Optoelectronic devices based on evaporated pentacene films

Aluminium/pentacene Schottky diodes have been fabricated on glass substrates. An ohmic, transparent front contact was prepared by sputtering a highly conductive indium-tin-oxide layer. The pentacene layer was evaporated in high vacuum of a pure (98%) commercially available source. All the samples were grown at room temperature and moderate deposition rates (<10 Å/s). Optical measurements evidence absorption peaks at energy positions 1.86, 1.97, 2.13, 2.3 and 2.5 eV, corresponding to singlet states of the lowest unoccupied molecular orbital of pentacene. The current-voltage characteristics show good rectifying behaviours. Finally, a clear antibatic response is observed in the external quantum efficiency of the photodiodes when illuminated through the indium-tin-oxide contact.