Exciplex dynamics in a blend of π-conjugated polymers with electron donating and accepting properties: MDMO-PPV and PCNEPV

The photophysical properties of a solution processed blend of two semiconducting polymers with electron donating and electron accepting properties, respectively, as used in polymer photovoltaic devices have been investigated. We show that in the binary mixture of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and poly[oxa-1,4-phenylene-(1-cyano-1,2-vinylene)-(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene)-1,2-(2-cyanovinylene)-1,4-phenylene] (PCNEPV) photoexcitation of either one of the polymers results in formation of a luminescent exciplex at the interface of the two materials. Photoinduced absorption spectroscopy shows that this exciplex can decay to the lowest triplet state (T1) of MDMO-PPV. Application of an electric field results in dissociation of the marginally stable exciplex into charge carriers, which provides the basis for the photovoltaic effect of this combination of materials. Spin allowed recombination of the charge carriers to the MDMO-PPV T1 state is invoked to explain the field-enhanced quantum yield for triplet formation observed by photoinduced reflection measurements on photovoltaic devices made from the composite films. The field enhanced triplet yield is identified as loss mechanism for the photovoltaic performance of this combination of materials.