Modified charge-transfer emission in perylene tetracarboxylic dianhydride-aluminum quinoline layer structures

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

Origin of the anomalous X-ray diffraction in phthalocyanine films

“…A key class of organic solids, such as quaterthiophene (4T), 3, 4, 9, 10 perylenetetracarboxylic dianhydride (PTCDA), 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), and others [15][16][17], are constructed 56001-p1 from highly anisotropic molecules and form layered structures at the same time. Here, we illustrate this submolecular data extraction procedure for the case of thin films made of anisotropic organic molecules belonging to the phthalocyanine (C 32 N 8 H 16 ) family.…”

Eliminating thermal effects in z-scan measurements of thin PTCDA films

“…The error is attributed to laser intensity instabilities and to long-term temperature fluctuations which cause slight changes in the laser pulse energy and in the pulse width. As mentioned before, we attribute the reduction of the TPA coefficients for repetition times shorter than 5 μs to the accumulation of heat in the PTCDA film which is predominantly caused by the non-radiative recombination of Frenkel excitons, excimers and charge transfer excitons [26][27][28]37,38]. Our interpretation is supported by comparing the onset of the heating effect for repetition times smaller than 5 μs with the thermal diffusion time t d of the accumulated heat within the laser focus area into the adjacent unexcited sample region in the PTCDA film.…”

“…The molecules align parallel to the (102) lattice plane, which is equal to the substrate surface. The distance between consecutive molecular planes is shorter as in graphite resulting in a large overlap of molecular π-orbitals [26][27][28].…”

Index matching in multilayered organic waveguides

“…Computer-controlled mechanical shutters at the Knudsen cells enable a few monolayer thickness control of organic PTCDA/Alq 3 multilayers. The optical quality of the pristine PTCDA and Alq 3 layers as well as of PTCDA/Alq 3 multilayers was studied with photoluminescence and transmission experiments as well as with x-ray measurements [31][32][33]36]. The structures of the two multilayers studied in the present work are sketched in figure 1.…”

“…The uniform orientation of molecules within all stacks results in a layered structure where the molecules are parallel to the (102) lattice plane, which itself is parallel to the substrate surface. The short distance of 0.32 nm between next neighbor (102) planes results in strong anisotropic optical and electrical properties [31][32][33][34]. In contrast to PTCDA, the propeller shaped Alq 3 molecules form quasi-amorphous films [35,36] with isotropic optical properties.…”