Balancing Charge Injection and Transport in Organic Light-emitting Diodes with a Transparent Conductive Tungsten Oxide L
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Balancing Charge Injection and Transport in Organic Light-emitting Diodes with a Transparent Conductive Tungsten Oxide Layer R. Acharya, X. M. Li, Y. Lu, and X. A. Cao Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506, USA ABSTRACT High-brightness green phosphorescent hybrid inorganic-organic light-emitting diodes (HyLEDs) and inverted HyLEDs (IHyLEDs) have been demonstrated. The devices comprised a transparent and conductive WO3 layer deposited by thermal evaporation, which improved both hole injection and transport, and led to more balanced charge injection and significant performance enhancement. At 20 mA/cm2, the HyLEDs had a low operation voltage of 6.1 V, 0.8 V lower than that of OLEDs with an organic hole transport layer. With an optimized layer structure, the HyLEDs reached 104 cd/m2 brightness at 7.3 V. At this brightness level, the current efficiency was 55.2 cd/A, 57% higher than that of the OLEDs. In the IHyLEDs, facile hole injection and transport through WO3 was balanced by electron injection from the indium-tinoxide (ITO) cathode overcoated with nanometer-thick Ca, leading to a low turn-on voltage of ~6 V. Brightness of 8133 cd/m2 was reached at 20 mA/cm2 and the corresponding current efficiency was 40 cd/A. The hybrid devices also exhibited markedly improved stability under constantcurrent stressing due to the robust WO3 hole transport layer. INTRODUCTION One important factor limiting the performance of organic light-emitting diodes (OLEDs) is unbalanced charge injection from the cathode and anode [1-6]. In typical bottom-anode topcathode OLEDs built on indium-tin-oxide (ITO)/glass substrates, a large energy barrier existing at the interface between the ITO anode and organic hole transport layer (HTL) causes inefficient hole injection, which does not balance electron injection from an Al/LiF cathode. The unbalanced charge injection causes high operation voltages and limits the luminous efficiencies of the OLEDs [1-6]. A large voltage drop at the ITO/HTL interface is needed to overcome the energy barrier during device operation, generating significant joule heat, which accelerates the crystallization of organic materials and thus device degradation [5,6]. One common strategy for enhancing hole injection in OLEDs is to insert a nanometer-thick organic or inorganic interlayer between the ITO and HTL as a hole injection layer (HIL) to creates a ladder-like or bended energy band structure that facilitates hole injection [1-4]. The optimal thickness of a typical HIL has been found be a few nanometers or less [2,3]. It is difficult to deposit such a thin material on ITO to form a uniform and robust interlayer. In addition, the processing adds substantially to the complexity of the device manufacturing procedure. Inverted OLEDs with a bottom-cathode top-anode architecture are better suited for integration with n-channel TFTs based on α-Si or amorphous oxide in large-size active-matrix OLED panels [7]. The unbalanced charge injection issue has been found
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