Basic aspects for improving the energy conversion efficiency of hetero-junction organic photovoltaic cells

Hetero-junction organic photovoltaic (OPV) cells consisting of donor (D) and acceptor (A) layers have been regarded as next-generation PV cells, because of their fascinating advantages, such as lightweight, low fabrication cost, resource free, and flexibility, when compared to those of conventional PV cells based on silicon and semiconductor compounds. However, the power conversion efficiency (η) of the OPV cells has been still around 8%, though more than 10% efficiency has been required for their practical use. To fully optimize these OPV cells, it is necessary that the low mobility of carriers/excitons in the OPV cells and the open circuit voltage (V (OC)), of which origin has not been understood well, should be improved. In this review, we address an improvement of the mobility of carriers/excitons by controlling the crystal structure of a donor layer and address how to increase the V (OC) for zinc octaethylporphyrin [Zn(OEP)]/C(60) hetero-junction OPV cells [ITO/Zn(OEP)/C(60)/Al]. It was found that crystallization of Zn(OEP) films increases the number of inter-molecular charge transfer (IMCT) excitons and enlarges the mobility of carriers and IMCT excitons, thus significantly improving the external quantum efficiency (EQE) under illumination of the photoabsorption band due to the IMCT excitons. Conversely, charge accumulation of photo-generated carriers in the vicinity of the donor/acceptor (D/A) interface was found to play a key role in determining the V (OC) for the OPV cells.