Highly Transparent and Conductive CdO Thin Films as Anodes for Organic Light-Emitting Diodes
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Highly Transparent and Conductive CdO Thin Films as Transparent Anodes for Organic Light-Emitting Diodes Yu Yang, Qinglan Huang, Andrew W. Metz, Shu Jin, Jun Ni, Lian Wang and Tobin J. Marks* Department of Chemistry and the Materials Research Center, Northwestern University, Evanston, IL 60208-3113, USA ABSTRACT In this paper, CdO thin films are used for the first time as transparent anodes for organic light-emitting diodes (OLEDs). Highly conductive and transparent CdO thin films have been grown on glass and on single-crystal MgO(100) by low pressure metal-organic chemical vapor deposition (MOCVD) at 400 oC, and were implemented in small-molecule OLED fabrication. Device response and applications potential have been investigated and compared with those of commercial ITO-based control devices. It is found that as-deposited CdO thin films are capable of injecting holes into such devices, rendering them promising anode materials for OLEDs. A maximum luminance of 32,000 cd/m2 and an external forward quantum efficiency of 1.4 %, with a turn-on voltage of 3.2 V are achieved on MgO(100)/CdO-based devices.
INTRODUCTION Organic light-emitting diodes (OLEDs) as a practical new display technology have been receiving increasing attention during the last two decades [1]. Compared to LCDs, OLEDs are self-luminous, consume less power, offer wide view angle, and can be made flexible. OLEDs are fabricated by sandwiching organic materials (hole transport material, emissive material, and electron transport material) between two electrodes, one of which is transparent through which light is emitted. Tin-doped indium oxide (ITO), with a typical electrical conductivity and visible range transparency of 3-5×103 S/cm and 85-90%, respectively, is currently the most widely used transparent anode material for OLEDs. However, it has many limitations such as high cost and limited availability, modest conductivity, chemical instability [2], and the relatively low work function (4.3-4.7 eV) [3]. In view of these deficiencies, extensive efforts are dedicated to finding ITO-alternative transparent electrodes that are cheap but possess superior electrical and optical properties. The development of improved TCO materials would significantly benefit the optoelectronic industry. CdO-based TCOs are of great interest due to their high carrier mobility, nearly metallic conductivity, and applications as electrodes for photovoltaic devices [4]. However, there have been no reported attempts to fabricate OLEDs using CdO-based TCOs as the electrodes. Although CdO is somewhat toxic and thus not suitable for all applications in the display area, this can be overcome to some degree by encapsulation, which is normally used to extend OLED display lifetimes. In this contribution, we report the growth of high-quality CdO thin films on glass and single-crystal MgO(100) by MOCVD, and the resulting OLED response characteristics using these films as anodes for small-molecule OLEDs. Device response and application potential are investigated and compared with th
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