Fabrication and performance of highly textured electrodeposited ZnO back reflector for nc-Si solar cells
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Fabrication and performance of highly textured electrodeposited ZnO back reflector for nc-Si solar cells Dinesh Attygalle, Qi Hua Fan, Shibin Zhang, William B. Ingler, Xianbo Liao, Xunming Deng Dept. Physics and Astronomy, Univ. Toledo, 2801 Bancroft Street, Toledo 43606
ABSTRACT To improve the cell efficiency of thin film solar cells textured back reflectors (BR) are widely used. This is particularly important in a-Si:H based solar cells due to low absorption coefficient at longer wavelengths. In this work we present a cost effective way to fabricate uniformly textured ZnO by using electrochemical methods. Further it was observed that Quantum Efficiency (QE) of shorter wavelengths also improved for highly textured ZnO BR. Together this resulted in more than 2mA increment in short circuit current density (Jsc) and 19% relative improvement in solar cell efficiency over sputter deposited BR. A possible mechanism responsible for the improved blue QE is also discussed.
INTRODUCTION Light trapping is a method commonly used in thin film solar cells to enhance the long wavelength absorption. In a substrate type thin film solar cells using a textured back reflector does this [3]. Common back reflectors consist of a reflecting metal layer such as Ag or Al and a TCO layer such as ZnO. It was shown numerically by X. Yang [2] that the texturing on ZnO surface is more effective than the texture on metal. ZnO can be deposited in numerous ways, such as RF sputtering, DC sputtering and PECVD. Alternatively the electrochemical deposition is also used for this purpose. A lot of research have been done to understand various aspects involved with ZnO electrodeposition [5,6]. The low cost associated with the electrodeposition techniques of ZnO attracted many researches and industrial community [1]. The non-uniformity was one of the major problem associated with electrochemical deposition of ZnO films, especially for large area applications like solar cells. In this paper we discuss fabrication of ZnO films at high rate, using an improved electrochemical procedure, on stainless steel substrate, which is pre-coated with Ag/ZnO by sputter deposition. We demonstrate that, better uniformity and control over the ZnO texture can be achieved using this novel method. In addition, though it is not the prime purpose of BR, the favorable feature size helps to improve the blue response of the solar cell. EXPERIMENT The substrates were stainless steel sheets coated with Ag and ZnO layers by RF magnetron sputtering at the facility of Xunlight Corporation. The Ag and ZnO layers were about 300nm and 420nm thick, respectively. The BR sheet was then cut into 2”x4” pieces. The ZnO electrodeposition was done on top of the sputter deposited Ag/ZnO layers. The electrodeposition
of ZnO can be directly done on Ag, how ever for comparison purposes Ag/ZnO sputter deposited substrates were used. Figure 1 shows the schematic diagram of the ZnO deposition setup.
Figure 1 Improved ZnO electrodeposition setup The anode is a 99.9+% pure Zn meta
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