Antireflective coatings for multijunction solar cells ... - OSA Publishing

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of antireflection coating design for multijunction solar cells and concentrator systems,” ..... materials, and therefore obtain efficiency increases at no extra cost.
Antireflective coatings for multijunction solar cells under wide-angle ray bundles Marta Victoria,* César Domínguez, Ignacio Antón, and Gabriel Sala Instituto de Energía Solar -Universidad Politécnica de Madrid, Ciudad Universitaria, 28040 Madrid, Spain *[email protected]

Abstract: Two important aspects must be considered when optimizing antireflection coatings (ARCs) for multijunction solar cells to be used in concentrators: the angular light distribution over the cell created by the particular concentration system and the wide spectral bandwidth the solar cell is sensitive to. In this article, a numerical optimization procedure and its results are presented. The potential efficiency enhancement by means of ARC optimization is calculated for several concentrating PV systems. In addition, two methods for ARCs direct characterization are presented. The results of these show that real ARCs slightly underperform theoretical predictions. ©2012 Optical Society of America OCIS codes: (350.6050) Solar energy; (220.1770) Concentrators; (310.1210) Antireflection coatings.

References and links 1. 2.

A. Luque, Solar cells and optics for photovoltaic concentration (Adam Hilger, 1989). M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, “Solar cell efficiency tables (Version 38),” Prog. Photovolt. Res. Appl. 19(5), 565–572 (2011). 3. R. Winston, J. C. Miñano, and P. Benítez, Nonimaging Optics (Elsevier, 2005). 4. D. J. Aiken, “Antireflection coating design for series interconnected multi-junction solar cells,” Prog. Photovolt. Res. Appl. 8(6), 563–570 (2000). 5. C. Algora and V. Díaz, “Modelling of GaAs solar cells under wide angle cones of homogeneus light,” Prog. Photovolt. Res. Appl. 7(5), 379–386 (1999). 6. V. Dı́az Luque and C. Algora del Valle, “On the effects of tilted light in a global prediction of AlGaAs/GaAs solar cell performance,” Solar Energy & Solar Cells 57(4), 313–322 (1999). 7. C. E. Valdivia, E. Desfonds, D. Masson, S. Fafard, A. Carlson, J. Cook, T. J. Hall, and K. Hinzer, “Optimization of antireflection coating design for multijunction solar cells and concentrator systems,” Proc. SPIE 7099, 709915, 709915-10 (2008). 8. S. Wojtczuk, P. Chiu, X. Zhang, D. Derkacs, C. Harris, D. Pulver, and M. Timmons, “InGaP/GaAs/InGaAs 41% concentrator cells using bi-facial epigrowth,” in Proceedings of the 35th IEEE Photovoltaic Specialists Conf. (2010) pp.1259–1264. 9. A. N. Matveev, Optics (Mir, 1988) 10. J. S. Rayleigh, “On the reflection of vibrations at the confines of two media between which the transition is gradual” in Proceedings London Math. Soc. 11 (1880) pp. 51–56. 11. D. Poitras and J. A. Dobrowolski, “Toward perfect antireflection coatings. 2. theory,” Appl. Opt. 43(6), 1286– 1295 (2004). 12. W. H. Southwell, “Coating design using very thin high- and low-index layers,” Appl. Opt. 24(4), 457–460 (1985). 13. D. J. Aiken, “High performance anti-reflection coatings for broadband multi-junction solar cells,” Sol. Energy Mater. Sol. Cells 64(4), 393–404 (2000). 14. M. F. Schubert, F. W. Mont, S. Chhajed, D. J. Poxson, J. K. Kim, and E. F. Schubert, “Design of multilayer antireflection coatings made from co-sputtered and low-refractive-index materials by genetic algorithm,” Opt. Express 16(8), 5290–5298 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-16-8-5290. 15. D. J. Poxson, M. F. Schubert, F. W. Mont, E. F. Schubert, and J. K. Kim, “Broadband omnidirectional antireflection coatings optimized by genetic algorithm,” Opt. Lett. 34(6), 728–730 (2009). 16. E. D. Palik, Handbook of Optical Constant of Solids (Academic Press, 1997). 17. Sopra materials database, www.sopra-sa.com. 18. D. Redfield, “Method for evaluation of antireflection coatings,” Solar Cells 3(1), 27–33 (1981). 19. M. Victoria, C. Domínguez, S. Askins, I. Antón, and G. Sala, “Optical characterization of FluidReflex concentrator,” in Proceedings of Int. Conf. on Concentrating Photovoltaic Systems (2010) pp. 118–121.

