Enhanced Charge Transfer in Quasi-2D Perovskite by Formamidinium Cation Gradient Incorporation for Efficient and...

Quasi-2D perovskites have attracted much attention in perovskite photovoltaics due to their excellent stability. However, their photoelectric conversion efficiency (PCE) still lags 3D counterparts, particularly with high short-circuit current (J_SC) loss. The quantum confinement effect is pointed out to be the sole reason, which introduces widened bandgap and poor exciton dissociation, and undermines the light capture and charge transport. Here, the gradient incorporation of formamidinium (FA) cations into quasi-2D perovskite is proposed to address this issue. It is observed that FA prefers to incorporate into the larger n value phases near the film surface compared to the smaller n value phases in the bulk, resulting in a narrow bandgap and gradient structure within the film. Through charge dynamic analysis using in situ light-dark Kelvin probe force microscopy and transient absorption spectroscopy, it is demonstrated that incorporating 10% FA significantly facilitates efficient charge transfer between low n-value phases in the bulk and high n-value nearby film surface, leading to reduced charge accumulation. Ultimately, the device based on (AA)₂(MA_0.9FA_0.1)₄Pb₅I₁₆, where AA represents n-amylamine renowned for its exceptional environmental stability as a bulky organic ligand, achieves an impressive power conversion efficiency (PCE) of 18.58% and demonstrates enhanced illumination and thermal stability.