In this study, Light and elevated Temperature Induced Degradation (LeTID) behaviors on Czochralski (Cz) p-type mono-crystalline silicon wafers were studied. For commercial solar cells on mono-crystalline p-type Si, Al2O3 films are usually used for passivation. Some study showed that forming gas (FGA) (N2 95% + 5 % H2) annealing at about 400~500oC is beneficial for the passivation ability. However, the typical high-temperature (above 800oC) co-firing cell process for metallization would lead to degradation. In this study, symmetrical carrier lifetime samples with thermally deposited Al2O3, and Al2O3 capped with 75 nm SiNx:H deposited by PECVD were prepared and annealed in FGA at 425 oC and 550 oC and then they underwent rapid thermal annealing (RTA) at different temperatures like 800 oC, 830 oC and 850 oC. The results turn out that for single layers both annealing temperatures boost lifetime, however, for the stack layers FGA at 425 oC improves lifetimes while FGA at 550 oC leads to degradation of lifetime. This is attributed to the dehydrogenation of the surface since high temperature will cause breaking of hydrogen bonds. After RTA lifetime studies show that the lifetime of sample passivated with Al2O3/SiNx:H stack is better than the lifetime of Al2O3 single layer after RTA since the high hydrogen content of SiNx: H cause more hydrogenation of Si/Al2O3 interface states unlike the low hydrogen content in the single layer. Although graphene oxide (GO) was found to provide surface passivation in our p-type Cz silicon wafers, FGA and RTA were not performed for this sample as they reduce its field effect passivation and it can best be used directly to boost the performance of commercial type solar cells after these annealing steps are already performed.
LeTID studies were done using an incandescent lamp set at 1000 W/m2 and a heater at 80 oC on samples described above after the RTA step. Lifetimes measured in generalized mode using the WCT 120 were used to observe both the degradation and recovery of the LeTID defects. Results show the same rate of degradation for both stack layers annealed with different FGA temperature and an improved lifetime of samples that received FGA treatments at 425 oC as compared to the samples with no forming gas annealing during the light soaking at high temperature. Samples with Al2O3/SiNx:H stacks shows a slow and long degradation while Al2O3 single layers shows a faster degradation and recovery, this is attributed to the high content of the hydrogen in the stacks where the more the hydrogen the greater the extent of LeTID. Likewise, we have found that the extent of LeTID is correlated with the increase in firing peak temperature since the degradation and recovery of lifetime is clearly pronounced when samples are fired at 830 oC and 850 oC as compared to firing at 800 oC.

Publication …………………………………………………………………… ii
Abstract …………………………………………………………………………iii
Acknowledgements ………………………………………………………………v
Contents …………………………………………………………………………vi
List of Figures ……………………………………………………………… viii
List of Tables …………………………………………………………………xi
Chapter 1 …………………………………………………………………………1
Introduction …………………………………………………………………… 1
1.1 Preface ……………………………………………………………… 1
1.2 Motivation …………………………………………………………… 5
1.3 Thesis Organization …………………………………………………6
Chapter 2: Introduction to LeTID …… ……………………………………7
2.1 What is LeTID …………………………………………………………7
2.2 Recovery of LeTID defects …………………………………………9
2.3 The Physics of LeTID ……………………………………………… 11
2.4 The role of passivation layers in LeTID ………………………13
Chapter 3: Methodology ……………………………………………………… 14
3.1 Fabrication of passivation films ……………………………… 14
3.1.1 Aluminum oxide (Al2O3) and Al2O3/SiNx stacks ……………… 14
3.1.2 Graphene oxide (GO)………………………………………………… 18
3.2 The WCT-120 lifetime tester ………………………………………20
3.2.1 Impact of Forming gas annealing on lifetime in silicon … 23
3.2.2 Rapid thermal annealing of silicon samples ………………… 25
3.3 Experimental setup for LeTID …………………………………… 27
Chapter 4: Lifetime before light soaking……………………………… 29
4.1 Surface passivation: Al2O3 and Al2O3/SiNx: H stacks……… 29
4.2 Thermal stability of the passivation layers………………… 33
4.3 surface Passivation: Graphene oxide (GO) ……………………35
Chapter 5: Lifetime after light soaking………………………………… 37
5.1 LeTID in Al2O3/SiNx: H stacks ………………………………… 37
5.2 LeTID in Al2O3, Al2O3/SiNx: H stacks and GO samples …… 39
5.3 The influence of firing temperature on LeTID ………………42
Conclusion ……………………………………………………………………… 45
References........................................................46

