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Department of MaterialsSemiconductor and Silicon Photovoltaics GroupDepartment of MaterialsSemiconductor and Silicon Photovoltaics Group
Extrinsic surface passivation of silicon solar cellsRuy Sebastian Bonilla
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Content• Surface recombination basics
1. Key aspects on the dielectric-silicon interface2. Consistent surface recombination metrics 3. Intrinsic vs Extrinsic surface passivation4. Potential of charge-assisted (field-effect)
passivation5. Damage free plasma hydrogenation
Extrinsic surface passivation [email protected] 2
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Surface recombination in silicon
Extrinsic surface passivation [email protected] 3
-+
-+
𝑅" = 𝑛𝑝 − 𝑛()*
𝑝 + 𝑝"𝜎-𝑁/𝑣/1
+ 𝑛 + 𝑛"𝜎2𝑁/𝑣/1
∝ 𝑁/× 𝑚𝑖𝑛𝑜𝑟𝑖𝑡𝑦
Shockley–Read–Hallrecombinationrate:
Department of MaterialsSemiconductor 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: ns, ps (CHARGE)
• Two other key aspects:– Nature of interface states– Their ability to capture carrier (𝜎n,p)
Extrinsic surface passivation [email protected] 4
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
The silicon-dielectric interface
Extrinsic surface passivation [email protected] 5
𝑅 ∝ 𝑁/×𝑝;R. S. Bonilla and P. R. Wilshaw J. Appl. Phys. 121, 135301 (2017)
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
The silicon-dielectric interface• The ability of states to capture carriers
Extrinsic surface passivation [email protected] 6
𝑅" = 𝑛𝑝 − 𝑛()*
𝑝 + 𝑝"𝜎-𝑁/𝑣/1
+ 𝑛 + 𝑛"𝜎2𝑁/𝑣/1
Shockley–Read–Hall recombination rate:
𝑆-= = 𝐷(/𝜎-𝑣/1𝑆2= = 𝐷(/𝜎2𝑣/1
𝑆)?? =1Δ𝑛
𝑛;𝑝; − 𝑛()*𝑝; + 𝑝"𝑆-=
+ 𝑛; + 𝑛"𝑆2=
At the surfaceSurface Recombination Velocity
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Surface recombination metrics
Extrinsic surface passivation [email protected] 7
𝐽=,;DE? =𝑞𝑛()*
𝑝; + 𝑝"𝑆-=
+ 𝑛; + 𝑛"𝑆2=
𝑆)?? =1Δ𝑛
𝑛;𝑝; − 𝑛()*𝑝; + 𝑛(𝑆-=
+ 𝑛; + 𝑛(𝑆2=
K. R. McIntosh and L. E. Black J. Appl. Phys. 116, 014503 (2014)
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Intrinsic vs Extrinsic passivation
Extrinsic surface passivation [email protected] 8
𝑄(/ = 10""𝑒/𝑐𝑚*
Single interface mid-gap defect:
𝑆)?? =1Δ𝑛
𝑛;𝑝; − 𝑛()*𝑝; + 𝑛(𝑆-=
+ 𝑛; + 𝑛(𝑆2=
Δ𝑛 = 10"LcmOP
+
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Surface Passivation
• CHARGE is essential to obtain good surface passivation!
• Intrinsic passivation– 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 [email protected] 9
𝑹𝒔 ∝ 𝑫𝒊𝒕×𝒑𝒔𝑹𝒔 ∝ 𝑫𝒊𝒕×𝝈𝒑×𝒑𝒔
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Potential of Extrinsic Passivation
• Externally added charge to the dielectric after deposition –e.g. Corona charge:
Extrinsic surface passivation [email protected] 10
1𝜏)??
