Functionalized nickel oxide as a hole transport layer for organic solar cells with simultaneous enhancement of efficiency and stability

Organic solar cells (OSCs) have achieved rapid progress, with the power conversion efficiency (PCE) exceeding 19% based on conventional device configuration, in which PEDOT:PSS is the most commonly used hole transport layer (HTL). However, the use of PEDOT:PSS is problematic for the longevity of OSCs due to its hygroscopic and acidic nature. Herein, we have developed an alternative HTL by employing a self-assembled monolayer (SAM) of phosphate to functionalize solution-processed nickel oxide (NiO_x) thin films, which are prevailing HTLs in state-of-the-art perovskite solar cells. As revealed by a combination of measurements, this SAM-modification can not only optimize band alignment by manipulating the work function of NiO_x films, but also suppress charge recombination by passivating surface traps on NiO_x films. With enhanced hole extraction efficiency and electron blocking ability, an impressive PCE of 16.7% is achieved in PM6:Y6-based OSCs with excellent reproducibility. Importantly, the optimized NiO_x-based OSCs exhibit both superior stability and higher PCE than the PEDOT:PSS-based counterparts. This study provides a facile method for modifying NiO_x to replace PEDOT:PSS as a reliable HTL for highly efficient and stable OSCs. © The Royal Society of Chemistry2024.