Anti Reflection (AR) Coating for Indium Gallium Nitride (InGaN) Solar Cells

Transcription

1 Anti Reflection (AR) Coating for Indium Gallium Nitride (InGaN) Solar Cells by N. C. Das, M. L. Reed, A. V. Sampath, H. Shen, M. Wraback, R. M. Farrell, M. Iza, S. C. Cruz, J. R. Lang, N. G. Young, Y. Terao, C. J. Neufeld, S. Keller, S. Nakamura, S. P. DenBaars, U. K. Mishra, and J. S. Speck ARL-TR-6124 August 2012 Approved for public release; distribution unlimited.

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3 Army Research Laboratory Adelphi, MD ARL-TR-6124 August 2012 Anti Reflection (AR) Coating for Indium Gallium Nitride (InGaN) Solar Cells N. C. Das, M. L. Reed, A. V. Sampath, H. Shen, and M. Wraback Sensors and Electron Devices Directorate, ARL R. M. Farrell, M. Iza, S. C. Cruz, J. R. Lang, N. G. Young, Y. Terao, S. Nakamura, S. P. DenBaars, and J. S. Speck Materials Department University of California Santa Barbara, California 93106, C. J. Neufeld, S. Keller, S. Nakamura, S. P. DenBaars, and U. K. Mishra Dept. of Electrical and Computer Engineering University of California Santa Barbara, CA Approved for public release; distribution unlimited.

4 REPORT DOCUMENTATION PAGE Form Approved OMB No Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports ( ), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 1. REPORT DATE (DD-MM-YYYY) August REPORT TYPE Final 4. TITLE AND SUBTITLE Anti Reflection (AR) Coating for Indium Gallium Nitride InGaN Solar Cells 3. DATES COVERED (From - To) June a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) N. C. Das, M. L. Reed, A. V. Sampath, H. Shen, M. Wraback, R. M. Farrell, M. Iza, S. C. Cruz, J. R. Lang, N. G. Young, Y. Terao, C. J. Neufeld, S. Keller, S. Nakamura, S. P. DenBaars, U. K. Mishra, and J. S. Speck 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) U.S. Army Research Laboratory ATTN: RDRL-SEE-M 2800 Powder Mill Road Adelphi, MD PERFORMING ORGANIZATION REPORT NUMBER ARL-TR SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR S ACRONYM(S) 11. SPONSOR/MONITOR'S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited. 13. SUPPLEMENTARY NOTES 14. Abstract: This report describes enhanced solar energy harvesting using an optimized anti reflection (AR) coating consisting of a double layer stack of silicon dioxide (SiO 2 ) and silicon nitride (Si 3 N 4 ) films. First, we made theoretical calculations for the minima of reflection curves and then performed experiments with different thicknesses of SiO 2 and Si 3 N 4 layers. The low temperature SiO 2 and Si 3 N 4 films were produced by low pressure chemical vapor deposition (LPCVD). Our experimental results agree qualitatively with the theoretical findings. The optimized SiO 2 and Si 3 N 4 layers were 500 and 750 Å, respectively. We also observed an increase of transmission in ultraviolet (UV) region from 75% for a indium gallium nitride (InGaN) substrate without AR coating to greater than 90% for the same layer with AR coating. By using an optimized AR coating we observed a 20% increase in InGaN solar cell efficiency. 15. SUBJECT TERMS AR coating, silicon oxide, silicon nitride, InGaN solar cell 16. SECURITY CLASSIFICATION OF: a. REPORT Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified 17. LIMITATION OF ABSTRACT UU 18. NUMBER OF PAGES 16 19a. NAME OF RESPONSIBLE PERSON Naresh C. Das 19b. TELEPHONE NUMBER (Include area code) (301) Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18 ii

5 Contents List of Figures iv 1. Introduction 1 2. Experimental 1 3. Results and Discussions 2 4. Conclusions 6 5. References 7 List of Symbols, Abbreviations, and Acronyms 8 Distribution List 9 iii

6 List of Figures Figure 1. Reflectance simulation curve for different combination of Si 3 N 4 and SiO 2 films less than 5% in the entire spectral region between 3000 to Å. The reflectance also decreases in the UV region of 3500 to 4000 Å, which is the operating region for InGaN solar cells....3 Figure 2. Experimental reflectance curves for different AR coating layers....4 Figure 3. Experimental transmission results for different AR coating layers...5 Figure 4. I-V curves for InGaN solar cell with and without AR coating...6 iv

