Synchronous surface and bulk composition management for red-shifted light absorption and suppressed interfacial recombination in perovskite solar cells

Abstract

Organic small molecules like ammonium halide salts (AHSs) are widely used to suppress the non-radiative charge recombination of solution-processed halide perovskite films with defective surface and unfavorable energy levels for enhanced photovoltage, but more often than not at a cost of enlarging the band gap of perovskite materials, which limits the light absorption and thus, the photocurrent density. In addition, there is still a lack of fundamental understanding and visual characterization of the surface and bulk composition change upon subsequent surface treatment with heterogeneous ingredients. Herein, we report and identify that AHS soaking treatment enables the formation of δ-FAPbI₃ on the surface of the perovskite film for defect passivation functions, thus reducing the interfacial recombination loss and increasing photovoltage. Meanwhile, the AHS molecules diffuse and alloy with the bulk perovskite material to reduce the band gap, red-shifting the absorption region and thus increasing the photocurrent. Blade-coated perovskite solar cells (PSCs) prepared via AHS treatment achieved a champion power conversion efficiency (PCE) of 21.9% with a high open-circuit voltage (V_oc) up to 1.18 V under AM 1.5G sun illumination, which is remarkably higher than the pristine PSCs without any treatment (19.2%), and outperforms other blade-coated PSCs, regardless of their perovskite compositions. Surface treatment of perovskite films with functional, organic small molecules is a promising strategy for rational composition management and favorable gradient distribution, which are beneficial for realizing efficient and stable PSCs.