Optical Enhancement by Textured Back Reflector in Amorphous and Nanocrystalline Silicon Based Solar Cells

AbstractWe present the results of systematic studies of optical enhancements by textured Ag/ZnO back reflectors in a-SiGe:H and nc-Si:H solar cells. First, the back reflector materials were characterized by AFM, XRD, and TEM. The results showed that the ZnO layers deposited by sputtering exhibit an increased surface texture with the deposition temperature and film thickness. The material structure showed a strong (002) preferential orientation. The large columnar crystal structure determines the surface texture. Second, the solar cell performance was correlated to the back reflector structure. We found that with a thin ZnO layer, a textured Ag layer results in more light scattering than a flat Ag layer. However, when a thick ZnO layer is used, a flat Ag layer can produce similar or more light scattering than a textured Ag layer. Third, we developed a method to estimate the optical enhancement for a-SiGe:H and nc-Si:H solar cells on various structures of Ag/ZnO back reflectors. Comparing the quantum efficiency data from solar cells made using the same recipe but one on a flat stainless steel substrate and another on a textured Ag/ZnO BR substrate revealed that the optical enhancement for the long wavelength light can be as high as 20 to 30. Compared to the theoretical value of 4n2, there is still scope for further improvement

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High efficiency amorphous and nanocrystalline silicon based multi-junction solar cells deposited at high rates on textured Ag/ZnO back reflectors

MRS Proceedings ◽

10.1557/proc-1153-a10-05 ◽

2009 ◽

Vol 1153 ◽

Cited By ~ 6

Author(s):

Guozhen Yue ◽

Laura Sivec ◽

Baojie Yan ◽

Jeff Yang ◽

Subhendu Guha

Keyword(s):

Solar Cells ◽

Triple Junction ◽

High Efficiency ◽

Nanocrystalline Silicon ◽

Open Circuit ◽

Very High Frequency ◽

Solar Cell Performance ◽

Single Junction ◽

Back Reflectors ◽

Back Reflector

AbstractWe report our recent progress on nc-Si:H single-junction and a-Si:H/nc-Si:H/nc-Si:H triple-junction cells made by a modified very-high-frequency (MVHF) technique at deposition rates of 10-15 Å/s. First, we studied the effect of substrate texture on the nc-Si:H single-junction solar cell performance. We found that nc-Si:H single-junction cells made on bare stainless steel (SS) have a good fill factor (FF) of ˜0.73, while it decreased to ˜0.65 when the cells were deposited on textured Ag/ZnO back reflectors. The open-circuit voltage (Voc) also decreased. We used dark current-voltage (J-V), Raman, and X-ray diffraction (XRD) measurements to characterize the material properties. The dark J-V measurement showed that the reverse saturated current was increased by a factor of ˜30 when a textured Ag/ZnO back reflector was used. Raman results revealed that the nc-Si:H intrinsic layers in the two solar cells have similar crystallinity. However, they showed a different crystallographic orientation as indicated in XRD patterns. The material grown on Ag/ZnO has more random orientation than that on SS. These experimental results suggested that the deterioration of FF in nc-Si:H solar cells on textured Ag/ZnO was caused by poor nc-Si:H quality. Based on this study, we have improved our Ag/ZnO back reflector and the quality of nc-Si:H component cells and achieved an initial and stable active-area efficiencies of 13.4% and 12.1%, respectively, in an a-Si:H/nc-Si:H/nc-Si:H triple-junction cell.

