Photonic Structures for Light Trapping in Thin Film Silicon Solar Cells: Design and Experiment

Abstract

Abstract:One of the foremost challenges in designing thin-film silicon solar cells (TFSC) is devising efficient light-trapping schemes due to the short optical path length imposed by the thin absorber thickness. The strategy relies on a combination of a high-performance back reflector and an optimized texture surface, which are commonly used to reflect and scatter light effectively within the absorption layer, respectively. In this paper, highly promising light-trapping structures based on a photonic crystal (PC) for TFSCs were investigated via simulation and experiment. Firstly, a highly-reflective one-dimensional photonic crystal (1D-PC) was designed and fabricated. Then, two types of 1D-PC-based back reflectors (BRs) were proposed: Flat 1D-PC with random-textured aluminum-doped zinc oxide (AZO) or random-textured 1D-PC with AZO. These two newly-designed BRs demonstrated not only high reflectivity and sufficient conductivity, but also a strong light scattering property, which made them efficient candidates as the electrical contact and back reflector since the intrinsic losses due to the surface plasmon modes of the rough metal BRs can be avoided. Secondly, conical two-dimensional photonic crystal (2D-PC)-based BRs were investigated and optimized for amorphous a-SiGe:H solar cells. The maximal absorption value can be obtained with an aspect ratio of 1/2 and a period of 0.75 µm. To improve the full-spectral optical properties of solar cells, a periodically-modulated PC back reflector was proposed and experimentally demonstrated in the a-SiGe:H solar cell. This periodically-modulated PC back reflector, also called the quasi-crystal structure (QCS), consists of a large periodic conical PC and a randomly-textured Ag layer with a feature size of 500-1000 nm. The large periodic conical PC enables conformal growth of the layer, while the small feature size of Ag can further enhance the light scattering. In summary, a comprehensive study of the design, simulation and fabrication of 1D-PC-and 2D-PC-based back reflectors for TFSCs was carried out. Total absorption and device performance enhancement were achieved with the novel PC light-trapping systems because of their high reflectivity or high scattering property. Further research is necessary to illuminate the optimal structure design of PC-based back reflectors and high solar cell efficiency.

Research Progress of Plasmonic Nanostructure-Enhanced Photovoltaic Solar Cells

“…Ding et al [ 28 ] investigated a nanocone grating with architecture with a high aspect ratio and a dense periodicity of about 500 nm. This configuration was applied at the front surface as an antireflection coating, and it was successfully demonstrated that a substantial increase in absorption could be obtained.…”

“…( B ) SEM images of MCPS (P3H0.5, P3H1.0, P3H1.5), MCPS coated with Ag/AZO, and a corresponding solar cell. ( C ) SEM images of microcone structure (MCS), quasicrystal structure (QCS), and MCS-based and QCS-based a-SiGe:H solar cells [ 28 ] (reprinted with permission from [ 28 ], 2017, Creative Commons CC-BY license).…”

“…For this purpose, in the industry, the main nanostructures used are inverted pyramid or upright structures [ 23 , 25 , 26 , 27 ] or random textures [ 28 , 29 , 30 ] with a distinctive typical period size of 3–10 µm applied primarily for c-Si solar cell texturing [ 31 ]. It was reported by Kumaravelu et al [ 32 ] that for thin-film solar cells, where the thickness of active layer itself is a few microns/hundreds of nanometers, micron-scale structuring is not advantageous because it requires deep etching and easily creates defects in the active layer.…”

“…There are numerous nanostructures with which light can be trapped in thin-film solar cells. The most used approaches for light trapping are plasmonic nanoparticle structures [ 37 , 38 ], random scattering surfaces [ 28 ], periodic nanograting [ 39 , 40 ], nanowires [ 41 ], and photonic crystal structures [ 28 , 42 ].…”

Soft-Microstructured Transparent Electrodes for Photonic-Enhanced Flexible Solar Cells

“…Its collection, conversion, and storage still require significant technological improvements in order to fully realize its potential as a fossil fuel replacement [1]. The current photovoltaic (PV) market is thus still dominated by first-generation crystalline silicon solar cells (~95% share), which can achieve a maximum PV conversion efficiency of ~26%, despite their high production and implementation costs [2][3][4]. Second-generation devices based on thin-film solar cells (TFSCs) have emerged due to the necessity of lowering production costs, reducing material expenditure, and increasing mechanical flexibility, therefore enabling a much wider range of applications.…”