Optimal wavelength scale diffraction gratings for light trapping in solar cells
TL;DR: In this paper, the potential performance of thin silicon solar cells with either silicon (Si) or titanium dioxide (TiO2) gratings using numerical simulations was examined, and the results showed that submicron symmetric and skewed pyramids of Si or TiO2 are a highly effective way of achieving light trapping in thin film solar cells.
Abstract: Dielectric gratings are a promising method of achieving light trapping for thin crystalline silicon solar cells. In this paper, we systematically examine the potential performance of thin silicon solar cells with either silicon (Si) or titanium dioxide (TiO2) gratings using numerical simulations. The square pyramid structure with silicon nitride coating provides the best light trapping among all the symmetric structures investigated, with 89% of the expected short circuit current density of the Lambertian case. For structures where the grating is at the rear of the cell, we show that the light trapping provided by the square pyramid and the checkerboard structure is almost identical. Introducing asymmetry into the grating structures can further improve their light trapping properties. An optimized Si skewed pyramid grating on the front surface of the solar cell results in a maximum short circuit current density, Jsc, of 33.4 mA cm−2, which is 91% of the Jsc expected from an ideal Lambertian scatterer. An optimized Si skewed pyramid grating on the rear performs as well as a rear Lambertian scatterer and an optimized TiO2 grating on the rear results in 84% of the Jsc expected from an optimized Si grating. The results show that submicron symmetric and skewed pyramids of Si or TiO2 are a highly effective way of achieving light trapping in thin film solar cells. TiO2 structures would have the additional advantage of not increasing recombination within the cell.
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TL;DR: In this article, the authors review the theory of nanophotonic light trapping, with experimental examples given where possible, focusing particularly on periodic structures, since this is where physical understanding is most developed, and where theory and experiment can be most directly compared.
Abstract: Nanophotonic light trapping for solar cells is an exciting field that has seen exponential growth in the last few years. There has been a growing appreciation for solar energy as a major solution to the world’s energy problems, and the need to reduce materials costs by the use of thinner solar cells. At the same time, we have the newly developed ability to fabricate controlled structures on the nanoscale quickly and cheaply, and the computational power to optimize the structures and extract physical insights. In this paper, we review the theory of nanophotonic light trapping, with experimental examples given where possible. We focus particularly on periodic structures, since this is where physical understanding is most developed, and where theory and experiment can be most directly compared. We also provide a discussion on the parasitic losses and electrical effects that need to be considered when designing nanophotonic solar cells.
286 citations
TL;DR: In this paper, the state-of-the-art ultrathin photovoltaic cells with thickness at least 10 times lower than conventional solar cells are compared with theoretical light-trapping models.
Abstract: Ultrathin solar cells with thicknesses at least 10 times lower than conventional solar cells could have the unique potential to efficiently convert solar energy into electricity while enabling material savings, shorter deposition times and improved carrier collection in defective absorber materials. Efficient light absorption and hence high power conversion efficiency could be retained in ultrathin absorbers using light-trapping structures that enhance the optical path. Nevertheless, several technical challenges prevent the realization of a practical device. Here we review the state-of-the-art of c-Si, GaAs and Cu(In,Ga)(S,Se)2 ultrathin solar cells and compare their optical performances against theoretical light-trapping models. We then address challenges in the fabrication of ultrathin absorber layers and in nanoscale patterning of light-trapping structures and discuss strategies to ensure efficient charge collection. Finally, we propose practical architectures for ultrathin solar cells that combine photonic and electrical constraints, and identify future research directions and potential applications of ultrathin photovoltaic technologies. Ultrathin solar cells attract interest for their relatively low cost and potential novel applications. Here, Massiot et al. discuss their performance and the challenges in the fabrication of ultrathin absorbers, patterning of light trapping structures and ensuring efficient charge-carrier collection.
135 citations
TL;DR: This letter proposes ultrathin a-Si/c-Si tandem solar cells with an efficient light trapping design, where a nanopyramid structure is introduced between the top and bottom cells, and the use of SiOx mixed-phase nanomaterial helps to provide the maximum light trapping without paying the price of reduced electrical performance.
Abstract: Recently, ultrathin crystalline silicon solar cells have gained tremendous interest because they are deemed to dramatically reduce material usage. However, the resulting conversion efficiency is still limited by the incomplete light absorption in such ultrathin devices. In this letter, we propose ultrathin a-Si/c-Si tandem solar cells with an efficient light trapping design, where a nanopyramid structure is introduced between the top and bottom cells. The superior light harvesting results in a 48% and 35% remarkable improvement of the short-circuit current density for the top and bottom cells, respectively. Meanwhile, the use of SiOx mixed-phase nanomaterial helps to provide the maximum light trapping without paying the price of reduced electrical performance, and conversion efficiencies of up to 13.3% have been achieved for the ultrathin tandem cell employing only 8 μm of silicon, which is 29% higher than the result obtained for the planar cell.
78 citations
TL;DR: In this paper, a new world record was achieved for the efficiency of single-junction microcrystalline silicon solar cells, with a conversion efficiency of 11.0%, independently confirmed by the Advanced Industrial Science and Technology (AIST) Characterization, Standards, and Measurement (CSM) team.
Abstract: In this paper, we present our latest results toward high-efficiency thin-film silicon solar cells. Owing to the superior light trapping capability of periodic textures combined with other technologies, a new world record was achieved for the efficiency of single-junction microcrystalline silicon solar cells, with a conversion efficiency of 11.0%, independently confirmed by the Advanced Industrial Science and Technology (AIST) Characterization, Standards, and Measurement (CSM) team.
