3. Results and discussion
3.3 Reactive ion etching (RIE) treatment for exposing nanowires
30
31
To improve efficiency, we changed the RIE time for exposing nanowire tips. Then, we measured the reflectance data of the exposed nanowire tips. Reflectance is proportional to the difference exposed length of the nanowire tips. The length of the exposed nanowire tips increases with RIE time. Figure 3.16 shows that the nanowire has a reflectance of 20% at an RIE time of 1200 s.
Figure. 3.17 Schematic illustration of BCB etching by RIE process for exposing nanowires.
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However, highly exposed nanowire tips leads to an unconnected ITO layer. So, the device with most exposed nanowire tips will not necessarily have the best efficiency. As shown in figure 3.18, we archive top efficiency at a reactive ion etching (RIE) time of 600 sec. When we do RIE treatment for 1200 s, we observe that the BCB layer is etched deeply around the nanowires, causing the ITO layer to be deposited ununiformly. An ununiformly deposited ITO layer leads to decreased electrical properties of the solar cells.
0.0 0.1 0.2 0.3 0.4
-30 -25 -20 -15 -10 -5 0 5 10
Current density (mA/cm2 )
Voltage (V)
200 s 600 s 1200 s
Figure. 3.18 J-V characteristics of InAs0.75P0.25/InP nanowire based solar cells with increasing reactive ion etching (RIE) times of 200, 600, 1200 s.
200 s 600 s 1200 s Jsc (mA/cm2) 16.3 24.95 22.18
Voc (V) 0.355 0.43 0.405
FF (%) 53 70 22.18
Efficiency (%) 3.0 7.5 5.2
Table. 3.4 J-V characteristics of nanowire based solar cells with increasing RIE time.
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500 1500 2500
2 3 4 5 6 7 8
Jsc (mA/cm2 )
Efficiency (%) Fill Factor (%)
Voc (V)
16 18 20 22 24 26 28
0.30 0.35 0.40 0.45 0.50
50 55 60 65 70 75
Figure. 3.19 Electrical properties of InAs0.75P0.25/InP nanowire based solar cells with increasing RIE time.
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3.4 Back surface field (BSF) effects
To obtain high efficiency solar cells, the surface carrier recombination should be low. There is a method for effective surface passivation, which is a built-in electric field at the back surface as a low- high junction which improves not only the Jsc current but also the Voc of a solar cell. Widely used back surface field dopants are aluminum and boron. Boron diffusion has advantages in that boron is more soluble than aluminum. Accordingly, the surface concentration of boron is higher than aluminum.
Furthermore, uniformity of boron diffusion yields is better than aluminum[35]. For these reasons, a boron-diffused back surface field (BSF) is more suitable than an aluminum-alloyed back surface field (BSF). We formed the back surface field (BSF) using the spin on dopant method.
p-Si 800 900 1000 1100
0 20 40 60 80 100
Temperature(C)
Sheet resistance (Ohm/sq)
0 1x1016 2x1016 3x1016 4x1016 5x1016 Sheet resistance
Sheet carrier density
Sheet carrier density (cm -2)
Figure. 3.20 Sheet resistance and sheet carrier density with increasing spin on dopant (SOD) annealing temperature.
We measured sheet resistance and sheet carrier density with increasing spin on dopant (SOD) annealing temperature. Figure 3.20 shows the dependence of the boron sheet resistivity on annealing temperature. The temperature was varied from 800 to 1100℃. The sheet resistivity for SODs (boron dopant solution B153, FILMTRONICS, INC.) shows the sheet resistivity decreases with increasing annealing temperature.
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0.0 0.1 0.2 0.3 0.4 0.5
-30 -25 -20 -15 -10 -5 0 5 10
Current density (mA/cm2 )
Voltage (V)
Bare Si After SOD
Figure. 3.21 Effect of boron SOD process on corresponding J-V performance.
Bare Si After SOD Jsc (mA/cm2) 28.22 29.66 Voc (V) 0.4066 0.4806
FF (%) 59.01 71.1
Efficiency (%) 6.7 10.1
Table. 3.5 Effect of boron SOD process on corresponding J-V performance.
