Resonance energy transfer from organic chromophores to fullerene molecules

The mechanism of charge separation in polymeric bulk heterojunction photovoltaic cells is usually described as electron transfer from the absorbing polymer to an electron acceptor material such as (6,6)-phenyl C-61 butyric acid methyl ester (PCBM). We consider the possibility of energy transfer to PCBM as another potential mechanism for charge separation. We demonstrate resonance energy transfer from a red-emitting organic chromophore (Nile red) to PCBM and measure a Forster radius of 3.1 nm. Using standard Forster energy transfer theory, we calculate a Forster radius (R-0) of around 2.7 nm for this donor-acceptor pair in polystyrene. Nile red has a similar emission spectrum to commonly used conjugated polymers used in polymer/PCBM photovoltaic cells. We consider the implications of an energy transfer mechanism on the design requirements for future photovoltaic cells. (C) 2006 American Institute of Physics.