Department of Materials Department of Materials Semiconductor and Silicon Photovoltaics Group Semiconductor and Silicon Photovoltaics Group Extrinsic surface passivation of silicon solar cells Ruy Sebastian Bonilla
Department of Materials Semiconductor and Silicon Photovoltaics Group Content • Surface recombination basics 1. Key aspects on the dielectric-silicon interface 2. Consistent surface recombination metrics 3. Intrinsic vs Extrinsic surface passivation 4. Potential of charge-assisted (field-effect) passivation 5. Damage free plasma hydrogenation Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 2
Department of Materials Semiconductor and Silicon Photovoltaics Group Surface recombination in silicon - + - + Shockley–Read–Hall recombination rate: * 𝑜𝑞 − 𝑜 () 𝑆 " = ∝ 𝑂 / × 𝑛𝑗𝑜𝑝𝑠𝑗𝑢𝑧 𝜏 - 𝑂 / 𝑤 /1 + 𝑜 + 𝑜 " 𝑞 + 𝑞 " 𝜏 2 𝑂 / 𝑤 /1 Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 3
Department of Materials Semiconductor and Silicon Photovoltaics Group The silicon-dielectric interface • So far the two typical key elements to recombination at the silicon surface are: – The concentration of trap states (CHEMISTRY) – The concentration of carriers: n s , p s (CHARGE) • Two other key aspects: – Nature of interface states – Their ability to capture carrier ( 𝜏 n,p ) Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 4
Department of Materials Semiconductor and Silicon Photovoltaics Group The silicon-dielectric interface 𝑆 ∝ 𝑂 / ×𝑞 ; R. S. Bonilla and P. R. Wilshaw J. Appl. Phys. 121, 135301 (2017) Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 5
Department of Materials Semiconductor and Silicon Photovoltaics Group The silicon-dielectric interface • The ability of states to capture carriers At the surface Shockley–Read–Hall Surface Recombination Velocity recombination rate: * 𝑜𝑞 − 𝑜 () * 𝑇 )?? = 1 𝑜 ; 𝑞 ; − 𝑜 () 𝑆 " = Δ𝑜 𝜏 - 𝑂 / 𝑤 /1 + 𝑜 + 𝑜 " 𝑞 + 𝑞 " 𝑞 ; + 𝑞 " + 𝑜 ; + 𝑜 " 𝜏 2 𝑂 / 𝑤 /1 𝑇 -= 𝑇 2= 𝑇 -= = 𝐸 (/ 𝜏 - 𝑤 /1 𝑇 2= = 𝐸 (/ 𝜏 2 𝑤 /1 Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 6
Department of Materials Semiconductor and Silicon Photovoltaics Group Surface recombination metrics * 𝑟𝑜 () * 𝑇 )?? = 1 𝑜 ; 𝑞 ; − 𝑜 () 𝐾 =,;DE? = Δ𝑜 𝑞 ; + 𝑞 " + 𝑜 ; + 𝑜 " 𝑞 ; + 𝑜 ( + 𝑜 ; + 𝑜 ( 𝑇 -= 𝑇 2= 𝑇 -= 𝑇 2= K. R. McIntosh and L. E. Black J. Appl. Phys. 116, 014503 (2014) Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 7
Department of Materials Semiconductor and Silicon Photovoltaics Group Intrinsic vs Extrinsic passivation + Single interface mid-gap defect: Δ𝑜 = 10 "L cm OP * 𝑇 )?? = 1 𝑜 ; 𝑞 ; − 𝑜 () Δ𝑜 𝑅 (/ = 10 "" 𝑓/𝑑𝑛 * 𝑞 ; + 𝑜 ( + 𝑜 ; + 𝑜 ( 𝑇 -= 𝑇 2= Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 8
Department of Materials Semiconductor and Silicon Photovoltaics Group Surface Passivation • CHARGE is essential to obtain good surface passivation! • Intrinsic passivation 𝑺 𝒕 ∝ 𝑬 𝒋𝒖 ×𝝉 𝒒 ×𝒒 𝒕 𝑺 𝒕 ∝ 𝑬 𝒋𝒖 ×𝒒 𝒕 – That due to the dielectric film in the as deposited state – Chemical and FEP (Charge-assisted) • Chemistry and charge difficult to optimise – Limited by the deposition process • If charge deposited after the film deposition ( extrinsic ) it can be optimised independent of chemistry Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 9
Department of Materials Semiconductor and Silicon Photovoltaics Group Potential of Extrinsic Passivation • Externally added charge to the dielectric after deposition –e.g. Corona charge: Sinton transient PCD 1 1 + 1 1 + 2𝑇 R.S. Bonilla et al. Applied = + 𝜐 )?? 𝜐 YZ[ 𝜐 \D 𝜐 ]Y^ 𝑋 1.42 cm/s @ ∆n=10 15 cm -3 Surface Science 412 (2017) 657–667 Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 10
Department of Materials Semiconductor and Silicon Photovoltaics Group Potential of Extrinsic Passivation SiN x a-TiO x SiN x SiO 2 SiO 2 SiO 2 SiO 2 a-Si n-Si 1Ωcm n-Si 1Ωcm n-Si 1Ωcm n-Si 1Ωcm 1.2 cm/s S eff = 0.15 cm/s 0.01 cm/s 1.42 cm/s 4%H anneal: 0.6 cm/s H passivation: 0.17 cm/s Δn=10 15 cm -3 Bonilla et. al. Phys. Status Solidi A 214, No. 7, 1700293 (2017) Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 11
Department of Materials Semiconductor and Silicon Photovoltaics Group Potential of Extrinsic Passivation a-Si/SiOx/SiNx SiN x SiO 2 a-Si C, G 20Hz-2MHz n-Si 1Ωcm 𝑺 𝒕 ∝ 𝑬 𝒋𝒖 ×𝝉 𝒒 ×𝒒 𝒕 R. S. Bonilla et al. Phys. Status Solidi RRL 11, No. 1 (2017) Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 12