7 1. Introduction Silicon (Si) solar cells have seen much improvement in performance in terms of power conversion efficiency and the cost per watt in production, and as such, are the leading photovoltaic device, enjoying 80% of current solar cells market share (1, 2). Recently many attempts have been made (3 5) to improve the solar cell performance by using plasmonic materials, nanostructures, and various anti-reflection (AR) coatings. However, silicon solar cell response in ultraviolet (UV) part of solar spectrum is limited. Indium gallium nitride (InGaN) quantum well (QW) structure is of special interest for solar cells because of its enhanced responsivity in UV region of the solar spectrum. Using a hybrid solar cell structure made with an InGaN QW structure on top and a Si solar cell in bottom is expected to enhance the solar energy conversion efficiency (6). Since the open circuit voltage (V oc ) for InGaN solar cell is about 2.0 V and about 0.5 V for an Si solar cell, combining four Si solar cells in series can enable voltage matching with an InGaN solar cell, creating a useful high voltage hybrid solar cell. In addition, the InGaN epitaxial layer is very resistant to environmental corrosion and hence can protect the Si solar cell when used as the top layer. The key to making such hybrid structures is to design and fabricate a top InGaN solar cell with the AR coating optimized for the entire solar spectral region. In this report, we present the experimental results of the design and development of various types of AR coatings on an InGaN solar cell structure. Both theoretical models and experimental results are presented. We discovered that a double-layer AR coating with silicon nitride (Si 3 N 4 ) on bottom and silicon dioxide (SiO 2 ) on top provides a minimum reflection. 2. Experimental InGaN/gallium nitride (GaN) multiple quantum well (MQW) and p-i-n solar cell structures were grown on double-side-polished 50-mm-diameter (0001) sapphire substrates by metal-organic chemical-vapor deposition (MOCVD). The InGaN solar cell structures consist of the following: a 1- m unintentionally doped GaN template layer, a 2- m Si-doped n-gan layer ([Si]=6x10 18 cm 3 ), a 10-nm highly Si-doped n + -GaN layer ([Si]=2x10 19 cm 3 ), a 30-period undoped MQW active region with 2.6-nm In 0.2 Ga 0.8 N QWs and 6.7-nm GaN barriers, 1

8 a 40-nm highly Mg-doped smooth p + -GaN layer ([Mg]=5x10 19 cm 3 ), a 30-nm Mg-doped smooth p-gan layer ([Mg]=2x10 19 cm 3 ), and a 15-nm highly Mg-doped p + -GaN contact layer. The device characteristics were measured by wafer-level probing at room temperature. Dark and illuminated current versus voltage (I-V) measurements were taken using an Agilent HP 4156 C parametric analyzer. An unfiltered Newport solar simulator provided broadband illumination with an equivalent AM1.5G illumination intensity of approximately 1.0 sun (100 mw/cm 2 ). 3. Results and Discussions AR coatings on solar cells consist of a thin layer of dielectric material of a specially chosen thickness so that interference effects in the coating cause the wave reflected from the AR coating s top surface to be out of phase with the wave reflected from the semiconductor surfaces. These out-of-phase reflected waves destructively interfere with one another, resulting in zero net reflected energy. The thickness of the AR coating is chosen such that the wavelength in the dielectric material is one quarter the wavelength of the incoming wave. For a quarter wavelength AR coating of a transparent material with a refractive index n 1 and light incident on the coating with a free-space wavelength λ 0, the thickness d 1, which causes minimum reflection, is calculated by Reflection is further minimized if the refractive index of the AR coating is the geometric mean of that of the materials on either side; that is, glass or air and the semiconductor. This is expressed by (1) The reflectance of the incident light is a function of thicknesses of the AR coating layer, the incidence angles, and the refractive index of the medium. We used the simulation model developed by Paul Shen to calculate the reflectance of AR coating layer with different materials and thicknesses. Figure 1 shows the simulated reflection coefficients for different Si 3 N 4 and SiO 2 film thicknesses. We observed minimum reflection for the AR coating layer consisting of 500 Å of Si 3 N 4 and 750 Å of SiO 2. Reflectance increases as the SiO 2 layer either increases or decreases (2) 2

9 for the same Si 3 N 4 layer thickness of 500 Å. The reflectance for this optimized double-layer coating is also decreased in UV region. Figure 1. Reflectance simulation curve for different combination of Si 3 N 4 and SiO 2 films less than 5% in the entire spectral region between 3000 to Å. The reflectance also decreases in the UV region of 3500 to 4000 Å, which is the operating region for InGaN solar cells. The experimental reflectance results of different AR coating layer are shown in figure 2. We used Perkin Elmer UV-visible-IR spectrometer to record the reflectance and transmission data. The AR coating layer consisting of only an Si 3 N 4 film with a 400-Å thickness has the highest reflectance whereas the double-layer AR coating with 400 Å of Si 3 N 4 and 550 Å SiO 2 has the least reflectance. Though not shown here, we also observed lower reflectance for the doublelayer AR coating with 400 Å of Si 3 N 4 and 750 Å of SiO 2. By comparing figures 2 and 3, it is observed that the theoretical reflectance curve has a lower value compared to the experimental curves. This may be due to the fixed refractive index value used in calculating the theoretical curves; in practice, it may be worth varying the value using different In composition in InGaN epitaxial layer. 3

10 100 SiN400 SiN400Sio SiN400SiO Wavelength nm Figure 2. Experimental reflectance curves for different AR coating layers. In figure 3, we show the experimental transmission data for different AR films. We observed highest transmission characteristics for double-layer AR film with 400 Å of Si 3 N 4 and 750 Å of SiO 2 films. Transmission for this optimized thickness is higher for entire spectral range of Å. As seen in figure 3, the transmission of visible light in the spectral region of 4500 to Å increases from 75% to more than 90% when the optimized double-layer AR coating is used. Also the transmission through AR1 film (Si 3 N Å and SiO Å) is higher than AR2 (Si 3 N Å and SiO Å). 4

