Low-temperature, solution-processed, layered V2O5 hydrate as the hole-transport layer for stable organic solar cells

Abstract

Layered V₂O₅ hydrate has been applied as the hole transport layer (HTL) in organic solar cells (OSCs). V₂O₅ is obtained from a sodium metavanadate solution in water under ambient conditions, resulting in a final thin film of formula V₂O₅·0.5H₂O. The 0.5 water molecules are not removed from the V₂O₅ layered structure unless the sample is heated above 250 °C, which makes the thin film highly stable under real working conditions. The HTL was used in OSCs in the normal and the inverted configurations, applying metallic Ag as the back-metal electrode in both cases. Fabrication of both OSC configurations completely by solution-processing printing methods in air is possible, since the Al electrode needed for the normal-configuration OSC is not required. The work function (WF) and band gap energy (BG) of the V₂O₅ thin films were assessed by XPS, UPS and optical analyses. Different WF values were observed for V₂O₅ prepared from a fresh V₂O₅–isopropanol (IPA) solution (5.15 eV) and that prepared from a 24 h-old solution (5.5 eV). This difference is due to the gradual reduction of vanadium (from V⁵⁺ to V⁴⁺) in IPA. The OSCs made with the V₂O₅ thin film obtained from the 24 h-old V₂O₅–IPA solution required photo-activation, whereas those made with the freshly obtained V₂O₅ did not. Outdoor stability analyses of sealed OSCs containing a V₂O₅ HTL in either configuration revealed high stability for both devices: the photovoltaic response at T₈₀ was retained for more than 1000 h.