Antireflection coatings for solar panel power output enhancement. Engineering, SUNY Polytechnic Institute, Albany, NY 12203, USA

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1 Mater. Res. Soc. Symp. Proc. Vol Materials Research Society DOI: /opl Antireflection coatings for solar panel power output enhancement Gopal G. Pethuraja 1,2, Roger E. Welser 1, John W. Zeller 1, Yash R. Puri 1, Ashok K. Sood 1, Harry Efstathiadis 2, Pradeep Haldar 2, and Jennifer L. Harvey 3 1 Magnolia Solar, Inc., 251 Fuller Road, Albany, NY 12203, USA 2 Energy and Environmental Applications Center (E2TAC), College of Nanoscale Science and Engineering, SUNY Polytechnic Institute, Albany, NY 12203, USA 3 NYSERDA, 17 Columbia Circle, Albany, NY 12203, USA ABSTRACT The impact of nanostructured broadband antireflection (AR) coatings on solar panel performance has been projected for a broad range of panel tilt angles at various locations. AR coated films have been integrated on test panels and the short-circuit current has been measured for the entire range of panel tilts. The integration of the AR coatings resulted in an increase in short-circuit current of the panels by eliminating front sheet reflection loss for a broad spectrum of light and wide angle of light incidence. The short-circuit current enhancement is 5% for normal light incidence and approximately 20% for off-angle light incidence. The National Renewable Energy Laboratory (NREL) System Advisor Model (SAM) predicts that this AR coating can yield at least 6.5% improvement in solar panel annual power output. The greatest enhancement, approximately 14%, is predicted for vertical panels. The AR coating s contributions to vertical mount panels and building-integrated solar panels are significant. This nanostructured broadband AR coating thus has the potential to lower the cost per watt of photovoltaic solar energy. INTRODUCTION Even though solar energy is inexhaustible, cost poses a major hindrance to its widespread utilization. Various approaches have been explored to improve the cost per watt of solar panels. Antireflection (AR) coatings are an inexpensive approach to increasing solar panel power output that can decrease the cost per watt of solar panels. Sunlight incident on the front surface of conventional panels undergoes Fresnel reflection due to the mismatch between the refractive indices of air and of the frontsheet of the panel. This reflection loss is typically around 4% at noon and can be greater than 40% at dawn or dusk. An optical interface layer with intermediate refractive indices at the air/front sheet interface can eliminate or greatly reduce the unwanted reflection. Designing the optical interface layer, i.e., antireflection structure, is challenging due to the unavailability of materials having the required intermediate refractive indices. Recent developments in nanostructured coatings have overcome this limitation and provide new avenues for novel antireflection structures[1][2]. The need for broadband and wider angle AR structures has been significantly amplified due to recent development of novel photovoltaic (PV) technologies, such as tandem cells that harvest the entire spectrum of sunlight with high efficiencies. However, most of the approaches previously developed to create broadband high-performance nanostructured AR coatings have experienced difficulties due to

2 limitations in tuning the refractive index of the coatingg materials and lack of controllability in achieving the desired thickness of the ultra-low refractive index material. Magnoliaa has developed a scalable self-assembled nanostructure process that overcomes these limitations [3][4][5][6]. Magnolia s processs has the ability to create ultra-low refractivee index (down to 1.08) material with controllability in both layer thicknesss and refractive index[7] ]. Magnolia developed nanostructured AR layers using an oblique angle deposition method and demonstrated ultra-high, omnidirectional transmittance over the entire accessible portion of the solar spectrum and at a wide range of optical incidence angles[7][3]. In this paper, we review our latest work on high-performancee nanostructure-based AR coatings, including recent efforts to deposit such AR coatings on large area substrates and test them with solar energy convertors. AR-coated frontsheets integrated on solar panels demonstrate 5% higher short-circuit current at normal incidence and 20% higher short-circuit current for light incident 80 from the normal. In addition, National Renewable Energy Laboratory (NREL) System Advisor Model (SAM) predictions based on experimental results are alsoo reported in this paper. EXPERIMENT Oblique-angl e deposition is a method of growing porous thin films[2] ], and hencee thin films with low-refractive index enabled by surface diffusion and self-shadowing effects during the deposition process. Random growth fluctuations onn the substrate produce a shadow region that incident vapor flux cannot reach, and a non-shadow region where incident flux deposits preferentially, thereby creating oriented rod-like structures with high porosity as illustrated in Figure 1. The deposition angle, defined as the angle between the normal to the sample surface and the incident vapor flux, affects the tilt of the nanorod structures relative to the sample surface. Because the gaps between the nanorods can be much smaller than the wavelength of visible and infrared light, the nanostructur ed layers act as a single homogenous film with a refractivee index intermediate between that of air and of the nanorod material, which decreases with increasing porosity. Figure 1: (a) Simplified schematic of the oblique-angle deposition process for synthesizing porous, nanostructur red films, showing (a) the initial formation of material islands at random locations across the substrate, followed by (b) the formation of self-shadowea non-normal deposition angle (θ) to the regions and nano- columnar growth when material vapor flux arrives at substrate.

