Probing charge transfer complex (CTC) states in organic solar cells using photocurrent spectroscopy

Diketopyrrolopyrrole (DPP) containing copolymers have generated considerable amount of interest in bulk heterojunction organic photovoltaics due their high power conversion efficiency (above 10 percent) and mobility. Within a bulk heterojunction solar cell, the combination of a donor and acceptor chromophores facilitates charge transfer from the donor to the acceptor and may result in the formation of interfacial electronic state at the donor-acceptor interface. Here, we present photocurrent studies to identify the interfacial charge complex states in five DPP based copolymer (donor): fullerene devices using Fourier transform photocurrent spectroscopy (FTPS) and monochromatic photocurrent spectroscopy. The optical band gap of DPP based copolymer ranges from 1.4-1.7eV. Our studies show that a larger optical band gap difference between the donor and the acceptor prohibit the formation of a stable charge transfer complex state. Further, we also observe that devices in which no charge transfer state was observed show a better efficiency than devices in which charge transfer state is observed.