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.
Open-Circuit Photovoltage Exceeding 950 mV with an 840 mV Average at Sb2S3–Thianthrene+/0 Junctions Enabled by Thioperylene Anhydride Back Contacts