Laser enhanced gettering of silicon substrates

Abstract

One challenge to the use of lightly-doped, high efficiency emitters on multicrystalline silicon wafers is the poor gettering efficiency of the diffusion processes used to fabricate them. With the photovoltaic industry highly reliant on heavily doped phosphorus diffusions as a source of gettering, the transition to selective emitter structures would require new alternative methods of impurity extraction. In this paper, a novel laser based method for gettering is investigated for its impact on commercially available silicon wafers used in the manufacturing of solar cells. Direct comparisons between laser enhanced gettering (LasEG) and lightly-doped emitter diffusion gettering demonstrate a 45% absolute improvement in bulk minority carrier lifetime when using the laser process. Although grain boundaries can be effective gettering sites in multicrystalline wafers, laser processing can substantially improve the performance of both grain boundary sites and intra-grain regions. This improvement is correlated with a factor of 6 further decrease in interstitial iron concentrations. The removal of such impurities from multicrystalline wafers using the laser process can result in intra-grain enhancements in implied open-circuit voltage of up to 40 mV. In instances where specific dopant profiles are required for a diffusion on one surface of a solar cell, and the diffusion process does not enable effective gettering, LasEG may enable improved gettering during the diffusion process.

Key words： gettering; multicystaline; silicon; impurities; laser doping

Acknowledgments

This program was supported by the Australian Government through the Australian Renewable Energy Agency (ARENA) and the Australian Centre for Advanced Photovoltaics (ACAP). The views expressed herein are not necessarily the views of the Australian Government, and the Australian Government does not accept responsibility for any information or advice contained herein.

References

1 International Technology Roadmap for Photovoltaic (ITRPV). International Technology Roadmap for Photovoltaic Results 2015, 7th edition. Frankfurt. 2016, http://www.itrpv.net

2 Weber E. Transition metals in silicon. Applied Physics A, Materials Science & Processing, 1983, 30(1): 1–22

3 Cuevas A, Macdonald D, Kerr M, Samundsett C, Sloan A, Shea S, Leo A, mrcarica M, Winderbaum S. Evidence of impurity gettering by industrial phosphorus diffusion.In: IEEE 28th Photovoltaic Specialists Conference (PVSC). 2000: 244–247

4 Khedher N, Hajji M, Hassen M, Ben Jaballah A, Ouertani B, Ezzaouia H, Bessais B, Selmi A, Bennaceur R. Gettering impurities from crystalline silicon by phosphorus diffusion using a porous silicon layer. Solar Energy Materials and Solar Cells, 2005, 87(1–4): 605–611

5 Bentzen A, Holt A, Kopecek R, Stokkan G, Christensen J, Svensson G. Gettering of transition metal impurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing. Journal of Applied Physics, 2006, 99(9): 093509

6 Mayandi J, Marstein E S, Syre M, Olaisen B R, Thomassen B, Holt A, Vines L. The effect of P concentration on gettering of multicrystalline Si wafers. European Photovoltaic Solar Energy Conference (EU PVSEC), Hamburg, Germany, 2009, 2160–2163

7 Phang S, Macdonald D. Effect of boron codoping and phosphorus concentration on phosphorus diffusion gettering. In: 2014 IEEE 40th Photovoltaics Specialist Conference (PVSC). Denver, USA, 2014, 64–69

8 Hahn G, Joos S. State-of-the-art industrial crystalline silicon solar cells. In: Willeke G P, Weber F R, ed. Advances in Photovoltaics: Part 3. Academic Press, 2014, 1–72

9 Yang K H, Schwuttke G H. Minority carrier lifetime improvement in silicon through laser damage gettering. physica status solidi (a), 1980, 58(1): 127–134

10 Hayafuji Y, Yanada T, Aoki Y. Laser damage gettering and its application to lifetime improvement in silicon. Journal of the Electrochemical Society, 1981, 128(9): 1975–1980

11 Matsuoka Y, Usami A, Shimazaki K. Effect of carrier lifetime on laser-induced damage in silicon. Journal of Physics D: Applied Physics, 1975, 8(1): 1–2

12 Ametowobla M, Bilger G, Köhler J R, Werner J H. Laser induced lifetime degradation in p-type crystalline silicon. Journal of Applied Physics, 2012, 111(11): 114515

13 Abbott M, Cousins P, Chen E, Cotter J. Laser-induced defects in crystalline silicon solar cells. In: 2005 IEEE 31st Photovoltaics Specialist Conference (PVSC). Lake Buena Vista, USA, 2005, 1241–1244

