Inorganic Nanowire Light-Emitting Diodes in Organic Films
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Inorganic nanowire light-emitting diodes embedded in organic films R. Könenkamp, Robert Word and Christoph Schlegel Physics Department Portland State University 1919 SW 10th Avenue Portland, OR 97201, USA
ABSTRACT We report room-temperature, white-color electroluminescence in ZnO nanowires embedded solidly in organic thin films. Excitonic luminescence around 380 nm is observed as a shoulder on a broader, defect-related band covering all of the visible range and centered at 620 nm. The ZnO nanowires are grown in a low-temperature process employing a technique that is suitable for large-area applications. The nanowires are robustly encapsulated in organic films deposited from high-molecular-weight solutions. Electron injection occurs through a transparent SnO2 layer, while hole injection is mediated by a p-doped polymer and an evaporated Au-contact. INTRODUCTION Over the last years organic thin films have been used extensively for the fabrication of lightemitting diodes. The main advantages of using organic materials as optically active components are inexpensive preparation and processing, the possibility to obtain a flexible device structure and to cover a broad region of the visible spectrum. A potential shortcoming may lie in the limited stability of organic materials with respect to chemical reactions and to light. Here we report the use of inorganic ZnO nano-wires as the optically active component in an LED-structure that uses organic polymers only as an insulator and as one of the injecting contacts. The growth of the inorganic component occurs at low temperatures (~80oC) and is therefore also compatible with glass or flexible polymeric substrates. The used ZnO nanowires are mono-crystalline and their electronic material properties are comparable to those found in continuous thin films (1). Due to the small size of these structures the overlap of the injected hole and electron carriers is excellent and efficient luminescence can be expected. The nanowire morphology also allows a robust embedding in a polymeric matrix that maintains structural stability and flexibility. While the development of this new device type is just at the beginning, it may eventually lead to flexible thin film LED structures with useful optical properties and a high long-term stability.
PREPARATION ZnO nanowires were deposited in a standard electrodeposition process using aqueous solutions and involving a three-electrode arrangement with Pt counter and reference electrodes. A solution of 5 x 10-4 M ZnCl2 and 0.1 M KCl in H2O was used with the KCl serving as supporting electrolyte. The electrodeposition was carried out in the range of -660 to -760 mV versus the Normal Hydrogen Reference Electrode potential at 80 °C under
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externally induced convection and oxygen bubbling. Typical deposition currents were 0.6-1 mA/cm2 on 5 x 5 cm2 fluorine-doped SnO2 glass substrates (2, 3). The ZnO formation proceeds in two chemical steps: first, oxygen is electro-reduced at the substrate surface. This increases the OH--concentration
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