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Abstract:
Light emission from thin films of low molecular weight organic semiconductors is dominated by excitons. Here, we study the nature of exciton emission in 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films, in PTCDA/aluminum-tris-hydroxyquinoline (Alq₃) multilayers, and in PTCDA/Alq₃ codeposited films by photoluminescence spectroscopy at temperatures between 10 and 80 K. The films are grown by organic molecular beam deposition on Si(001) substrates covered with a natural oxide. In PTCDA/Alq₃ structures, we observe a redshift and an enhancement of the charge-transfer exciton emission (CT2) between stacked PTCDA molecules. The modification of the CT2 emission is attributed to compressive strain fields generated by tilted or distorted crystallites within the PTCDA layers. The internal strain increases the CT2 exciton trapping probability and its binding energy. The interpretation of the modified CT2 emission is supported by x-ray diffraction measurements and Fourier transform infrared spectroscopy revealing an enhancement of out-of-plane disorder of PTCDA molecules in PTCDA/Alq₃ multilayers and codeposited structures.