14 Thorstensen J, Foss E. Investigation of depth of laser damage to silicon as function of wavelength and pulse duration. Energy Procedia, 2013, 38: 794–800

15 Sinton R A, Cuevas A. A quasi-steady-state open-circuit voltage method for solar cell characterization. In: 16th European Photovoltaic Solar Energy Conference (EU PVSEC). Glasgow, UK, 2000: 1–4

16 Hameiri Z, Borojevic N, Mai L, Nandakumar N, Kim K, Winderbaum S. Should the refractive index at 633 nm be used to characterize silicon nitride films? In: Proceedings of the 43rd IEEE Photovoltaic Specialists Conference (PVSC). Portland, USA, 2016

17 Sinton R A, Cuevas A. Contactless determination of current–voltage characteristics and minority-carrier lifetimes in semiconductors from quasi-steady-state photoconductance data. Applied Physics Letters, 1996, 69(17): 2510–2512

18 Richter A, Werner F, Cuevas A, Schmidt J, Glunz S W. Improved parameterization of Auger recombination in silicon. Energy Procedia, 2012, 27: 88–94

19 Cuevas A, Macdonald D. Measuring and interpreting the lifetime of silicon wafers. Solar Energy, 2004, 76(1–3): 255–262

20 Macdonald D, Tan J, Trupke T. Imaging interstitial iron concentrations in boron-doped crystalline silicon using photoluminescence. Journal of Applied Physics, 2008, 103(7): 073710

21 Birkholz J E, Bothe K, Macdonald D, Schmidt J. Electronic properties of iron-boron pairs in crystalline silicon by temperature- and injection-level-dependent lifetime measurements. Journal of Applied Physics, 2005, 97(10): 103708

22 Macdonald D, Geerligs L J, Azzizi A. Iron detection in crystalline silicon by carrier lifetime measurements for arbitrary injection and doping. Journal of Applied Physics, 2004, 95(3): 1021–1028

23 Trupke T, Bardos R, Schubert M, Warta W. Photoluminescence imaging of silicon wafers. Applied Physics Letters, 2006, 89(4): 044107

24 Payne D, Wenham S. Evaluating the accuracy of point spread function deconvolutions applied to luminescence images. In: Proceedings of the 43rd IEEE Photovoltaic Specialists Conference (PVSC). Portland, USA, 2016

25 Teal A, Juhl M. Correcting the inherent distortion in luminescence images of silicon solar cells. In: Proceedings of the 42nd IEEE Photovoltaic Specialists Conference (PVSC). New Orleans, USA, 2015

26 Liu A, Walter D, Phang S P, Macdonald d. Investigating internal gettering of iron at grain boundaries in multicrystalline silicon via photoluminescence imaging. IEEE Journal of Photovoltaics, 2012, 2(4): 479–484

27 Hallam B, Tjahjono B, Trupke T, Wenham S. Photoluminescence imaging for determining the spatially resolved implied open circuit voltage of silicon solar cells. Journal of Applied Physics, 2014, 115(4): 044901

28 Reiss J H, King R R, Mitchell K W. Characterization of diffusion length degradation in Czochralski silicon solar cells. Applied Physics Letters, 1996, 68(23): 3302–3304

29 Macdonald D, Cuevas A, Kinomura A, Nakano Y, Geerligs L J. Transition-metal profiles in a multicrystalline silicon ingot. Journal of Applied Physics, 2005, 97(3): 033523

30 Seibt M, Kveder V. Gettering processes and the role of extended defects. In Advanced Silicon Materials for Photovoltaic Applications, 2012: 127–188

31 Nouri H, Bouaïcha M, Ben Rabha M, Bessaïs B. Gettering effect in grain boundaries of multi-crystalline silicon. Physica Status Solidi. C, Current Topics in Solid State Physics, 2012, 9(10–11): 1937–1941

32 Phang S P, Macdonald d. Direct comparison of boron, phosphorus, and aluminum gettering of iron in crystalline silicon. Journal of Applied Physics, 2011, 109(7): 073521

33 Abbott M, Poplavskyy D, Scardera G, Inns D, Lemmi F, McIntosh K, Baker-Finch C S. Iron contamination in silicon solar cell production environments. In: Proceedings of the 40th IEEE Photovoltaic Specialists Conference (PVSC). Denver, USA, 2014