Department of Materials Semiconductor and Silicon Photovoltaics Group State-of-the-art A. Cuevas et al. IEEE PVSC 2015, 6pp Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 13
Department of Materials Semiconductor and Silicon Photovoltaics Group State-of-the-art Proof of concept: Corona Discharge R.S. Bonilla et al. / Applied Surface Science 412 (2017) 657–667 Bonilla et. al. Phys. Status Solidi A 214, No. 7, 1700293 (2017) Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 14
Department of Materials Semiconductor and Silicon Photovoltaics Group Extrinsic Field Effect Passivation • Charge added to the dielectric after deposition greatly improves passivation. • It allows optimisation of FEP independently of chemical passivation • How important is it for a solar cell? Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 15
Department of Materials Semiconductor and Silicon Photovoltaics Group Field Effect Passivation in cell performance (Quokka) R.S. Bonilla et al. / Applied Surface Science 412 (2017) 657–667 Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 16
Department of Materials Semiconductor and Silicon Photovoltaics Group But… Is this Charge Stable? • Corona discharge ... Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 17
Department of Materials Semiconductor and Silicon Photovoltaics Group Ionic field effect passivation • Charge is introduced into dielectric films at high temperature and then permanently quenched in place by cooling to room temperature Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 18
Department of Materials Semiconductor and Silicon Photovoltaics Group Ionic field effect passivation • Diffusion of Potassium ions into SiO2 Bonilla et al. Solid State Phenomena Vol. 242 (2016) pp 67-72 Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 19
Department of Materials Semiconductor and Silicon Photovoltaics Group Diffusion + Drift of Potassium in SiO 2 Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 20
Department of Materials Semiconductor and Silicon Photovoltaics Group Long term stability of ion-charged SiO 2 Diffusion Diffusion + Drift Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 21
Department of Materials Semiconductor and Silicon Photovoltaics Group Long term stability of ion-charged SiO 2 Diffusion + Drift Direct measurement of charge concentration using kelvin probe and capacitance-voltage Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 22
Department of Materials Semiconductor and Silicon Photovoltaics Group Long term stability of ion-charged SiO 2 Exceeding 10k days ~ 30 Years Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 23
Department of Materials Semiconductor and Silicon Photovoltaics Group Towards industrially compatible extrinsic passivation (fast and cost-effective) • Field effect – Stabilise charge using ions: lab conditions >4 years, likely indefinite . But, as yet, slightly worse passivation – Working conditions stability: to be tested – Compatibility of process: K ions, others possible – Industrial deposition technique for ions – Process temperature: 450-550 C – Speed of process: currently 1-2 mins, but possible in seconds Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 24
Department of Materials Semiconductor and Silicon Photovoltaics Group Extrinsic Hydrogen Passivation • Hydrogen is effective at passivating defects and dangling bonds at the surface or in the bulk of silicon wafers • Industrially – dielectrics + firing • Research – Forming Gas anneals, Remote Hydrogen Plasma Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 25
Department of Materials Semiconductor and Silicon Photovoltaics Group Shielded Hydrogen Passivation • Uses a plasma source of atomic hydrogen • A thin palladium “shield” is inserted between the plasma and the sample • Protects against UV, high energy particles • Damage free plasma hydrogenation P Hamer, et al. Phys. Status Solidi RRL 11, 2017 Extrinsic surface passivation sebastian.bonilla@materials.ox.ac.uk 26
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