8 October 2005 Delayed electroluminescence: a new tool for studies of OLED fundamental properties
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We measured delayed electroluminescence in abrupt heterojunction undoped and doped small molecule organic light emitting diodes (OLEDs) based on NPB and AlQ3 hole and electron transport and emitter molecules, after the excitation currents are switched off and reverse bias applied to the sample. The experiments indicate that delayed light emission is a result of two distinct processes: emissive excited singlet state generation by either triplet-triplet annihilation or recombination of trapped positive and negative charges in the device. Under reverse device bias these two mechanisms have distinctly different signatures. Undoped devices show dominant light emission contribution from triplet-triplet annihilation, while in rubrene and coumarine doped devices delayed light emission comes predominantly from recombination of trapped charge. Therefore these molecules act as recombination centers when doped into AlQ3.
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Zoran D. Popovic and Hany Aziz "Delayed electroluminescence: a new tool for studies of OLED fundamental properties", Proc. SPIE 5937, Organic Light-Emitting Materials and Devices IX, 59370S (8 October 2005); doi: 10.1117/12.626159; https://doi.org/10.1117/12.626159

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