Construction of the energy diagram of an organic semiconductor film on SnO2:F by surface photovoltage spectroscopy

The work functions of solid layers of the hole conductors diketopyrrolopyrrole (DPP) and dithioketopyrrolopyrrole (DTPP) on SnO₂:F were measured by the Kelvin probe technique. From the dependence of the work function on the layer thickness, the width of the space charge layer at the interface and the total voltage drop over the organic layer were deduced. The presence of two photoactive areas, one at the air/D(T)PP interface and one at the D(T)PP/SnO₂:F interface, was demonstrated by measuring the surface photovoltage (SPV) spectra under front and back side illumination, and their dependence on the layer thickness. Calculation of the contributions of each of these areas to the SPV permitted the construction of the energy diagram of the DTPP/SnO₂:F junction. The spectral dependence of the SPV for D(T)PP layers is weak. For thick layers of D(T)PP, the SPV spectrum under front side illumination follows the absorption spectrum, while the one under back side illumination is different in shape. For thinner layers the SPV spectrum is a superposition of the front side signal and back side signal, modulated by an internal filter effect of the organic layer itself.