Resonance energy transfer from organic chromophores to fullerene molecules

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Abstract
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 C61 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 Förster radius of 3.1nm. Using standard Förster energy transfer theory, we calculate a Förster radius (R0) of around 2.7nm 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.