Oxidized Carbon Materials as Hole Transport Layers for High-Performance Organic Photovoltaics

Organic photovoltaics (OPVs) have recently advanced as promising solar cells owing to their many advantages, including the possibility of a low-temperature solution process, lightweight, and flexibility. In OPVs, the role of the interlayer is important for efficient charge transport from the photoactive layer to electrodes. However, as a hole-transporting layer (HTL), molybdenum oxide and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) are still used for commercialization despite their drawbacks. The development of novel materials with suitable energy levels and solvent orthogonality is required to enhance hole extraction and optimize the film morphology. Herein, oxidized carbon soot (OCS) and OCS-functionalized octadecylamine (OCS-ODA) nanoparticles as HTL materials are synthesized. As a large number of oxygen functional groups are produced via Hummer's method, the devices with the OCS-ODA exhibit high hole extraction ability. The ODA long alkyl chains functionalized by facile process also improve film morphology to minimize contact resistance and charge recombination. The small-area (SA), large-area, and flexible (F) OPVs with OCS-ODA show power conversion efficiencies of 15.04%, 14.57%, and 12.73%, respectively. In particular, OPVs with OCS-ODA are further demonstrated to possess storage stability in SA-OPVs (71% retention after 450 h) and mechanical stability in F-OPVs (78% retention after 1000 bendings).