Promising Cobalt Oxide Hole Transport Layer for Efficient and Stable Inverted Perovskite Solar Cells

Abstract The inverted perovskite solar cells (PSCs) based on inorganic metal oxide hole transport layers (HTLs) bear the merits of high device stability and low material cost. However, the existence of metal‐vacancy defects on the surface of the metal oxide layer is a key constraint on achieving high efficiency and stability, like the case of the well‐known nickel oxide (NiO_x) HTL. Here, a cobalt oxide (CoO_x) HTL with the defect‐less surface is prepared by a solution process using Co(OH)₂ as the cobalt source and water as an eco‐friendly solvent for the first time. The PSCs based on CoO_x HTL show superior thermal and ultraviolet stability over the conventional NiO_x counterparts. Theoretical calculations reveal that CoO_x has higher formation energy of metal‐vacancy defect as well as higher interfacial adhesion energy than NiO_x, resulting in a chemically stable HTL/perovskite interface. After further manipulating the microstructure and the electronic properties of the CoO_x HTL via magnesium acetate doping, a 22.35% efficiency is achieved with an ambient‐processed FA_0.4MA_0.6I₃ light‐absorbing layer. Such an efficiency exceeds all of the existing results reported for CoO_x‐based PSCs and a higher value of >24% is attainable via facile interface modification. The according device also demonstrates robust operational stability in the air without encapsulation.