ELECTROCHEMICAL CONTACT SEPARATION FOR PVD ALUMINUM BACK CONTACT SOLAR CELLS

Transcription

1 ELECTROCHEMICAL CONTACT SEPARATION FOR PVD ALUMINUM BACK CONTACT SOLAR CELLS M. Kam, A. Maywald, J. Bartsch, R. Efinger, R. Keding, M. Glatthaar, S. W. Glunz, I. Krossing Fraunhofer Institute for Solar Energy Systems ISE 5 th Metallization Worksho Constance, October 20 th

2 AGENDA Metallization of back contact solar cells Aluminum anodizing Industrial alications Electrochemical reaction Local anodizing of aluminum Mask based aroaches Novel maskless aroaches Summary 2

3 Metallization of IBC Solar Cells Motivation Back contact solar cells 3 High efficiencies reached Panasonic 25.6% 1 SunPower 25.0% 2 Prorietary rocess from SunPower Aluminum as contacting material Metal contact searation challenging Alternative contact searation rocess necessary [1] Panasonic HIT Solar Cell Achieves World`s Highest Energy Conversion Efficiency of 25,6% at Research Level, [2] D. D. Smith et. al. IEEE Journal of Photovoltaics, 2014.

4 Metallization of IBC Solar Cells Comarison of Process Stes Established Process 3 1 PVD-Aluminum 2 PVD-TiW 3 PVD-Coer 4 Resist rinting n n 5 Plating 6 Resist removal 7 Metal etching 4 [3] W. P. Mulligan, Solar cell and method of manufacture, US B1, [4] Kam et al, Solar Energy Materials & Solar Cells, 2014

5 Metallization of IBC Solar Cells Comarison of Process Stes Established Process 3 Alternative Maskless Process 1 PVD-Aluminum 2 PVD-TiW 3 PVD-Coer 4 Resist rinting 5 Plating 1 PVD-Aluminum 2 Al structuring??? 3 Zincate Process 4 4 Plating 6 Resist removal 7 Metal etching n n 5 [3] W. P. Mulligan, Solar cell and method of manufacture, US B1, [4] Kam et al, Solar Energy Materials & Solar Cells, 2014

6 Metallization of IBC Solar Cells Comarison of Process Stes Alternative Maskless Process 1 PVD-Aluminum 2 Al structuring??? 3 Zincate Process 4 4 Plating n n 6 [3] W. P. Mulligan, Solar cell and method of manufacture, US B1, [4] Kam et al, Solar Energy Materials & Solar Cells, 2014

7 Aluminum Anodizing Industrial Alications and Proerties of AAO Aluminum can be converted into electrical isolating aluminum oxide by anodizing Structure of anodic aluminum oxide (AAO) first reorted in Industrially transferred a few years later Today s main alications Anti corrosion treatment of Al alloys (aircraft and automotive) Decorative surfaces (color coating) Electronics industry Temlate for nanowire fabrication Idealized structure of anodic orous alumina 6 Anodized and dyed carabiners 7 7 [5] F. Keller, M. S. Hunter, D. L. Robinson, J. Electrochem. Soc. 100, 411, 1953 [6] G. D. Sulka, Nanostructured Materials in Electrochemistry, 2008 [7] Anodizing, en.wikiedia.org/wiki/anodizing, 2014

8 Al andoe Cathode Aluminum Anodizing Electrochemical Reaction Main anodizing reaction Cathode: 6 H 3 O + + 6e - 3 H H 2 O Anode: 2 Al 2 Al e - 2 Al H 2 O Al 2 O H 3 O + Formation of Anions OH - and O 2- Overall: 2 Al + 3 H 2 O Al 2 O 3 + 3H Electrolyte Al Al 2 O 3 H + +e - H 2 O 2- and OH - ions formation at the oxide/elektrolyte interface from water interaction with absorbed SO 4 2- anions 6 8 [6] G. D. Sulka, Nanostructured Materials in Electrochemistry, 2008

