Plasmonic Light Trapping in Amorphous Si Solar Cells Using Periodic Ag Nanodisk Structures
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Plasmonic Light Trapping in Amorphous Si Solar Cells Using Periodic Ag Nanodisk Structures Hidenori Mizuno, Hitoshi Sai, Koji Matsubara, and Michio Kondo Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology, Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan ABSTRACT This paper describes light trapping in superstrate-type amorphous Si solar cells incorporated with Ag nanostructures (nanodisks) fabricated by a transfer-printing approach. The changes in external quantum efficiency (EQE) and current-voltage characteristics were investigated by changing the position and size (thickness) of the Ag nanodisks in the cells fabricated on flat superstrates. It was confirmed that the optimized Ag nanodisk-configuration led to the enhanced EQE (20%) in the 600-800 nm wavelength range, and the enhanced EQE led to the improved overall conversion efficiency (7.5%) compared to the cell without Ag nanodisks (7.2%). However, the integration of the optimized Ag nanodisk-configuration with the cells fabricated on textured superstrates did not result in the enhanced EQE and conversion efficiency, suggesting further optical designs are necessary to exploit both texture- and plasmon-mediated light trapping effects. INTRODUCTION Thin-film Si materials, such as hydrogenated amorphous Si (a-Si:H) and microcrystalline Si (μc-Si:H), are attractive choices for solar cell applications due to the high natural abundance and low toxicity of Si. However, these thin-film Si-based solar cells require advanced light trapping systems to compensate the weak light absorption especially in the near-infrared region. While texture-mediated light trapping plays a dominant role in thin-film Si solar cells [1], the use of plasmonic effects derived from nanoscale metal structures have also been explored as a potential tool for light trapping [2]. The ability to introduce controlled metal nanostructure inside solar cells is thus increasingly important to design efficient devices, and with this respect, we recently proposed a simple transfer-printing approach to integrate Ag-based periodic nanodisk structures with μc-Si:H solar cells [3]. As a follow-up study on the use of such transfer-printed Ag nanodisks, we herein show the plasmonic light trapping effect applied in superstrate-type a-Si:H solar cells. EXPERIMENT The a-Si:H solar cells investigated in this study (Figure 1) were fabricated as follows. Glass substrates coated with flat or textured SnO2:F films were utilized as superstrates (flat ones were prepared by polishing commercially available textured ones, Asahi-VU). Plasma-enhanced chemical vapor deposition was performed on the superstrates to sequentially form a-Si:H p-i and
μc-Si:H n layers. The thickness of p, i, and n layers were 10, 250 and 40 nm, respectively. ZnO:Ga spacer layers with variable thickness (x) were then sputtered on the μc-Si:H n layer, and Ag nanodisks with variable thickness (y) were introduced on the μc-Si:H n layers (when x = 0 nm) or ZnO:Ga spacer layers (when
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