LESR study on PPV-PPE/PCBM composites for organic photovoltaics

Photoinduced charge carrier generation and charge transfer in the new conjugated poly(phenylene vinylene)-poly(phenylene ethynylene) (PPV-PPE) copolymers mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were studied by LESR technique at 77 K. The effective charge separation has been verified by two ESR signals belonging to positive polarons on the polymer chains and to radical anions of PCBM molecules, respectively. The relaxation times T1 and T2 of the anion radical of PCBM were estimated by anisotropic linewidth analysis and microwave power saturation studies. The low temperature recombination of long living charge carrier pairs after cessation of photoexcitation was studied and described by a tunnel model in which the recombination time depends on the distances between charge carrier pairs. Results on PPV-PPE/PCBM were compared with those of a poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV)/PCBM composite.