Impact of Photo-Induced Doping of Spiro-OMeTAD as HTL on Perovskite Solar Cell Hysteresis Dynamics

The doping strategy employed for enhancing the hole conductivity of spiro-OMeTAD as a hole transport layer (HTL) in perovskite solar cells (PSCs) is rarely specified in the literature. However, as revealed in this work, the specifics of the doping process with respect to the type of storage and its duration can have important consequences for the evolution of hysteresis dynamics in PSCs. The variability in photovoltaic metrics and hysteresis loci in current-voltage (J-V) curves are studied before and after light soaking, as well as at different scan rates, to determine operational differences as a result of a given doping process. With the aid of X-ray photoelectron spectroscopy (XPS) results showing the formation of deleterious compounds such as Li2O and LiF, next to proposed operational band alignments, the evolution of hysteresis trends for different doping strategies is explained. Finally, utilizing electrochemical impedance spectroscopy (EIS), signatures of HTL bulk and interfacial conductivity modulation are identified. The propagation of a third semicircle in the low-frequency part of the Nyquist plot is ascribed to performance loss caused by unfavorable interfacial electrochemical reactions, while the shrinkage of the high-frequency arc is linked with performance gain due to increased HTL bulk conductivity. This work underlines the important correlation between the spiro-OMeTAD doping strategy with hysteresis evolution as well as loss and gain in PSC performance under operational conditions.