Energy Gap between Photoluminescence and Electroluminescence as Recombination Indicator in Organic Small-Molecule Photodiodes

Charge transfer (CT) states at the donor-acceptor interfaces play an important role in organic optoelectronic devices, yielding photocarrier generation or recombination losses. In this study, we fabricate and characterize vacuum-deposited organic photodiodes (OPDs) composed of SubPc:C60 with various active-layer thicknesses and mixing ratios in terms of the charge separation efficiency (CSE) and charge collection efficiency (CCE). We demonstrate that the combined field-assisted quenching study using both photoluminescence (PL) and electroluminescence (EL) reveals detailed information on the device physics of bulk heterojunction photodiodes as related to the CT state. Our modified PL quenching efficiency approach allows us to reasonably evaluate the CSE. In addition, we find that the EL energy is closely related to the recombination loss factor, and the energy gap between PL and EL exhibits a strong linear relationship with the CCE. As a result, the energy gap is proved to be a meaningful indicator of the carrier transport properties, which allows prediction of the CCE in organic bulk heterojunctions.