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Radiative recombination of trapped excitons in Alq3 films: Importance of intermolecular interactions

A. M. Ajward1, X. Wang1, N. Wickremasinghe1, L. A. A. DeSilva2, and H. P. Wagner1,*

  • 1Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, USA
  • 2Department of Physics, University of West Georgia, Carrollton, Georgia 30118, USA

  • *Corresponding author: wagnerhp@uc.edu
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Phys. Rev. B 88, 045205 – Published 23 July, 2013

DOI: https://doi.org/10.1103/PhysRevB.88.045205

Abstract

We investigate the light emission of optically excited tris(8-hydroxyquinolinato) aluminum (Alq3) films by temperature-dependent, time-integrated, as well as time-resolved photoluminescence (PL) at various photon densities. The Alq3 films are deposited on Si (001) substrate using organic molecular beam deposition. At high excitation densities, the PL efficiency decreases when the temperature is reduced from 170 to 15 K. At low laser intensities, the PL efficiency is nearly temperature independent. The observed PL quenching at high-excitation densities is assigned to singlet-singlet annihilation revealing a low-temperature bimolecular quenching coefficient that is more than two orders of magnitude higher than previously reported at room temperature. The observed strong bimolecular interaction at low temperature is attributed to an enhanced local (microscopic) density of captured excitons in extended traps. The reduction of the exciton annihilation with increasing temperature is assigned to a thermally activated occupation of nonquenchable localized exciton states. Above 190 K, the PL efficiency starts to decrease independently from the excitation level which is ascribed to a thermally activated detrapping of excitons and subsequent migration to nonradiative centers outside the traps. A coupled-rate equation model, including bimolecular quenching, thermally activated occupation of nonquenchable states, and detrapping of excitons at higher temperatures, supports these interpretations.

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Phys. Rev. B 88, 045205– Published 23 July, 2013
  • Received 17 December 2012

©2013 American Physical Society

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