74 citations
TL;DR: This work reports on the fabrication of both planar and patterned ultrathin c-Si solar cells on glass using low temperature, low-cost, and scalable techniques and reveals that the low photon escape probability of 25% is the key factor in the light trapping mechanism.
Abstract: Ultrathin c-Si solar cells have the potential to drastically reduce costs by saving raw material while maintaining good efficiencies thanks to the excellent quality of monocrystalline silicon. However, efficient light trapping strategies must be implemented to achieve high short-circuit currents. We report on the fabrication of both planar and patterned ultrathin c-Si solar cells on glass using low temperature (T < 275 °C), low-cost, and scalable techniques. Epitaxial c-Si layers are grown by PECVD at 160 °C and transferred on a glass substrate by anodic bonding and mechanical cleavage. A silver back mirror is combined with a front texturation based on an inverted nanopyramid array fabricated by nanoimprint lithography and wet etching. We demonstrate a short-circuit current density of 25.3 mA/cm2 for an equivalent thickness of only 2.75 μm. External quantum efficiency (EQE) measurements are in very good agreement with FDTD simulations. We infer an optical path enhancement of 10 in the long wavelength rang...
73 citations
References
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01 Jan 2003
TL;DR: In this article, the p-n junction Monocrystalline solar cells and thin film solar cells managing light over the limit: Strategies for Higher Efficiency are discussed. And the basic principles of PV Electrons and Holes in Semiconductors Generation and Recombination Junctions Analysis of the P-n Junction Mon-Cylindrical Solar Cells
Abstract: Photons In, Electrons Out: Basic Principles of PV Electrons and Holes in Semiconductors Generation and Recombination Junctions Analysis of the p-n Junction Monocrystalline Solar Cells Thin Film Solar Cells Managing Light Over the Limit: Strategies for Higher Efficiency.
2,252 citations
Additional excerpts
...for mass production [1, 2]....
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TL;DR: In this paper, the light trapping properties of textured optical sheets have become of recent interest in photovoltaic energy conversion since light trapping allows a significant reduction in the thickness of active solar cell material.
Abstract: The light trapping properties of textured optical sheets have become of recent interest in photovoltaic energy conversion since light trapping allows a significant reduction in the thickness of active solar cell material. Previous analyses have concentrated on sheets with randomly textured (Lambertian) surfaces. The texturing of crystalline silicon substrates with anisotropic etches to give surfaces covered by square based pyramids defined by intersecting (111) crystallographic planes is a widely used technique for reflection control in silicon solar cells. This paper analyzes the light trapping properties of substrates with such pyramidally textured surfaces. Important differences are found from the case of Lambertian surfaces with practical consequences for the design of high efficiency silicon solar cells.
919 citations
"Optimal wavelength scale diffractio..." refers methods in this paper
...The standard approach for wafer-based cells is to use pyramid structures on a silicon surface which are usually several microns or more in size to trap light within the solar cell [6]....
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TL;DR: The detailed balance method for calculating the radiative recombination limit to the performance of solar cells has been extended to include free carrier absorption and Auger recombination in addition to radiative losses.
Abstract: The detailed balance method for calculating the radiative recombination limit to the performance of solar cells has been extended to include free carrier absorption and Auger recombination in addition to radiative losses. This method has been applied to crystalline silicon solar cells where the limiting efficiency is found to be 29.8 percent under AM1.5, based on the measured optical absorption spectrum and published values of the Auger and free carrier absorption coefficients. The silicon is assumed to be textured for maximum benefit from light-trapping effects.
831 citations
"Optimal wavelength scale diffractio..." refers background in this paper
...potential for increasing open circuit voltage (Voc) if a high level of optical confinement can be achieved [4]....
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...to the potential for lower bulk recombination [3, 4] and the...
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TL;DR: Here, a photonic crystal-based light-trapping approach is analyzed and compared to previous approaches for c-Si thin film solar cells, which gives rise to weak absorption of one-third of usable solar photons.
Abstract: Most photovoltaic (solar) cells are made from crystalline silicon (c-Si), which has an indirect band gap. This gives rise to weak absorption of one-third of usable solar photons. Therefore, improved light trapping schemes are needed, particularly for c-Si thin film solar cells. Here, a photonic crystal-based light-trapping approach is analyzed and compared to previous approaches. For a solar cell made of a 2 µm thin film of c-Si and a 6 bilayer distributed Bragg reflector (DBR) in the back, power generation can be enhanced by a relative amount of 24.0% by adding a 1D grating, 26.3% by replacing the DBR with a six-period triangular photonic crystal made of air holes in silicon, 31.3% by a DBR plus 2D grating, and 26.5% by replacing it with an eight-period inverse opal photonic crystal.
715 citations
"Optimal wavelength scale diffractio..." refers background in this paper
...However, there are 36% of photons with energies above the bandgap of Si in this wavelength range [5]....
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TL;DR: In this article, an efficient light-trapping scheme was developed for solar cells that can enhance the optical path length by several orders of magnitude using a textured photonic crystal as a backside reflector.
Abstract: An efficient light-trapping scheme is developed for solar cells that can enhance the optical path length by several orders of magnitude using a textured photonic crystal as a backside reflector. It comprises a reflection grating etched on the backside of the substrate and a one-dimensional photonic crystal deposited on the grating. Top-contacted crystalline Si solar cells integrated with the textured photonic crystal back reflector were designed and fabricated. External quantum efficiency was significantly improved between the wavelengths of 1000 and 1200nm (enhancement up to 135 times), and the overall power conversion efficiency was considerably increased.
364 citations