Simple and Low‐Cost Vanadyl Oxalate as Hole Transporting Layer Enables Efficient Organic Solar Cells

Abstract Organic solar cells (OSCs) with the conventional configuration usually use polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS) as the hole‐transporting layer (HTL); however, its acidity tends to affect the performance and long‐term stability of the devices. Therefore, replacing PEDOT:PSS with other more stable HTLs is essential for realizing the practical applications of OSCs. To achieve this goal, a simple and low‐cost vanadyl oxalate (VOC 2 O 4 ) is identified as a HTL to facilitate high power conversion efficiencies (PCEs), good stability, and high thickness tolerance to be achieved in OSCs. The VOC 2 O 4 thin film can be easily prepared by spin‐coating from its aqueous solution onto ITO/glass substrate and thermally annealed at 100 °C to exhibit high transmittance, conductivity, and work function. It can be applied as a robust HTL with wide processing conditions, especially after being heated at 200 °C and treated with UV‐ozone (UVO) to afford a very high PCE of 18.94% in OSCs. This value is among the highest PCEs obtained for binary OSCs. In addition, the derived OSCs exhibit high thickness tolerance and better stability than those based on PEDOT:PSS as HTL. These results reveal that VOC 2 O 4 is an excellent HTL for OSCs, having great potential for large‐area device applications.