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20. I. Rey-Stolle and C. Algora, “Optimum antireflection coatings for heteroface AlGaAs/GaAs solar cells-Part II: The influence of uncertainties in the parameters of window and antireflection coatings,” J. Electron. Mater. 29(7), 992–999 (2000). 21. M. Victoria, C. Domínguez, I. Antón, and G. Sala, “Comparative analysis of different secondary optical elements for aspheric primary lenses,” Opt. Express 17(8), 6487–6492 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-17-8-6487. 22. S. R. Kurtz and M. J. O’Neill, “Estimating and controlling chromatic aberration losses for two-junction, twoterminal devices in refractive concentrator systems,” in Proceedings of the 25th IEEE Photovoltaic Specialists Conf. (1996) pp. 361–364. 23. L. W. James, “Effects of concentrator chromatic aberration on multi-junction cells,” in Proceedings of the 24th IEEE Photovoltaic Specialists Conf. (1994) pp. 1799–1802. 24. I. García, C. Algora, I. Rey-Stolle, and B. Galiana, “Study of non-uniform light profiles on high concentration solar cells using quasi-3D distributed models,” in Proceedings of the 33rd IEEE Photovoltaic Specialists Conf. (2008) pp. 1–6. 25. K. Nishioka, T. Takamoto, and W. Nakajima, “Analysis of triple-junction solar cell under concentration by SPICE,” in Proceedings of the 3rdWorld Conf. on Photovoltaic Energy Conversion (2003) pp. 869–872. 26. M. Victoria, R. Herrero, C. Domínguez, I. Antón, S. Askins and, G. Sala, “Characterization of the spatial distribution of irradiance and spectrum in concentrating photovoltaic systems and their effect on multi-junction solar cells,” Prog. Photovolt: Res. Appl. (2011) published online DOI: 10.1002/pip.1183. 27. J. Jaus, P. Nitz, G. Peharz, G. Siefer, T. Schult, O. Wolf, M. Passig, T. Gandy, and A. W. Bett, “Second stage reflective and refractive optics for concentrator photovoltaics,” in Proceedings of the 33rd IEEE Photovoltaic Specialist Conf. (2008) pp. 1–5. 28. V. Díaz, J. M. Ruíz, C. Algora, and J. Alonso, “Outdoor characterization of GaAs solar cell under tilted light for its encapsulation inside optic concentrator,” in Proceedings of the 27th Eur.Photovoltaic Sol. Energy Conf., (2001). 29. C. Domínguez, I. Antón, and G. Sala, “Solar simulator for concentrator photovoltaic systems,” Opt. Express 16(19), 14894–14901 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-16-19-14894. 30. C. Stevenson, P. R. Denton, G. Sadkhin, and V. Fridman, “Stability and Repeatability of 2-Layer Anti-Reflection Coatings,” Denton Vacuum technical paper, http://www.dentonvacuum.com/PDFs/Tech_papers/stab.pdf

1. Introduction Antireflection coatings (ARCs) have been widely investigated since the beginning of photovoltaics [1]. The high refractive indexes of semiconductors (n~4) will lead to unacceptable reflection losses if no ARC is used. Multijunction (MJ) solar cells have shown the highest efficiency for a photovoltaic device ever measured at 43.5% [2]. ARC design for III-V semiconductor MJ solar cells is especially challenging as these cells are typically sensitive to a wider spectral bandwidth than silicon. In addition, their subcells are connected in series. Hence, if the ARC performs worse in a particular region of the spectrum, photogenerated current for the corresponding subcell will be limited and, as a consequence, total cell efficiency will significantly decrease. In terrestrial applications, MJ solar cells are only economically viable when used in concentrator systems. Concentration implies that the bundle of rays impinging the cell consists of a cone with large interior angle, in some systems as wide as ± 70°. Hence, besides considering the wide spectral sensitivity and the series connected structure, the angular distribution of the incident light over the cell must be taken into account to properly design an ARC for a concentrator cell. When using optics to concentrate light, the product of concentration and acceptance angle, known as CAP, is the most significant figure of merit as it indicates how close a real concentrator is to ideality [3]. Attainable CAP is limited by the étendue conservation theorem. In rotationally symmetric optical systems in air it can be stated as:

X geom sin 2 (θ entrance ) ≤ n2 2 sin 2 (θ cell )

(1)

where Xgeom represents the ratio of the optical input area to the exit area, or concentration ratio and θentrance represents the system acceptance angle. The acceptance angle is the angle of incidence of the extreme light ray: the most inclined ray that continues to reach exit of the optical system. Symbols n2 and θcell stand respectively for the refractive index and the extreme ray’s angle at the system exit. To maximize attainable CAP, the refractive index at the exit must be higher than unity, for example by using a dielectric SOE over the cell, but this is not

#163093 - $15.00 USD Received 15 Feb 2012; revised 17 Mar 2012; accepted 19 Mar 2012; published 22 Mar 2012

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the scope of this article. Furthermore, the angle at the exit of the optical system, that is, over the cell, must be as wide as possible, ideally 90°. Nevertheless, it must be considered that reflection losses at the cell aperture increase with θcell and, in point-focus systems the largest incident angles contain the majority of the energy. Therefore, as CPV systems approach ideality, this necessarily translates into higher CAPs and wider ray-bundles, and therefore a need for an ARC design which takes into account these angular light distributions. Previous work has only considered one of these two aspects at time when approaching ARC optimization for MJ concentrator cells. On the one hand, Aiken [4] described the increased complexity when designing ARC for MJ solar cells considering normal incidence and he proposed the ARC design as a subcells current matching technique. On the other hand, Algora et al. [5-6] optimized the thicknesses of a single and double layer ARC for a cone of light with semi-angle between 0° and 90° impinging a single junction GaAs solar cell. Their approach consisted on performing a simultaneous optimization of all the materials thicknesses (not only those of the ARC but also those of the semiconductor structure). Their simulated results showed that only the ARC and the window layers optimum values depends on the semi-angle of the incident cone of light. Additionally, the resulting optimum ARC thickness does not change appreciable for cones narrower than ± 60°. Valdivia et al. [7] presented a numerical optimization of ARCs composed of 1 to 4 layers when a MJ solar cell is illuminated by the angular distribution created by a CPV system composed of a Fresnel lens and a refractive secondary optical element (SOE). However, for this particular CPV system most of the rays are contained in a narrow cone of light (semi-angle