Abstract
A high frequency rectangular alternating voltage was applied to organic light emitting diodes (OLEDs) with the structure ITO/TPD/Alq3/Al and ITO/CoPc/Alq3/Al, where ITO is indium-tin-oxide, TPD is 4,4′-bis[N-phenyl-N-(m-tolyl)amino]biphenyl, CoPc is cobalt phthalocyanine, and Alq3 is Tris(8-quinolinolato)aluminum, and the effect on emission of the reverse bias was examined. The results reveal that the emission intensity under an alternating reverse-forward bias is greater than that under an alternating zero-forward bias. The difference in the emission intensity (∆I) increased both for decreasing frequency and increasing voltage level of the reverse bias. In particular, the change in emission intensity was proportional to the voltage level of the reverse bias given the same frequency. To understand ΔI, this paper proposes a model in which an OLED works as a capacitor under reverse bias, where positive and negative charges accumulate on the metal/organic interfaces. In this model, the emission enhancement that occurs during the alternating reverse-forward bias is rationalized as a result of the charge accumulation at the organic/metal interfaces during the reverse bias, which possibly modulates the vacuum level shifts at the organic/metal interfaces to reduce both the hole injection barrier at the organic/ITO interface and the electron injection barrier at the organic/Al interface under forward bias.
Highlights
About 25 years after the first report of electroluminescence from a single crystal of anthracene under a high applied voltage [1], Tang proposed an organic light emitting diode (OLED) composed of a double-layer structure of organic thin films [2]
By using a time-resolved optical second harmonic generation measurement while applying a rectangular alternating voltage to typical OLEDs, Iwamoto et al reported that charges accumulate at the organic/organic interface under forward bias with high frequency, and they proposed a model of emission induced by the accumulated charges at the organic/organic interface [5,6,7,8]
We propose another model in which an works as a capacitor under cobalt phthalocyanine. We propose another model in which an OLED works as a capacitor under reverse bias, bias, and and the the accumulated accumulated charges charges at at the theorganic/metal organic/metal interfaces induce an an enhancement enhancement of of reverse interfaces induce emission under under the the alternating alternating reverse-forward reverse-forward bias
Summary
About 25 years after the first report of electroluminescence from a single crystal of anthracene under a high applied voltage [1], Tang proposed an organic light emitting diode (OLED) composed of a double-layer structure of organic thin films [2]. By using a time-resolved optical second harmonic generation measurement while applying a rectangular alternating voltage to typical OLEDs, Iwamoto et al reported that charges accumulate at the organic/organic interface under forward bias with high frequency, and they proposed a model of emission induced by the accumulated charges at the organic/organic interface [5,6,7,8] These studies signify that, to understand the OLED emission mechanism and control its performance, we must focus on the electronic states at the interfaces as well as the work functions and molecular orbital energy levels.
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