3 SiO2 coatingss are well-kn nown for theeir long-term m stability aand high trannsmittance oover a wide spectral range. However, conventional c l dense SiO2 has a refracctive index oof about 1.466, and thus is no ot an effective AR mateerial for optical windowss having a rrefractive inddex near On the otherr hand, the refractive r ind dex of porou us SiO2 can be reduced to values off 1.1 or low wer by increasin ng the poro osity[1]. The use of porous p nanoomaterials ffabricated bby oblique-angle depositio on offers un nique advan ntages comp pared to othher methodss such as ttunability oof the refractivee index, flex xibility in maaterial choicce, simplicityy of a physiccal vapor deeposition proocess, and the ability a to opttimize the co oating for an ny substrate-aambient matterial system m. Figure 2 sshows a cross-sectional scaanning electrron micrograaph of typicaal porous naanomaterial thin films ggrown by obliqu ue-angle dep position usin ng silicon dio oxide. Figure 2. 2 Cross-secctional scan nning electro on micrograaph of a m multi-layeredd nanostrucctured optical th hin film synth hesized by th he oblique angle deposittion of silicoon dioxide. In n this work, AR coating gs with specu ular surfacess comprisedd of multiplee layers of poorous SiO2 hav ve been depo osited on 6 x x6 glass plaates, UV stabbilized polycarbonate, aand UV stabiilized polyethylene terephth halate (PET)) films. The specific targget thicknesss and refracctive index vvalues for the step-graded AR coatin ng structuree were chossen to minnimize an uunwanted dip in transmitttance near 550 nm due to t interferen nce effects oobserved in our earlier w work[3]. Siinglecell test solar panels have been n fabricated using comm mmercially prrocured 6 mono-crystaalline silicon so olar cells. Figure 3 show ws an optical image of thee test solar ppanel. Figure 3.. Photograph h of the test solar panel used for evaaluation of thhe effects off the AR coatting.

4 Short-circuit current in the test panel has been measured under illumination for solar panel tilt angles ranging from -80 to +80. In order to evaluate the effect off the AR coating, the change in the short-circuit current has been measured after ntegrating AR coated and uncoated PET films on the front glass of a testt panel using an index-matching fluid. NREL s SAM has been used to predict the enhancement in annual power output of a solar panel due to the integrated AR coating. RESULTS AND DISCUSSION The AR coating was found to yield the following enhancements in solar panel short- light incidence. This is shown in Figure 4, which plotss the short-circuit current of a test panel circuit current: 5% more current at normal light incidence, and 20% more current at off-angle measured before and after integrating uncoated and AR coated PET films on its front sheet. The utilization of the same test panel for integrating uncoated and AR coated PET films eliminates the cell-to-cell performance variations. Integration of the uncoated PET film lowers the short- between PET and glass. The AR-coated PET-film-integrated test panel provides higher short- circuit current due to optical absorption in the PET film and reflection due to index mismatch circuit current compared to uncoated PET-film-integrated test panels due to elimination of the front surface reflection loss for a broader spectrum of light and wider angles of light incidence. The AR coating enables 5% higher short-circuit currantt at 0 tilt and approximately 20% more current at 80 tilt. Figure 4. Normalized short-circuit current vs. solar panel tilt angle for a test PV panel after attaching uncoated PET film and after attaching AR coated PET film. The uncoated and AR coated films were attached using index-matching fluid between the panel and the film. AR coating yields 5% and 20% higher short-circuit current at normal and off-angle light incidence, respectively.

