Hot Pure Oxygen Accelerated Oxidation of Spiro-OMeTAD for Efficient Perovskite Solar Cells with a Record Certified Fill Factor Exceeding 87%

2,2′,7,7′-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene (spiro-OMeTAD) is an irreplaceable hole transport material (HTM) for high-performance n-i-p perovskite solar cells (PSCs), and an oxidation process is necessary to improve its conductivity and hole mobility. The conventional oxidation process takes overnight or even longer in dry air with low reproducibility. Herein, we develop a hot pure oxygen treatment (HPOT) strategy to shorten the oxidation process to 5 h. Compared to the conventional method, HPOT intrinsically improves the conductivity and hole mobility of the spiro-OMeTAD film, resulting in a lower series resistance for the PSC devices. With the downshift of the Fermi level, the hole injection from the perovskite layer to the spiro-OMeTAD was facilitated. As a result, the conventional n-i-p PSC devices based on HPOT-spiro-OMeTAD deliver a certified power conversion efficiency (PCE) of 25.34% with a record certified fill factor (FF) reaching 87.02%. Meanwhile, the HPOT-spiro-OMeTAD layer affords enhanced light stability to the PSC devices.