In this work, optical properties of organic nanostructures are investigated. In temperature dependent (10 – 300 K) absorption studies of PTCDA and PTCDA/Alq3 multilayers a red shift and a line narrowing of vibrational Frenkel exciton bands is observed when the temperature is decreased. The reduced transition energy is explained by a thermal contraction along the PTCDA molecular stacks that cause an increased inter-molecular overlap between PTCDA molecules leading to an enhanced environmental shift. The reduction of the inhomogeneous broadening of the bands is explained by a reduced population of internal and external vibrational levels of the electronic ground state. The reduced temperature shift in multilayer is attributed to a reduced thermal contraction in the PTCDA crystallites due to adjacent Alq3 interlayers that possess a smaller thermal contraction than PTCDA.
The exciton emission in PTCDA thin films, PTCDA/Alq3 multilayers and co-deposited PTCDA/Alq3 layers is studied by temperature dependent (10 – 300 K) PL measurements. The different recombination channels arising from Frenkel excitons and self-trapped excitons (charge-transfer excitons CT1-nr, CT1, CT2 and excimers) that were observed earlier in PTCDA single crystals also appear in films, multilayers and in co-deposited layers. In PTCDA/Alq 3 multilayers, an unknown low energy line dominates the emission spectrum up to 200 K. In accordance to investigations using X-ray diffraction, FTIR absorption and strain dependent PL measurements the new channel is attributed to a modified CT2 transition due the compressive strain between stacked molecules.
Temperature dependent (10 – 300 K) PL measurements of Alq 3 layers are performed. An exciton trapping model which includes the formation of self-trapped excitons is proposed to explain the observed temperature dependent PL intensity enhancement and the spectral red-shift of the PL spectrum (at ~ 180 K). Alq3 based OLED structures are fabricated and electro-optical measurements are performed. The I-V measurements reveal a trap charge limited current behavior. The EL efficiency of the device shows similar temperature dependence as the PL intensity obtained from the Alq 3 film. Furthermore, both the EL and PL spectra reveal a maximum redshift at 180 K which is tentatively attributed to the formation of self-trapped excitons within the Alq3 layer.
|School:||University of Cincinnati|
|School Location:||United States -- Ohio|
|Source:||DAI-B 79/10(E), Dissertation Abstracts International|
|Keywords:||Absorption, Alq3, Electroluminescence, Oled, Photoluminescence, Ptcda|
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