[1] https://commons.wikimedia.org/wiki/File:PnJunction-PV-E.PNG

[2] Hermle, Martin, et al. "Passivating contacts and tandem concepts: Approaches for the highest silicon-based solar cell efficiencies." Appl. Phys. Rev.7.2 (2020): 021305. https://doi.org/10.1063/1.5139202

[3]https://www.ee.co.za/article/minor-improvements-to-silicon-crystalline-pv-cells-increase-efficiency.html

[4] F. Fertig et al. “Light-Induced Degradation of Silicon Solar Cells with Aluminium oxide Passivated Rear Side.” Energ. Procedia, 5th Internl. Conference on Silicon Photovoltaics, SiliconPV 2015, 77 (August 1, 2015): 599–606. https://doi.org/10.1016/j.egypro.2015.07.086

[5] F. Fertig .et al. “Mass Production of P-Type Cz Silicon Solar Cells Approaching Average Stable Conversion Efficiencies of 22 %.” Energy Procedia, 7th International Conference on Silicon Photovoltaics, SiliconPV 3-5 April 2017, Freiburg, Germany, 124 (September 1, 2017): 338–4 . https://doi.org/10.1016/j.egypro.2017.09.308.
[6] https://www.qcells.com/en/main/about/why_qcells/technology/QANTUM-TECHNOLOGY.html

[7] B. Veith, et al. "Comparison of the thermal stability of single Al2O3 layers and Al2O3/SiNx stacks for the surface passivation of silicon." Energy Procedia 8 (2011): 307-312. https://doi.org/10.1016/j.egypro.2011.06.141

[8] Pan, Huang-Wei, et al. "Silicon nitride films fabricated by a plasma-enhanced chemical vapor deposition method for coatings of the laser interferometer gravitational wave detector." Physical Review D 97.2 (2018): 022004. doi.org/10.1103/PhysRevD.97.022004

[9] C. H. Lin et al. “Commercial Solar Cells With Graphene Oxide as the Passivation Film.” IEEE J. of Photovoltaics, Vol.11, No.4, pp.873-877, May 2021. 10.1109/JPHOTOV.2021.3075869

[10] K. Ramspeck et al. “Light induced degradation of rear passivated Mc-Si solar cells.” Proc. 27th Eur. Photovolt. Sol. Conf. Exhib. Frankfurt, Ger, 2012, pp. 861–865.

[11] D. Ballutaud et al. “Study of the composition of hydrogenated silicon nitride SiNx:H for efficient surface and bulk passivation of silicon.” Sol. Energ. Mat. Sol. Cells 93 (2009) 1281–1289 https://doi.org/10.1016/j.solmat.2009.01.023

[12] F. Kersten et al. “A new mc-Si degradation effect called LeTID.” 2015 IEEE 42nd Photovolt. Spec. Conf. PVSC 2015, 2015 https://doi:10.1109/PVSC.2015.7355684.

[13] K. Hubener, F. Kersten, et al. Dependence of LeTID on brick height for different wafer suppliers with several resistivities and dopants, 9th Int. Work Cryst Silicon Sol Cells. 2016.

