In situ reactive modification strategy to enhance the performance of n–i–p perovskite solar cells

Abstract

Rational modification of perovskite surfaces using organic ammonium salts can considerably improve the performance of perovskite solar cells (PSCs). However, the impact of the cation–anion bonding strength within organic ammonium salts on the resulting modified perovskite films is still quite unclear. Herein, a series of phenylethylammonium salts are systematically explored for surface passivation: iodide (PEAI), trifluoroacetate (PEATFA) and acetate (PEAAc), which show increasing cation–anion bonding strength. The bonding strength was found to govern the chemical and supramolecular interactions of phenylethylammonium salts with the perovskite components. While PEAI and PEATFA show weak interactions with FAI and PbI₂, PEAAc reveals surprisingly high reactivity and forms chemical species through unprecedented pathways. NMR spectroscopy and advanced infrared near-field optical microscopy (IR s-SNOM) unravel that PEAI and PEATFA are passivators mainly targeting the perovskite grain boundaries. By contrast, the reactive PEAAc-modification achieves simultaneous defect passivation through the formation of a lead acetate shell on the grain surface and accumulation of PEAI at the grain boundaries. Increasing the cation–anion bonding strength is also found to reduce the reactivity of phenylethylammonium salts to form two-dimensional perovskites. While PEAI-modification leads to little gain in power conversion efficiency (PCE) of PSCs, PEATFA-modification obviously improves the PCE and stability of PSCs. In particular, PEAAc-modification effectively chelates the undercoordinated Pb²⁺ and enables the most favorable energy band alignment to enhance the hole extraction at the perovskite/hole transport layer heterointerface. The photovoltaic performance and stability of corresponding PSCs with n–i–p architecture are improved considerably and a champion PCE of 24.9% is achieved. The presented “smart passivation” strategy using reactive modifiers also provides principally new opportunities in the development of efficient and stable PSCs.