Understanding and avoiding the formation of voids for rear passivated silicon solar cells

Transcription

1 Understanding and avoiding the formation of voids for rear passivated silicon solar cells Elias URREJOLAnow with 2, Kristian PETER 1, Heiko PLAGWITZ 2, Gunnar SCHUBERT 2 1 International Solar Energy Research Center - ISC - Konstanz, Germany 2 SUNWAYS AG Photovoltaic Technology, Konstanz, Germany 3rd Workshop on Metallization for Crystalline Silicon Solar Cells Wednesday, 26 October 2011

2 Part I. Motivation Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 2

3 Solar Cell Structures: Total vs. Local Total: Al-BSF Solar Cell Local: i-perc [1] p-si BSF L-BSF Al rear side BSF L-BSF [1] G. Agostinelli et al. 20th EUPVSEC, Barcelona pp (2005). Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 3

4 Motivation: Void instead of Eutectic El-Analysis? FF ~ 78.6% L-BSF void formation uncertain origin? [1 [1-4] Rear side completely passivated [1] F. Huster, 20 th EUPVSEC, Barcelona, p (2005) [2] F.S. Grasso et al, 2 nd Workshop on Metallization, Konstanz, pp (2010) [3] T. Lauermann et al, 35 th IEEE PVSC, pp (2010) [4] S. Gatz et al., Energy Procedia, vol. 8, 318 (2011) Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 4

5 Part II. Understanding Void Formation Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 5

6 Contact Formation: Total vs. Local Standard Process [1] Local Al-Si Contact Al Si [2, 5, 6] Al-Si phase diagram (equilibrium) [3] Peak Cooling temperature liquid heating Solid down 850 C T peak Si diffusion in Al [4] (a) (b) (c) (a,b) (c) (a) Al-rich solidus in lamellar structure (b) Eutectic (Al % Si) (c) BSF: Si-rich solidus D Si D Al [1] F. Huster, 20 th EUPVSEC, Barcelona, p (2005). [2] F.S. Grasso et al. 2nd Workshop on Metallization, Konstanz, pp (2010). [3] J.L. Murray and A.J. McAlister, Bulletin of Alloy Phase Diagrams 5, (1984). [4] E. Urrejola et al. Applied Physics Letters 98, (2011). [5] V.A. Popovich et al. 24 th EUPVSEC, Hamburg, pp (2009) [6] M. Rauer et al. Energy Procedia vol 8, 200 (2011). Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 6

7 Silicon Diffusion in Aluminum [1] [1] E. Urrejola, et al. Applied Physics Letters,, vol. 98, (2011) d 2 : Si diffusion in Al d2 d1 d 1 d 1 : local contact opening Al p-si 3 [µm/[ m/ C] Spread of Si in the Al matrix d 2 increases by 3 [µm/[ m/ C] within T peak : { } C does not depend on the contact area Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 7

8 Impact of Al Amount on Mass Transport [1] [1] E. Urrejola, et al. Journal of Applied Physics vol. 107, (2010) Al responsible for mass transport Si diffuses faster than Al in Al-Si alloys (DSi > DAl)[2] saturation [2] J.O. McCaldin and H. Sankur, Applied Physics Letters, vol. 19, 524 (1971). Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 8

9 Impact of Pitch on Void Formation d2 Al d1 L p L p : contact spacing p-si L p Lp = 2 mm Lp < 0.25 mm influence on void formation? Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 9

10 Impact of Pitch on Void Formations Same Al paste, same firing Difference? Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 10

11 Profile of Si Distribution in Al (EDX Line) Si Small contact spacing Si almost homogeneously distributed in the Al matrix Si Si Large contact spacing ~ no Si in middle part! Si Si Void: Si directly obove the opening < Si overlying the dielectric barrier Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 11

12 Comparable with Literature C. J. Kircher,, Journal of Applied Physics, vol. 47, 5394 (1976). J. O. McCaldin,, Applied Physics Letters, vol. 20, 171 (1972). DURING COOLING: The reduced Si concentration in the Al adjacent to contact area is due to Si diffusing back and regrowing on the Si surface DURING COOLING: 1. dissolved Si is unable to go the distance to the substrate and remains as precipitates 2. Si at the Al-Si interface can easily diffuse to the substrate Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 12

13 Concentration of Si in Al (EDX/EDS) L-BSF Al - 84% Si - 16% Large V Al - 69% Si - 12% O - 19% Small V Al - 62% Si - 18% O - 20% Large V Al - 72% Si - 10% O - 18% Small V Al - 65% Si - 12% O - 23% Volume contained in void distributed in the Al matrix Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 13

14 Hypothesis of Void Formation Non equilibrium process! void complete formed D Si > D Al void with BSF At the T peak : Al/Si melt transferred into Al matrix void is formed During cooling: Si tries to diffuse back and solidifies in Al (high concentration of Si at the interface) no BSF was found (?) During cooling down: Si recrystallizes epitaxially + Al Si (BSF) If cooling is faster than recrystallization and D Si Si does not reach the interface and void is formed. Rest of Si solidifies in Al. Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 14

15 Understanding the Formation of Voids void no BSF eutectic below Al void + BSF + eutectic formations L-BSF Al - 87% Si - 13% eutectic below Al voids appear at T peak and / or during cooling down Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 15

16 Part III. How to avoid the voids? Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 16

17 1. Effect of gravity on the Al-Si morphology [1] [1] E. Urrejola, et al. Journal of Applied Physics, vol. 110, (2011) Due to Al 2 O 3 the Al paste does not move during the alloying process [2] thus this phenomenon must be due to gravity [1] [2], Case A Case B [2] 1 L. Sardi et al. Solar Cells, vol 11, 51 (1984). Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 17

18 2. Optimization of Contact Spacing [1] [1] E. Urrejola, et al. Energy Procedia,, vol. 8, 331 (2011) Lp [µm] Eutectic or void depth [µm][ BSF [µm] Voids/eutectic [%] ± ± ± ±2-100 Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 18

19 3. Others from Literature Reducing D Si Al [1, (e.g. Si in the Al paste 2] well formed L-BSFL reduced recombination [3] Paste development to optimize Si Al mass transfer [4] Firing optimization [5] [3] ) [1] 1 M. Rauer et al. IEEE Electron Device Letters vol. 32, 916 (2011) [2] A. Urueña et al. 24 th EUPVSEC, pp (2009). [3] J. Müller et al. IEEE Transactions on Electron Devices vol. 58, 3239 (2011). [4] V. Meemongkolkiat, 4 th WCPEC, Hawaii pp (2006). [5] T. Lauermann et al. 26 th EUPVSEC, Hamburg pp (2011) Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 19

20 Summary Si diffusion into Al must be controlled Voids formation depends on: Diffusion of Si into Al and temperature: D Si > D Al Concentration and distribution of Si in the Al layer Amount of Al and contact spacing Others: paste, firing parameters, surface ablation Avoiding voids : Al on top during firing Reducing D Si Al (e.g. Si in Al before firing) Contact spacing optimization Paste development to optimize Si Al mass transfer Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 20

21 Acknowledgements The authors gratefully acknowledge the financial support by the German Federal Ministry of Education and Research within the framework of the Leading-Edge Cluster Competition and the research cluster Solarvalley Central Germany under contract No. 03SSF0335I, and Merck KGaA for the kind supply of the etching paste. Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 21

22 THANK YOU FOR YOUR ATTENTION Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola 22