docAl-Si alloy formation in narrow p-Si contact areas
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Al-Si alloy formation in narrow p-Si contact areas
Al-Si alloy formation in narrow p-Si contact areas Elias URREJOLA1, Kristian PETER1, Heiko PLAGWITZ2, Gunnar SCHUBERT2 1 International Solar Energy Research Center - ISC - Konstanz, Germany 2 Sunways AG, Konstanz, Germany 2nd Workshop on Metallization for Crystalline Silicon Solar Cells Session II: Contact Formation II 14.04.2010 Outline • Motivation: local contact rear side PERC • Result: contact resistivity (TLM) • Analysis: micrographs SEM • Solar cell results and summary 2 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Motivation – PERC cell Local Al contact analysis for the next generation solar cells laser opening, alloying process: Pyramids 60 µm Extended analysis of the Al-Si alloying formation Ref: G. Beaucarne, 1st Workshop on Metallization for Crystalline Silicon Solar Cells, (2008). Uruena, et al. 24th EUPSEC (2009), p. 1483. Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola 3 ISCISC-Konstanz Motivation – PERC cell i-PERC (2D) Ag contact passivated emitter dielectric stack alloyed junction p- Si Al contact • high efficiency • reduced Si thickness • dielectric passivation at the rear • local contact formation (LBSF) high internal reflection lower recombination p-Si d1 dielectric barrier d2 screen printed Al fingers on dielectric openings 4 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Motivation – PERC cell Model of the local contact formation PERC (rear) Analysis: Contact resistance and contact formation p-Si passivation p-Si etching lines (d1) d1 dielectric barrier d2 d1: dielectric barrier opening width [µm] d2: screen printed Al finger width [µm] screen printed Al-finger (d2) firing (alloying) TLM, SEM d2 >> d1 5 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Contact resistivity result (TLM) Screen printed Al finger, d2 = 700 µm p-Si d1 dielectric barrier d2 10% covered area FF loss 1% Contact resistivity variation: not expected? ρc doping, T contact area? 6 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz The Al-Si alloying process RT: After drying RT: After firing heating cooling down Alloying by rapid thermal annealing 7 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Reduction of the contact area from Reference Screen printed 700 µm Al finger width on p-Si surface to PERC d1 < 100µm, d2 > 500 µm h = 50µm Screen printed 700 µm Al finger width on 100 µm dielectric opening width lower contact resistivity (less FF loss) more passivated area (lower recombination) 8 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Al-Si alloy junction d2 dielectric barrier d1 p-Si (SEM/EDX) Al matrix (Al-17%Si) Al-12.6%Si 20µm BSF (Al-1%Si) 9 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Analysis: Al-Si alloy formation d2 Screen printed Al finger, d2 = 700 µm dielectric barrier d1 p-Si d1 = 80µm 20µm 40µm 10 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Analysis: Al-Si alloy formation d2 Screen printed Al finger, d2 = 700 µm d1 = 80µm dielectric barrier d1 p-Si d1 = 175µm 40µm 20µm 50µm 11 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Analysis: Al-Si alloy formation d2 Screen printed Al finger, d2 = 700 µm d1 = 80µm d1 = 175µm a b 20µm 40µm dielectric barrier d1 p-Si d1 = 250µm 50µm b same Al mass, material, firing, printing conditions > Contact resistivity dependence Al-Si alloy geometry a 12 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Analysis: Al-Si alloy formation d2 Screen printed Al finger, d2 = 700 µm 50µm dielectric barrier d1 p-Si High overlapping of Al strong alloy HCl 13 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Discussion: contact resistivity d2 a dielectric barrier d1 p-Si d1a d1 = 2 d1a = 2 d1b 20µm ∆d1c a d1 d1 + d1c d1 + d1c‘ d1 + d1c‘‘ High contact resistivity d1c planar surface 14 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Summary of experiments d2 dielectric barrier d1 d1 p-Si p-Si High overlapping of paste amount deep Al-Si alloy at the edge. p-Si 15 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz At the solar cell level Gruppe N° i-PERC Reference Eta [%] VOC ISC FF [mV] [mA/cm²] [%] Rs [Ωcm2] best PERC 16,0 619,7 33,7 76,8 1,1 mean PERC 15,8 616,9 33,4 76,9 1,1 Reference 15,6 612,3 32,9 77,6 0,9 mean Reference 15,6 611,1 32,8 77,6 0,9 best mc-Si wafers gain: ƞ: 0.4 %, Jsc: 1 mA/cm2, Voc: 7 mV 16 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Summary • Increasing Ratio: Al finger [µm] / opening [µm] deep Al-Si alloy formation minimization of contact resistivity ρc • Increasing opening width identical alloying forms at the edges planar surface increasing with Ratio • Shallow dielectric barrier lowest ρc of 8 mΩcm2 lower FF losses • Solar cell level d1<100µm (opening), d2>600µm (Al mass) less metallization more passivated area (less recombination) gain: ƞ: 0.4 %, Jsc: 1 mA/cm2, Voc: 7 mV 17 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Local rear contacts PERC solar cell: narrow openings but high material overlapping c b p-Si d1 dielectric barrier a d2 Thank you! elias.urrejola@isc-konstanz.de Acknowledgements: German Federal Ministry of Education and Research (contract no. 03SSF0335I) Merck for the kind supply of the etching paste Appendix 1 200 µm 600 µm 900µm 100 µm 100µm 200 µm 200µm c b p-Si d1 dielectric barrier a d2 20 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Appendix 2 • BSF strongly dependence on the temperature • Homogeneous eutectic at high temperatures 1. T: 620°C, Al: 800µm BSF: 4µm, opening: 150µm, thickness: 20µm 2. T: 710°C, Al: 800µm BSF: 7µm, opening: 150µm, thickness: 20µm better Rc. 21 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz Appendix 3 Transmission line model (TLM): Two probes are used to send a current through the strip while the voltage across two other probes is measured. A plot of this voltage as a function of distance yields a transfer length Lt. The contact resistivity is related to Lt and the sheet resistivity Rs of the diffused layer Ref: H. Plagwitz, Thesis, (2007). 22 Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola. Urrejola ISCISC-Konstanz