5 NREL s SAM model predicts that the AR coating can yield 6.5% to 14% more annual power output depending on panel location and tilt. Figure 5 showss the SAM prediction of the effect of the AR coating on enhancement of solar panel annual power output for the entire range of panel tilt at two different geographic locations. The prediction shows that the AR coating can yield at least 6.5% increased solar panel annual powerr output. For flat panel installation, AR coating can yield 7.5% more current for panel location inn Tucson, AZ and 8.5% more current for panel location in Albany, NY. The increasee in annual power output is significantly higher for vertical mount panels. Figure 5. NREL s SAM prediction of AR coating effectt on enhancement of solar panel annual power output for panel tilts ranging from flat panel installation (0 ) to vertical panel installation (90 ). The predictions for panel location in Tucson, AZZ are presented with open squares, and those for panel location in Albany, NY are presented with open triangles. The AR coating can yield 6.5% or more annual power output. Table I: AR coating effect on enhancement in annual power output for vertical panel Annual power output Albany, NY Tucson, AZ Honolulu, HI Percent enhancement 10.6% 11.2% 13.9% Table I compares the effect of AR coating on vertically mounted solar panels for locations in Albany, Tucson, and Honolulu. The AR coating can provide a greater than 10% increase in power output compared to a panel without AR coating. Therefore, our AR coatings can significantly influence the performance of vertically mounted panels and building-integrated panels. CONCLUSIONS Nanostructure ed broadband AR coatings significantly improve the performance of solar panels. The AR coatings increase the short-circuit current of solar panels by eliminating reflection loss at the front surface for a broad spectrum of light and wide range of light incident angles. The short-circuit current enhancement is 5% for normal light incidence and 20% for offyield angle light incidence. Predictions based on NREL s SAM indicate that the AR coatings can

6 at least 6.5% enhancement in annual solar panel power output. The influence of these AR coatings is significant for vertically mounted panels and can increase their annual power output up to 14%. The nanostructured broadband AR coatings in particular can significantly influence the performance of vertical panels and building-integrated solar panels. ACKNOWLEDGMENTS The authors wish to thank the New York State Energy Research and Development Authority (NYSERDA) for supporting this work via Contract No. ERDA The authors would also like to thank Randy Stewart, Paul Hauser, Jose Castillo-Aguilella and Eric Aspnes at Prism Solar for their help and support with testing. REFERENCES [1] XiJ.-Q., M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S.-Y. Lin, LiuW., and S. A., Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection, Nat. Photonics, vol. 1, no. 3, pp , Mar [2] D. J. Poxson, F. W. Mont, M. F. Schubert, J. K. Kim, and E. F. Schubert, Quantification of porosity and deposition rate of nanoporous films grown by oblique-angle deposition, Appl. Phys. Lett., vol. 93, no. 10, p , [3] R. E. Welser, A. W. Sood, A. K. Sood, D. J. Poxson, S. Chhajed, J. Cho, E. F. Schubert, D. L. Polla, and N. K. Dhar, Ultra-high transmittance through nanostructure-coated glass for solar cell applications, in Proc. of SPIE, 2011, vol. 8035, p X 80350X 7. [4] R. E. Welser, A. W. Sood, G. G. Pethuraja, A. K. Sood, X. Yan, D. J. Poxson, J. Cho, E. Fred Schubert, and J. L. Harvey, Broadband nanostructured antireflection coating on glass for photovoltaic applications, Photovoltaic Specialists Conference (PVSC), th IEEE. pp , [5] A. K. Sood, A. W. Sood, R. E. Welser, G. G. Pethuraja, Y. R. Puri, X. Yan, D. J. Poxson, J. Cho, E. F. Schubert, N. K. Dhar, D. L. Polla, P. Haldar, and J. L. Harvey, Development of Nanostructured Antireflection Coatings for EO/IR Sensor and Solar Cell Applications, Mater. Sci. Appl. VO - 03, no. 09, p. 633, [6] G. G. Pethuraja, A. Sood, R. Welser, A. K. Sood, H. Efstathiadis, P. Haldar, and J. L. Harvey, Large-area nanostructured self-assembled antireflection coatings for photovoltaic devices, Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th. pp , [7] S. Chhajed, D. J. Poxson, X. Yan, J. Cho, E. F. Schubert, R. E. Welser, A. K. Sood, and J. K. Kim, Nanostructured Multilayer Tailored-Refractive-Index Antireflection Coating for Glass with Broadband and Omnidirectional Characteristics, Applied Physics Express, vol. 4, no. 5. p , 2011.