We demonstrate solution-processed tungsten trioxide (WO3) 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 WO3 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 WO3 buffer layer in normal as well as inverted architecture solar cells. Normal architecture devices based on WO3 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 cm2) and excellent diode rectification underline the charge selectivity of the solution-processed WO3 layers.