High Fill Factor Polymer Solar Cells Incorporating a Low Temperature Solution Processed WO₃ Hole Extraction Layer

We demonstrate solution‐processed tungsten trioxide (WO 3 ) incorporated as hole extraction layer (HEL) in polymer solar cells (PSCs) with active layers comprising either poly(3‐hexylthiophene) (P3HT) or poly[(4,4'‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole)‐5,50‐diyl] (Si‐PCPDTBT) mixed with a fullerene derivative. The WO 3 layers are deposited from an alcohol‐based, surfactant‐free nanoparticle solution. A short, low‐temperature (80 °C) annealing is sufficient to result in fully functional films without the need for an oxygen‐plasma treatment. This allows the application of the WO 3 buffer layer in normal as well as inverted architecture solar cells. Normal architecture devices based on WO 3 HELs show comparable performance to the PEDOT:PSS reference devices with slightly better fill factors and open circuit voltages. Very high shunt resistances (over 1 MΩ cm 2 ) and excellent diode rectification underline the charge selectivity of the solution‐processed WO 3 layers.