Electrical prosperties of reactive ion etached black silicon- Shaozhou Wang

Faculty of Engineering School of Photovoltaic and Renewable Energy Engineering Electrical Properties of Reactive Ion Etched Black Silicon Shaozhou Wang PhD candidate Research team Malcolm D. Abbott1,2, Bram Hoex1, David N. R. Payne1,3, Giuseppe Scardera1， Tsun H. Fung1, Muhammad Umair Khan1,Yu Zhang1, Fajun Ma1 1 University of New South Wales, Sydney, NSW, Australia 2 PV Lighthouse, Coledale, NSW, Australia 3 Macquarie University, Sydney, NSW, Australia 2 Many Different Textures Studied Studying both black silicon MCCE and RIE and conventional textures Surface features and optical properties thoroughly studied and compared 3 Holistic Approach Structural Optical Characterization Opto-electrical Simulations Process Integration Optimization Energy Yield Analysis Why study RIE b-Si Nano-texturing is industry trend. Near-zero-reflectance b-Si can potentially improve solar cell efficiency to a new level. Symmetric RIE structure is ideal for the fundamental study of near-zero reflectance b-Si Challenges with b-Si for solar cell integration inferior electrical properties Characteristic Electrical Properties of RIE b-Si Guide the improvement on industrial MACE b-Si Fung, TH et al., Solar Energy Materials and Solar Cells, 2019 submitted RIE b-Si and random pyramid samples Emitter optimization Surface passivation Supported experiments Importance of field- effect passivation Simulation Minority carrier distribution in nano-texturing Collection efficiency Characterization SEM-DCI to determine junction depth 4 Samples for Electrical Property Study Extreme RIE b-Si area factor 4.2 was compared by random pyramid area factor 1.4 6 different diffusion recipes different oxidation conditions were used Surface passivated by ALD SiO2/Al2O3 stack and enhanced by corona charge deposition Characterization collection efficiency QE and J0e Fung, TH et al., Solar Energy Materials and Solar Cells, 2019 submitted 5 Fixed charges Qf repel surface minority carriers 6 Seff effective surface recombination velocity Surface Passivation Mechanisms -10 -5 0 5 10 10 0 10 1 10 2 10 3 10 4 1/ Q f 2 No rmal ize d S e ff Q f 10 12 cm -2 1/ Q f 2 Reduction of recombination centres Dit 10 8 10 9 10 10 10 11 10 12 10 13 10 14 10 15 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 No pa s s i v ati on No rmal ize d S e ff D it eV -1 cm -2 Be s t p oss i bl e Seff scales with 1/Qf2 and is proportional with Dit, i.e. Qfis a more powerful “knob” to turn Surface Passivation Improvement by Increased Charge Passivation by ALD SiO2/Al2O3 stack Strong field-effect passivation can improve J0e effectively Fung, TH et al., Solar Energy Materials and Solar Cells, 2019 submitted 7 Challenges from Enhanced Surface Area Higher J0e We got the lowest J0e among the reported results in the literature Our low J0e can be attributed to improved profile and excellent surface passivation However, J0e of b-Si is still inferior to random pyramids and polished surface, i.e. there still room for optimisation Fung, TH et al., Solar Energy Materials and Solar Cells, 2019 submitted 8 Why Field-effect Passivation is Significant 1E 14 1E 15 1E 16 1E 17 1E -04 1E -03 1E -02 Al2O3 o n Plan ar Al2O3 o n 2 min RIE b- Si Al2O3 o n 6 min RIE b- Si Mino rity Ca rr ier Life time s Mino rity Ca rr ier Density cm -3 1E 14 1E 15 1E 16 1E 17 1E -04 1E -03 1E -02 HfO2 on Plana r HfO2 on 2 min RIE b-Si HfO2 on 6 min RIE b-Si Mino rity Ca rr ier Life time s Mino rity Ca rr ier Density cm -3 6 min RIE2 min RIE 500 nm Area factors are 4.32 and 4.16 determined by AFM Strong field-effect passivation can passivate the large surface area effectivelyALD films with different interfacial properties 9 3.5 0E 01 1 7.0 0E 01 1 1.0 5E 01 2 1.4 0E 01 2 0.0 0E 00 0 5.0 0E 01 0 1.0 0E 01 1 1.5 0E 01 1 2.0 0E 01 1 Al2O3 o n HfO2 on Midg ap Dit e V -1 cm -2 Nega tiv e Q f / q cm -2 Characteristic Carrier Distribution in Nano-texturing Valley Peak Peak Valley Simulated by Sentaurus TCAD Undiffused RIE b-Si with area factor 5.1 Low Sn0 and Sp0 Doping Injection 1e15 cm-3 10 200 nm 500 nm 2D/3D nano-scale simulations to verify hypothesises of field-effect passivation on b-Si Dopant Profile for Black Silicon Advanced doping characterization method Scanning Electron Microscope Dopant Contrast Image SEM-DCI Valley of the feature has the same junction depth as RP Peak of the feature has a much deeper junction depth SEM-DCI images of random pyramid and RIE b-Si with the same diffusion recipe Fung, TH et al., Solar Energy Materials and Solar Cells, 2019 submitted 11 Challenges from Enhanced Surface Area Lower QE Advanced QE measurement photoluminescence- based spectro-response PL-SR method Our optimized diffusion recipe can improve QE of b- Si, but it is still much lower than the referenced random pyramid at short wavelength Paduthol, A et al., IEEE J. Photovoltaics, 2018 Fung, TH et al., Solar Energy Materials and Solar Cells, 2019 submitted 12 Increasing the oxygen concentration can improve the diffusion for b-Si, but the J0e and QE are still not comparable to conventional texturing wafers b-Si J0e improved via emitter optimization and enhanced charge For non-diffused b-Si, the film with strong Qf are necessary as Seff has a quadratic dependence on Qf 2D/3D nano-scale simulations used to monitor impact of charge on minority carrier distributions Contactless QE technique used to characterise collection efficiency of extreme b-Si Dopant contrast imaging used to characterize doping in extreme b-Si features Conclusion 13 Questions Acknowledgement Shaozhou Wang shaozhou.wangunsw.edu.au 14