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 diketopyrrol opyrrole (DPP) and dithioketopyrrolopyrrole (DTPP) on SnO2:F were meas ured by the Kelvin probe technique. From the dependence of the work fu nction on the layer thickness, the width of the space charge layer at the interface and the total voltage drop over the organic layer were d educed. The presence of two photoactive areas, one at the air/D(T)PP i nterface and one at the D(T)PP/SnO2:F interface, was demonstrated by m easuring the surface photovoltage (SPV) spectra under front and back s ide illumination, and their dependence on the layer thickness. Calcula tion of the contributions of each of these areas to the SPV permitted the construction of the energy diagram of the DTPP/SnO2:F junction. Th e spectral dependence of the SPV for D(T)PP lavers is weak. For thick layers of D(T)PP, the SPV spectrum under front side illumination follo ws the absorption spectrum, while the one under back side illumination is different in shape. For thinner layers the SPV spectrum is a super position of the front side signal and back side signal, modulated by a n internal filter effect of the organic layer itself. (C) 1998 Elsevie r Science B.V.