodynamically induced tensile stress in the perovskite film will lead to the formation of atomic vacancies, seriously destroying the photovoltaic efficiency stability of the perovskite solar cells (PSCs). Among them, cations and halide anions vacancies are unavoidable; these point vacancies are considered to be a major source of the ionic migration and perovskite degradation at the crystal boundary and surface of the perovskite films. Here, we use choline bromide to modify the perovskite film by occupying the atomic defects in the CsPbBr perovskite film. The results show that the zwitterion quaternary ammonium ions and bromide ions in choline bromide can simultaneously occupy the Cs cation and Br anions vacancies in the perovskite film by the ionic bonding effect, for which the defect-state density on the surface of the perovskite film can be significantly reduced, leading to the effective enhancement of carrier lifetime. In addition, the residual stress at the crystal boundary can be effectively reduced by lowering the Young's modulus in the CsPbBr perovskite film. As a result, the optimized device achieves a photoelectric conversion efficiency (PCE) of 9.06% with an increase of 41.1% compared to the control device with a PCE of 6.42%. Most importantly, the newborn thermal stress due to thermal expansion during heat working conditions can be transferred from the polycrystalline perovskite to the carbon layer by the matched Young's modulus, thus resulting in improved stability perovskite film under environmental conditions. The work provides new insights for preparing high-quality perovskite films with low defect-state density and residual stress.