[14] F. Kersten et al. “Degradation of multicrystalline silicon solar cells and modules after illumination at elevated temperature.” Sol. Energ. Mat. Sol. Cells 142 (2015) 83-86. https://doi.org/10.1016/j.solmat.2015.06.015

[15] C. Sen, C. Chan, Hamer P, et al. “Annealing prior to contact firing: a potential new approach to suppress LeTID,” Sol. Energ. Mat. Sol. Cells 200 (2019) 109938. https://doi.org/10.1016/j.solmat.2019.109938

[16] D. Chen et al. “Evidence of an identical firing-activated carrier-induced defect in monocrystalline and multicrystalline silicon.” Sol. Energ. Mat. Sol. Cells 172 (2017) 293-300. https://doi.org/10.1016/j.solmat.2017.08.003

[17] K. Ramspeck et al. “Light induced degradation of rear passivated mc-Si solar cells,” in: Proc. 27th Eur. Photovolt. Sol. Energy Conf., 2012: pp. 861–865. https://doi.org/10.4229/ 27THEUPVSEC2012-2DO.3.4

[18] D. Chen et al. "Progress in the understanding of light‐and elevated temperature‐induced degradation in silicon solar cells: A review." Prog. Photovolt. Res. Appl. (2020) 1-22. https://doi.org/10.1002/pip.3362

[19] D. Chen et al. “Hydrogen-induced degradation: Explaining the mechanism behind light-and elevated temperature-induced degradation in n-and p-type silicon.” Sol. Energ. Mat. Sol. Cells 207 (2020) 110353. https://doi.org/10.1016/j.solmat.2019.110353

[20] J. Schmidt et al. “Effective surface passivation of crystalline silicon using ultrathin Al2O3 films and Al2O3/SiNx stacks.” Phys. Status Solidi Rapid Res. Lett. 3(9) (2009) 287-289. https://doi.org/10.1002/pssr.200903272

[21] D. Bredemeier et al. “Lifetime degradation and regeneration in multicrystalline silicon under illumination at elevated temperature.” Aip Advances 6(3) (2016) 035119. https://doi.org/10.1063/1.4944839

[22] K. Nakayashiki et al. “Engineering solutions and root-cause analysis for light-induced degradation in p-type multicrystalline silicon PERC modules.” IEEE j. Photovolt. 6(4) (2016) 860-868. 10.1109/JPHOTOV.2016.2556981

[23] W. Kwapil et al. “Kinetics of carrier-induced degradation at elevated temperature in multicrystalline silicon solar cells.” Sol. Energ. Mat. Sol. Cells 173 (2017) 80-84. https://doi.org/10.1016/j.solmat.2017.05.066

[24] R. Sharma et al. “Role of post-metallization anneal sequence and forming gas anneal to mitigate light and elevated temperature induced degradation of multicrystalline silicon solar cells.” Sol. Energ. Mat. Sol. Cells 195 (2019) 160-167. https://doi.org/10.1016/j.solmat.2019.02.036

[25] B. Sopori, et al. “Mechanism (s) of hydrogen diffusion in silicon solar cells during forming gas anneal.” Renew. Energ. 25–30 September (1997). 10.1109/PVSC.1997.653916

[26] C. Boehme et al. “Dissociation reactions of hydrogen in remote plasma-enhanced chemical-vapor-deposition silicon nitride.” Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 19(5) (2001) 2622-2628.https://doi.org/10.1116/1.1398538

[27] U. Varshneyet al. “Evaluating the impact of SiNx thickness on lifetime degradation in silicon.” IEEE J. Photovolt. 1–7 (2019) 2896671. 10.1109/JPHOTOV.2019.2945199

[28] C. Vargas et al. “Carrier induced degradation in multicrystalline silicon: dependence on the silicon nitride passivation layer and hydrogen released during firing.” IEEE J. Photovolt. 8 (2018) 413–420 https://doi:10.1109/JPHOTOV.2017.2783851.

[29] C. Boehme et al. “H loss mechanism during anneal of silicon nitride: chemical dissociation.” J. Appl. Phys. 88 (2000) 6055–6059 https://doi:10.1063/1.
1321730. https://doi.org/10.1063/1.3525573

[30] B. Hallam et al. “The role of hydrogenation and gettering in enhancing the efficiency of next-generation Si solar cells: an industrial perspective.” Phys. StatusA Solidi Appl. Mat. Sci. 214 (2017) https://doi:10.1002/pssa.201700305.

