ZnO-based transparent anodes for organic light-emitting devices
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ZnO-based transparent anodes for organic light-emitting devices K. Sivaramakrishnan, N. Bakken, and T. L. Alford School of Materials and Flexible Display Center at ASU, Arizona State University, Tempe, Arizona 85287, USA ABSTRACT ZnO/Au/ZnO (ZAZ) electrodes grown on flexible PEN substrates were evaluated as transparent electrodes for organic light-emitting devices (OLEDs). OLEDs fabricated with the ZAZ electrodes showed reduced leakage in contrast to control OLEDs on ITO and reduced ohmic losses at high current densities. At a luminance of 25000 cd/m2, the lum/W efficiency of the ZAZ electrode based device was 5% greater than for the device on ITO. The ZAZ electrodes also allow for a broader spectral output in the green wavelength region of peak photopic sensitivity compared to ITO. The results have implications for electrode choice in display technology.
INTRODUCTION Transparent conducting oxides (TCOs) are a class of direct wide band gap semiconductors[1] that have generated interest for use as transparent electrodes in flexible flatpanel displays, solar cells, and organic light-emitting devices (OLEDs) in the past few years.[24] Indium tin oxide (ITO) is the most commonly used electrode material because of its good electrical conductivity and transparency in the visible region.[5] However, there is an increase in demand for TCOs having lower resistivity while retaining optical transparency.[6] Highly doped ITO is limited in its conductivity by ionized impurity scattering.[7] Typical ITO films have a conductivity about 5×103 (:-cm)-1 which is inadequate for technologies such as flat-panel displays, and other high brightness light-emitting devices.[8]. This inherent limitation in the resistivity of ITO cannot be circumvented by changing geometry as increased thickness increases the absorbance, leading to poorer optical performance. Attempts have therefore been made to use ITO-based multilayers to lower resistivity of the electrodes.[9] However, there are other drawbacks to ITO such as high cost due to indium scarcity and strong dependence of performance on deposition conditions.[1] There have been reports of deleterious diffusion of indium and tin into proximate organic charge transporting or emissive layers. [10-13] Other complications for ITO include that the electrical and optical properties depend on dopant concentration, defects, and vacancies.[14] The lifetime and efficiency of OLEDs using ITO can also be strongly dependent on the interface quality [15] and cleaning procedures used prior to deposition of the organic layers[16]. As a consequence, ITO electrodes can require varying degrees of post-deposition processing. [17,18] There is also a need for a conductor with better transparency across the visible spectrum, especially in the green-blue region [19]. These challenges motivate exploration of transparent conducting materials systems that could be implemented as improvements or alternatives to the ITO standard. Research on replacements for ITO electrodes include other conducting oxides, carbon nanotubes,
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