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Fabrication of Photonic Crystal based Back Reflectors for Light Management and Enhanced Absorption in Amorphous Silicon Solar Cells

MRS Proceedings ◽

10.1557/proc-1153-a07-15 ◽

2009 ◽

Vol 1153 ◽

Cited By ~ 2

Author(s):

Benjamin Curtin ◽

Rana Biswas ◽

Vikram Dalal

Keyword(s):

Solar Cells ◽

Photonic Crystal ◽

Steel Substrate ◽

Back Surface ◽

Enhanced Absorption ◽

Reference Device ◽

Back Reflectors ◽

Light Management ◽

Back Reflector ◽

Microscopy Images

AbstractPhotonic crystal based back-reflectors are an attractive solution for light management and enhancing optical absorption in thin film solar cells, without undesirable losses. We have fabricated prototype photonic crystal back-reflectors using photolithographic methods and reactive-ion etching. The photonic crystal back-reflector has a triangular lattice symmetry, a thickness of 250 nm, and a pitch of 765 nm. Scanning electron microscopy images demonstrate high quality long range periodicity. An a-Si:H solar cell device was grown on this back-reflector using standard PECVD techniques. Measurements demonstrate strong diffraction of light and high diffuse reflectance by the photonic crystal back-reflector. The photonic crystal back-reflector increases the average photon collection by ˜9% in terms of normalized external quantum efficiency, relative to a reference device on a stainless steel substrate with an Ag coated back surface.

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Nanomolded buried light-scattering (BLiS) back-reflectors using dielectric nanoparticles for light harvesting in thin-film silicon solar cells

EPJ Photovoltaics ◽

10.1051/epjpv/2019011 ◽

2020 ◽

Vol 11 ◽

pp. 2

Author(s):

Derese Desta ◽

Rita Rizzoli ◽

Caterina Summonte ◽

Rui N. Pereira ◽

Arne Nylandsted Larsen ◽

...

Keyword(s):

Solar Cells ◽

Light Scattering ◽

Silicon Nanoparticles ◽

Short Circuit ◽

Spatial Separation ◽

Short Circuit Current ◽

Silicon Solar Cells ◽

Growth Interface ◽

Inverted Pyramid ◽

Back Reflector

The article presents a nanoparticle-based buried light-scattering (BLiS) back-reflector design realized through a simplified nanofabrication technique for the purpose of light-management in solar cells. The BLiS structure consists of a flat silver back-reflector with an overlying light-scattering bilayer which is made of a TiO2 dielectric nanoparticles layer with micron-sized inverted pyramidal cavities, buried under a flat-topped silicon nanoparticles layer. The optical properties of this BLiS back-reflector show high broadband and wide angular distribution of diffuse light-scattering. The efficient light-scattering by the buried inverted pyramid back-reflector is shown to effectively improve the short-circuit-current density and efficiency of the overlying n-i-p amorphous silicon solar cells up to 14% and 17.5%, respectively, compared to the reference flat solar cells. A layer of TiO2 nanoparticles with exposed inverted pyramid microstructures shows equivalent light scattering but poor fill factors in the solar cells, indicating that the overlying smooth growth interface in the BLiS back-reflector helps to maintain a good fill factor. The study demonstrates the advantage of spatial separation of the light-trapping and the semiconductor growth layers in the photovoltaic back-reflector without sacrificing the optical benefit.

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Light trapping in hydrogenated amorphous and nano-crystalline silicon thin film solar cells

MRS Proceedings ◽

10.1557/proc-1153-a13-02 ◽

2009 ◽

Vol 1153 ◽

Cited By ~ 11

Author(s):

Jeffrey Yang ◽

Baojie Yan ◽

Guozhen Yue ◽

Subhendu Guha

Keyword(s):

