Pseudo-halide anion engineering for efficient quasi-2D Ruddlesden-Popper tin perovskite solar cells

Abstract

Quasi-two-dimensional (2D) organic-inorganic hybrid tin perovskites have emerged as promising alternatives to lead-based perovskites in thin-film photovoltaics because of their reduced toxicity and improved stability. However, the undesired small n-value 2D phases and disordered crystal orientation enormously restrict the efficiency of quasi-2D tin perovskite solar cells (PSCs) due to uncontrollable nucleation and crystal growth processes. Here, we propose a mixed pseudo-halide anion engineering approach by using acetate (Ac − ) and tetrafluoroborate (BF 4 − ) anions to make quasi-2D Ruddlesden-Popper perovskites with a target formula of PEA 2 FA 4 Sn 5 I 16 . We find that the mixed Ac − and BF 4 − anions can not only promote homogeneously distributed PEA + cations in the precursor by effectively breaking the PEA + ···PEA + stacking but also retard the crystallization process by coordinating with unbonded SnI 2 , thereby, significantly reducing small n-value 2D phases, improving the crystal orientation, and suppressing the Sn 2+ oxidation. The resulting PSC exhibits up to 9% power conversion efficiency and 400 h stability. • Efficiency up to 9% is achieved for 2D tin perovskite solar cells with good stability • The crystallization process is successfully controlled by pseudo-halide engineering • Vertical orientation and reduced small n-value 2D phases are achieved The poor stability of tin perovskites greatly hinders their photovoltaic applications. Li et al. propose a pseudo-halide engineering strategy to grow 2D Ruddlesden-Popper tin perovskites with reduced small n-value 2D phases and improved crystal orientation. Efficiency up to 9% and 400 h stability is achieved.