[31] D. Chenet al. “<700 mV open-circuit voltages on defect-engineered P-type silicon heterojunction solar cells on Czochralski and multicrystalline Wafers.” 2018 IEEE 7th World Conf. Photov. Energy conversion, WCPEC 2018 - A Jt. Conf. 45th IEEE PVSC, 28th PVSEC 34th EU PVSEC, 2018, pp. 1677–1681
https://doi:10.1109/PVSC.2018.8548239.

[32] P. Zheng et al. “21.63% industrial screen-printed multicrystalline Si solar cell” Phys. Status Solidi Rapid Res. Lett. 11 (2017) 1–4 https://doi:10.1002/ pssr.201600453.

[33] C. Sen et al. “Assessing the impact of thermal profiles on the elimination of light and elevated-temperature-induced degradation.” IEEE J. Photovolt. 9 (2018) 40–48. 10.1109/JPHOTOV.2018.2874769

[34] M. Stutzmann et al. “States of hydrogen in crystalline Silicon.” Phys. B Phys. Condens.” Matter. 170 (1991) 240–244 https://doi:10.1016/ 0921-4526(91)90130-7.

[35] C.E. Chan et al. “Rapid stabilization of high-performance multicrystalline P-type silicon PERC cells.” IEEE J. Photovolt. 6 (2016) 1473–1479 https://doi:10.1109/JPHOTOV.2016.2606704.

[36] J. Hong et al. “Influence of the high-temperature ‘firing’ step on high-rate plasma
deposited silicon nitride films used as bulk passivating antireflection coatings on
silicon solar cells.” J. Vac. Sci. Technol. B Microelectron. Nanom. Struct. 21 (2003)
2123 https://doi:10.1116/1.1609481.

[37] F. Kersten et al. “Influence of Al2O3 and SiNx passivation layers on LeTID.” 32 nd Eur. Photovolt. Sol. Energy Conf. Exhib. vol. 92 (2016) pp. 507–509
https://doi:10.4229/EUPVSEC20162016-2DO.2.6.

[38] F. Fertig et al. “Light-induced degradation of PECVD aluminium oxide
passivated silicon solar cells.” Phys. Status Solidi Rapid Res. Lett. 9 (2015) 41–46
https://doi:10.1002/pssr.201409424.

[39] https://www.samco.co.jp/en/products/uploads/Schematic-of-ALD-process-flow.jpg

[40] P.O. Oviroh et al. “New development of atomic layer deposition: processes, methods and applications.” Sci. technol. Adv. Mat. 20(1) (2018) 465-496. https://doi.org/10.1080/14686996.2019.1599694

[41] H. Sio et al. “Light and elevated temperature induced degradation in p-type and n-type cast-grown multicrystalline and mono-like silicon.” Sol. Energ. Mat. Sol. Cells, 182 (2018) 98-104. https://doi.org/10.1016/j.solmat.2018.03.002

[42] H. Kharkwal et al. “Fabrication and Characterization of Layered Graphene Oxide Biocompatible Nano-Film by Various Methods,” International Journal of Biochemistry and Biophysics, vol. 6, no.1, pp. 1-19.

[43] Wu, Xuelian, et al. "Preparation of Bi-based photocatalysts in the form of powdered particles and thin films: a review." Journal of Materials Chemistry A 8.31 (2020): 15302-15318.
[44] C.H. Lin et al. “Metal-insulator-semiconductor photodetectors with different coverage ratios of graphene oxide,” IEEE J. Sel. Top. Quantum Electron, vol. 20, no. 1, pp.25-29, January 2014. 10.1109/JSTQE.2013.2265218

[45] https://www.pveducation.org/pvcdrom/pn-junctions/lifetime
[46] B. Liu et al. “Improved evaluation of saturation currents and bulk lifetime in industrial Si solar cells by the quasi steady state photo-conductance decay method.” Sol. Energ. Mat. Sol. Cells 149 (2016) 258-265. https://doi.org/10.1016/j.solmat.2016.01.032

