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.
Highlights
Efficiency improvement and cost reduction are everlasting schemes for photovoltaic (PV) research and development
The current PV market is dominated by crystalline silicon (c-Si) solar cells because of their high efficiency and steadily decreasing manufacturing cost [1,2], thin-film silicon solar cells (TFSC) including hydrogenated amorphous silicon (a-Si:H), amorphous silicon germanium (a-SiGe:H) and microcrystalline silicon are still promising candidates for special applications
Conventional 1D-photonic crystal (PC) back reflectors (BRs) usually were treated as distributed Bragg reflectors (DBR), in which the individual layer thickness was calculated by the quarter-wavelength basis for the Bragg wavelength λ0 [22,23,24,25,26,27]
Summary
Efficiency improvement and cost reduction are everlasting schemes for photovoltaic (PV) research and development. Decreasing the thickness-contrast of the absorptive layer with respect to the non-absorptive layer is expected to be a good method to improve the performance of the 1D-PC without changing the PBG for light-trapping [27] It is well-known that a highly efficient BR should simultaneously have high reflectivity, strong scattering of incident light and sufficient conductivity. The moderately random textures of post-chemically etched AZO films result in less microvoids and less undesirable metastable defects, which are favorable for high-quality thin silicon layers and high efficiency solar cells [29]. Several photonic crystal-based light-trapping approaches were developed and integrated into a-Si:H or a-SiGe:H solar cells These newly-designed composite structures included periodic 1D-PC, random-textured AZO and Ag, periodic conical 2D-PC and their combination. 520–800 the range from 520–800 nm to enhance absorption in a-Si:H cells
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