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Enhancing the light absorption in thin-film silicon solar cells is important for improving efficiency and reducing cost. In this paper, we propose a Fourier-series based periodic array (FSPA) for light trapping texture on the front surface of thin film silicon solar cells. The optimized texture with 300nm thickness yields a photocurrent of 27.05 mA/cm2 at a silicon thickness of 1μm. The spectral analysis shows the texture exhibits better than inverted pyramid and cosine surface arrays in near-infrared region. The angular analysis shows that the texture outperforms inverted pyramid and cosine surface arrays at all incidence angles. For incidence angles within about [0°, 65°], the short circuit current density has no obvious decrease.
© 2017 Optical Society of America
1. Introduction
Currently wafers are produced by expensive silicon purification, ingot growth and dicing processes. The cost of the wafer is about 40-50% of the cost of the finished solar module [1], which hampers large-scale commercial application of crystalline silicon solar cells. Use of thin film silicon is one of the ways to reduce the cost of solar cells, but this brings the problem of low conversion efficiency owing to small absorption length and low absorption coefficient in near-infrared light region [2]. Light trapping nanostructures as top or bottom textures can effectively enhance the efficiency of thin film solar energy by scattering light and increasing the optical path length of photons in silicon, including textured transparent conduct oxide(TCO), metallic particles, periodic photonic structures, etc.. Textured TCO is commonly made by plasma etching [3,4], which leads to random surface. Randomly textured TCO has wide angular response but with small interaction between light and silicon. Plasmonic nanoparticles have been proposed to improve the interaction and scattering cross section [5]. Due to intrinsic absorption in metallic particles, there are only a limited number of cases where the beneficial impact has been experimentally demonstrated on realistic cell designs [6]. In recent years, periodic photonic structures were suggested as light trapping textures to confine the electromagnetic energy in silicon, such as nanowire [7], nanocone [8], and nanopyramid [9], and cosine surface array [10]. In [7] and [8], nanowire and nanocone array were fabricated by self-assembly method. The self-assembly method typically makes a pattern with an area of square millimeter, which is not suitable for commercial purpose. Pyramid array can be only obtained on monocrystalline silicon substrate by anisotropic wet etching. To avoid the shortcomings mentioned above, cosine surface array was proposed [10]. The cosine surface texture can be achieved on any silicon substrate and has comparable performance to inverted pyramid. In this paper, we propose a FSPA texture that has better optical performance than pyramid and cosine surface array. The texture can be easily fabricated by interference lithography and transferred into any silicon cells by plasma etching. In the following, we describe how to design and fabricate the texture as well as demonstrate its performance.
2. Design of FSPA
In principle, the texture can be mathematically expressed by a Fourier-series based function that is defined by:
where (ai, bi) are the Fourier coefficients and Λ is the periodicity. C0 is zero-order component in connection with the texture thickness. By optimizing (C0, Λ, a1, b1, …, aN, bN), one can obtain an ideal light trapping texture. It is observed that the texture is composed of cosine components so that it can be formed by interference lithography, a low-cost large-area fabrication technique. In order for easy fabrication, we set N to be 2 and thus the optimized parameters are reduced to (C0, Λ, a1, b1, a2, b2).The solar cell devices for optimization originate from [11] as illustrated in Fig. 1. From top to bottom, it consists of TCO, 1 μm crystalline silicon (Si), and a perfect metallic reflector on the backside. The bottom interface between silicon and reflector is flat, and only the top interface between Si and TCO is textured with our proposed structures. As a consequence, parasitic plasmonic losses of the metal back contact were not considered. For the sake of simplicity, the silicon was considered to be intrinsic and the effects of p and n regions were neglected. The TCO material was set to Zinc oxide. Its optical constant was also obtained from [11]. The optical constants of silicon were extracted from handbook of optical constants [12]. Optical simulations are based on the Rigorous Coupled-Wave Analysis technique that is a fast and efficient technique to analyze the light propagation in thin-film devices with nanotextured interfaces [13].
