Modification of Energy Level Alignment for Boosting Carbon‐Based CsPbI2Br Solar Cells with 14% Certified Efficiency

Abstract Hole transfer material (HTM)‐free, carbon‐based all‐inorganic perovskite solar cells (C‐PSCs) are promising alternatives to conventional organic–inorganic hybrid PSCs in addressing thermal and moisture instability issues. However, the energy level mismatch between the inorganic perovskite and carbon electrode coupled, together with the incapability of the carbon electrode to reflect incident light for reabsorption, limits the power conversion efficiency (PCE) of C‐PSCs. To address these issues, herein, a new strategy of a hexyltrimethylammonium bromide (HTAB)‐modified CsPbI 2 Br perovskite surface is devised to reduce this energy offset from 0.70 to 0.32 eV and increase the built‐in potential by 70 mV for the final devices. Additionally, a CsPbI 2 Br perovskite film with a thickness of up to 800 nm is realized via a hot‐flow‐assisted spin coating approach in an ambient atmosphere with humidity of less than 80%. Reduced energy offset coupled with suppressed charge recombination and thick perovskite layer boosts the champion PCE of CsPbI 2 Br C‐PSCs to 14.3% ( J sc = 14.1 mA cm −2 , V oc = 1.26 V, and fill factor = 0.806), and the average PCE to 13.9% under one sun illumination. A new certified efficiency record of 14.0% is obtained for HTM‐free inorganic C‐PSCs. Meanwhile, the moisture‐resistant barrier from the alkyl chain in HTAB improves the stability of the final devices.