CdSe Sensitized ZnO nanowire/polymer based p-i-n Heterojunction Solar Cell
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CdSe Sensitized ZnO nanowire/polymer based p-i-n Heterojunction Solar Cell Athavan Nadarajah, Robert C Word and Rolf Koenenkamp Physics Department Portland State University 1719 SW 10th Avenue, Portland, OR. 97201, USA ABSTRACT: We report a novel hybrid p-i-n heterojunction solar cell consisting of an undoped CdSe quantum dot film sandwiched between electrodeposited n-type ZnO nanowires on a compact ZnO thin film/FTO and a spin-coated hole-conducting layer. Microscopic studies show the conversion of CdSe quantum dots into conformal and continuous polycrystalline thin film coatings on ZnO nanowires upon annealing in CdCl2/ambient air. The morphology change of the CdSe quantum dot layers then provided excellent charge transfer between the absorber layer and the contiguous layers. The quantum-dot-sensitized ZnO nanostructured solar cells exhibited short-circuit current densities ranging from 5 to 10 mA/cm2 and open-circuit voltages of 0.4−0.6 V when illuminated with an 85 mW/cm2 quartz-halogen spectrum. External quantum efficiencies as high as 55−60% were also achieved. INTRODUCTION: ZnO nanowires have gained significant interest in solar cell research due, in part, to its direct wide bandgap (3.37eV), specific optoelectronic properties [1], and a high mobility pathway for electron transport from the absorber region to the front contact layer. They can be grown with numerous techniques including electrodeposition [2, 3], chemical vapor deposition [4], and metalorganic vapor phase epitaxy [5]. Among them, electrodeposition is attractive as a low-cost and low-temperature method to grow ZnO nanowires on conductive transparent electrodes. Quantum dot sensitized ZnO nanowires have attracted considerable interest in lowcost photovoltaic devices as an electron collecting window layer [6-8]. The solar cell stability, light harvesting capability, and the total cost are determined by the photosensitizer materials. In recent years, there has been significant studies towards the use of relatively low band-gap inorganic semiconducting quantum dots such as CdS [9], CdSe [6, 10, 11], InP [12], InAs [13] and Bi2S3 [14] as photosensitizer materials due to their tunable band edge that offer new opportunities for harvesting light energy in the visible region of the solar spectrum. Among them, nanocrystalline CdSe has a bandgap of 1.7 eV and suitable valence and conduction band positions for photo-generated carriers transfer to the n-ZnO and p-type material. As a result, the CdSe quantum dots-sensitized ZnO nanowires solar cell could offer new and enhanced opportunities to harvest light energy in the whole visible region of solar light. In this study, we describe the preparation and properties of solar cells based on CdSe quantum dots, ZnO nanowires and a p-type layer as the main components. All layers are made using cost-efficient electrochemical and solution deposition techniques. Prior to the ZnO electrodeposition, a transparent conducting fluorine-doped SnO2 substrate with a ZnO thin continuous layer of ZnO by spray pyro
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