The back surface field (BSF) layer generates a surface field that repels the minority carriers away from the back surface. For the as-fabricated InAs0.75P0.25/InP nanowire solar cells, the measured electrical properties are 28.22mA/cm2 in JSC, 0.4066V in VOC, 59.01% in FF, and 6.7% in efficiency. For the InAs0.75P0.25/InP nanowire solar cell using a post SOD processed wafer, the values are 29.66mA/cm2 in JSC, 0.4806V in VOC, 71.10% in FF, and 10.1% in efficiency. The significant increase in the electrical properties may be due to reduced carrier recombination. Decreased the carrier recombination that attributed to the enhancement in electrical properties with 18% increase in VOC,[36] hence overall improvement of 50% in efficiency.
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3.5 Grid pattern design for top metal contact
In solar cell technology, the improvement of conversion efficiency not only depends on structure and materials but also optimization of the front finger pattern design. If the spacing between grid lines is too narrow or too wide, this can cause large power loss as the current density generated by the solar cell is too high. If the spacing is too narrow, the loss of grid shadowing will be larger, whereas, if the spacing is too wide, the loss of series resistance will be larger. The losses associated with the grid directly influence the conversion efficiency of solar cells. Sheet resistivity, the power loss due to the emitter resistance can be calculated as a function of finger spacing in the top contact. The current can be collected from the base close to the finger and therefore has only a short distance to flow to the finger or, alternatively, if the current enters the emitter between the fingers, then the length of the resistive path seen by such a carrier is half the grid spacing. We monitored the conversion efficiency of solar cells with two types of grid pattern, which have finger distances (F/D) of 940μm and 440μm with a constant tapered 300-60µm bus bar. Grid pattern 1 has a finger distance (F/D) of 940μm and grid pattern 2 has finger distance (F/D) of 440μm.
Figure. 3.22 Two type of grid patterns with finger. (Grid pattern 1: finger distance (F/D) of 940μm, Grid pattern 2: finger distance (F/D) of 440μm)
37 Finger distance NO grid
pattern
Bus bar length
Finger length
F/D 940μm 19 300-60 60-30
F/D 440μm 39 300-60 60-30
Table. 3.6 Specifications of grid pattern with difference finger distances.
We fabricated grid patterns on InAs0.75P0.25 / InP nanowire based solar cells, of size 2cm×2cm.
The grid pattern was deposited using a shadow mask. Figure 23 shows that grid pattern 2, with a finger distance of 440μm, lead to increased electrical properties, hence overall improvement of efficiency to 11.4%.
0.0 0.1 0.2 0.3 0.4 0.5 0.6
-40 -35 -30 -25 -20 -15 -10 -5 0 5 10
Current density (mA/cm2 )
Voltage (V)
No grid pattern F/D 940m grid pattern F/D 440m grid pattern
Figure. 3.23 J-V performance with two type of grid patterns with difference finger distance.
(Grid pattern 1: finger distances (F/D) of 940μm, Grid pattern 2: finger distance (F/D) of 440μm)
38 NO grid
pattern
F/D 940μm
F/D 440μm Jsc (mA/cm2) 31.56 32.78 34.45
Voc (V) 0.522 0.566 0.545
FF (%) 21.45 44.12 60.61
Efficiency (%) 3.4 7.8 11.4
Table. 3.7 J-V performance with two type of grid patterns with difference finger distance.
(Grid pattern 1: finger distance (F/D) of 940μm, Grid pattern 2: finger distance (F/D) of 440μm)
NO grid pattern
F/D 940um grid pattern
F/D 440um grid pattern 1
2 3 4 5 6 7 8 9 10 11 12
Jsc (mA/cm2 )
Efficiency (%) Fill Factor (%)
Voc (V)
29 30 31 32 33 34 35
0.5 0.6 0.7
15 30 45 60
Figure. 3.24 Electrical properties of InAs0.75P0.25/InP nanowire based solar cells with different grid pattern finger distances.
