Injection photoconductivity and photovoltage in metal/tetracene/metal sandwich systems

Abstract

Steady state and pulsed photoconduction ( i ) and photovoltaic ( U oc , i sc measurements were made on vacuum-evaporated tetracene films sandwiched between two aluminium or two gold semitransparent electrodes. The properties of the cells are shown to be strongly dependent on whether the substrate (S) or the non-substrate (NS) electrode is illuminated. A theoretical model is presented to explain the photoconduction and photovoltaic characteristics of the cells. The model is based on the assumption that charge carriers (holes) are injected into the band and to surface traps at the contacts by triplet excitons created by the exciting light in the tetracene layer. The charge introduced into the sample, in turn, affects the diffusion flux of triplet excitons reaching the electrodes. The energy and space distributions of traps are different at the S and the NS electrode. This causes triplet excitons to be more strongly quenched on their way to the S electrode than to the NS electrode. Predictions of the model are shown to agree with the experimental action spectra and the dependence of i , U oc and i sc on the light intensity I o . In particular, it allows the sublinear dependence of the emission-limited photocurrent on I o and the characteristic reversals of sign in the action spectra and in the dependence of U oc on I o to be explained. The cells studied entirely under vacuum (2 x 10 -3 Pa) showed no chane in the shape of the action spectra but the value of U oc decreased by a maximum of about one order of magnitude. The influence of the temperature could be rationalized by a temperature-induced variation in the trap parameter l and a change in the ratio of the dark to the photoinjection efficiency.