Theory of injection photovoltages in organic insulators

Abstract

Development of an open-circuit photovoltage, U, in an organic insulator by photoinjection of charge carriers from the electrodes is treated theoretically. In the single-carrier case (both electrodes injecting the same carrier) it is shown that, in the absence of surface traps, U increases at a rate of 60 mV per decade of light intensity, II, above a threshold value of II. Photoinjection from the back (unilluminated) electrode by incompletely absorbed light causes U to become independent of ll at high light intensity. The same process may also cause U to change sign as the wavelength approaches an absorption minimum of the organic. Traps in the bulk of the insulator do not affect the single-carrier photovoltage, but traps at the surface may complicate the intensity dependence of U if they are involved in the injection mechanism. They may for example cause U to decrease and change sign at high n. Only shallow surface traps are considered. Possible effects of surface states are discussed briefly. The major assumption of this paper--neglect of all but injected charge carriers-breaks down in principle in the two-carrier case. However, if there are no sources of photovaltage in the bulk of the insulator the two-carrier case gives a stronger dependence of U on II, and no saturation or possibility of change of sign with wavelength variation. Predictions of the model are shown to agree with the results for a number of systems reported in the literature.