Influence of Interface Doping on Charge-Carrier Mobilities and Sub-Bandgap Absorption in Organic Solar Cells

P3HT:PCBM (poly(3-hexylthiophene-2,5-diyl):([6,6]-phenyl-C61-butyric acid methyl ester)-based bulk heterojunctions (BHJs) were doped by using 4-toluenesulfonic acid (TSA) as dopant. This approach was inspired by the well-known interfacial doping of the active layer via the electron-blocking layer PEDOT:PSS (poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate)) at its interface. TSA is amphiphilic, acidic, and structurally very similar to the monomeric building block of PSS. Upon TSA doping, a notable increase in the light absorption in the sub-bandgap region of pristine P3HT was observed. These features are assigned to polaron transitions within P3HT; however, the TSA impact on polaron absorption features in the BHJ is rather small. Although, for small TSA concentrations and thick active layers (∼220 nm) the fill factor of the solar cells improved dramatically with increasing TSA content in the active layer, which is discussed in terms of contact resistances at interfaces in the present paper. For 0.5...