Cross Self-n-Doping and Electron Transfer Model in a Stable and Highly Conductive Fullerene Ammonium Iodide: A Promising Cathode Interlayer in Organic Solar Cells

Self-n-doped stable and highly conductive fullerene ammoniums with excellent thickness-tolerance could act as promising cathode interlayers to facilitate electron transfer and improve power conversion efficiencies (PCEs) of large-area organic solar cells (OSCs). Herein, systematic studies on electronic and spatial structure of fullerene ammonium iodide (PCBANI) have been performed to elucidate the cross self-n-doping mechanism. In PCBANI, partial electron transfer from iodide to the core fullerene could result in n-doping and high conductivity. This doping process forms strong anion-pi interactions between iodides and fullerene cores accompanied by side-chain's head-to-tail cation-pi interactions that contribute to the stabilization of the n-doped fullerene. Moreover, two possible pathways of the cross self-n-doping involving intermolecular exchange and transfer of iodide have been verified by experiment combined with computational modeling. Based on all of the solid evidence, we propose an electron transfer model for PCBANI in which the iodide sandwiched in the n-doped fullerene core acts as a shuttle to transfer electrons via redox processes. This finding provides a strategy for electrically doping and assembling fullerenes to improve their performance in photovoltaic devices and endow them with new functionalities that could be applied to optoelectronics and organic electronics.