[47] WCT-120 Photoconductance Lifetime Tester and optional Suns-Voc User Manual

[48] J. Ha rko nen, V-P. Lempinen, T. Juvonen, J. Kylmaluoma, Sol Energ. Mat. Sol. Cells 73 (2002) 125-130. https://doi.org/10.1016/S0927-0248(01)00117-9

[49] S. Dhungel et al. “Effect of pressure on surface passivation of silicon solar cell by forming gas annealing.” Mat. Sci. in semiconductor proc. 7 (2004) 427-431. https://doi.org/10.1016/j.mssp.2004.09.101

[50] P. Sana et al. “Gettering and hydrogen passivation of edge‐defined film‐fed grown multicrystalline silicon solar cells by Al diffusion and forming gas anneal.” App. Phys. Lett. 64(1) (1994) 97-99. https://doi.org/10.1063/1.110880

[51] L. Ammor et al. “Influence of the Decoration by Dislocations on Grain Boundary Passivation by Hydrogen in Silicon.” MRS Online Proc. Lib. 106, 95 (1987). https://doi.org/10.1557/PROC-106-95
[52] C. Yang et al.. “Characteristic study of silicon nitride films deposited by LPCVD and PECVD.” Silicon, 10(6) (2018) 2561-2567. https://doi.org/10.1007/s12633-018-9791-6
[53] J. Benick et al. “Thermal stability of the Al2O3 passivation on p‐type silicon surfaces for solar cell applications. .” Phys. Status Solidi Rapid Res. Lett. 3(7‐8) (2009) 233-235. https://doi.org/10.1002/pssr.200903209

[54] https://www.beltfurnaces.com/silicon.html

[55]http://www.interpv.net/tech/tech_view.asp?idx=726&part_code=020080031&page=1


[56] T. Lüder et al. “Annealing behavior of Al2O3 thin films grown on crystalline silicon by atomic layer deposition.” In Proceedings of the 25th (2010). EU Photovolt. Sol. Energ. Conf. Valencia, Spain (pp. 213-2140).

[57] Hsu, Chia-Hsun, et al. "Enhanced Si passivation and PERC solar cell efficiency by atomic layer deposited aluminum oxide with two-step post annealing." Nan.res. lett. 14.1 (2019): 1-10. https://doi.org/10.1186/s11671-019-2969-z

[58] F. Fertig et al. “Mass production of p-type Cz silicon solar cells approaching average stable conversion efficiencies of 22%. “Energy Procedia 124 (2017) 338-345. https://doi.org/10.1016/j.egypro.2017.09.308

[59] B. Liao et al. “The effect of light soaking on crystalline silicon surface passivation by atomic layer deposited Al2O3.” J. Appl. Phys. 113(2) (2013) 024509. https://doi.org/10.1063/1.4775595

[60] B. Hoex et al. “Silicon surface passivation by atomic layer deposited Al2O3.” J. Appl. Phys. 104(4) (2008) 044903. https://doi.org/10.1063/1.2963707

[61] F. Jiang et al. “Hydrogenation of Si from SiN x (H) films: Characterization of H introduced into the Si.” Appl. Phys. Lett. 83(5) (2003). 931-933.

[62] B Liao et al. “The effect of light soaking on crystalline silicon surface passivation by atomic layer deposited Al2O3.” J. Appl. Phys., 113(2) (2013) 024509. https://doi.org/10.1063/1.4775595

[63] C. Vargas et al. “Carrier-induced degradation in multicrystalline silicon: Dependence on the silicon nitride passivation layer and hydrogen released during firing.” IEEE j. Photov. 8(2) (2018)413-420. 10.1109/JPHOTOV.2017.2783851

[64] D. Sperber et al. “On improved passivation stability on highly-doped crystalline silicon and the long-term stability of regenerated Cz-Si.” Sol. Energ. Mat. Sol. Cells 185 (2018) 277-282. https://doi.org/10.1016/j.solmat.2018.05.031