9 Aluminum Anodizing Pore Formation Stages of ore formation Potential develoing deends on the formation stages Crack initiation in the oxide barrier layer Local increase of current density results in ore formation Al 2 O 3 Si 500 nm To: Kinetics of orous oxide growth in galvanostatic regimes 6 Bottom: Stages of anodic orous oxide develoment 6 9 [6] G. D. Sulka, Nanostructured Materials in Electrochemistry, 2008

10 Aluminum Anodizing Pore Formation Stages of ore formation Potential develoing deends on the formation stages Crack initiation in the oxide barrier layer Local increase of current density results in ore formation Volume exansion Volume exansion observed during anodization of aluminum 6 Anodized aluminum on textured silicon surface 10 [6] G. D. Sulka, Nanostructured Materials in Electrochemistry, 2008

11 Local Anodizing Previous Aroaches for Contact Searation Well known local anodizing rocesses by masking (SiO 2 as mask 8,9, PL organic resists 9,10 ) Disadvantages 11 SiO 2 has to be structured exensively before anodizing Patterned rinted resists has to be removed in an extra ste Adhesion of rinted resist could fail by volume exansion Process seed is limited Local and fast anodizing rocess is desired to establish AAO in PV [8] P. Verlinden, Multilevel Metallization for Large Area Point-Contact Solar Cells, 1988 [9] J. Park, Fabrication of Aluminum/Alumina Patterns using Localized Anodization of Al., 2012 [10] P. H. Lu, Selective Anodization, 40th IEEE, 2014 Anodized masked samle after 20 min 9 Peeling resist Printed resist after anodizing 9 Resist 5 µm 50µm

12 Local Anodizing Previous Aroaches for Contact Searation 1 PVD-Aluminum 2 PVD-TiW 3 PVD-Coer 4 Resist rinting 5 Plating 1 PVD-Aluminum 2 Local Anodizing?? 3 Zincate Process 4 4 Plating 6 Resist removal 7 Metal etching n n 12

13 Novel Maskless Aroaches Metallic Elevations Stam Wetting of elevations Difficult rocess control Exanded anodizing Risk of short circuit Short rocess duration (< 5 sec) Al 2 O 3 s/c Al Si Anode Al 13

14 Novel Maskless Aroaches Local Sealing by Rubber Aluminum is locally shielded by a rubber seal Narrow lines are reached (< 70 µm) Short rocess duration (< 5 sec) Force alication to wafer necessary Al 2 O 3 Al Al Si Al 2 O 3 Anode 50 µm 14

15 Novel Maskless Aroaches Electrochemical Screen Printing Advantages Screen rinting established in PV Fast rocess (< 5 sec) Narrow anodized line widths are realized ( µm) Challenges Secial screen to rovide current transort Secial emulsion (chemically resistive against acid astes) Acidic astes still have to be otimized for rinting Screen (cathode) Emulsion Aluminum (anode) Silicon Al Electrolyte Aluminum oxide Al 2 O 3 Al Si 100µm 15

16 Novel Maskless Aroaches Electrochemical Disensing Advantages Small reaction zone Excellent rocess control Challenges for further exeriments Line width > 200 µm Line interutions 16

17 Local Anodizing Previous Aroaches for Contact Searation Relacement of three rocess stes by maskless local anodizing 1 PVD-Aluminum 2 PVD-TiW 3 PVD-Coer 4 Resist rinting 5 Plating 6 Resist removal 7 Metal etching 1 PVD-Aluminum 2 Local Anodizing 3 Zincate Process 4 4 Plating 17

18 Summary Alternative efficient metal atterning roces s es for backcontact solar cells are needed, esecially for aluminum Anodizing of Aluminum introduced in general, full layer localized anodizing suitable for PV alication In-s itu anodizing via screenrinting or disensing rove to be romising methods for high through-ut industrial alication 18

19 Thank you for your Attention! Fraunhofer Institute for Solar Energy Systems ISE Mathias Kam 19