Comparison of different electrochemical deposits for contact metallization of silicon solar cells

Transcription

1 Metallization Workshop October 1 st 2008 Utrecht Comparison of different electrochemical deposits for contact metallization of silicon solar cells Caroline Boulord

2 Overview I Ni-P Electroless Deposition II Jet Metal TM III Conclusion

3 Overview I Ni-P Electroless Deposition II Jet Metal TM III Conclusion

4 I - Ni-P Electroless Deposition Principle Autocatalytic deposition of Nickel-Phosphorous (Ni-P) by immersion of a substrate in the plating bath: - oxidation of the sodium hypophosphite (NaH 2 PO 2 ) - reduction of the nickel ions (Ni 2+ ) Ni + 2H 2PO2 + 2H 2O Ni + 2H 2PO3 + 2H + H 2

5 I - Ni-P Electroless Deposition Description of the studied baths Bath 1 : Ni-P acidic Bath 2 : Ni-P alkaline ph Température Agitation 4,5 5,5 85 C yes C yes Deposition rate ~ 10 µm/h ~ 7 µm/h

6 I - Ni-P Electroless Deposition Sample preparation Monocrystalline silicon subtrates (area : 4 cm² ) : p-type : <100> oriented, 300 µm thick, 1 Ω.cm, texturised. n-type : <100> oriented, 300 µm thick, texturised diffused in POCl3 with emitter sheet resistance : 40 Ω/

7 I - Ni-P Electroless Deposition Experimental Substrate Activation Aspect Adherence Bath 1 : acidic N-type Type p with illumination By contact with a catalytic metal Porous Good for thickness < ~ 5 µm Bath 2 : alkalyne N-type and P-type None Homogeneous Good for thickness < ~ 5 µm Antireflection coating (SiNx:H PECVD) No deposition Particles of metal observed Application Deposition on screenprinted contacts Good seed layer for an electrolytic deposition of copper

8 I - Ni-P Electroless Deposition Experimental Deposition time : 1 min Thickness : ~ 0,2 µm Deposition time : 2 min Thickness : ~ 0,3 µm Substrate after immersion in acidic Ni-P bath

9 I - Ni-P Electroless Deposition Experimental Deposition time : 1 min Thickness : ~ 0,1 µm Deposition time : 1 min Thickness : ~ 0,1 µm Substrate after immersion in alkaline Ni-P bath

10 I - Ni-P Electroless Deposition Experimental Antireflection coating (SiNx:H PECVD) after immersion in the NiP alkaline bath Deposition time : 1 h

11 I - Ni-P Electroless Deposition Results Alkaline NiP bath : ρ c measured by TLM on n-type samples rapid thermal fired in N 2 atmosphere during 30 seconds at 450 C Emitter sheet resistivity (Ohm/ ) Surface donor concentration N D (cm -3 ) Contact resisitivityρ c (Ω.cm²) 40 ~ ~ Thickening with copper of Ni-P electroless layers on monocrystalline silicon texturised

12 I - Ni-P Electroless Deposition Results Acidic NiP bath : Deposition of 1 µm Ni-P layer on silver screen-printed contacts of 8 standard monocrystalline solar cells (area ~ 1 cm²) Decrease of Rs : ~ 0,08 Ohm.cm² Optical characteristics of the antireflection coating not deteriorated

13 Interest of the deposits - soldering facilitated I - Ni-P Electroless Deposition Results - improvement of the conductivity of the finger - effective penetration of the plated Ni-P down into the pores of the screen-printed Ag-paste useful before an electrolytic thickening step. Example Thickening of silver screen-printed contacts of standard solar cells with Ni/Cu

14 Overview I Ni-P Electroless Deposition II Jet Metal TM III Conclusion

15 II - Jet Metal TM Principle Liquid film Metallic film Metallic solution Compressed air Substrate Reductor solution G. Stremsdoerfer, A. Fares Karam, French patent, N (1997)

16 II - Jet Metal TM Experimental Samples: N-type and P-type monocrystalline texturised Si Deposits: Ag and NiB Aspect: Homogeneous Good contact resistivity SEM Micrographs of a Ni-B deposit on N-type Si

17 Overview I Ni-P Electroless Deposition II Jet Metal TM III Conclusion

18 III - Conclusion NiP Electroless deposits - acidic bath: suitable for deposition on Ag or Ag/Alu screen-printed contacts - alkaline bath: good seed layer for an electrolytic deposition of Cu or Ag on silicon Ag and NiB JetMetal TM deposits Suitable for solar cells back contact metallization on silicon

19 III Conclusion Perspectives Standard Cells Rear Contact Cells