The photophysical properties of [6,6]-phenyl C-61 butyric acid methyl ester (PCBM) are found to differ depending on its concentration in thin films. A film of dispersed PCBM molecules in polystyrene (PS) showed only weak absorption in the visible region (400-700 nm) with a single S-1 -> S-0 emission band at around 700 nm. By comparison, a film of pristine PCBM showed heightened absorption in the visible region and the presence of an additional higher energy emission at similar to 500 nm assigned to the presence of charge transfer (CT) transitions in crystalline PCBM. The energy of the CT exciton is estimated to be similar to 2.8 eV. (C) 2007 Elsevier B.V. All rights reserved.