Approaching the Fill Factor Limit in Dopant-Free Hole Transporting Layer-Based All-Inorganic Perovskite Solar Cells

As an important part of perovskite solar cells (PSCs), hole transporting layer (HTL) has a critical impact on the performance and stability of the devices. In an attempt to alleviate the moisture and thermal stability issues from the commonly used HTL Spiro-OMeTAD with dopant, it is urgent to develop novel HTLs with high stability. In this study, a new class of polymers D18 and D18-Cl are applied as undoped HTL for CsPbI₂Br-based PSCs. In addition to the excellent hole transporting properties, we unveil that D18 and D18-Cl with larger thermal expansion coefficient than that of CsPbI₂Br could impose a compressive stress onto the CsPbI₂Br film upon thermal treatment, which could release the residual tensile stress in the film. As a result, the efficiency of CsPbI₂Br-based PSCs with D18-Cl as HTL reaches 16.73%, and the fill factor (FF) exceeds 85%, which is one of the highest FF records for the conventional-structured device to date. The devices also show impressive thermal stability with over 80% of the initial PCE retained after 85 °C heating for 1500 h.