Low level optical absorption measurements on organic semiconductors

Abstract The optical absorption of n-type (C 60 and PTCDA) and p-type (CuPc and pentacene) organic semiconductors is investigated by optical transmission and photothermal deflection spectroscopy. The usual absorption bands related to HOMO-LUMO transitions are observed in the high absorption region of transmission spectra. Photothermal deflection spectroscopy also evidences exponential absorption shoulders with characteristic energies 47 meV for CuPc, 38 meV for pentacene, 50 meV for PTCDA and 87 meV for C 60 . In addition, broad bands in the low absorption level are observed for C 60 and PTCDA. These bands have been attributed to contamination due to air exposure. On the other hand, in CuPc a clear absorption peak at 1.12 eV is observed with smaller features at 1.04 eV, 1.20 eV and 1.33 eV. These peaks are attributed to transitions between the Pc levels of CuPc ions. Finally, the optical absorption expected in blends of organic semiconductors is estimated by an effective media approximation.