Understanding the correlation between energy-state mismatching and open-circuit voltage loss in bulk heterojunction solar cells

Photoinduced intermolecular charge transfer (PICT) determines the voltage loss in bulk heterojunction (BHJ) organic photovoltaics (OPVs), and this voltage loss can be minimized by inducing efficient PICT, which requires energy-state matching between the donor and acceptor at the BHJ interfaces. Thus, both geometrically and energetically accessible delocalized state matching at the hot energy level is crucial for achieving efficient PICT. In this study, an effective method for quantifying the hot state matching of OPVs was developed. The degree of energy-state matching between the electron donor and acceptor at BHJ interfaces was quantified using a mismatching factor (MF) calculated from the modified optical density of the BHJ. Furthermore, the correlation between the open-circuit voltage (V_oc) of the OPV device and energy-state matching at the BHJ interface was investigated using the calculated MF. The OPVs with small absolute MF values exhibited high V_oc values. This result clearly indicates that the energy-state matching between the donor and acceptor is crucial for achieving a high V_oc in OPVs. Because the MF indicates the degree of energy-state matching, which is a critical factor for suppressing energy loss, it can be used to estimate the V_oc loss in OPVs.