The document discusses several metallization techniques for improving the efficiency of high efficiency solar cells, including: 1) Advanced screen printing techniques that can reduce line widths and increase short circuit current. 2) Two-layer metallization approaches using a seed layer and light-induced plating to thicken the contact layer and improve aspect ratios. 3) Emerging techniques like pad printing, metal aerosol jetting, and laser sintering of metal powders that can print even narrower line widths compared to screen printing.

1. Metallization techniques for High Efficiency Solar Cells Mehul Raval [email_address]

2. Outline Current Industrial Solar Cell Efficiencies Advanced Screen Printing techniques for higher efficiency solar cells Two layer metallization approaches with Light Induced Plating(LIP) Conclusions

3. Industrial Solar Cell Efficiencies Q. Cells Q6LMX3 : Full Square Monocrystalline Solar Cell has a max efficiency of 17.4% . Parameters: 1) Format-156mmx156mm ± 0.5mm. 2) Avg thickness- 200µm ± 30µm. 3) Voc = 614 mV, Isc = 8.68A, Pmax = 4.23W 4) FF value- 79.27% 5) Screen Printed front contacts.

4. Isofoton 156 family : Pseudo Square Monocrystalline cell has max efficiency of 18.3% . Parameters: 1) Format-156mmx156mm ± 0.5mm. 2) Avg thickness- 200um ± 30µm, 220µm ± 30µm. 3) Voc = 627 mV, Isc = 8.78A, Pmax = 4.35-4.40W 4) FF value- 79% 5) Screen Printed front contacts

5. Sunways AH508400F : Pseudo Square Monocrystalline cell has efficiency of 18.3% . Parameters: 1) Format-156mmx156mm ± 0.5mm. 2) Avg thickness- 200µm ± 40µm 3) Voc = 626 mV, Isc = 8.8A, Pmax = 4.37W 4) FF value- 79.32% 5) Screen Printed front contacts During 25 th EU PVSEC, Valencia they announced that with screen printed metallization they achieved 19.1% efficiency on 6” cells(wow!)

6. IndoSolar I6MU1660 : Square Multicrystalline cell has efficiency of 16.6% ± 0.2% . Parameters: 1) Format-156mmx156mm. 2) Avg thickness- 220µm ± 30µm 3) Voc = 621 mV, Isc = 8.39A, Pmax = 4.03W 4) FF value- 77.34% 5) Screen Printed front contacts thickened by silver plating.

7. Advanced Screen Printing Techniques for higher efficiencies* Typical line widths are in the range: 110-150µm with line openings in the stencil: 100-120µm. Higher Voc & Isc can be obtained with high sheet resistance emitters . Reduction of line thickness is necessary to decrease shadowing losses. Reduced cross section increases the line resistance and also the higher risk of line breakage below 80µm. *Advanced screen printing technique for high definition front side metallization of crystalline silicon solar cells -Denis Erath , et al. Solar Energy Materials & Solar Cells 94(2010) 57–61.

8. In the work, line width reduced to 90um and for fine line printing(< 60µm) a low Emulsion Over Mesh(EOM) used with heated printed chuck to vary substrate temperature(Room Temp- 100°C). Chuck section heated and preheating of cell done for 3secs before transferring to chuck to meet industry’s cycle time of below 3secs. Direct contact of screen with hot wafe r and proximity with chuck lead to heating of screen. No screen clogging or paste drying was observed even with breaks exceeding 10 mins between prints.

9. Fig 1. Printed Line width and paste quantity v/s substrate temp for multicrystalline solar cell Fig 3. Printed Line width and Jsc v/s substrate temp for monocrystalline solar cell Fig 2. Microscopic Image for screen printed lines(Ws = 90 µ m); left @ Ambient Temp, right @ Higher Temp Observations: Line width reduced from 112 µ m @RT to 95 µ m at higher temp. Absolute gain in Jsc of 0.3mA/ sq.cm. For optimized no of lines, an absolute gain of 0.2% was obtained wrt those with line width of 100 µ m.

10. For fine line printing a EOM below 10um used as compare to the standard of around 20 µ m. Avg height of around 9 µ m and so the aspect ratio = 0.15. Faster evaporation of solvent and major effect of induced lower paste viscosity affects the release behavior of paste when squeezed out. Compared to release behavior @ RT, more paste remains in the screen. Fig 4. 3-D & microscopic picture of fine screen printed line on textured Cz-Si. Table: I-V Parameters for alkaline textured Cz-Si solar cells of size 125x125 sq.mm, w s = 100µm Table: Average w l after co-firing with w s = 40µm & 50µm Substrate Condition Voc(mV) Jsc(mA/cm 2 ) FF(%) η(%) Rs(Ωcm 2 ) Unheated(avg out of 8 cells) 612 ± 1.4 34.0 ± 0.3 78.4± 0.4 16.3 ± 0.2 0.7 ± 0.02 Heated(avg out of 40 cells) 613 ± 1.6 34.4 ± 0.1 78.1± 0.3 16.5 ± 0.1 0.8 ± 0.03 Substrate Temp Ts Room Temperature High Temperature Substrate w s ( µ m) w l ( µ m) w l ( µ m) Cz-Si Textured 40 X 52 50 71 62 mc-Si Textured 40 X 46 50 64 52

11. Two layer metallization Approaches* Screen Printing has many undesirable features like poor aspect ratio , high line resistance , high doping concentration required for underlying emitter for low contact resistance. Two step metallization: - 1 st seed layer on Si with good mechanical and electrical contact. - Subsequent growth step to thicken the line by plating to increase conductivity. *High-Efficiency Crystalline Silicon Solar Cells- S.W. Glunz,Fraunhofer Institute for Solar Energy Systems, Heidenhofstrasse 2, 79110 Freiburg, Germany.Advances in OptoElectronics Volume 2007, Article ID 97370, 15 pages.

