%0 Journal Article
%@ 1932-7447
%A Tregnago, G
%A Wykes, M
%A Paterno, GM
%A Beljonne, D
%A Cacialli, F
%D 2015
%F discovery:1471747
%J Journal of Physical Chemistry C
%N 21
%P 11846-11851
%T Low-Temperature Photoluminescence Spectroscopy of Solvent-Free PCBM Single-Crystals
%U https://discovery.ucl.ac.uk/id/eprint/1471747/
%V 119
%X PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) is a highly soluble C60 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 ∼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 Herzberg–Teller 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 T1g and Hg excited states of C60 and hence that the Hg state plays a larger role in the relaxed excited state of PCBM than in that of C60.
%Z ACS AuthorChoice - This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.