π-Conjugated polymers and molecules enabling small photon energy loss simultaneously with high efficiency in organic photovoltaics

Organic photovoltaics (OPVs) are a topic of significant research interest in the field of renewable energy as well as organic electronics. The crucial issue in OPVs is the improvement of the power conversion efficiency (PCE). In addressing this issue, one of the most important factors is the photon energy loss (Eloss), which is defined as the difference between the bandgap of the materials and the energy corresponding to the open-circuit voltage. Typically, the Eloss for OPVs is considerably larger than that for inorganic and perovskite photovoltaics, which has prevented OPVs from generating larger photovoltages. In parallel, reducing the Eloss for OPVs causes a loss of driving-force energy for charge generation, which is detrimental to the generation of photocurrent. Thus, OPVs have been facing a trade-off between photocurrent and photovoltage. However, a number of recently developed π-conjugated materials for use as p-type and n-type organic semiconductors have been shown to enable small Eloss values that are close to those for inorganic systems, simultaneously with efficient charge generation. Here, we summarize recent progress in π-conjugated polymers and molecules that enable small Eloss and high PCE at the same time. We hope that this review will be of help to chemists and materials scientists who are involved in the design of materials and blends with an eye toward highly efficient OPVs.