Charge selective interlayers are of critical importance in order for solar cells based on low mobility materials, such as polymer-fullerene blends, to perform well. Commonly used anode interlayers consist of high work function transition metal oxides, with molybdenum trioxide (MoO3) being arguably the most used. Here, it is shown that a thin interlayer of MoO3 causes unintentional bulk doping in solar cells based on polymers and polymer-fullerene blends. The doping concentrations determined from capacitance-voltage measurements are larger than 10(16) cm(-3) and are seen to increase closer to the anode, reference devices without MoO3 are undoped. Using time of flight secondary ion mass spectroscopy it is furthermore shown that molybdenum is present on the surface of all films with an interfacial layer of MoO3 beneath the active layer. Doping concentrations of this magnitude are detrimental for device performance, especially for active layers > 100 nm.