Formation Mechanism of Fullerene Cation in Bulk Heterojunction Polymer Solar Cells

The charge carrier dynamics in blend films of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and conjugated polymers with different ionization potentials are measured using transient absorption spectroscopy to study the formation mechanism of PCBM radical cation, which was previously discovered for blend films of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and PCBM. On a nanosecond time scale after photoexcitation, polymer hole polaron and PCBM radical anion are observed but no PCBM radical cation is found in the blends. Subsequently, the fraction of polymer hole polarons decreases and that of PCBM radical cations increases with time. Finally, the fraction of PCBM radical cations becomes constant on a microsecond time scale. The final fraction of PCBM radical cation is dependent on the ionization potential of polymers but independent of the excitation wavelength. These findings show that the formation of PCBM radical cation is due to hole injection from polymer to PCBM domains. Furthermore, the energetic conditions for such hole injection in polymer/PCBM blend films are discussed on the basis of Monte Carlo analysis for hole hopping in a disordered donor/acceptor heterojunction with varying energetic parameters.