Managing Defects Density and Interfacial Strain via Underlayer Engineering for Inverted CsPbI2Br Perovskite Solar Cells with All‐Layer Dopant‐Free

Abstract Inorganic perovskite CsPbI 2 Br has advantages of excellent thermal stability and reasonable bandgap, which make it suitable for top layer of tandem solar cells. Nevertheless, solution‐processed all‐inorganic perovskites generally suffer from high‐density defects as well as significant tensile strain near underlayer/perovskite interface, both leading to compromised device efficiency and stability. In this work, the defect density as well as interfacial tensile strain in inverted CsPbI 2 Br perovskite solar cells (PeSCs) is remarkably reduced by using a bilayer underlayer composed of dopant‐free 2,2′,7,7′‐tetrakis( N,N‐ dip‐methoxyphenylamine)‐9,9′‐spirobifluorene (Spiro‐OMeTAD) and copper phthalocyanine 3,4′,4″,4′″‐tetrasulfonated acid tetrasodium salt (TS‐CuPc) nanoparticles. As compared to control devices with pristine Spiro‐OMeTAD, devices based on Spiro‐OMeTAD/TS‐CuPc exhibit remarkably improved photovoltaic performance and enhanced thermal/humidity stability due to the better perovskite crystallization, improved interfacial passivation, and hole‐collection as well as efficient interfacial strain release. As a result, a champion efficiency of 14.85% can be achieved, which is approaching to the best reported for dopant‐free and inverted all‐inorganic PeSCs. The work thus provides an efficient strategy to simultaneously regulate the defects density and strain issue related to inorganic perovskites.