Understanding different oxidation methods for the hole transport layer Spiro-OMeTAD to improve perovskite solar cell performance

The preparation of a high-performance hole transport layer is a pivotal factor in achieving efficiency and stable perovskite solar cells. 2,2’,7,7’-Tetrakis[N, N-di(4-methoxyphenyl)amino]−9,9’-spirobifluorene (Spiro-OMeTAD) currently stands as the most widely employed hole transport material in high-performance perovskite solar cells. The current methodologies for its preparation primarily revolve around three techniques: O2 oxidation, cobalt salt doping, and CO2 bubbled doping. In this study, we systematically investigated and analyzed Spiro-OMeTAD prepared through these three methods, from solution and film to device. The CO2-bubbled method and Co-doped method allow for faster and more complete oxidation of Spiro-OMeTAD while maintaining conductivity and energy level matching. Therefore, the film of both methods shows better carrier extract capabilities and defect states than that of O2-oxidized. In particular, the film of the CO2-bubbled method had better hydrophobicity and thermal stability, showing the least degradation at 85 °C annealing, which can be attributed to the removal of hydrophilic Li+. This study could inspire further optimization of Spiro-OMeTAD film fabrication processes in perovskite solar cells.