(Invited) Reducing the Photon Energy Loss in Polymer Solar Cells

Abstract

A crucial issue facing polymer-based solar cells (PSCs) is how to manage the energetics of the polymer/fullerene blends to maximize short-circuit current density (J SC) and open-circuit voltage (V OC) at the same time and thus the power conversion efficiency (PCE). A key to resolving the issue is to reduce the relatively large photon energy loss (E loss), which is defined by E g– eV OC, where E gis the optical bandgap of the material. One example is a semiconducting polymer based on naphthobisoxadiazole (PNOz4T). PNOz4T, with a narrow E gof 1.52 eV, led to high V OCs of approximately 1 V and high PCEs of more than 8%, when combined with PC71BM. As a result, the E losswas found to be 0.52–0.56 eV, which is much smaller than that of typical polymer systems (0.7–1.0 eV) and approaches the values for inorganic and perovskite solar cells. The small E lossis due to the very small energy offset that is a driving force for the photoinduced charge separation, i.e., the offset of the LUMO energy levels between PNOz4T and PC71BM was 0.1 eV. It is also interesting to note that PCEs of the PNOz4T system are the highest values among the PSCs, that used fullerene derivatives as the electron acceptor, having such small E losss. We also show ternary blend system with well controlled energetic alignment can give small E losss with less than 0.5 eV.