ABSTRACT
Organic π-conjugated polymers and oligomers, due to their potential in the development of plastic solar cells that are lightweight, mechanically flexible, and low cost, have drawn significant attention as active materials in organic photovoltaic (OPV) devices.1-3 Because of the discovery of rapid and efficient photoinduced electron transfer from poly(2-methoxy-5-(2′-ethyl-hexyloxy)-para-phenylenevinylene (MEHPPV) to C60 in the early 1990s,4,5 the use of fullerene derivatives as electron acceptors in OPVs has become ubiquitous. The so-called bulk-heterojunction (BHJ) OPVs have been the most studied, using a single-layer blend of a conjugated polymer and a fullerene derivative, such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). In this type of device, appropriate phase segregation enables the formation of a BHJ at the interface of the donor and acceptor components. Current state-of-the-art single-layer conjugated polymer-based OPVs achieve power conversion efficiencies in the range of ~7%, and the highest reported efficiency of 6.5% is for a tandem solar cell.6-15 Solar cells with optimized performance have been developed using P3HT/PC61BM blends,16 and these cells feature nearly ideal photon-to-current quantum efficiency (IPCE) in the mid-visible region. However, to produce highly efficient OPVs, light absorption by the active layer must be extended into the near-infrared region while, at the same time, preserving the high IPCE and open circuit voltage. In principle, the overall power conversion efficiency of the device is mainly determined by several individual efficiencies: light-harvesting efficiency across the visible and near-infrared regions, exciton diffusion to the donor-acceptor interface, photoinduced charge separation, and the mobility of the charge carriers produced by photoinduced charge separation and charge collection.1
