Reliability Study of Organic Light-emitting Diodes by Continuous-wave and Pulsed Current Stressing
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Reliability Study of Organic Light-emitting Diodes by Continuous-wave and Pulsed Current Stressing X. M. Li, R. Acharya, Y. Q. Zhang, X. A. Cao Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506, USA ABSTRACT The stability of green phosphorescent OLEDs with different structures was evaluated through constant-current stressing. Through the modifications of the ITO anode by different plasma treatments and the hole transport layer (HTL) by incorporating inorganic dopants, we proved that energy level misalignment at the ITO/HTL interface leads to localized joule heating, accelerating defect generation and luminescence decay. Pulsed current stressing was then employed to suppress the joule-heating effect so as to differentiate the thermal and nonthermal factors governing the device degradation. For OLEDs with a large energy barrier at the ITO/HTL interface, the effective lifetime was markedly increased under pulsed operation, whereas in OLEDs with an appropriate interfacial energy level alignment, pulsed stressing with 10% duty cycle only improved the effective half life by ~15% as compared to continuous-wave stressing, indicating a minor role played by joule heating. . INTRODUCTION The generally poor reliability of organic light-emitting diodes (OLEDs) presents a challenge to their widespread acceptance for use in large-area displays and solid-state lighting [1-5]. Recent efforts to improve the material purity, device architecture design, and packaging techniques have led to greatly improved operational lifetime of OLEDs. These strategies effectively reduce extrinsic sources of degradation, including metallic impurities, residual water, and other environmental contaminants. However, various intrinsic factors which affect the device stability by causing deterioration of the organic materials and electrodes have not been fully investigated and understood [5-11]. A greater understanding of the intrinsic degradation mechanisms would help to further improve the reliability of OLEDs particularly at high brightness levels by optimizing the material selection and structural design, and pave the way for their broader applications as lighting sources. Intrinsic degradation of OLEDs, which manifests as a gradual decrease in the brightness without any obvious change in the device appearance, is more complicated and a bigger obstacle to overcome. Various mechanisms have been proposed to explain the intrinsic degradation behaviors of OLEDs, including thermal instability of organic materials [6], charge migration and accumulation [7], and electrochemical decomposition [8,9]. The resulting products may screen the local electric field or act as nonradiative recombination centers and luminescence quenchers [6-9]. All these processes can be thermally enhanced and accelerated by joule heating under a large driving current [10,11]. Therefore, current-induced thermal effects, which are particularly pronounced at localized regions like heterointerfaces where a large voltage drop exists, c
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