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.