Anion-Modulated Chemical Doping of Organic Hole Conductor Boosts Efficiency and Stability of Perovskite Solar Cells

Chemical doping of organic semiconductors enables significant progress in improving their optoelectronic performance. However, the correlation between doping counter ions and charge-transport mechanism has not been yet well-understood. In this study, it is discovered that the anion-dependent degree of delocalization (DOD) of lithium-based dopants significantly determines the doping kinetics as well as the conductivity of organic hole transport layer (HTL), leading to large variation in solar cell efficiency and device stability. Specifically, the incorporation of bis(pentafluoroethanesulfonyl) imide (PFSI-) as the anion with a high DOD results in one order of magnitude higher film conductivity and thus an elevated power conversion efficiency (PCE) exceeding 22.1%, much higher than the state-of-the-art lithium bis(trifluoromethane)sulfonimide (LiTFSI) (21.1%) and lithium hexafluorophosphate (LiPF6) (20.0%). Moreover, the dopant LiPF6 with a smaller DOD produces higher doping yield of HTL accompanied by stronger light-induced PCE fluctuation. Structural analysis reveals anion-modulated ion exchange kinetics determine the hole-transport mechanism and device photostability. To mitigate these detrimental effects, a versatile strategy of Li+ solvation is developed to modulate the anion dissociation, enabling simultaneous improvement of device efficiency and stability. This study elucidates an intriguing and generally applicable doping mechanism, and envisages a bright future to further developing efficient and stable organic electronics.

Automated summary

Chemical doping of organic semiconductors can greatly improve their optoelectronic performance, but the correlation between doping counter ions and charge-transport mechanism is not well understood. This study reveals that the anion-dependent degree of delocalization of lithium-based dopants plays a significant role in determining the doping kinetics and conductivity of organic hole transport layer. By modulating the anion dissociation, the efficiency and stability of organic electronics can be simultaneously improved.