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[References: 37 tit.]

In this work, indium oxide (In2O3) thin films were obtained by the thermal oxidation method of metallic indium thin films. The effect of thermal annealing on the morphology, structure, optical properties of In2O3 and also on the photovoltaic characteristics of organic solar cells (OSCs) was studied. Annealing of the films was carried out in the temperature range of 200–500 °C. The rise of annealing temperature leads to the increase in the grain size and to the widening of the In2O3 bandgap. Based on the In2O3 films with various annealing temperatures, inverted organic solar cells (OSCs) with FTO/In2O3/P3HT:ICMA/MoOx/Ag structure were assembled. OSCs with In2O3 oxidized at 200 °C generated low power conversion efficiency (PCE) of 0.45%. The increase in the oxidation temperature up to 300 °C results in a significant improvement in PCE, which has reached 1.38%. However, a further rise of the annealing temperature deteriorates the photovoltaic performance of OSCs. The dynamics of charge carrier transfer in OSCs based on In2O3 were investigated by the impedance spectroscopy (IS) technique. It follows from the IS data that the growth of In2O3 oxidation temperature leads to the improvement in the In2O3 conductivity, however, it also results in enhanced recombination rate of charge carriers at the In2O3/P3HT:ICMA interface, which indicates increased surface defects in In2O3 with the rise in the oxidation annealing temperature.