Thin film dielectric stacks for defect passivation in silicon solar cells-Utkarshaa Varshney

Faculty of Engineering School of Photovoltaic and Renewable Energy Engineering Thin film dielectric stacks for defect passivation in silicon solar cells 15th China SoG Silicon and PV Power Conference Shanghai, China- Nov 21th 2019 Utkarshaa Varshney Malcolm Abbott, Alison Ciesla, Phillip Hamer, Daniel Chen, Chandany Sen, Shaoyang Liu, Aref Samadi, Moonyong Kim, Catherine Chan, Bram Hoex 2 • Light and elevated temperature induced degradation (LeTID) • Influence of SiNx:H thickness on LeTID • Controlling LeTID with thin film barrier layers • Implications on industrial PERC solar cells • Conclusions Outline • LeTID was first reported in 2012 in mc-Si PERC cells [1]. • Exists also in Cz-Si, FZ-Si and n-type silicon wafers. • It can result in up to 16%rel. power loss. • Big focus on understanding its behavior and providing mitigation strategies. Light- and elevated Temperature- Induced Degradation 3[1] K. Ramspeck et al., 27th Eur. Photovolt. Sol. Energy Conf. Exhib., vol. 1, pp. 861–865, 2012 Pander et al., 36th EUPVSEC, 2019. Chen et al., Sol. Energy Mater. Sol. Cells, vol. 172, pp. 293–300, 2017. • Root Cause-Still unknown! • Depends on processing parameters. • A growing consensus on the involvement of hydrogen! • Surface dielectric layers are the predominant hydrogen source. What causes LeTID? 4 Chan et al., IEEE J. Photovoltaics, vol. 6, no. 6, pp. 1473–1479, 2016 Jensen et al., J. Appl. Phys., vol. 124, no. 8, p. 085701, 2018 5 • Light and elevated temperature induced degradation (LeTID) • Influence of SiNx:H thickness on LeTID • Controlling LeTID with thin film barrier layers • Implications on industrial PERC solar cells • Conclusions Outline Process Flow 6 FZ-Si (4 Ω·cm, 280 m) (A) (B) (D)(C) SiNx SiNx SiNx Ellipsometry & FTIR p-type mc-Si (1.7 Ω·cm, 190 m) POCl3 diffusion: n+ emitter (~ 60 Ω/sq) SiNx Fast firing – 770 ±2 °C (B) (D)(C) Dark and illuminated annealing FTIR (Note: )SiNx ~ 50.2 nm (A) SiNx SiNx SiNxSiNx SiNx SiNxSiNx SiNxSiNxSiNx SiNxSiNxSiNxSiNx SiNx SiNx Hydrogen content in SiNx:H 7 • Fourier transform infrared spectroscopy (FTIR) was used to characterize SiNx:H films. • Infrared absorbance corresponding H-bonds increase with SiNx:H thickness. • The hydrogen content in SiNx:H increases with thickness. [3] Verlaan et al., Phys. Rev. B, vol. 73, no. 19, 2006 Evolution of LeTID during illuminated annealing 8Normalized Defect Density (NDD) implies degradation p-type mc-Si SiNx n+ p-type mc-Si p-type mc-Si p-type mc-Si Evolution of LeTID during dark annealing 9Normalized Defect Density (NDD) implies degradation p-type mc-Si SiNx n+ p-type mc-Si p-type mc-Si p-type mc-Si Importance of SiNx:H thickness on LeTID 10 • Extent of LeTID directly proportional with SiNx:H thickness. • The trend is similar while illuminated annealing and dark annealing, with different offsets. • Optimum SiNx:H thickness will be a trade-off between passivation quality and degradation. Varshney et al., “Evaluating the Impact of SiNx Thickness on Lifetime Degradation in Silicon,” IEEE J. Photovoltaics, pp. 1–7, 2019 11 • Light and elevated temperature induced degradation (LeTID) • Influence of SiNx:H thickness on LeTID • Controlling LeTID with thin film barrier layers • Implications on industrial PERC solar cells • Conclusions Outline Experimental Methodology 12 SiNx:H only SiNx:H/AlOx AlOx/SiNx:H Varshney et al., “Controlling Light- and Elevated-Temperature-Induced Degradation With Thin Film Barrier Layers”, IEEE J. Photovoltaics, 2019 (in press) • Strong influence of surface layers on the extent of degradation. • AlOx/SiNx:H sample - Lowest NDD • SiNx:H/AlOx sample - Highest NDD Importance of surface passivation on extent of LeTID 13 p-type mc-Si SiNx:H SiNx:H p-type mc-Si SiNx:H SiNx:H ALD AlOx ALD AlOx p-type mc-Si SiNx:H ALD AlOx ALD AlOx SiNx:H Varshney et al., “Controlling Light- and Elevated-Temperature-Induced Degradation With Thin Film Barrier Layers”, IEEE J. Photovoltaics, 2019 (in press) Hypothesis- ALD AlOx blocks hydrogen 14 p-type Si p-type Si AlOx p-type Si AlOx SiNx Hydrogen • High temperature firing • During ramp up Hydrogen diffuses into the bulk • During cooldown Hydrogen effuses in the ambient • Role of AlOx varies with the deposition technique: • ALD AlOx - dense and hydrogen lean: Less LeTID • PECVD AlOx - Pinholes and hydrogen rich: More LeTID Comparison between PECVD AlOx and ALD AlOx 15 ALDPECVD Varshney et al., “Controlling Light- and Elevated-Temperature-Induced Degradation With Thin Film Barrier Layers”, IEEE J. Photovoltaics, 2019 (in press) 16 • Light and elevated temperature induced degradation (LeTID) • Influence of SiNx:H thickness on LeTID • Controlling LeTID with thin film barrier layers • Implications on industrial PERC solar cells • Conclusions Outline 17 • PERC is highly susceptible to LeTID. • Additional rear surface layers can potentially explain higher degradation in PERC. • Replacing PECVD AlOx with ALD AlOx can possibly reduce LeTID. n+ Implications on industrial PERC- Design considerations SiNx SiNx AlOx PERC Conclusions • Results are consistent with the involvement of hydrogen in LeTID. • The extent of LeTID proportionally increases with SiNx:H thickness. • Degradation extent can be controlled by the placement of thin barrier layers. • ALD AlOx- acts as a hydrogen barrier • PECVD AlOx- acts as a hydrogen source 18 Acknowledgements And all the industry partners of UNSW Advanced Hydrogenation Consortium u.varshney@unsw.edu.auUtkarshaa Varshney • The variation in reflection is small and couldn’t explain the difference in the extent of degradation. • Cannot attribute the difference in degradation to optics! Understanding the trends 19