Fermi level positioning in organic semiconductor phase mixed composites: The internal interface charge transfer doping model

► We introduce the internal interface charge transfer doping model. ► SXPS on TCNQ doped CuPc and on TCNQ/CuPc interfaces show similar Fermi level shifts. ► SXPS on WO3 doped CuPc and on WO3/CuPc interfaces show similar Fermi level shifts. ► The similar Fermi level shifts are explained by the formation of internal interfaces. ► STEM on WO3 doped CuPc directly shows the presence of internal interfaces.