Synergistic effect of ionic liquid-doped spiro-OMeTAD: simultaneous management of energy level alignment and interfacial traps in perovskite solar cells

Abstract

An organic 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), in conjunction with the additives lithium bis(trifluoromethanesulfonyl)imide salt (Li-TFSI) and tert-butylpyridine (t-BP), is the most common hole transporting layer (HTL) used in perovskite solar cells (PSCs) with n–i–p structures. Nevertheless, the HTL exhibits poor stability under atmospheric conditions, and the perovskite surface frequently contains multiple defects, thereby impeding the stability and efficacy of PSCs. Therefore, it is necessary to adopt effective approaches that can concurrently stabilize spiro-OMeTAD and provide passivation to the perovskite (PVK) surface. Herein, we synthesized butylamine bis(trifluoromethyl)sulfonylimine (BATFSI) ionic liquids, which play roles as dual dopants in the HTL. On the one hand, the conductivity of the HTL, as well as the energy band alignment between the PVK and HTL, are improved owing to the generation of more spiro-OMeTAD radicals. On the other hand, the migration of butylammonium (BA⁺) ions from the HTL towards the surface of the PVK could alleviate the traps at the PVK/HTL interface. Ultimately, the BATFSI-doped PSCs achieved the best PCE of 24.47%, much higher than that of un-doped PSCs (22.11%). Moreover, due to the hydrophobicity of the ionic liquids, the devices still retain over 80% of their original PCE after 40 days of air exposure. The successful introduction of the BATFSI ionic liquid has proven to be effective as a doping material for the HTL, demonstrating its excellent application prospects.