The FSPA textures were optimized by maximizing the short circuit current density Jsc over the entire silicon absorption spectrum (300nm-1100nm) if we assume that all the electron-hole pairs contribute to the photocurrent [14],
where q is the elementary charge, λ is the incident wavelength, h is Planck constant, c is the speed of light, A(λ) is the silicon absorption spectrum and IAM1.5G(λ) is the incident AM1.5G solar spectral irradiance. The absorption of the silicon layer can be expressed as A(λ) = 1-R(λ). By changing the parameters (C0, Λ, a1, b1, a2, b2), one can achieve different A(λ) and further affect Jsc.If the periodicity and thickness of the texture can be found at around reasonable values, the amount of calculation will be greatly reduced. According to Yu model [7], one can obtain large absorption enhancement at specific wavelength comparable to the grating periodicity. Therefore, selection of the FSPA periodicity should make sure that sub-periodicities Λ/i are located within the silicon absorption spectrum. Each periodicity generates an incoupling resonance and thus multiple incoupling resonances result in large absorption increase. Obviously, the periodicity of the texture is at least larger than 600nm in N = 2 case, suggesting that the periodicity should start from 600nm in global optimization. Figure 2 shows Jsc as a function of periodicity at different thicknesses and incident angles. In Fig. 2(a) with 100nm thickness case, it can be seen at each periodicity there is an optimized texture. For example, the optimized coefficients associated with the periodicity of 600nm are (2, 2, 0, 3), as indicated under the curves. At normal incidence, the Jsc at 600nm periodicity is about 20mA/cm2, whereas the planar structure only obtains a Jsc of 13.60mA/cm2. There is 47.6% increase using FSPA. At 900nm periodicity, maximum Jsc is achieved and then the associated Jsc decrease with the increase of the periodicity. Under oblique incidence, the averaged Jsc decreases slightly. At 30° incidence, the averaged Jsc decreases with 0.41mA/cm2. It is interesting that the averaged Jsc decreases with only 0.43mA/cm2 when the incidence angle increases to 60°, which means the FSPA texture has very wide angular response.
Fig. 2 Short circuit current density as a function of the FSPA periodicity at different thicknesses and incident angles. (a), (b) and (c) correspond to the texture thickness of 100nm, 300nm, 500nm, respectively.
With the increase of the texture thickness, Jsc is improved quickly. For 300nm thickness as shown in Fig. 2(b), the averaged Jsc under normal incidence is improved by 35.57%, up to 25.67 mA/cm2. At 30° and 60° incidence angles, the increase percent are 35.82% and 37.06%, respectively. For 500nm thickness(Fig. 2(c)), the averaged Jsc continue increase. In contrast to 300nm thickness case, the averaged Jsc at normal incidence increase by 7.65%. At 30° and 60° incidences, the increase percent are 6.84% and 5.53% respectively. However, we have to make a compromise choice of the texture thickness because deep nanostructures are not easy to realize by plasma etching while shallow ones cause small interaction. The reported nanotextures typically have a height distribution from 100nm to 300nm [15]. Considering the fabrication feasibility and cell efficiency, we chose 300nm texture thickness for further analysis. Back to Fig. 2(b), we can see that the associated Jsc achieves maximum at the periodicity of 1000nm where the Jsc is 27.05 mA/cm2, indicating 1000nm is the proper periodicity of FSPA for light trapping. The optimized FSPA is shown in Fig. 3. From the viewpoint of surface profile, it is more disordered than pyramid and cosine surface array, which results in wide angular response. However, it is compound periodic array, which contributes to incoupling of more light into the silicon layer and thus improving light absorption.
3. Fabrication of FSPA
It is worth pointing out that FSPA can be easily made by two-beam interference. As described in Eq. (1), the texture is actually composed of two cosine components in x direction and two cosine components in y direction. Thus, we have to exposure the photoresist four times, that is, two times along x direction and two times along y direction. The periodicity of the components can be controlled by changing the clip angle of two beams. We used a laser with 405nm wavelength. For 1000nm periodicity, the clip angle of two beams is 23.4°. For sub-periodicity of 500nm, the clip angle is 47.8°. The photoresist used is AZ6112. The exposure dose(or time) determines the depth of each component. The dose ratio in four exposures is in accord with the Fourier coefficients (4, 2, 3, 2). The first exposure and development time are 10 seconds and 12 seconds, respectively. Finally, we obtained the texture on photoresist as shown in Fig. 4(a). The cross section is presented in Fig. 4(b). It is observed that the experimental surface profile marked in red is close to the simulated one in black. The tiny displacement between the two curves is due to limited overlaying accuracy of different cosine components. The FSPA texture can be transferred into the silicon devices by plasma etching.
4. Performance analysis
In Fig. 5, we compared the FSPA texture with inverted pyramid and cosine surface array. For fair comparison, the pyramid and cosine surface array have the same thickness as FSPA and both textures were optimized. The periodicities of the optimized pyramid and cosine surface array are 424nm and 900nm, respectively. Figure 5(a) presents the spectral response at normal incidence for the textured and planar devices in which the data was smoothed. It can be seen that the three textured devices behave better than the planar device in entire silicon absorption spectrum. The pyramid and cosine surface arrays exhibit similar. In the wavelength region of 600nm to 900nm, the absorption in the FSPA case is obviously higher than the others, which greatly increases near-IR light absorption. At other wavelengths, the absorption in three cases are similar. The average enhancement factor [7] in the FSPA case relative to single pass through the substrate is 31.94, whereas those for pyramid and cosine cases are 11.70 and 12.89, respectively. Figure 5(b) shows the angular response. At normal incidence, for the planar device, the Jsc is less than 13.26mA/cm2. The pyramid and cosine surface textures improve the Jsc to 24.93mA/cm2 and 24.30mA/cm2, respectively. When the incidence angle is greater than 40°, the Jsc in cosine texture case exceeds that in pyramid case. It is pronounced that the Jsc for FSPA are always better than those for pyramid and cosine surface array at all incidence angles. From 0° to 65°, the Jsc has no obvious decrease. Even at near 80° incidence, the Jsc still remains at around 20 mA/cm2. In this regard, the FSPA texture is similar to a solar tracking system.
