Reducing the Energy Loss to Achieve High Open‐circuit Voltage and Efficiency by Coordinating Energy‐Level Matching in Sn–Pb Binary Perovskite Solar Cells

Tin–lead (Sn–Pb) binary low‐bandgap perovskites are more environmentally friendly than conventional Pb‐based perovskites and promise to deliver high photovoltaic performance by constructing tandem solar cells. However, the energy‐level mismatch between functional layers and tremendous trap states in perovskite films make it challenging to reduce the high open‐circuit voltage (Voc) loss in Sn–Pb binary perovskite solar cells (PSCs). Herein, energy loss reduction at the hole collection interface in Sn–Pb binary PSCs is demonstrated using nickel oxide (NiOx) as the hole transport material (HTM) with optimal poly[(9,9‐bis(3′‐(N,N‐dimethylamino)propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctyfluorene)] (PFN) modification, which enables a significantly enhanced Voc compared to the traditional poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)‐based devices. The NiOx/PFN bilayer has a downward‐shifted valence band compared to PEDOT:PSS, providing well‐matched energy‐level alignment with the perovskite material, resulting in more fluent charge transfer and reduced Voc losses. The optimized device has a high Voc of 0.88 V and an efficiency of 19.80%, surpassing the previous results reported for NiOx‐based Sn–Pb PSCs. Moreover, the robust NiOx/PFN substrate and the high‐quality perovskite film grown on it make the device less vulnerable to ambient exposure. This work highlights the significance of ideal hole conductors and interface engineering in efficient and stable Sn–Pb low‐bandgap PSCs. NiOx is used as the hole transport layer with the synergistic effect of poly[(9,9‐bis(3′‐(N,N‐dimethylamino)propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctyfluorene)] (PFN) to shift the valence band downward, leading to reduced open circuit voltage (Voc) loss and improved efficiency. The champion perovskite solar cell has a remarkable Voc of 0.88 V, surpassing the previous results reported for NiOx‐based Sn‐Pb PSCs.