Screen Printed Al-Pastes for LFC Solar Cells C. Schwab 1, B. Thaidigsmann 1, M. Linse 1, A. Wolf 1, F. Clement 1, A. Prince 2, R. Young 2, P. Weigand 3 1 Fraunhofer Institute for Solar Energy Systems ISE 2 DuPont (U.K.) Limited 3 DuPont de Nemours GmbH 3rd Metallization Workshop Charleroi, 26th October 2011 www.ise.fraunhofer.de
Motivation Requirements on screen printed Al-Pastes for LFC 1 Cells Passivation layer Screen printed Al-Paste p-type Si Good adhesion Preserve surface passivation quality Suited for Laser fired contact 1 formation High lateral conductance 2 [1] Schneiderlöchner et al., Proc. 17th EUPVSEC, 2001, 1303-6
Outline Experimental Al-paste evaluation (Paste A and PV361) LFC resistance Solar cell performance Investigated passivation layers Thermal SiO 2 /SiN X passivation PECVD Al 2 O 3 /SiN X passivation Influence paste lay down on device performance Latest solar cell results 3
Experimental Device fabrication and structure Screen printed Ag-contacts SiN X Emitter p-type Cz Si ~2 cm Passivation stack Screen printed Al-Paste LFC Applied Al-Pastes Passivation stacks Paste A SiN x Therm. SiO 2 Solamet PV361 SiN x Al2O3 4
Experimental Device characterization Sreen Printing Fast firing LFC formation inline FG annealing QE-measurement p-type Cz Si ~2 cm Allows for a rating of the penetration of the paste through the passivation 5
Experimental Device characterization PVD Al p-type Cz Si ~2 cm Sreen Printing Fast firing LFC formation inline FG annealing QE-measurement Adaption of LFC process for each rear side using test structure: Contact size Contact resistance 6
Experimental Device characterization p-type Cz Si ~2 cm Sreen Printing Fast firing LFC formation inline FG annealing QE-measurement Effect of LFC formation on V oc 7
Experimental Device characterization p-type Cz Si ~2 cm Sreen Printing Fast firing LFC formation inline FG annealing QE-measurement Effect of forming gas anneal 8
Experimental Device characterization p-type Cz Si ~2 cm Sreen Printing Fast firing LFC formation inline FG annealing QE-measurement Analysis of red response 9
Results LFC-resistance Contact radius r (µm) Resistance per LFC R LFC (Ω) Variation R LFC from theorie (%) SiO 2 /SiN x Paste A 38 141 ± 3 2.82 PV361 36 150 ± 7 2.69 Al 2 O 3 /SiN x Paste A 36 183 ± 14 30.38 PV361 37 170 ± 6 22.47 Adapted LFC process for each rear side: Comparabel Contact sizes Comaparabel Contact resistances on each passivation Increased R LFC for Al 2 O 3 /SiN x passivation p-type Cz Si ~2 cm 10
Results on SiO 2 /SiN x Passivation IV-Data SiN x Therm. SiO 2 Paste (#cells) FFO-Temp ( C) η (%) J SC * (ma/cm 2 ) V OC (mv) FF (%) Passivation: SiO 2 /SiN x Paste A (10) PV361 (10) median max median max 860 860 17.7 18.0 18.2 18.6 36.5 36.5 36.8 38.0 623 629 630 629 77.8 78.4 78.2 77.7 ~ 7mV difference in V oc! Cell tester measurement (unconfirmed) as processed, no mismatch-correction *low J sc and η due to increased shading by front grid 11
Global IQE Results on SiO 2 /SiN x Passivation IQE Measurement SiN x Therm. SiO 2 1.0 PTQ110164 0.8 1.0 Paste A PV361 Effective diffusion length L eff (µm) 0.6 0.9 SiO 2 /SiN x 0.4 0.8 0.7 Paste A 590 ± 80 PV361 1020 ± 90 0.2 0.6 SiO 2 /SiN x 0.0 0.5 850 900 950 1000 1050 1100 1150 average of 3 cells, best FFO-Peak-Temps. 400 600 800 1000 1200 Wavelength (nm) Reduced diffusion length for Paste A Best FFO-Peak-Temps., averaged over 3 cells 12
V OC (mv) Results on SiO 2 /SiN x Passivation V oc after FFO SiN x Therm. SiO 2 635 630 Paste A PV361 625 620 615 Significant difference in V oc already after FFO: 610 605 PTQ110164 after FFO after LFC SiO 2 /SiN x after FGA Paste A shows a stronger firing through behaviour 880 C FFO-Peak-Temp. 13
