Role of the effective mass and interfacial dipoles on exciton dissociation in organic donor-acceptor solar cells

Abstract

Efficient exciton dissociation at a donor-acceptor interface is the crucial, yet not fully understood, step for obtaining high efficiency organic solar cells. Recent theoretical work suggested an influence of polymer conjugation length and of interfacial dipoles on the exciton dissociation yield. This necessitates experimental verification. To this end, we measured the dissociation yield of several polymer/C${}_{60}$ planar heterojunction solar cells up to high electric fields. The results indeed prove that the yield of exciton dissociation depends strongly on the conjugation length of the polymers. Complementary photoemission experiments were carried out to assess the importance of dipoles at the donor-acceptor interfaces. Comparison of exciton dissociation models with experimental data shows that the widely used Onsager-Braun approach is unsuitable to explain photodissociation in polymer/C${}_{60}$ cells. Better agreement can be obtained using ``effective mass'' models that incorporate conjugation length effects by considering a reduced effective mass of the hole on the polymer and that include dielectric screening effects by interfacial dipoles. However, successful modeling of the photocurrent field dependence over a broad field range, in particular for less efficient solar cell compounds, requires that the dissociation at localized acceptor sites is also taken into account.