Ordered growth of Cs2AgBiBr6 double perovskites on PEIE-decorated SnO2 for efficient planar solar cells

Abstract

Cs₂AgBiBr₆ double perovskites are considered potential alternatives to lead-based perovskites for solar cell applications owing to their advantages of stability and non-toxicity. However, the disordered arrangement of Ag/Bi octahedra leads to the formation of self-trapped excitons (STEs) and hinders carrier transport, which is a severe problem in planar solar cells due to inferior charge extraction to mesoporous solar cells. Herein, we employ ethoxylated polyethylenimine (PEIE) to regulate the growth of Cs₂AgBiBr₆ films by sandwiching it between an SnO₂ electron transport layer and a perovskite layer. On the one hand, the introduction of PEIE delays the growth of Cs₂AgBiBr₆ and improves the ordering extent of the Ag/Bi octahedra. This inhibits the formation of STEs and facilitates carrier transport, which profoundly boosts the short-circuit current density. On the other hand, PEIE modulates the energy level of the SnO₂ electron transport layer to match that of Cs₂AgBiBr₆, which reduces the open-circuit voltage loss. Consequently, the power conversion efficiency (PCE) of the planar solar cell increases by 250% after PEIE modification from 0.59% to 1.52%. In addition, the PEIE-decorated device maintains over 90% of its initial PCE for 30 days under ambient conditions, demonstrating good environmental stability.