Low-Temperature Photoluminescence Spectroscopy of Solvent-Free PCBM Single-Crystals

PCBM ([6,6]-phenyl-C-61-butyric acid methyl ester) is a highly soluble C-60 derivative that is extensively used in organic solar cells, enabling power conversion efficiencies above 10%. Here we report, for the first time to the best of our knowledge, the photoluminescence of high-quality solvent-free PCBM crystals between room temperature and 4 K. Interestingly, the PL spectra of these crystals become increasingly structured as the temperature is lowered, with extremely well-resolved emission lines (and a minimum line width of similar to 1.3 meV at 1.73 eV). We are able to account for such a structured emission by means of a vibronic coupling model including Franck-Condon, Jahn-Teller and Herzberg-Teller effects. Although optical transitions are not formally forbidden from the low-lying excited states of PCBM, the high symmetry of the electronically active fullerene core limits the intensity of the 0-0 transition, such that HerzbergTeller transitions which borrow intensity from higher-lying states represent a large part of the observed spectrum. Our simulations suggest that the emissive state of PCBM can be considered as a mixture of the T-1g and Hg excited states of C-60 and hence that the H-g state plays a larger role in the relaxed excited state of PCBM than in that of C-60.