Investigation of metal contacts via thermal treatment at Interfaces between low temperature Ag pastes and TCO layer for HIT solar cell

Transcription

1 Investigation of metal contacts via thermal treatment at Interfaces between low temperature Ag pastes and TCO layer for HIT solar cell Ming-Shiou Lin, Kuang-Yang Kuo, Yong-Han Lin, Yueh-Lin Lee, Liang-Pin Chen and Chi-Chun Li Advance Technology Development Department Motech Industries, Inc. 1

2 Outline Introduction Experimental Cell precursor Printing performance Annealing experiment Conclusion 2

3 High Efficiency Si Solar Cells HJT cells top the efficiency charts Ref: Solar cell efficiency tables (version 47) 3

4 Silicon Heterojunction Solar Cells Band diagram of a Si heterojunction cell Schematic cross-section of the solar cell Features: Very high efficiency Good temp. coefficients Thin wafer possible Bifacial design possible 4

5 Low Temperature Ag Paste Requirements: Good printability Low line resistance Low contact resistance Good adhesion Low cost 5

6 Outline Introduction Experimental Cell precursor Printing performance Annealing experiment Conclusion 6

7 Cell Structure and Process Flow Texture a-si deposition TCO deposition Ag printing and soft baking Annealing Schematic cross-section of the solar cell Cell process flow 7

8 Cell Precursor Processes Random pyramid texture a-si ITO Si substrate a-si deposition TCO deposition 8

9 Printing Improvement W: 120 m H: 30 m W: 85 m H: 30 m Double printing improved finger profile W: 70 m H: 30 m Screen design change high aspect ratio 9

10 Effect of Thermal Treatment Strong influence of annealing on Ag line and Ag-TCO contact 10

11 Annealing Experiments Annealing with different temperatures and durations A B C Recipe Temperature ( ) Duration (min) r c (mω.cm 2 ) SEM image A Close contact to ITO B Non-contacted area C Large cavity and residue Insufficient annealing poor contact 11

12 Characterization of Residue EDS showed strong carbon signal from residue of Sample C A B C Elements Atomic% Si Ag O Elements Atomic% Si Ag C 29.6 O

13 Annealing Experiments Discussion Thermal budget constrained by passivation quality of a-si 200, <40min 230, <20min 260, degradation Recipe Temperature ( ) Duration (min) Comment A OK(?) B Insufficient sintering C Incomplete organics removal 13

14 Effect of Soft Baking Sample 1 Front Rear Sample 2 Front Rear Sample 3 Front Rear Rsheet (Ω/sq) Rsheet rhoc r c (mω.cm 2 ) Front side was printed first soft baked twice Both r c and R-line are superior for the front side TCO Rsheet the same R-line (Ω/cm) Shrinking finger dimension lowered rear r c Line resistivity ~ 5 μω.cm 14

15 Metallization Optimization Simulation of cell performance for 3BB, 4BB, 5BB Efficiency improvements Thermal treatment: +1% abs Fine line printing: +0.7% Grid optimization: +0.3% 15

16 Jun-14 Jul-14 Aug-14 Sep-14 Oct-14 Nov-14 Dec-14 Jan-15 Feb-15 Mar-15 Apr-15 May-15 Trend Chart of Cell Performance Efficiency (%) Project Target Date Acknowledgement: One-year grant from Southern Taiwan Science Park Bureau Co-worked with Nano Device Laboratories for thin film depositions 16

17 Performance of Peak Cell 22.35% efficiency certificated by Solar cell Calibration and Characterization Laboratory (SCCL) of ITRI in Taiwan 17

18 Outline Introduction Experimental Cell precursor Printing performance Annealing experiment Conclusion 18

19 Conclusion Process Window of thermal treatment identified for low T Ag paste to achieve r c ~ 2 mω.cm 2 Ag line resistivity ~ 5 μω.cm No Voc degradation Confirmed efficiency of the best HJT cell: 22.35% Future: Ag paste usage reduction BB less Cu 19

20 Thank You for Your Attention CC Li, Metallization Workshop, May