Mixed thin films of donor- and acceptor-type molecular semiconductors were prepared by physical vapour deposition and studied by conduction measurements in the dark during film growth and under illumination at three distinct wavelengths across the visible range of the solar spectrum. The molecules were chosen to provide both, good spectral coverage of the solar spectrum and appropriate differences in the electronic energy levels to allow charge carrier separation. Photocurrents were observed that indicated the expected contributions to the net photoconduction but also anomal, negative contributions were found, leading to a decreased conduction under illumination, showing the presence of isolated clusters. Ripening of the mixed films in vacuum and conditioning the films at air were studied subsequently and consequences on conduction and photoconduction discussed. The applicability of such films in evaporated organic bulk heterojunctions as photoconductors or in photovoltaic cells is discussed.
Vacuum chamber for simultaneous evaporation of pure and mixed organic thin films equipped with in situ (photo)-conduction measurements.