5. Conclusion
In conclusion, we proposed a FSPA texture that delivers better optical performance than pyramid and cosine surface array in spectral and angular responses. The texture can be easily fabricated by interference lithography, which enables low cost and large area (>1m2) fabrication. It is possible to achieve broader and wider absorption if the Fourier series are further reasonably truncated. Thus, our proposed texture for improved light absorption is a promising solution for economically viable photovoltaic devices.
Funding
This work was supported by the Science-Technology Foundation of Sichuan Province, China(No. 2016GZ0399).
References and links
1. B. Rech, T. Repmann, M. N. van den Donker, M. Berginski, T. Kilper, J. Hüpkes, S. Calnan, H. Stiebig, and S. Wieder, “Challenges in microcrystalline silicon based solar cell technology,” Thin Solid Films 511–512(14), 548–555 (2006). [CrossRef]
2. A. V. Shah, H. Schade, M. Vanecek, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz, and J. Bailat, “Thin-film silicon solar cell technology,” Prog. Photovolt. Res. Appl. 12(23), 113–142 (2004). [CrossRef]
3. J. Müller, B. Rech, J. Springer, and M. Vanecek, “TCO and light trapping in silicon thin film solar cells,” Sol. Energy 77(6), 917–930 (2004). [CrossRef]
4. M. Berginski, J. Hüpkes, M. Schulte, G. Schöpe, H. Stiebig, B. Rech, and M. Wuttig, “The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells,” Appl. Phys. (Berl.) 101(7), 074903 (2007). [CrossRef]
5. D. Derkacs, S. H. Lim, P. Matheu, W. Mar, and E. T. Yu, “Improved performance of amorphous silicon solar cells via scattering from surface plasmon polaritons in nearby metallic nanoparticles,” Appl. Phys. Lett. 89(9), 093103 (2006). [CrossRef]
6. M. L. Brongersma, Y. Cui, and S. Fan, “Light management for photovoltaics using high-index nanostructures,” Nat. Mater. 13(5), 451–460 (2014). [CrossRef] [PubMed]
7. E. Garnett and P. Yang, “Light trapping in silicon nanowire solar cells,” Nano Lett. 10(3), 1082–1087 (2010). [CrossRef] [PubMed]
8. S. Jeong, M. D. McGehee, and Y. Cui, “All-back-contact ultra-thin silicon nanocone solar cells with 13.7% power conversion efficiency,” Nat. Commun. 4(4), 2950 (2013). [PubMed]
9. M. S. Branham, W. C. Hsu, S. Yerci, J. Loomis, S. V. Boriskina, B. R. Hoard, S. E. Han, and G. Chen, “15.7% Efficient 10-μm-Thick Crystalline Silicon Solar Cells Using Periodic Nanostructures,” Adv. Mater. 27(13), 2182–2188 (2015). [CrossRef] [PubMed]
10. X. Guo, Y. Zhou, B. Liu, and Y. Li, “Cosine light-trapping nanostructures for thin film solar cells,” Opt. Lett. 40(16), 3866–3868 (2015). [CrossRef] [PubMed]
11. C. Haase and H. Stiebig, “Thin-film silicon solar cells with efficient periodic light trapping texture,” Appl. Phys. Lett. 91(6), 061116 (2007). [CrossRef]
12. E. D. Palik, Handbook of Optical Constants of Solids (Academic,1985).
13. V. Liu and S. Fan, “S4: A free electromagnetic solver for layered periodic structures,” Comput. Phys. Commun. 183(10), 2233–2244 (2012). [CrossRef]
14. R. Dewan, V. Jovanov, S. Hamraz, and D. Knipp, “Analyzing periodic and random textured silicon thin film solar cells by Rigorous Coupled Wave Analysis,” Sci. Rep. 4, 6029 (2014). [CrossRef] [PubMed]
15. C. Battaglia, C. M. Hsu, K. Söderström, J. Escarré, F. J. Haug, M. Charrière, M. Boccard, M. Despeisse, D. T. L. Alexander, M. Cantoni, Y. Cui, and C. Ballif, “Light Trapping in Solar Cells: Can Periodic Beat Random?” ACS Nano 6(3), 2790–2797 (2012). [CrossRef] [PubMed]
