Green Solvent Accelerates Spiro‐OMeTAD Oxidation for Efficient Perovskite Solar Cells

As one of the most widely used hole transport materials (HTMs) in optoelectronics devices, 2,2’,7,7’‐tetrakis[ N , N ‐di(4‐methoxyphenyl)amino]‐9,9‐spirobifluorene (spiro‐OMeTAD) has endowed perovskite solar cells (PSCs) with record power conversion efficiencies (PCEs). However, the long oxidation time in the ambient atmosphere is time‐consuming. Furthermore, commonly used solvents for spiro‐OMeTAD film processing such as chlorobenzene (CB) are toxic, which severely hinders the commercialization of PSCs. Herein, ethyl acetate (EA) serves as a superior green solvent alternative to the typical processing solvent, which substantially reduces the oxidation time of spiro‐OMeTAD. Without postoxidation treatment and any other additional oxidizing agents, the resultant device delivers a high PCE of 23.3%, significantly outperforming the CB‐based device. It is also the highest efficiency obtained from nonhalogenated solvent‐processed spiro‐OMeTAD. The electrostatic potential mapping demonstrates that the electron density at nitrogen atoms of EA‐processed spiro‐OMeTAD is more delocalized, leading to a faster oxidation. Furthermore, the processing solvent EA is a green solvent with low toxicity, and the amount of HTMs used in the fabrication of solar cells can be dramatically reduced. This study provides a new solution for processing spiro‐OMeTAD, which is ideal for the mass production of PSCs.