11 SiN400SiO700 SiN400SiO900 InGan-noAR Wavelength Figure 3. Experimental transmission results for different AR coating layers. The I-V characteristics of InGaN solar cells under illumination, with and without AR coating, are shown in figure 4. We used an unfiltered Newport solar simulator with broadband illumination with an equivalent AM1.5G illumination intensity of approximately 1.0 sun (100 mw/cm 2 ). We used 1-mm 2 InGaN solar cell grid-structured top p-metal. While both the devices exhibit an V oc of 2.0 V independent of the AR coating, the short circuit current (I sc ) is about 20% higher for the device with the AR coating compared to the device without the AR coating due to the increase in absorption in the structure. The observed photovoltaic response for these devices is comparable to the results from similar devices reported by Farrell et al. (7). 5

12 InGaNwith AR InGaN without AR Voltage (V) Figure 4. I-V curves for InGaN solar cell with and without AR coating. 4. Conclusions We simulated the double-layer AR coating characteristics with minima in the UV region for enhanced InGaN solar cell performance. Experimentally, we found that the double-layer AR coating consist of 400 Å of Si 3 N 4 and 750 Å of SiO 2 is suitable for AR coating for a broad range of solar energy absorption. We observed about a 20% increase in the short circuit voltage when using the optimized device structure. 6

13 5. References 1. Hezel, R. Progress in Manufacturable High-Efficiency Silicon Solar Cells. Adv. in Solid State Phys. 2004, 44, Green, M. A. The Future of Crystalline Silicon Solar Cell. Prog. Photovolt. Res. Appl. 2000, 8, Vanecek, M.; Babchenko, O.; Purkrt, A.; Holovsky, J.; Neykova, N.; Poruba, A.; Remes, Z.; Meier, J.; Kroll, U. Nanostructured Three-dimensional Thin Film Silicon Solar Cells with Very High Efficiency Potential. Applied Physics Letters 2011, 98, Chen, C.; Jia, R.; Yue, H.; Li, H.; Liu, X.; Ye, T.; Kasai, S.; Tamotsu, H.; Wu, N.; Wang, S.; Chu, J.; Xu, B. Silicon Nanostructure Solar Cells With Excellent Photon Harvesting. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures 2011, 29 (2), Shui-Yang, L.; Dong-Sing, W.; Wen-Chang, Y.; Jun-Chin, L. Tri-layer Antireflection Coatings (SiO 2 /SiO 2 TiO 2 /TiO 2 ) for Silicon Solar Cells Using a Sol gel technique. Solar Energy Materials & Solar Cells 2006, 90, Das, N. C. et al. Heterogeneous Integration of InGaN and Silicon Solar Cells for Enhanced Energy Harvesting. 38 th PVSC conference proceedings, Austin, TX, June Farrell, R. M.; Neufeld, C. J.; Cruz, S. C.; Lang, J. R.; Iza, M.; Keller, I. S.; Nakamura, S.; DenBaars, S. P.; Mishra, U. K.; Speck, J. S. Applied Physics Letters 2011, 98,

14 List of Symbols, Abbreviations, and Acronyms AR GaN InGaN I sc I-V MOCVD MQW QW Si Si 3 N 4 SiO 2 UV V oc anti-reflection gallium nitride indium gallium nitride short circuit current illuminated current versus voltage metal-organic chemical-vapor deposition multiple quantum well quantum well silicon silicon nitride silicon dioxide ultraviolet circuit voltage 8

15 1 DEFENSE TECHNICAL (PDF INFORMATION CTR only) DTIC OCA 8725 JOHN J KINGMAN RD STE 0944 FORT BELVOIR VA DIRECTOR US ARMY RESEARCH LAB IMAL HRA 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB RDRL CIO LL 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB RDRL CIO LT 2800 POWDER MILL RD ADELPHI MD US ARMY RESEARCH LABORATORY ATTN RDRE SEE P FOLKES ATTN RDRL SEE P SEMENDY FRED ATTN RDRL SEE M STOUT LAWRENCE ATTN RDRL SEE M MIKE WRABACK ATTN RDRL SEE M M REED ATTN RDRL SEE M A SAMPATH ATTN RDRL SEE M G GARRETT ATTN RDRL SEE M G DANG ATTN RDRL SEE M P SHEN ATTN RDRL SEE M G METCALFE ATTN RDRL SEE I S SVENSSON ATTN RDRL SEE I P UPPAL ATTN RDRL SEE I W SARNEY ATTN RDRL SEE I P. TAYLOR ATTN RDRL SER L M DUBEY ATTN RDRL SER L M ERVIN ATTN RDRL SER P AMIRTHARAJ ATTN RDRL SEE I P WIJEWARNASURIYA ATTN RDRL SEE P GILLESPIE ATTN RDRL SEE I R FU ADELPHI MD

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