The purpose of present thesis was to optimize processing of molybdenum oxide film as a hole-transporting layer in organic solar cells. Values of the transmission and reflection coefficients were measured for the various thicknesses of raw and annealed layers. Afterwards, complete solar cells with MoOx buffer layer were constructed and the current-voltage characteristics were measured. Analysis of the results showed that annealing slightly deteriorates the optical properties but improves the electrical properties of the obtained layers of molybdenum oxide. The most suitable MoOx buffer layer for the use in organic solar cells is 35nm thick layer annealed at 180°C.