Solution processed WO3 layer for the replacement of PEDOT:PSS layer in organic photovoltaic cells

Abstract

Tungsten oxide layer is formed uniformly by a sol-gel technique on top of indium tin oxide as a neutral and photo-stable hole extraction layer (HEL). The solution processed tungsten oxide layer (sWO(3)) is fully characterized by UV-Vis, XPS, UPS, XRD, AFM, and TEM. Optical transmission of ITO/sWO(3) substrates is nearly identical to ITOs. In addition, the sWO(3) layer induces nearly ohmic contact to P3HT as PEDOT: PSS layer does, which is determined by UPS measurement. In case that an optimized thickness (similar to 10 nm) of the sWO(3) layer is incorporated in the organic photovoltaic devices (OPVs) with a structure of ITO/sWO(3)/P3HT: PCBM/Al, the power conversion efficiency (PCE) is 3.4%, comparable to that of devices utilizing PEDOT: PSS as HEL. Furthermore, the stability of OPV utilizing sWO3 is significantly enhanced due to the air-and photo-stability of the sWO(3) layer itself. PCEs are decreased to 40% and 0% of initial values, when PEDOT: PSS layers are exposed to air and light for 192 h, respectively. In contrast, PCEs are maintained to 90% and 87% of initial PCEs respectively, when sWO(3) layers are exposed to the same conditions. Conclusively, we find that solution processed tungsten oxide layers can be prepared easily, act as an efficient hole extraction layer, and afford a much higher stability than PEDOT: PSS layers.