Ion-Modulated Radical Doping of Spiro-OMeTAD

Spiro-OMeTAD is one of the most studied and suitable hole transport layer materials (HTL) due to its facile implementation and high performance in organic-inorganic electronic devices. Conventional doping of spiro-OMeTAD with hygroscopic lithium salts and volatile 4-<i>tert</i>-butylpyridine is a time-consuming process and also leads to poor device stability. Additionally, the complex in-situ oxidation processes makes it challenging to understand the mechanism of conventional spiro-OMeTAD doping, and further limit the development of stable HTLs with high PCEs.¹ Here, I introduce a clean and free-post-oxidization doping recipe for spiro-OMeTAD by using stable organic radicals as the dopant and ionic salts as the doping modulator (referred to as ion-modulated (IM) radical doping).² In this doping strategy, the radicals provide hole polarons that instantly increase the conductivity and work function, and ionic salts further modulate the work function (WF) by affecting the energetics of the hole polarons. More attractively, the IM radical doping strategy provides a facile yet effective approach to optimizing separately the conductivity and WF of organic semiconductors for a variety of optoelectronic applications. The doped spiro-OMeTAD based on our IM radical doping strategy delivered a high power conversion efficiency (PCE) of perovskite-based solar cells (PSCs) over 25% and excellent stability (T₈₀ for ~ 1200 h under 70±5% relative humidity and T₈₀ for ~ 800 h under 70±3 °C without encapsulation), minimizing the trade-off between efficiency and stability of PSCs.<br/><br/>1, J. J. Yoo, et al. <i>Nature</i>, 2021, 590(7847), 587-593.<br/>2, T. Zhang, F. Wang,* et al. <i>Science</i>, 2022, 377(6605), 495-501.