Tuning of the excited-state properties and photovoltaic performance in PPV-based polymer blends

The authors use solvents with different boiling points and a mixture of these solvents to tune the morphology of blends formed from poly [2,5-dimethoxy-1,4-phertylenevinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (M3EH-PPV) and poly[oxa-1,4-phenylene-1,2-(1-cyano)-ethylene-2,5-dioctyloxy-1,4-phenylene-1,2-(2-cyano)-ethylene-1,4-phenylene] (CN-ether-PPV). In photoluminescence (PL), the emission of as-prepared films spin-coated from chloroform (CF) is entirely dominated by an exciplex, with no evidence for the radiative decay of either the M3EH-PPV or the CN-ether-PPV exciton. Evidently these intrachain excited species dissociate rapidly by intermolecular charge transfer, pointing to a highly intermixed blend morphology. On the other hand, the PL of films deposited from 1,2,4-trichlorobenzene (TCB) exhibits predominant emission from the M3EH-PPV exciton, indicating the presence of rather pure M3EH-PPV domains in the phase-separated polymers layers. The blend morphology is shown to have a large influence on the solar cell properties and particularly on the fill factor. For an annealed layer coated from a 1:4 TCB:CF mixture, a fill factor of 44% was achieved, which is among the highest values reported for polymer-polymer blends. For all blends the photocurrent rises linearly with light intensity, implying that bimolecular recombination and the formation of space charge from the photogenerated carriers is of minor importance. Charge carrier mobilities and bimolecular recombination coefficients were measured on the very same polymer blends used for solar cell fabrication, utilizing the method of charge carrier extraction by linearly increasing voltage technique (photo-CELIV). These studies did not reveal a significant effect of the blend morphology on the bulk carrier transport and recombination. The authors conclude that the photovoltaic properties are mainly determined by processes on the very local scale, namely the competition between the field-induced dissociation and recombination of the initially formed polaron pairs. It is proposed that the nanomorphology has a profound effect on the initial separation of these Coulombically bound electron-hole pairs and on the probability that they recombine via the formation of interfacial exciplexes or, possibly, via intrachain triplet excitons.