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4. Conclusion
III-V nanowire based solar cells hold great promise for third-generation solar cells and has potential for powering nanoscale device. For these reasons, III-V nanowire based solar cells have studied by researcher. Especially, III-V nanowires can control band gap from 0.35 eV to 1.4 eV by ratio of compound materials. Nanowire grown as strain induced mechanism do not need noble patterning process. Moreover, Si is abundant and the most valuable platform in the earth. It allows for huge improvement in the performance of solar cells. We optimized that solar cell fabrication process toward high efficiency using an InAs0.75P0.25/InP nanowires grown by strain induced method. First, we study morphology of InAs0.75P0.25/InP nanowires grown by strain induced method. It is very important to nanowires density, length and thickness for high conversion efficiency. We notice that nanowires tend to merge at density over 75×106 /cm2 after BCB coating process. It leads to decrease open circuit voltage (Voc), hence decrease conversion efficiency of solar cell. For these reasons, we get best conversion efficiency at length of 3.5-4.0 μm, thickness of 150-200 nm. Second, we try to deposit ITO, which has low Resistivity and high transmittance. When ITO layer is too thin under 400 nm, it leads to increase series resistance. Thus, conversion efficiency also decrease with Voc and Jsc. Otherwise, when the ITO layer is too thick over 400 nm, the decrease in the conversion efficiency is due to the increase of light absorption at ITO layer. The optimum condition of ITO layer is tilting angle of 40 °, deposition temperature of 240 ℃, thickness of 400 nm. Third, we study RIE etching process for exposing nanowire tips. Benzocyclobutene (BCB) resist need for support the vertically standing nanowires on the substrate.
We find that most exposed tip of nanowire has not necessarily best efficiency due to unconnected ITO layer. Fourth, we adopt back surface filed effect using SOD process for high conversion efficiency. Back surface field (BSF) of p type layer generate a surface field that repels the minority carriers away from the back surface. Finally, Short-circuit current is increased by grid pattern for top contact. Grid pattern reduce sheet resistance. We fabricated InAs0.75P0.25/InP nanowire based solar cells, which have conversion efficiency of 11.4 % at cell area of 4 cm2(finger distance: 440 μm).
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References
1. Ginley, D., Green, M, A. and Collins, R. Solar Energy Conversion Toward 1 Terawatt.
Harnessing Materials for Energy, 2008. 33: p. 355-371.
2. World Energy Council, World Energy Resources 2013 Survey. United Kingdom, 2013.
3. Fang , Z., Wang, X, C., We, H, C. and Zhao, C, Z. Achievements and Challenges of CdS/CdTe Solar Cells, 2011. 2011: p. 8-15.
4. Everett, V., Blakers, A., Weber, K., & Franklin, E. Very High Efficiency Solar Cells. In:
ANZSES 45th Annual Conference, Alice Spring, Australia. 2007.
5. Shaheen, S. E., Radspinner, R., Peyghambarian, N., & Jabbour, G. E. Fabrication of bulk heterojunction plastic solar cells by screen printing. Applied Physics Letters, 2001. 79(18): p.
2996-2998.
6. Chopra1, K, L., Paulson, P, D. and Dutta, V. Thin-Film Solar Cells: An Overview. PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS, 2004. 12(2-3): p. 69-92.
7. Nathan S. Lewis., et al., Toward Cost-Effective Solar Energy Use, Science, 2007. 315(798): p.
798-801.
8. Fukui, T., Yoshimura, M., Nakai, E. and Tomioka, K. III-V Compound Semiconductor Nanowire Solar Cells. Technical Digest, 2013. 41(2): p. 119-124.
9. Choubey, P. C., Oudhia, A. and Dewangan, R. A review: Solar cell current scenario and future trends. Recent Research in Science and Technology, 2012. 4(8).
10. Lundgren, C., Lopez, R., Redwing, J., and Melde, K. FDTD modeling of solar energy absorption in silicon branched nanowires. Optics express, 2013. 21(103): p. A392-A400.
11. Tomioka, K., Yoshimura, M., and Fukui, T. A. III-V nanowire channel on silicon for high- performance vertical transistors. Nature, 2012. 488(7410): p. 189-192.
12. Vurgaftman, I., Meyer, J. R., and Ram-Mohan, L. R. Band parameters for III–V compound semiconductors and their alloys. Journal of applied physics, 2012. 89(11): p. 5815-5875.
13. Sun, B. Solar & Alternative Energy Hybrid photovoltaics for eco-friendly solar cells.
41
14. Mariani, G., Scofield, A. C., Hung, C. H., & Huffaker, D. L. GaAs nanopillar-array solar cells employing in situ surface passivation. Nature communications, 2013. 4: p. 1497.
15. Czaban, J. A., Thompson, D. A., and LaPierre, R. R. GaAs core− shell nanowires for photovoltaic applications. Nano letters, 2008. 9(1): p. 148-154.
