In situ photoluminescence investigation of doped Alq

We report the photoluminescence (PL) properties measured in situ from vacuum-deposited organic films of tris-(8-hydroxyquinoline) aluminum (Alq) doped with 4-(dicyanomethylene)-2-methyl-6(p-dimethylaminostyryl)-4H-pyran (DCM), where the red emission from the guest molecules is due to Förster energy transfer of excited state energy from host to guest. Both bare DCM-doped Alq (Alq:DCM) and bilayer Alq/Alq:DCM films have been studied, with the thickness of the Alq overlayer continuously varied in the latter case. The PL spectra from the bilayer structure contain no Alq contribution when its thickness is below 2.4 nm. Taking the value as the maximum distance for which the Alq exciton can travel in the film and still transfer its energy to a DCM molecule, the minimum DCM concentration in Alq:DCM necessary to produce red emission only can be estimated at 0.31 wt%. The most efficient red emission appears at the DCM concentration of about 1.7 wt%, at which more than 90% Alq-originated excitons are involved in the Förster energy transfer process. There exists a gradual red shift of the PL spectra with increasing DCM concentration, which can be attributed to the polarization and accompanying aggregation tendency of DCM molecules in Alq during the formation of the doped films.