Charge separation in polyflourene composites with internal donor/acceptor heterojunctions

The photosensitivity of semiconducting polymers can be enhanced by blending donor (D) and acceptor (A) polymers to optimise photoinduced charge separation. The photoluminescence of two such polymers, an acceptor poly(9,9-dioctylfluorene-co-benzothidiadiozole) (BT) and a donor poly(9,9-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenylbenzidine) (BFB) are quenched in their mutual blend indicative of rapid and efficient separation of photogenerated electron–hole pairs with electrons in the BT acceptor and holes in the BFB donor. In order to be useful for photovoltaic applications, the excitons have to reach a D/A interface before they decay radiatively or non-radiatively. Whereas charge separation seems to be efficient in these polymer networks, charge transport still remains a limiting factor due to the imperfect network structures and low charge carrier mobilities. This charge transport can be improved by controlling the morphology of the blend. Although the initial power conversion efficiency (PCE) was low for our composites (0.12%), the blends show promising photovoltaic characteristics; the efficiency is 220 times higher than for diodes made with pure BFB. We report work aimed at improving the efficiency via control of the blend morphology and the use of polymeric anodes.