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

Abstract

Using electro-optical pump and probe measurements, we studied optical properties of a thin film of tris-(8-hydroxyquinoline) aluminum $({\mathrm{Alq}}_{\mathrm{3}})$ doped with DCM2 laser dye under pulsed injection currents as high as 300 ${\mathrm{A/cm}}^{\mathrm{2}}.$ 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 ${\mathrm{Alq}}_{\mathrm{3}}\mathrm{:DCM2}$ 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.