V OC (mv) Results on SiO 2 /SiN x Passivation V oc after FFO and LFC SiN x Therm. SiO 2 635 630 Paste A PV361 625 620 615 LFC reduces V oc due to local metallization 610 605 PTQ110164 after FFO after LFC SiO 2 /SiN x after FGA 880 C FFO-Peak-Temp. 14
V OC (mv) Results on SiO 2 /SiN x Passivation V oc after FFO, LFC and FGA SiN x Therm. SiO 2 635 630 Paste A PV361 625 620 615 610 605 PTQ110164 after FFO after LFC SiO 2 /SiN x after FGA FGA beneficial to V oc : Improved contacts Improved passivation 880 C FFO-Peak-Temp. 15
Results on Al 2 O 3 /SiN x Passivation IV-Data Paste (#cells) FFO-Temp ( C) η (%) J SC * (ma/cm 2 ) V OC (mv) FF (%) SiN x Al2O3 Passivation: Al 2 O 3 /SiN x Paste A (10) PV361 (10) median max median max 880 900 18.0 18.7 18.3 18.7 36.6 37.9 37.2 38.1 631 633 632 632 77.8 77.7 77.8 77.9 Similar V oc! Cell tester measurement (unconfirmed) as processed, no mismatch-correction *low J sc and η due to increased shading by front grid 16
Global IQE Results on Al 2 O 3 /SiN x Passivation IQE Measurement SiN x Al2O3 1.0 PTQ110164 0.8 1.0 Paste A PV361 Effective diffusion length L eff (µm) 0.6 0.9 SiO 2 /SiN x Paste A 590 ± 80 0.4 0.2 0.0 0.8 0.7 0.6 0.5 Al 2 O 3 /SiN x 850 900 950 1000 1050 1100 1150 average of 3 cells, best FFO-Peak-Temps. 400 600 800 1000 1200 PV361 1020 ± 90 Al 2 O 3 /SiN x Paste A 1690 ± 210 PV361 1590 ± 170 Wavelength (nm) Similar Diffiusion legth for both pastes Best FFO-Peak-Temps., averaged over 3 cells 17
V OC (mv) Results on Al 2 O 3 /SiN x Passivation V oc after FFO SiN x Al2O3 640 638 636 Paste A PV361 634 632 630 628 626 Both pastes show same V oc level after FFO: 624 622 PTQ110164 after FFO after LFC Al 2 O 3 /SiN x after FGA Thicker SiN x Capping and/or Al 2 O 3 layer prevents or reduces damage induced by Paste A 880 C FFO-Peak-Temp. 18
V OC (mv) Results on Al 2 O 3 /SiN x Passivation V oc after FFO, LFC and FGA SiN x Al2O3 640 638 636 Paste A PV361 634 632 630 628 LFC reduces V oc 626 FGA increases V oc 624 622 PTQ110164 after FFO after LFC Al 2 O 3 /SiN x after FGA V oc after FFO and after FGA on the same level 880 C FFO-Peak-Temp. 19
Results on SiO 2 /SiN x Passivation Lay Down Variation PV361 Lay Down in mg/cm² (#cells) η (%) J SC (ma/cm 2 ) V OC (mv) FF (%) 4.5 (10) median max 18.1 18.6 36.9 37.7 630 632 77.9 78.1 6 (10) median max 18.2 18.6 36.8 38.0 630 629 78.2 77.7 8 (10) median max 18.1 18.5 36.7 37.5 630 631 78.4 78.5 Best FFO-Peak-Temps., Comparabel efficiencies for all lay downs Less material consumption 20
Latest Cell Results TOPAS 1 solar cell structure Sreen printed Ag-contact p-typ Cz-Si ~ 2 cm LFC PV361 SiN X Therm. Oxide Emitter Highly doped Emitter Therm. Oxide PECVD-Capping SP best group median (6) η (%) J sc (ma/cm 2 ) V oc (mv) FF (%) pff (%) 19.4 39.0 649 76.7 82.8 best cell 19.5 39.1 650 76.9 82.9 Cell tester measurement (unconfirmed) as processed, no mismatch-correction [1] Mack et al., Proc. 35th IEEE PVSC, 2010 21 GRIPS
Summary and conclusion Evaluation of Al-Pastes for LFC-PERC: Paste A and PV361 Reduced damage to SiO 2 /SiN x -Passivation by PV361 7 mv higher V oc Similar performance of both pastes for Al 2 O 3 /SiN x passivation Thicker SiN x and/or Al 2 O 3 layer prevent penetration by Paste A Similar efficiency for laydowns as low as 4.5 mg/cm² using PV361 Reduced material consumption Paste and passivation have to be matched and optimized together 22
Thank You Very Much for Your Attention! Fraunhofer Institute for Solar Energy Systems ISE Christoph Schwab www.ise.fraunhofer.de christoph.schwab@ise.fraunhofer.de 23
Paste Evaluation Adhesion and resistivity Adhesion Specific Resistance (µωm) Paste A OK 0.12 PV361 OK 0.27 Adhesion by tape test Resistivity by Sheet resistance from 4 Point probe method Paste thickness from confocale microscope higher resistivity of PV361, reduction of ~0.1%abs in FF expected 24