δ‐Phase Management of FAPbBr 3 for Semitransparent Solar Cells

Formamidinium lead bromide (FAPbBr 3 ) perovskite owing to suitable wide band gap has promising application in the fields of semitransparent and tandem solar cells. However, the effect of photoinactive δ phase (δ‐FAPbBr 3 ) on the film and device properties is still unclear and rarely investigated. In this work, the authors find that the growth of δ‐FAPbBr 3 has a strong relationship with the property of the underlayer. On the hydrophilic underlayer, crystalline δ‐FAPbBr 3 is uniformly distributed in the bulk of the α‐FAPbBr 3 film, which has a lower defect density and better carrier transport. A power conversion efficiency of 9.12% is achieved by the management of δ‐FAPbBr 3 and using a phosphonate/phosphine oxide dyad additive, which is the highest value among the inverted FAPbBr 3 ‐based perovskite solar cells (PSCs). Moreover, the light utilization efficiency of a semitransparent device reaches 3.15%. This work provides new insights and methods for the realization of high‐performance FAPbBr 3 ‐based PSCs.