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Distribution of Silicon iin the Allumiinum Matriix for Rear Passiivated Sollar Celllls

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Solar cell concepts with passivated, locally contacted rear side have gained much interest over the last years due to the development of advanced passivation techniques. However, there are still open questions regarding the understanding of the local Al-Si contact formation and its influence on the local back-surface-field formation for industrial screen-printed aluminum pastes. In this work we show that the geometry of the rear pattern (contact size
and contact spacing) strongly influences the Al-Si interaction in the local contact openings: Si diffuses in Al through narrow dielectric barrier openings and spreads laterally to a determined limit determined by the firing temperature. We found that the contact spacing can affect the tendency for formation of Kirkendall voids below the Al-Si contacts instead of an eutectic layer. When decreasing the contact spacing and therefore the overlap of Al on each side of the local opening, the melt saturates faster and the formation of a high-quality local back-surface-field is increased, minimizing the presence of voids. This physical observation can enhance the design of the rear pattern for rear passivated solar cells and gives further understanding of the Al-Si interaction during the firing process.

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Distribution of Silicon iin the Allumiinum Matriix for Rear Passiivated Sollar Celllls

  1. 1. Distribution of Silicon in the AluminumDistribution of Silicon in the Aluminum Matrix for Rear Passivated Solar CellsMatrix for Rear Passivated Solar Cells Elias URREJOLAElias URREJOLA11 , Kristian PETER, Kristian PETER11 ,, HeikoHeiko PLAGWITZPLAGWITZ22 , Gunnar SCHUBERT, Gunnar SCHUBERT22 11 International Solar Energy Research CenterInternational Solar Energy Research Center -- ISCISC -- Konstanz, GermanyKonstanz, Germany 22 SUNWAYS AG Photovoltaic Technology, Konstanz, GermanySUNWAYS AG Photovoltaic Technology, Konstanz, Germany SILICONSILICON PVPV Tuesday, 19 April 2011Tuesday, 19 April 2011
  2. 2. Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells - Elias Urrejola 2 IntroductionIntroduction Standard p-Si solar cell Al rear side Rear passivated solar cell Higher efficiency: • Rear Passivation • Local contact
  3. 3. Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells - Elias Urrejola 3 Motivation: Local AlMotivation: Local Al--Si InteractionSi Interaction rear passivated solar cell is theis the SiSi diffusiondiffusion limitedlimited in the Al matrixin the Al matrix ?? Al cross-section p-Si Al L-BSF Al Si
  4. 4. Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells - Elias Urrejola 5 Diffusion of Si in the Al matrixDiffusion of Si in the Al matrix dd22 dd11 L-BSF Total spread ofTotal spread of SiSi in the Al matrix din the Al matrix d22 depends linearly on firing temperaturedepends linearly on firing temperature E. UrrejolaE. Urrejola et al.et al. ApplAppl. Phys.. Phys. LettLett. 98, 153508 (2011). 98, 153508 (2011) d1: local contact opening d2: spread of Si in Al
  5. 5. Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells - Elias Urrejola 7 Influence of Contact SpacingInfluence of Contact Spacing LLpp = 2 mm= 2 mm LLpp < 0.25 mm< 0.25 mm Contact spacing limitsContact spacing limits SiSi diffusiondiffusion influence on contact formation?influence on contact formation? LLpp dd22 dd11 Al p-Si Lp: contact spacing
  6. 6. Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells - Elias Urrejola 8 Influence of Contact SpacingInfluence of Contact Spacing Si diffusion inSi diffusion in AlAl large contact spacing large spread of Si voids instead of eutectic Si diffusion limited by contact spacing Al p-Si
  7. 7. Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells - Elias Urrejola 9 35 m KirkendallKirkendall voidvoid Kirkendall effect: Difference in Interdiffusion rates Si diffuses much faster in Al than Al in Si. Large finger spacing high overlap of melted Al (unsaturated) enhanced Si mass transfer Kirkendall void!
  8. 8. Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells - Elias Urrejola 10 Influence of Contact SpacingInfluence of Contact Spacing 100100--2727 2211 87874.14.1 –– 4.54.52323 220.70.7 25254.54.5 –– 4.74.72323 220.250.25 885.75.7 –– 771414 220.10.1 VoidsVoids [%][%] BSFBSF [[ m]m] EutecticEutectic depth [depth [ m]m] LpLp [mm][mm] Small finger spacing Large finger spacing 35 m
  9. 9. Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells - Elias Urrejola 11 Saturated Al EDS/EDX AnalysisEDS/EDX Analysis Kirkendall voids by high firing temperatureT = 950°C E. UrrejolaE. Urrejola et al.et al. ApplAppl. Phys.. Phys. LettLett. 98, 153508 (2011). 98, 153508 (2011) High T [°C] Kirkendall voids, BSF formed eutectic L-BSF
  10. 10. Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells - Elias Urrejola 12 SummarySummary Study on the diffusion of Si in screenStudy on the diffusion of Si in screen--printed Al:printed Al: –– Rear side pattern influences contact formationRear side pattern influences contact formation –– Spread of Si depends on firing temperature but notSpread of Si depends on firing temperature but not on contact size (don contact size (d11)) –– Contact pitch (Contact pitch (LLpp) limits the contact formation and) limits the contact formation and LL--BSFBSF –– KirkendallKirkendall voids form at higher contact spacing,voids form at higher contact spacing, wider local contact openings and too fast coolingwider local contact openings and too fast cooling
  11. 11. Acknowledgements:Acknowledgements: German Federal Ministry of Education and ResearchGerman Federal Ministry of Education and Research (contract no. 03SSF0335I).(contract no. 03SSF0335I). Thank you!Thank you! elias.urrejola@iscelias.urrejola@isc--konstanz.dekonstanz.de
  12. 12. Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells - Elias Urrejola 14 npv workshop www.npv-workshop.com - 230 scientists - industry and institutes
  13. 13. Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells - Elias Urrejola 15 Diffusion of Si in Al matrixDiffusion of Si in Al matrix Broader openings distinct forms at the edges E. Urrejola,E. Urrejola, et al.et al. J.J. ApplAppl. Phys.107, 124516 (2010).. Phys.107, 124516 (2010).

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