Formation of methanofullerene cation in bulk heterojunction polymer solar cells studied by transient absorption spectroscopy

Photogenerated charge carriers for blend films of poly[2-methoxy-5-(3,7-dimetliyloctyloxy)- 1,4-phenyleneviiiyleiieJ (MDMO-PPV) and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) have been investigated by transient absorption spectroscopy. The blend film with a low PCBM fraction (<10wt %) exhibits a wide absorption that ranges from 900 to 1000nm, which is characteristic of the MDMO-PPV hole polaron and PCBM radical anion. On the other hand, the blend film with a higher PCBM fraction (> 30wt %) exhibits a major absorption band at similar to 900nm, which is characteristic of the PCBM radical cation. For identification of charge carriers, the absorption spectrum and molar absorption coefficient of each charged species have been evaluated separately using various combinations of electron donor and acceptor materials. Consequently, the MDMO-PPV hole polaron has been found to have a broad absorption at similar to 950 nm and the PCBM radical anion and cation show a distinct absorption at 1020 and 890 nm, respectively. On the basis of these absorption spectra, the transient spectra observed for the blend films have been simulated. The spectrum for a low PCBM fraction is well reproduced by superposition of the absorption spectra of the MDMO-PPV hole polaron and PCBM radical anion. On the other hand, the spectrum for a high PCBM fraction is well reproduced by superposition of the absorption spectra of the MDMO-PPV hole polaron, PCBM radical anion, and PCBM radical cation, which indicates that the PCBM radical cation is formed in the blend films with PCBM at a high concentration. Possible mechanisms for the formation of the PCBM. radical cation in the blend are also discussed.