The development of optimal backside reflectors (BSRs) is crucial for future low cost and high efficiency silicon (Si) thin-film solar cells. In this work, nanostructured polymer substrates with aluminum coatings intended as BSRs were produced by positive and negative nanoimprint lithography (NIL) techniques, and hydrogenated amorphous silicon (a-Si:H) was deposited hereon as absorbing layers. The relationship between optical properties and geometry of front textures was studied by combining experimental reflectance spectra and theoretical simulations. It was found that a significant height variation on front textures plays a critical role for light-trapping enhancement in solar cell applications. As a part of sample preparation, a transfer NIL process was developed to overcome the problem of low heat deflection temperature of polymer substrates during solar cell fabrication.

Nanoimprinted backside reflectors for a-Si:H thin-film solar cells: critical role of absorber front textures.

Theoretical analysis of improved efficiency of silicon-wafer solar cells with textured nanotriangular grating structure

In this work, a shape-optimized periodic pattern design is employed to boost the short circuit current of solar cells. A decent result of an additional 16.1% enhancement in short circuit current is achieved by solely pattern-wise optimization, compared to the baseline structure that is already under full parameter optimization. The underlying physics is that the shape-optimized pattern leads to optimal quasi-guided mode excitations. As a result of the pattern design, a single strongly confined quasi-guided mode is replaced with several weakly confined modes, to cover a broader spectral range. Previous works of optimized periodic gratings result in gradually varied grating heights and require grayscale lithography leading to high process complexity. Using randomized pattern for isotropic Lambertian light trapping, on the other hand, leads to an overly large simulation domain. The proposed pattern design methodology achieves the optimal balance between the slow-light enhancement strength and the enhancement spectral range for nanophotonic light trapping using quasi-guided modes.

/pdf/toward-ultimate-nanophotonic-light-trapping-using-pattern-3or029toc5.pdf

Toward ultimate nanophotonic light trapping using pattern-designed quasi-guided mode excitations

The geometry and dimension design is the most critical part for the success in nano-photonic devices. The choices of the geometrical parameters dramatically affect the device performance. Most of the time, simulation is conducted to locate the suitable geometry, but in many cases simulation can be ineffective. The most pronounced examples are large-area randomized patterns for solar cells, light emitting diode (LED), and thermophtovoltaics (TPV). The large random pattern is nearly impossible to calculate and optimize due to the extended CPU runtime and the memory limitation. Other scenarios that numerical simulations become ineffective include three-dimensional complex structures with anisotropic dielectric response. This leads to extended simulation time especially for the repeated runs during its geometry optimization. In this paper, we show that by incorporating genetic algorithm (GA) into real-world experiments, shortened trial-and-error time can be achieved. More importantly, this scheme can be used for many photonic design problems that are unsuitable for simulation-based optimizations. Moreover, the experimentally implemented genetic algorithm (Exp-GA) has the additional advantage that the resultant objective value is a real one rather than a theoretical one. This prevents the gaps between the modeling and the fabrication due to the process variation or inaccurate numerical models. Using TPV emitters as an example, 22% enhancement in the mean objective value is achieved.

https://ir.nctu.edu.tw:443/bitstream/11536/129432/1/277926353f5c7df2fe6760afa4f55134.pdf

Experimentally-implemented genetic algorithm (Exp-GA): toward fully optimal photovoltaics.

Nanostructured metallic backside reflectors (BSRs) are crucial for enhanced optical absorption in thin-film amorphous silicon solar cells. The structural fabrication based on rapid aluminum (Al) anodization has a potential for roll-to-roll processes, which are promising for low-cost and large-scale fabrication of BSRs. In this regard, the short fabrication time via appropriate choice of acid, acid concentration, temperature, and time-dependent voltage control is very important. In addition, we first demonstrate the trimming of structural height by using conventional Al anodization, so the best pore size of BSRs with the same structural height can be determined. According to integrated external quantum efficiency (IEQE) calculations, the nanostructured BSR with 430-nm pore size shows 51.6 % IEQE improvement compared with the value of the flat BSR. Moreover, it is interesting that the absorption spectra of a-Si:H on nanostructured Al BSRs with and without the 100-nm Ag coating are highly similar even if the optical properties of Al and Ag are different.

Rapid fabrication and trimming of nanostructured backside reflectors for enhanced optical absorption in a -Si:H solar cells

The optimized random reflector is highly preferred for solar cells due to its superiority over an un-optimized totally random surface and its potential to exceed the Lambertian limit. There are some obstacles to overcome for realizing optimized random reflectors, including the feasibility from the process viewpoint and the intensive computational demand for large-scale random reflector design. Here a binary random grating is proposed which can be easily fabricated using common lithographic techniques. By using a global optimization algorithm and three-dimensional (3D) EMW simulation, the solar cell structure with 4 4 quasi-random binary grating can provide 23% higher integrated absorbance than its periodic grating counterpart and 103.5% higher integrated absorbance than a planar cell, approaching the Lambertian limit. Broad-band transmission improvement at short wavelength and a broad-band waveguiding effect at long wavelength is observed for the optimized 3D geometry. Additionally, the optimized random grating surpasses the periodic grating at all incident angles. The absorbance of the large-scale, fully optimized binary pattern can potentially exceed the Lambertian limit while its computational demand is shown to be manageable.

http://albertlin.weebly.com/uploads/8/9/2/2/8922931/jo.pdf

Lithographically fabricable, optimized three-dimensional solar cell random structure

Solar cell with grating structure is used frequently for increased optical absorption, but it can lead to electrical degradation. Here coupled simulation is conducted to clarify the physical reason causing this J-V degradation.

/pdf/the-coupled-electrical-optical-simulation-of-solar-cell-j-v-2wexkq7itb.pdf

The coupled electrical-optical simulation of solar cell J-V degradation with grating heights

Diffuse reflectors can be beneficial for solar cells, due to no plasmonic loss, higher reflectance, decent light scattering, and low cost. Experimental and theoretical works are presented here to demonstrate its feasibility for photovoltaics.

/pdf/low-cost-all-dielectric-thin-film-solar-cell-using-diffuse-1vx623zvs0.pdf

Low cost all-dielectric thin-film solar cell using diffuse medium reflectors

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Lithographically fabricable, optimized three-dimensional solar cell random structure

The coupled electrical-optical simulation of solar cell J-V degradation with grating heights

Low cost all-dielectric thin-film solar cell using diffuse medium reflectors