The surface photovoltage of polymethine semiconducting films

Abstract

Recent solar cell work on Al/merocyanine/Ag sandwich devices has prompted the examination of the surface photovoltage of thin films (∼500 Å) of merocyanine and of four other closely related polymethine type dyes deposited on Ag rear contacts. All of these materials exhibited space charge depletion layers at the front surface with energy band bending changes resulting from a laser pulse ranging from 1 to 100 mV, and corresponding photovoltaic relaxation time constants on the order of 30 msec. The spectral dependence of the observed photovoltage was dominated by the nominally ’’forbidden’’ S0→T1 transition in the near IR. Much less photovoltage is observed at wavelengths corresponding to the allowed S0→S1 transition, even though light of this photon energy is strongly absorbed. This discrepancy between the relative intensity in the absorption and photovoltage spectra could be due to decreased dissociation of the bound excitons, and/or to increased recombination of the free carriers at the organic semiconductor free surface.