Development of a Complex High-Conductivity Hole Transport Layer with Energy-Level Control for High-Efficiency Organic Solar Cells by the Solution Process

In this study, organic solar cells (OSCs) with a photoactive layer based on the bulk heterojunction structure of the high-performance polymer donor PM6 and non-fullerene acceptor BTP-eC9 were developed. Hydrogen molybdenum bronze (HXMoO3) was introduced as a hole transport layer (HTL) in lieu of poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS), a common HTL in OSCs, for energy-level control and enhanced charge transport. UV photoelectron spectroscopy revealed that the highest occupied molecular orbital (HOMO) level of HXMoO3 (−5.10 eV) was lower than that of PEDOT:PSS (−5.00 eV). The VOC of the OSC with HXMoO3 as the HTL (0.818 V) was higher than that of the device with PEDOT:PSS (0.636 V). By the energy-level alignment with enhanced VOC, the energy conversion efficiency of the device with HXMoO3 (13.1%) was found to be higher than that of the device with PEDOT:PSS (8.4%). Ag nanowires (NWs) were added to the HTLs to improve their conductivity, and enhancements in electrical properties owing to increased intercontact due to the high conductivity of the NWs and their molecular rearrangement with HXMoO3 were determined. The increases in the short-circuit current density, fill factor, and open-circuit voltage of the fabricated PM6:BTP-eC9 device contributed to its outstanding properties, as demonstrated by its enhanced power conversion efficiency of 14.0%. The secondary binding and alignment of HXMoO3 and Ag NWs were determined by X-ray photoelectron spectroscopy and field-emission scanning electron microscopy, and the structure of the OSC was shown to improve carrier transport and interfacial contact. This work can provide a solution to address the low energy conversion efficiency of current OSCs and develop various applications.