Synthesis of ZnO Nanowires by Hydrothermal Technique for Integration Into Chalcopyrite Thin Films
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Synthesis of ZnO Nanowires by Hydrothermal Technique for Integration Into Chalcopyrite Thin Films H. Karaagac1, M. Parlak2 and M. Saif Islam1 1 Department of Electrical and Computer Engineering, University of California at Davis, Davis CA 95616, USA 2 Department of Physics, Middle East Technical University, 06531 Ankara, Turkey ABSTRACT Vertically oriented, highly dense ZnO nanowires (NWs) array was successfully grown on both glass and silicon substrates using hydrothermal technique. A systematic study was carried out to investigate the effects of growth parameters including growth time and thickness of ZnO seed layer on the quality of ZnO NWs in terms of their homogeneity and orientation in the vertical direction. The diameter as well as the length of grown ZnO NWs was found to be closely dependent on the thickness of the pre-coated ZnO seed layer. The structures of ZnO NWs and electron-beam evaporated AgGa0.5In0.5Se2 (AGIS) thin film have been characterized by X-ray diffraction measurements and optical properties were measured by transmission measurement. The optic band gap of AGIS thin film was found to be almost optimum (1.56 eV) to match the abundant part of solar cell spectrum. AGIS thin film was deposited on the synthesized ZnO NWs to form p-n heterojunction based inorganic solar cell, which exhibited photovoltaic behavior with a power conversion efficiency of 0.37 % under A.M (1.5) illumination. INTRODUCTION In the last two decades, a number of investigations have been carried out on producing cost effective highly efficient thin film solar cells. In this regard, chalcopyrite (I-III-VI2) compounds are promising candidates for the realization of highly-efficient solar cells based on the construction of the p-n heterojunction with II-VI semiconductors. Cu-III-VI2 and AgIII-VI2 based chalcopyrites are currently of great importance especially in the photovoltaic community due to their unique electrical, structural and optical properties [1, 2]. Beside thin films, recently, a considerable interest has also emerged in the fabrication of nano-structured based solar cells due to their intriguing optical, electrical and optical properties. Among the one- dimensional structures, nanowires and nanopillars are particularly important for the fabrication of a new-generation of solar cells [3]. A solar cell consisting of nanowires (NWs) embedded in thin films can be regarded as semiconductor sensitized nanostructured solar cells, an offshoot of the dye-sensitized solar cell. In other words, an inorganic semiconductor is used to replace organic material in the molecular dye-sensitized solar cell structure [4]. It is expected that such devices will enable us to overcome the various difficulties associated with absorbing light and collection efficiency of photo-generated minority-carriers [5]. ZnO, being a II-VI semiconductor with a large band gap (3.37 eV) and exciton binding energy (60 meV) at room temperature [6], is regarded as one of the most important functional semiconductor oxide due to its excellent properties, such
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