Stainless Steel ◽

Solar Cells ◽

Triple Junction ◽

Crystalline Silicon ◽

Light Trapping ◽

Nano Crystalline ◽

Back Reflectors ◽

Back Reflector ◽

Junction Structure ◽

Hydrogenated Amorphous

AbstractLight trapping effect in hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) and nano-crystalline silicon (nc-Si:H) thin film solar cells deposited on stainless steel substrates with various back reflectors is reviewed. Structural and optical properties of the Ag/ZnO back reflectors are systematically characterized and correlated to solar cell performance, especially the enhancement in photocurrent. The light trapping method used in our current production lines employing an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure consists of a bi-layer of Al/ZnO back reflector with relatively thin Al and ZnO layers. Such Al/ZnO back reflectors enhance the short-circuit current density, Jsc, by ˜20% compared to bare stainless steel. In the laboratory, we use Ag/ZnO back reflector for higher Jsc and efficiency. The gain in Jsc is about ˜30% for an a-SiGe:H single-junction cell used in the bottom cell of a multi-junction structure. In recent years, we have also worked on the optimization of Ag/ZnO back reflectors for nano-crystalline silicon (nc-Si:H) solar cells. We have carried out a systematic study on the effect of texture for Ag and ZnO. We found that for a thin ZnO layer, a textured Ag layer is necessary to increase Jsc, even though the parasitic loss is higher at the Ag and ZnO interface due to the textured Ag. However, a flat Ag can be used for a thick ZnO to reduce the parasitic loss, while the light scattering is provided by the textured ZnO. The gain in Jsc for nc-Si:H solar cells on Ag/ZnO back reflectors is in the range of ˜60-75% compared to cells deposited on bare stainless steel, which is much larger than the enhancement observed for a-SiGe:H cells. The highest total current density achieved in an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure on Ag/ZnO back reflector is 28.6 mA/cm2, while it is 26.9 mA/cm2 for a high efficiency a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cell.

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Light-trapping in Thin Film Silicon Solar Cells with a Combination of Periodic and Randomly Textured Back-reflectors

MRS Proceedings ◽

10.1557/opl.2012.888 ◽

2012 ◽

Vol 1426 ◽

pp. 117-123 ◽

Cited By ~ 1

Author(s):

Sambit Pattnaik ◽

Nayan Chakravarty ◽

Rana Biswas ◽

D. Slafer ◽

Vikram Dalal

Keyword(s):

Thin Film ◽

Solar Cells ◽

Light Absorption ◽

Nanoimprint Lithography ◽

Light Trapping ◽

Silicon Solar Cells ◽

Long Wavelength ◽

Thin Film Silicon ◽

Back Reflectors ◽

Back Reflector

ABSTRACTLight trapping is essential to harvest long wavelength red and near-infrared photons in thin film silicon solar cells. Traditionally light trapping has been achieved with a randomly roughened Ag/ZnO back reflector, which scatters incoming light uniformly through all angles, and enhances currents and cell efficiencies over a flat back reflector. A new approach using periodically textured photonic-plasmonic arrays has been recently shown to be very promising for harvesting long wavelength photons, through diffraction of light and plasmonic light concentration. Here we investigate the combination of these two approaches of random scattering and plasmonic effects to increase cell performance even further. An array of periodic conical back reflectors was fabricated by nanoimprint lithography and coated with Ag. These back reflectors were systematically annealed to generate different amounts of random texture, at smaller spatial scales, superimposed on a larger scale periodic texture. nc-Si solar cells were grown on flat, periodic photonic-plasmonic substrates, and randomly roughened photonic-plasmonic substrates. There were large improvements (>20%) in the current and light absorption of the photonic-plasmonic substrates relative to flat. The additional random features introduced on the photonic-plasmonic substrates did not improve the current and light absorption further, over a large range of randomization features.

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Material structure and metastability of hydrogenated nanocrystalline silicon solar cells

Applied Physics Letters ◽

10.1063/1.2216022 ◽

2006 ◽

Vol 88 (26) ◽

pp. 263507 ◽

Cited By ~ 59

Author(s):

Guozhen Yue ◽

Baojie Yan ◽

Gautam Ganguly ◽

Jeffrey Yang ◽

Subhendu Guha ◽

...

Keyword(s):

Solar Cells ◽

Nanocrystalline Silicon ◽

Silicon Solar Cells ◽

Material Structure

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Photonic Crystal Back Reflectors for Enhanced Absorption in Amorphous Silicon Solar Cells

MRS Proceedings ◽

10.1557/proc-1245-a07-21 ◽

2010 ◽

Vol 1245 ◽

Author(s):

Benjamin Curtin ◽

Rana Biswas ◽

Vikram Dalal

Keyword(s):

