Brush painted V2O5 hole transport layer for efficient and air-stable polymer solar cells

As an alternative hole transport layer (HTL) to poly(3,4-ethylendioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), we investigated V2O5 thin films fabricated by simple brush painting without any post-treatments. Polymer solar cells (PSCs) with V2O5 fabricated by brush painting showed superior performance to that of conventional PEDOT:PSS based system; in particular, higher FF was obtained. Because more hydrophobic oxide surface induces better physical contact and better wetting of active blends, lower RS is obtained; thus, the resulting device shows higher performance. PSC with brush painted V2O5 at 50 °C using 0.99 vol% vanadium (V) oxytriisopropoxide in isopropyl alcohol exhibited the best power conversion efficiency (PCE) of 3.83%. Furthermore, employment of V2O5 as HTL leads to significant improvement in long-term stability in air (dropping of PCE of conventional PEDOT:PSS system to ~0% and retention of PCE of V2O5 based PSC to over 60% after exposure in air for 96 h without any encapsulation).