Protic ionic liquids as p‐dopant for organic hole transporting materials and their application in high efficiency hybrid solar cells

Chemical doping is a powerful method to improve the charge transport and to control the conductivity in organic semi-conductors (OSs) for a wide range of electronic devices. We demonstrate protic ionic liquids (PILs) as effective p-dopant in both polymeric and small molecule OSs. In particular, we show that PILs promote single electron oxidation, which increases the hole concentration in the semiconducting film. The illustrated PIL-doping mechanism is compatible with materials processed by solution and is stable in air. We report the use of PIL-doping in hybrid solar cells based on triarylamine hole transporting materials, such as 2,2′,7,7′-tetrakis(N,N-di-p- methoxyphenyl-amine)9,9′-spirobifluorene (spiro-OMeTAD). We show improved power conversion efficiency by replacing lithium salts, typical p-dopants for spiro-OMeTAD, with PILs. We use photovoltage−photocurrent decay and photoinduced absorption spectroscopy to establish that significantly improved device performance is mainly due to reduced charge transport resistance in the hole-transporting layer, as potentiated by PIL-doping.