Solar Cells ◽

Photonic Crystal ◽

Amorphous Silicon ◽

Light Absorption ◽

Short Circuit ◽

Silicon Solar Cells ◽

Etch Depth ◽

Enhanced Absorption ◽

Back Reflectors ◽

Back Reflector

AbstractPhotonic crystal back reflectors offer enhanced optical absorption in thin-film solar cells, without undesirable losses. Rigorous simulations of photonic crystal back reflectors predicted maximized light absorption in amorphous silicon solar cells for a pitch of 700-800 nm. Simulations also predict that for typical 250 nm i-layer cells, the periodic photonic crystal back reflector can improve absorption over the ideal randomly roughened back reflector (or the ‘4n2classical limit') at wavelengths near the band edge. The PC back reflector provides even higher enhancement than roughened back reflectors for cells with even thinner i-layers. Using these simulated designs, we fabricated metallic photonic crystal back reflectors with different etch depths and i-layer thicknesses. The photonic crystals had a pitch of 760 nm and triangular lattice symmetry. The average light absorption increased with the PC back reflectors, but the greatest improvement (7-8%) in short circuit current was found for thinner i-layers. We have studied the dependence of cell performance on the etch depth of the photonic crystal. The photonic crystal back reflector strongly diffracts light and increases optical path lengths of solar photons.

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Effect of Surface Morphology in ZnO:Al/Ag Back Reflectors for Flexible Silicon Thin Film Solar Cells on Light Scattering Properties

Korean Journal of Materials Research ◽

10.3740/mrsk.2010.20.10.501 ◽

2010 ◽

Vol 20 (10) ◽

pp. 501-507 ◽

Cited By ~ 2

Author(s):

Sang-Hun Beak ◽

Jeong-Chul Lee ◽

Sang-Hyun Park ◽

Jin-Soo Song ◽

Kyung-Hoon Yoon ◽

...

Keyword(s):

Thin Film ◽

Solar Cells ◽

Light Scattering ◽

Surface Morphology ◽

Thin Film Solar Cells ◽

Silicon Thin Film ◽

Back Reflectors ◽

Effect Of Surface

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Light Trapping in Hydrogenated Amorphous and Nanocrystalline Silicon Based Thin-Film Solar Cells With Ag/ZnO Back Reflectors

MRS Proceedings ◽

10.1557/opl.2012.528 ◽

2012 ◽

Vol 1391 ◽

Author(s):

Baojie Yan ◽

Guozhen Yue ◽

Laura Sivec ◽

Jessica Owens-Mawson ◽

Jeffrey Yang ◽

...

Keyword(s):

Solar Cells ◽

Classical Limit ◽

Light Trapping ◽

Wavelength Region ◽

Nanocrystalline Silicon ◽

Future Research ◽

Single Junction ◽

Back Reflectors ◽

Top Contact ◽

The Difference

ABSTRACTWe report on our systematic study of light trapping effects using Ag/ZnO BRs for nc-Si:H solar cells. The texture of Ag and ZnO was optimized to achieve enhancement in photocurrent. The light trapping effect on photocurrent enhancement in solar cells was carefully investigated. Comparing to single-junction solar cells deposited on flat stainless steel substrates, the gain in Jsc by using Ag/ZnO BRs is 57% for nc-Si:H solar cells. This gain in Jsc is much higher than what has been achieved by advanced light trapping approaches using photonic structures or plasmonic light trapping reported in the literature. We achieved a Jsc of 29-30 mA/cm2 in a nc-Si:H single-junction solar cell with an intrinsic layer thickness of ∼2.5 μm. We compared the quantum efficiency of single-junction cells to the classical limit of fully randomized scattering and found that there is a 6-7 mA/cm2 difference between the measured Jsc and the classical limit, in which 3-4 mA/cm2 is in the long wavelength region. However, by taking into consideration the losses from reflection of the top contact, absorption in the doped layers, and imperfect reflection in the BRs, the difference disappears. This implies we have reached the practical limit if the scattering from randomly textured substrates is the only mechanism of light trapping. Therefore, we believe future research for improving photocurrent should be directed toward reducing (i) reflection loss by the top contact, the absorption in ZnO and at the Ag/ZnO interface, and (ii) p layer absorption.

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