Combined optical and electrical modeling of polymer:fullerene bulk heterojunction solar cells

Optical interference effects are important for the total absorption as well as the profile of the exciton generation rate in polymer:fullerene bulk heterojunction solar cells. For solar cells with an active layer of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] as electron donor and [6,6]-phenyl C61 butyric acid methyl ester as electron acceptor, the total exciton generation rate can be directly extracted from the saturated photocurrent. It is demonstrated that for solar cells with an active layer thickness smaller than 250 nm, a constant exciton generation profile, based on this extracted total rate, gives identical electrical characteristics as compared to exciton generation profiles from an optical model. For thicker cells interference effects have to be taken into account, since a uniform generation profile leads to an overestimation of recombination losses and space-charge formation.