Morphology dependent optical properties of ZnO/SiNWs nanocomposites
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Morphology dependent optical properties of ZnO/SiNWs nanocomposites Aliaksandr Sharstniou1, Stanislau Niauzorau1, Eugene Chubenko1, Bruno P. Azeredo2 and. Vitaly Bondarenko1 1 Belarusian State University of Informatics and Radioelectronics, 6 P. Brovki str., Minsk, Belarus. 2 Arizona State University, The Polytechnic School, Mesa, AZ, USA. ABSTRACT Zinc oxide/silicon nanowires (ZnO/SiNWs) nanocomposites is a promising material for heterojunction solar cells. They combine the low-reflectivity of SiNWs, where photogenerated charge carriers are produced and harvested, and the high transparency of ZnO, which serves as a functional transparent conductive electrode. In this paper, we present a study of the antireflective properties of ZnO/SiNWs core-shell nanostructures. SiNWs were fabricated by a twostep metal-assisted chemical etching and coated with ZnO by electrochemical deposition. Particularly, the change in the specular reflectance of ZnO/SiNWs nanocomposites as a function of thermal annealing temperature under ambient atmosphere is investigated. First, it was shown that the reflectance in the wavelength range of 400-1000 nm of as-synthesized ZnO/SiNWs nanocomposites increases when compared to the bare SiNWs formed from Si wafers with resistivity of 0.3 and 12 Ω∙cm by an 0.51 % and 0.47 %, respectively. Second, it was found that annealed ZnO/SiNWs had a 0.26 % and 0.17 % lower reflectance in the wavelength range of 400-1000 nm than as-synthesized ZnO/SiNWs and yet higher than bare SiNWs. Potential causes such results are discussed in the context of existing literature. INTRODUCTION Recently, research efforts have been focused on increasing the efficiency of Si-based heterojunction solar cells to provide the lowest cost per watt [1]. One of the ways to increase Sibased heterojunction solar cell efficiency is to reduce its reflection by introducing surface nanostructures such as SiNWs that effectively work as a broadband anti-reflective layer. Among all methods for nanotexturing silicon, metal-assisted chemical etching (MACE) is a simple and potentially low-cost method based solely on wet chemistry that do not involve dry-etching and lithography [2]. There are several reports on the fabrication of solar cells based on n-type SiNWs and p-type amorphous Si (a-Si) forming core-shell structures [3, 4] that highlight the longstanding effort to build improved energy harvesting devices. Though, such solar cells exhibit quite low efficiency compared to single-crystal silicon solar cells, this difference can be explained by the relative high reflectance of SiNWs/a-Si nanocomposites (approximately 15 %) [4]. Additionally, application of a-Si layers deposited by PECVD has two main disadvantages: the increase in solar cells production cost and due to a rather low electrical conductivity of the aSi layer which has to be covered by a transparent conductive oxide, such as Al-doped ZnO, to reduce the series resistance of the solar cell. Moreover, intrinsic ZnO is a low-resistance n-type transparent semiconductor [5], which can be us
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