16. Cui, Y., Wang, J., Plissard, S. R., Cavalli, A., Vu, T. T., van Veldhoven, R. P., Gao, L., Trainor, M., Verheijen, M. A., Haverkort, J. E., and Bakkers, E. P. Efficiency enhancement of InP nanowire solar cells by surface cleaning. Nano letters, 2013. 13(9): p. 4113-4117.
17. Mariani, G., Zhou, Z., Scofield, A., and Huffaker, D. L. Direct-Bandgap Epitaxial Core–
Multishell Nanopillar Photovoltaics Featuring Subwavelength Optical Concentrators. Nano letters, 2013. 13(4): p. 1632-1637.
18. Algamdi, A., Aloriany, H., Almjaishi, S., Almalki, F., Alsolami, E., Alsolami, Z. and Al- jawhari, H. Photovoltaics Technology & Its applications in Saudi Arabia. 2007.
19. Choubey, P, C., Oudhia, A. ,.and Dewangan, R. A review: Solar cell current scenario and future trends, Recent Research in Science and Technology, 2012. 4(8): p. 99-101.
20. BP p.l.c., BP Energy Outlook 2030, London, United Kingdom, 2012
21. Battey, K., Cohen, J., Fulton, S., Leach, A., Miller, A., Newman, J., Parsons, K., Ruben-Salama, C., Tisch, R. H.R. 1945: Energy for our Future Act “To improve energy efficiency in the United States”, 2007.
22. Fraas, L., Partain, L., Solar Cells and Their Application, second edition.
23. Liu, Z., Third Generation Solar Cell Research, 2012 International Conference on Solid State and Materials
24. Savenije, T, J., ORGANIC SOLAR CELLS, Delft University of Technology.
25. Wen, L., Li, X., Zhao, Z., Bu, S., Zeng, X., Huang, J. H., and Wang, Y. Theoretical consideration of III–V nanowire/Si triple-junction solar cells. Nanotechnology, 2012. 23(50), 505202.
26. Woodall, J, M. Ill-V Compounds and Alloys: An Update. Science, 1980. 208(23): p. 908-915.
27. Scharber, M, C. and Sariciftci, N, S. Efficiency of bulk-heterojunction organic solar cells, Progress in Polymer Science, 2013, 38: p. 1929-1940.
42
28. Flammersberger, H. Experimental study of Cu2ZnSnS4 thin films for solar cells, 2010.
29. . Kumar, V. Characterization of large area cadmium telluride films and solar cells deposited on moving substrates by close spaced sublimation (Doctoral dissertation, University of South Florida), 2003.
30. Garnett, E. C., Brongersma, M. L., Cui, Y., and McGehee, M. D. Nanowire solar cells. Annual.
31. Wallentin, J., Anttu, N., Asoli, D., Huffman, M., Å berg, I., Magnusson, M. H., and Borgström, M. T. InP nanowire array solar cells achieving 13.8% efficiency by exceeding the ray optics limit. Science, 2013. 339(6123): p. 1057-1060.
32. Shin, J. C., Lee, A., Mohseni, P. K., Kim, D.Y., Yu, L., Kim, J. H., Kim, H. J., Choi, W. J.;
Wasserman, D. M., Choi, K. J., Li, X. Wafer-Scale Production of Uniform InAsyP1-y Nanowire Array on Silicon for Heterogeneous Integration. ACS Nano 2013, 7(6): p. 5463-5471.
33. Leem, J, W. and Yu, J, S. Glancing angle deposited ITO films for efficiency enhancement of a- Si:H/μc-Si:H tandem thin film solar cells, OPTICS EXPRESS, 2011, 19(38): p. A258-A268.
34. Tomioka, K., Yoshimura, M., and Fukui, T. A III-V nanowire channel on silicon for high- performance vertical transistors. Nature, 2012. 488(7410): p. 189-192
35. Lee, J. Y., and Lee, S. H. Boron back surface field using spin-on dopants by rapid thermal processing. JOURNAL-KOREAN PHYSICAL SOCIETY, 2004. 44: p. 1581-1586.
36. Charlie, C. D., Wallace, C. H., Dixon, D. S., and Wei, E. I. Optical and electrical effects of gold nanoparticles in the active layer of polymer solar cells. Journal of Materials Chemistry, 2012. 22(3): p. 1206-1211.