Optical properties of molecular organic semiconductor thin films under intense electrical excitation

Using electro-optical pump and probe measurements, we studied optical properties of a thin film of tris-(8-hydroxyquinoline) aluminum (Alq(3)) doped with DCM2 laser dye under pulsed injection currents as high as 300 A/cm(2). Pump-induced transparency and absorption were observed in the spectral ranges of 570-610 nm and 620-705 nm, respectively. We attribute these optical effects to molecular polarons formed under electrical excitation. Polaron absorption dominates the optical properties of electrically pumped Alq(3):DCM2 films in the spectral range where optical gain is anticipated (i.e., from 630 to 700 nm). Polaronic effects may present a significant obstacle for realization of organic diode lasers based on this material system. (C) 1999 American Institute of Physics. [S0003-6951(99)00108-4].