12. Light Induced Plating used for thickening of the seed layer and utilizes the photovoltaic effect of the cell. Results in highly conductive contacts and aspect ratio close to 1:2 . Absolute efficiency increase of 0.3-0.5% has been achieved on 15.6x15.6 sq.cm mc-Si solar cell with saving in silver paste*. Full potential of LIP can be achieved if the seed layer has better properties then screen-printed contacts. *A. Mette, et al. “Increasing the efficiency of screen-printed silicon solar cells by light-induced silver plating,” in Proceedings of the 4thWorld Conference on Photovoltaic Energy Conversion, vol. 1, pp. 1056–1059,Waikoloa, Hawaii, USA, May 2006.

13. Pad Printing: - Structures with size less than 50um can be printed with modified screen-printing pastes(less viscosity & resin added to make paste tacky ). - Height of the lines is reduced and hence LIP can be used to increase the conductivity. -Promising results obtained with hot-melt pastes.* *A. Mette, et al. “Hot melt ink for the front side Metallisation of silicon solar cells,” in Proceedings of the 20th European Photovoltaic Solar Energy Conference, Barcelona, Spain, June 2005. Table: Result of 100x100 sqmm Cz-Si solar cell. Line width = 50 µ m. Paste Voc(mV) Jsc(mA/sq.cm) FF(%) η(%) Hot Melt 624 36.1 79.7 17.9 Conventional 627 36.2 76.9 17.4

14. Metal Aerosol Jetting: - Metal Aerosol is generated and conducted into a specially designed print head. Ring-shaped gas flow focuses the aerosol and print lines with 50um width can be achieved.

15. Modified commercial pastes were used and line widths of 50-60µm were achieved. Efficiencies of 17.8% were obtained as compared to 17.2% with conventional screen printing.* *A. Mette, et al.“Metal aerosol jet printing for solar cell metallization,” Progress in Photovoltaics: Research and Applications 15(7) ,pp 621-627. Table:Results of 125×125mm2 monocrystalline solar cells with Al-BSF and SiNx front passivation. Table:Results of 50×50mm2 multicrystalline solar cells. Finger width Voc(mV) Jsc(mA/sq.cm) FF(%) η (%) 90um 620 35.8 80.2 17.8 Finger width Voc(mV) Jsc(mA/sq.cm) FF(%) η(%) 160um 617 32.7 79.4 16 70um 618 34.2 77.4 16.4

16. Ni Plating : - Nickel Plating is used in laser buried contacts process of BP solar cells. - Electroless Ni plating can be used for seed layer. - Process with reduced steps for high efficiency solar cell with Ni plating reported*. - Laser ablation of ARC  Selective deposition of metal  Electroplating to thicken the contact. - Simulation studies done for increased ablation at tips & pyramids. - Efficiency of 19.1% achieved on 20x20 sq.mm FZ wafer. *Annerose Knorz, et al.&quot;Selective Laser Ablation of SiNx Layers on Textured Surfaces for Low Temperature Front Side Metallizations&quot;,PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS,Prog. Photovolt: Res. Appl. 2009; 17:127–136.

17. Recent paper(Valencia Conference 2010) used Etch resist to pattern the ARC and then Ni plating along with LIP used to make the contacts*. Work of Vikrant on Ni plating cited in the paper! *FORMATION OF A LOW OHMIC CONTACT NICKEL SILICIDE LAYER ON TEXTURED SILICON WAFERS USING ELECTROLESS NICKEL PLATING A. Nguyen, et al.Energy and Environmental Technology Applications Center (E2TAC), College of Nanoscale Science and Engineering, University at Albany, State University of New York, Albany, NY 12205. L. Michaelson, et al.Technic Inc., One Spectacle St., Cranston, RI 02910

18. Laser sintering of metal powder:* - Deposition of metal powder and melting locally by laser. Fig 1.Lase micro sintering of metal powder Fig 2. SEM image of laser microsintering thickened by LIP Efficiency of 14.1%, Voc of 622mV & pseudo FF of 78% on heavily doped emitter on 1x1 sq.cm cell. *M. Alem´an, et al.“Laser microsintering as a new metallization technique for silicon solar cells,” in Proceedings of the 21st European Photovoltaic Solar Energy Conference and Exhibition, Dresden, Germany, September 2006.

19. Metallization Trends in future* *Future Photovoltaics May 2010

20. Conclusions Advanced screen printing techniques can be used for improving cell efficiencies. Options like Pad-printing & Ni Plating (for two layer approach with LIP) are promising methods for high efficiency solar cells. Experience of pad-printing from microelectronics & our work on Ni plating can be utilized.

21. Pad Printing: - Photopolymer plate used.

22. Metal Aerosol Printing: