Charge transfer complexes in polymer: fullerene bulk heterojunction solar cells

Abstract

For an efficient conversion of a flux of solar photons into an electric current by organic materials, the presence of a material interface between an electron donating and electron accepting material is crucial. Most successful active layers for organic solar cells comprise a blend of conjugated polymers as electron donors and fullerenes as electron acceptors, exhibiting power conversion efficiencies higher than 6%, nowadays. In order to find pathways to increase this efficiency further, properties of the electronic states at the donor/acceptor interface and their role in determining the overall power conversion efficiency, are investigated in this work. To probe these interfacial properties, the fast and highly sensitive technique Fourier-Transform Photocurrent Spectroscopy (FTPS) is used to detect the weak absorption caused by ground state interaction of polymers and fullerenes, forming a charge transfer complex (CTC). Optical excitation of this donor/acceptor CTC by light with photon energies lower than the optical gap of both the donor and acceptor materials, results in the creation of a charge transfer (CT) exciton or CT state, comprising an electron in the acceptor phase, coulombically bound to a hole on the donor phase. In our study, such a CT transition within the optical gap of both pure materials was detected in all polymer:fullerene solar cells exhibiting a significant photovoltaic effect. In these cases, the energy of the CT state is lower than the energy of the excited states of the pure blend constituents, and can efficiently be populated. The competition between geminate recombination and field dependent dissociation of CT excitons can still limit photocurrent production in some polymer:fullerene material combinations. ...