Comparison and characterization of different tunnel layers, suitable for passivated contact formation Gordon LING(1), Zheng XIN(1), Cangming KE(1), Kitz Jammaal BUATIS(1), Shubham DUTTAGUPTA(1), Jae Sung LEE(2), Archon LAI(2), Adam HSU(2), Johannes ROSTAN(2), Rolf STANGL(1) (1) Solar Energy Research Institute of Singapore (SERIS) (2) REC Solar Ltd. Pte. Singapore PVSEC-26 26 Oct 2016

Outline Introduction / motivation for passivated contacts Criteria for efficient hole-extracting passivated contacts Evaluation of tunnel oxide candidates Conclusion Future work Acknowledgements

Introduction Pushing the solar cell efficiencies limits Address key performance losses Continuous reduction of recombination losses predicted [1] Recombination losses Solar Cell % Optical losses Resistance losses [1] International Technology Roadmap for Photovoltaic 2016. Available at ITRPV.net

over un-passivated contact Motivation for passivated contacts Reducing contact recombination Metal Doped capping layer tunnel oxide layer n-Si wafer Metal n-Si wafer Jo,rear reduces significantly Tunnel layer used VOC (mV) JSC (mA/cm2) FF (%) Eff % improvement over un-passivated contact Published year Ref SiO2 718 42.1 83.2 25.1 N.A 2015 1 719 - 24.9 2 691 38.4 82.1 21.8 11.3 2014 3 AlOx 673 40.3 79.9 21.7 2.8 2011 4 683 39.6 78.1 21.2 2016 5 [1] S.W. Glunz et al., 31st EUPVSEC, 259-263 (2015). [2] A. Moldovan et al., Solar Energy Materials and Solar Cells, 142, 123-127 (2015). [3] F. Feldmann et al., Solar Energy Materials and Solar Cells, 120, Part A, 270-274 (2014) [4] D. Zielke et al., physica status solidi (RRL) – Rapid Research Letters, 5(8), 298-300 (2011) [5] Y. Tao et al., Progress in Photovoltaics: Research and Applications, 24(6), 830-835 (2016)

field-effect passivation Hole-extracting passivated contacts Key requirements for tunnel oxide layer Doped capping layer tunnel oxide layer n-Si wafer - + Tunneling relevant thickness (< 2 nm) Efficient hole extraction - High negative Qf Low interface Dit Efficient field-effect passivation Efficient chemical passivation Efficient hole extraction

Our work Evaluate tunnel oxide candidates for hole-extraction Asymmetrical lifetime samples were utilized Focus on front-side tunnel oxide candidates wet-SiOx (RCA oxide) Ozone-SiOx (UV photo-oxidation) ALD-AlOx (Spatial atomic layer deposition) Compare to a symmetrically passivated sample (Ref) n-Si wafer wet-SiOx SiNx n-Si wafer wet-SiOx SiNx ozone-SiOx n-Si wafer wet-SiOx SiNx ALD AlOx Si wafer resistivity : 2.7 Ω-cm Thickness : 140 µm n-Si wafer wet-SiOx SiNx

Our work Industrial, high-throughput ALD AlOx tunnel layer processing Uniform deposition rate: ~ 0.13 nm per ALD cycle 100 µs 80 µs 60 µs High-throughput industrial-scale thermal ALD reactor (Solaytec) 40 µs 20 µs 1.5 nm AlOx 0 µs

Significant boost in tunnel layer passivation quality with ALD AlOx Enhanced passivation with AlOx Effective carrier lifetime Comparable thickness for wet-SiOx, ozone-SiOx and AlOx (1.5 - 2.1 nm) τeff (AlOx) ≈ 100 τeff (wet-SiOx) τeff (AlOx) ≈ 100 τeff (ozone-SiOx) Significant boost in tunnel layer passivation quality with ALD AlOx n-Si wafer wet-SiOx SiNx n-Si wafer wet-SiOx SiNx ozone-SiOx n-Si wafer wet-SiOx SiNx ALD AlOx n-Si wafer wet-SiOx SiNx WHY ?  High negative interface charge

High negative fixed interface charge Qf Interface fixed charge density High Qf (-61012 cm-2), one order higher than wet-SiOx and ozone-SiOx Reduced surface recombination Efficient hole extraction AlOx n-Si + - 0.6V SiOx n-Si + - 0.07V + + [9] O.V. Aleksandrov et al., Semiconductors, 45(4), 467-473 (2011). [10] F. Gaspard et al., Electrical properties of thin anodic silicon dioxide layers grown in pure water. Vol. 22. 1987. 65-69. [11] S.C. Vitkavage et al., Journal of Applied Physics, 68(10), 5262-5272 (1990). [12] N.E. Grant et al., IEEE Electron Device Letters, 30(9), 922-924 (2009). [13] N.E. Grant et al., ECS Journal of Solid State Science and Technology, 3(2), P13-P16 (2014). [14] K. Imamura et al., Journal of Applied Physics, 107(5), 054503 (2010).

Comparable interface defect density Dit Comparable Dit for our wet-SiOx, ozone-SiOx and AlOx Dit lower than previously reported values for wet-SiOx / ozone-SiOx Thermal SiOx (>100 nm) Low Dit Too thick for contact passivation [11] S.C. Vitkavage et al., Journal of Applied Physics, 68(10), 5262-5272 (1990).   [15] H. Angermann, Applied Surface Science, 254(24), 8067-8074 (2008). [16] H. Angermann et al., Applied Surface Science, 258(21), 8387-8396 (2012). [17] H. Angermann, Applied Surface Science, 312, 3-16 (2014).

Comparison of Dit(E) distribution Dit,min for ALD AlOx is approximately at c-Si midgap energy Dit,min for wet-SiOx and ozone-SiOx are further off-midgap According to Angermann [16], this could be due to an increased micro surface roughness, leading to an increase in dangling bond defects Implication Need to improve the processing approach for the wet-SiOx and ozone-SiOx tunnel layers [16] H. Angermann et al., Applied Surface Science, 258(21), 8387-8396 (2012).

Deposition of capping layers First initial results Capping Layers Investigated Metal poly-Si(p) n-Si wafer Metal PEDOT:PSS tunnel oxide layer n-Si wafer Initial results PEDOT:PSS p-Si wafer Design Tunnel layer lifetime [us] (after capping) iVoc [mV] (after capping) Joe [fA/cm2] (after capping) I wet-SiOx 349.53 680 45 II ALD AlOx 151.7 655 133

Conclusion Efficient hole extracting passivated contacts should have: Ultra-thin tunnel relevant highly passivating oxide layer High negative fixed interface charge We can measure Qf and Dit(E) of ultra-thin tunnel layers AlOx seems to outperform wet-SiOx and ozone-SiOx Benefits from a high negative Qf (1 order higher) Efficient field-effect passivation (2 orders higher τeff) Efficient band bending (0.6V versus 0.07V) Organic capping layer significantly reduces Joe However: detailed investigations after various capping layer depositions are still pending!

Future work Acknowledgement Integration with different doped capping layers Integration at solar cell device level Acknowledgement This research work is supported by the EIRP-07 project “Passivated contacts for high-efficiency silicon wafer based solar cells” NRF2014EWT-EIRP001-006, in a joint collaboration between SERIS and REC Solar Pte. Ltd.

Thank you for your attention! More information www.seris.sg Dr Gordon LING gordon.ling@nus.edu.sg