PCBM single crystals: remarkably enhanced band-like charge transport, broadband UV-visible-NIR photo-responsivity and improved long-term air-stability†
We report the charge transport mechanism, long-term stability and UV-visible-NIR photo-responsivity of single crystals of PCBM (phenyl-C61-butyric acid methyl ester) – a dominant acceptor material in organic photovoltaics. Despite PCBM's paramount role in such devices, its intrinsic properties are largely unknown because it forms highly disordered solution-processed films, the electron transport mechanism remains ill-defined, and the long-term stability is poor – posing a major bottleneck for advancing cell efficiency and stability. We employed a liquid–liquid interfacial precipitation strategy to grow single crystals of PCBM, which allowed us to experimentally elucidate its electron transport properties, long-term stability and photo-responsivity. Temperature-dependent mobility studies enabled us to reveal its charge transport mechanism. Promisingly, PCBM single crystals were found to exhibit a more favorable band-like charge transport mechanism at room temperature and present electron mobility exceeding that of their thin-film counterparts by two orders of magnitude. Photodetectors based on single crystals show broadband photo-responsivity from the UV and visible to NIR regions. Long-term stability tests showed that the performance of devices based on single crystals remained 80% after 480-hour aging, whereas the performance of thin film devices dropped by over 80% under the same conditions. Our findings underscore single crystals as a key strategy to achieve breakthroughs in highly efficient and stable devices.