Copper as Conducting Layer in the Front Side Metallization of Crystalline Silicon Solar Cells

Transcription

1 Copper as Conducting Layer in the Front Side Metallization of Crystalline Silicon Solar Cells Processes, Challenges & Characterization Jonas Bartsch Fraunhofer Institute for Solar Energy Systems ISE 2 nd Workshop on Solar Cell Metallization Konstanz, 15 th of April

2 Introduction Concept of seed and plate Efficiency advantage compared to screen print Flexibility 2

3 Introduction Concept of seed and plate Efficiency advantage compared to screen print Flexibility 3

4 Introduction Motivation Silver material main driver (~30 %) for cell processing costs Silver and copper equally conductive Copper much cheaper Ag Cu Ni Conductivity [10 6 S/m] 61,35 59,1 13,9 ρ [g/cm³] 10,49 8,92 8,90 4

5 Introduction Motivation Silver material main driver (~30 %) for cell processing costs Silver and copper equally conductive Copper much cheaper Why do we still use silver? Ag Cu Ni Conductivity [10 6 S/m] 61,35 59,1 13,9 ρ [g/cm³] 10,49 8,92 8,90 5

6 Introduction Copper migration in silicon Copper migrates very easily in silicon (even at ambient T) Diffusion coefficient [cm²/s] Eicke R. Weber: Transition Metals in Silicon, Appl. Phys. A, 30, 1-22 (1983) 1000/T [K -1 ] 6

7 Introduction Copper migration in silicon Copper migrates very easily in silicon (even at ambient T) Defect energy levels close to middle of bandgap effective centre for recombination Middle of BG Introduction of diffusion barrier necessary 7

8 Introduction Nickel as diffusion barrier Already known from semiconductor industry Cheap Possible as contact material (low barrier height) Presentation of A. Mondon, later this morning 8

9 Process Approaches Plating Ni/Cu/Sn on printed and fired seed layers Close to current process Direct plating of Ni for contact formation More process development necessary Seeding + Firing Examples: Sn Si Printed seed layer Cu-Plating Ni-Diffusion barrier SiN x 9

10 Process Sequence Fired seed SiN x Ni-Plating SiN x Cu Plating SiN x Sn Plating SiN x Si Si Si Si 10

11 Process Resulting Solar cell contacts EDX-Analysis Width ~55µm, height ~ 15µm Tin Copper Nickel Silver 11

12 Solar Cell Results Random pyramids, fine line seed 2x2 cm² FZ Seed-Ni-Cu (best cell) V OC [mv] J SC [ma/cm²] FF [%] η [%] 646,4 38,86 80,8 20,3 Passivated rear, LFC Demonstrates the potential of the technology. Comparable to silver (as expected) 12

13 Even more important: Long term stability Guaranteed module lifetime: years How will the contact behave? Copper Seedlayer & metal semiconductor-contact SiN x n + Characterization needed Quick measurement of Cu penetration depth / junction quality via pff Cell not damaged yet p SCR 13

14 Even more important: Long term stability Guaranteed module lifetime: years How will the contact behave? Copper Seedlayer & metal semiconductor-contact SiN x Characterization needed Quick measurement of Cu penetration depth / junction quality via pff n + SCR p Damage increased J 02, reduced FF 14

15 Long term stability consideration Concept of characterization Cell damage if copper gets to SCR We can t wait for 25 years Acceleration of diffusion by thermal stress Pseudo fill-factor pff is monitored by Suns-V OC Diffusion basics E A L = D t D = D0 exp kbt L t 2 = D = D 0 E exp kbt 2 EA 1000 L ln( { t) = + ln k y B { T D a x A b Key assumption: Comparable loss in pff means comparable diffusion state (or penetration depth) 15

16 Long term stability consideration Degradation behaviour under thermal stress Quick loss of pff without diffusion barrier 5% pff-loss assumed critical Effect of Ni-layer clearly visible Estimation of lifetime: Time / Temperaturepairs needed 2 EA 1000 L ln( { t) = + ln k y B { T D a x b On fine-line printed seed layer Introduction of Nickel barrier layer 16

17 Long term stability consideration Arrhenius-plot Time/temperature pairs with equal pff-loss Insert data into Arrhenius plot Cells with Cu plated directly on seed Extraction of effective E A of diffusion 17 Extrapolation to module exposure conditions Method submitted for publication ln( { t) y 2 EA 1000 L = + ln kb { T D a x b

18 Long term stability considerations Estimation of lifetime Assumed module T: 80 C Extrapolation Robust process preferable Good data basis needed At least three points 175 C good lower T for samples with barrier layer Well suited for quick estimation Time/Temperature-pairs of 5% pff-loss 18

19 Long term stability considerations Estimation of lifetime Assumed module T: 80 C Extrapolation Robust process preferable Good data basis needed At least three points 175 C good lower T for samples with barrier layer Well suited for quick estimation Time/Temperature-pairs of 5% pff-loss 19

20 Long term stability considerations Estimation of lifetime Assumed module T: 80 C Extrapolation Robust process preferable Good data basis needed At least three points 175 C good lower T for samples with barrier layer Well suited for quick estimation Time/Temperature-pairs of 5% pff-loss 20

21 Follow-up Work Backing the estimation with more data Comparison to climate chamber test Identification of the path for copper penetration Tackling adhesion issues Results already very promising On thick printed seed layer 21

22 Follow-up Work Backing the estimation with more data Comparison to climate chamber test Identification of the path for copper penetration Tackling adhesion issues Results already very promising On thick printed seed layer 22

23 Acknowledgement Thanks to: Andrew Mondon, Katharina Bay, Elisabeth Schäffer, Daniel Schmidt, Aleksander Filipovic, Jan Specht, Jan Nekarda, Denis Erath, Christian Schetter, Matthias Hörteis, Stefan Glunz. 23

24 Thank You Very Much for Your Attention! Cu SiNx Ni Seed layer Fraunhofer Institute for Solar Energy Systems ISE Jonas Bartsch 24

25 Cost comparison Silver/Copper Metallization Assumed System: Aerosol-Seed + Silver vs. Aerosol-Seed + Ni + Cu + Sn Rough comparison, conservative assumptions Aerosol Nickel- Process Conducting layer Tin- Process Total Silver Process Same cost - Material Same cost c/wafer 10.5 c/wafer c/wafer Copper Process Same cost 2.5 c/wafer Material Same cost 0.01 c/wafer 2.5 c/wafer 0.12 c/wafer 2.5 c/wafer 0.02 c/wafer 7.65 c/wafer 25

26 Plating Equipment Manual plating tool for Nickel, Copper and tin Inline-Machine for Copper 26