Metallization & Interconnection Workshop th Workshop on Metallization & Interconnection for Crystalline Silicon Solar Cells.

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1 Abasifreke Ebong, Nirupama Bezawada, Veysel Unsur, Ren Keming and Ahrar Chowdhury Department of Electrical and Computer Engineering University of North Carolina at Charlotte 9201 University City Blvd, Charlotte NC , USA

2 (i) R 1 metal-semiconductor back contact (ii) R 2 bulk semiconductor (iii) R 3 emitter between two gridlines (iv) R 4 metal semiconductor contact on gridline (v) R 5 gridline (vi) R 6 - busbar 2

3 Sheet Resistance (ohm/sq.) ITRPV 2017 Expected trend for sheet resistance 2024; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; Year

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5 ; ; ; 160 Gridline width (um) ; ; ITRPV 2017 Expected trend for gridline width 2010; ; ; ; ; ; ; ; Year

6 2-5% 5-10% 85-90% [1] R. Prunchak, US patent 7,736,546B2, [2] Carroll et al, US Patent, 8,889,980 B2 [3] R. G. Rajendran, US 2013/ A1

7 (i) SS (wwwww) + 2PPO (ggggg) SSO 2(ggggg) + 2PP (ii) SS (wwwww) + 2AA 2 O (ggggg) SSO 2 (ggggg) + 4AA (iii)ss 3 N 4(dddddddddd oo wwwww) + 6AA 2 O (ggggg) 3SSO 2 (ggggg) + 12AA + 2N 2 [1] C. Ballif, et al, App. Phys. Lett., 82 (12), , [2] Schubert et al, Solar Energy Materials and Solar Cells, 90, , [3] Hilali et al, J. Electrochem. Soc. 153, A5, [4] Li et al, J. Appl. Phys. 105, , 2009 [5] Eberstein et al, Energy Proceedia 27, , [6] Tai et al, RSC Advances, 5, , 2015 p.7

8 TEM micrograph: overfired Ag/Si contact. The intermediate SiNx layer has been dissolved in the firing process. Ballif et al : Appl. Phys. Lett., Vol. 82, No. 12, 24 March 2003 p.8

9 SEM-picture of the interface of a silver thick film-finger on [1 0 0] orientated silicon. Silver crystallites grown into the silicon are clearly visible. Schubert et al., Solar Energy Materials & Solar Cells 90 (2006) p.9

10 SEM micrographs of: contact layer, finger layer, full layer and elemental analysis Ag Crystallites Contact layers Top view of contact and finger layers after sintering Contact and Finger layers Ag Crystallites Si

11 Paste particle size Paste ID D10 D50 D90 Series Resistance (Ω-cm 2 ) Contact Resistance (Ω-cm 2 ) Fill factor (%) A B C D E F G H I J K L CP p.11

12 Contact characterization Paste D 34/66 PbO/TeO 2 Paste J 50/50 Pbo/TeO 2 SEM/EDX for Ag paste D and J after drying at 200 o C for 2-mins

13 EDX analyses of Contact Paste D Element Wt% At% CK OK MgK AlK SiK PbM TeL Matrix Correction ZAF After 2-min drying at 200 o C Paste J Element Wt% At% CK OK MgK SiK PbM TeL Matrix Correction ZAF

14 Contact characterization Ag + Te + Pb Ag + Pb + Te K_Count Si Paste D K_Count Si Paste J SEM/EDX for Ag paste D and J after contact co-firing at 815 o C peak firing temperature in IR belt furnace.

15 EDX analyses of Contact Paste D Paste J After contact co-firing at 815 o C peak firing temperature in IR belt furnace.

16 Contact characterization 350 (111) 350 (111) Paste D after 200 o C dry Paste D Paste J after 200 o C dry Paste J Count per Sec Counts per sec (200) (200) (220) Si peak (311) Paste D after 815 o C sintering (222) θ (degree) (220) Paste J after 815 o C firing (311) Si Peak (222) Ѳ (degree) XRD pattern for Ag paste D and J after drying at 200 o C for 2-mins and after contact cofiring at 815 o C peak firing temperature in IR belt furnace. Before: All Ag phases are similar After: (111) phase in D doubles due to PbO/TeO 2 ratio difference

17 Raman Spectra of contact Si Intensity 124-D-HNO Ag 2 Te TeO 2 Paste D 124-D-HNO D-HNO D-HF D-HF Wavenumber

18 1. Nano-sized metallic Zn additives - uniformly etch the non-conductive SiNx:H layer 2. TeO 2 additive - uniform etching of sinx:h layer - Decreases viscosity of molten glass and causes uniform wetting of SiNx:H - No Te after glass removal with HF (i) Ag, Te and Si form Ag-Te, Ag-Si and Ag-Te-Si alloys. (ii) Formation of Ag 2 Te a semimetal increases conductivity of glass low contact resistance and gridline resistance. 3. Both 66PbO-34TeO 2 and the 50PbO-50TeO 2 glasses enable higher isothermal conductivity. The resulting low melting nature of the glass facilitate diffusion process introducing ionic conductivity leading to high isothermal conductivity. 1. Li et al., J. Appl. Phys. 110, (2011) 2. Ionkin et al, ACS Appl. Mater. Interfaces 3, 606 (2011) 3. Ebong et al, JJAP, 56, 08MB07 (2017) 4. ] R. Prunchak, US patent 7,736,546B2, Vithal et al., J. Appl. Phys. 81 (12), , (1997) p.18

19 Abasifreke Ebong, Nirupama Bezawada, Veysel Unsur, Ren Keming and Ahrar Chowdhury Department of Electrical and Computer Engineering University of North Carolina at Charlotte 9201 University City Blvd, Charlotte NC , USA

20 Belt speed: 230 pm Peak Temp: 815 o C