=1
𝜏YZ[+
1𝜏\D
+1
𝜏]Y^+2𝑆𝑊 1.42 cm/s @ ∆n=1015 cm-3
SintontransientPCD
R.S. Bonilla et al. AppliedSurface Science 412 (2017) 657–667
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Potential of Extrinsic Passivation
Extrinsic surface passivation [email protected] 11
n-Si1Ωcm
1.42 cm/s
4%H anneal: 0.6 cm/s
H passivation: 0.17 cm/s
0.15 cm/s 0.01 cm/s
Bonilla et. al. Phys. Status Solidi A 214, No. 7, 1700293 (2017)
SiO2
n-Si1Ωcm
SiO2
SiNx
n-Si1Ωcm
SiO2
a-TiOx
n-Si1Ωcm
SiO2
a-Si
1.2 cm/s
SiNx
Δn=1015 cm-3
Seff=
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Potential of Extrinsic Passivation
Extrinsic surface passivation [email protected] 12
a-Si/SiOx/SiNx
R. S. Bonilla et al. Phys. Status Solidi RRL 11, No. 1 (2017)
𝑹𝒔 ∝ 𝑫𝒊𝒕×𝝈𝒑×𝒑𝒔
n-Si1Ωcm
SiO2a-Si
SiNx
C,G20Hz-2MHz
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
State-of-the-art
Extrinsic surface passivation [email protected] 13
A. Cuevas et al. IEEE PVSC 2015, 6pp
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
State-of-the-art
Extrinsic surface passivation [email protected] 14
Proof of concept:Corona Discharge
Bonilla et. al. Phys. Status Solidi A 214, No. 7, 1700293 (2017)
R.S. Bonilla et al. / Applied Surface Science 412 (2017) 657–667
Department of MaterialsSemiconductor 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 [email protected] 15
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Field Effect Passivation in cell performance (Quokka)
Extrinsic surface passivation [email protected] 16
R.S. Bonilla et al. / Applied Surface Science 412 (2017) 657–667
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
But… Is this Charge Stable?
• Corona discharge ...
Extrinsic surface passivation [email protected] 17
Department of MaterialsSemiconductor 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 [email protected] 18
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Ionic field effect passivation• Diffusion of Potassium ions into SiO2
Extrinsic surface passivation [email protected] 19
Bonilla et al. Solid State Phenomena Vol. 242 (2016) pp 67-72
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Diffusion + Drift of Potassium in SiO2
Extrinsic surface passivation [email protected] 20
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Long term stability of ion-charged SiO2
Extrinsic surface passivation [email protected] 21
Diffusion Diffusion + Drift
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Long term stability of ion-charged SiO2
Extrinsic surface passivation [email protected] 22
Diffusion + Drift
Direct measurement of charge concentration using kelvin probe and capacitance-voltage
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Long term stability of ion-charged SiO2
Extrinsic surface passivation [email protected] 23
Exceeding 10k days ~ 30 Years
Department of MaterialsSemiconductor 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 [email protected] 24
Department of MaterialsSemiconductor 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
[email protected] surface passivation
Department of MaterialsSemiconductor 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
Extrinsic surface passivation
P Hamer, et al. Phys. Status Solidi RRL 11, 2017
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
SHP - Results
eCV measurements of active boronconcentration vs depth after 20 minexposure at 200 oC.
Extrinsic surface passivation
P Hamer, et al. Phys. Status Solidi RRL 11, 2017
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Poisoning and Thicker Foils
eCV plots using leaves with andwithout “poisoning”.
eCV plots using a 100 nm thick“leaf” and a 10 µm thick Pd/Ag“foil”.
Extrinsic surface passivation
Bourret-Sicotte, et al. Phys. Status SolidiA 214, No. 7 (2017)
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Industrial Application
• Potentialforin-lineprocessing• Quick,damage-freehydrogen
exposure• Potentialfor:
• lowtemperatureprocessing,
• passivationwithoutfiringdielectrics,
• passivationofcarrierselectivecontacts.
Extrinsic surface passivation
Bourret-Sicotte, et al. Phys. Status SolidiA 214, No. 7 (2017)
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Summary• Understanding of dielectric surface passivation• Extrinsic FEP can be very effective. SRV<0.1
cm/s– It is also independent from the chemical and optical
properties of the dielectric.– Possible combination with damage free hydrogenation
• Progress towards stable, fast, commercial, extrinsic field effect passivation.
Extrinsic surface passivation [email protected] 30
Funded by:EP/M022196/1EP/M024911/1
In collaboration with:
Department of MaterialsSemiconductor and Silicon Photovoltaics Group
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells
journal homepage: www.elsevier.com/locate/solmat
Passivation of all-angle black surfaces for silicon solar cells
Tasmiat Rahmana,⁎, Ruy S. Bonillab, Amirjan Nawabjana, Peter R. Wilshawb, Stuart A. Bodena
a Electronics & Computer Science, University of Southampton, Southampton, SO17 1BJ, United Kingdomb Department of Materials, University of Oxford, Oxford, OX1 3PH, United Kingdom
A R T I C L E I N F O
Keywords:NanowireBlack siliconSurface passivationSilicon solar cellsDielectric thin filmsCorona discharge
A B S T R A C T
Optical losses at the front surface of a silicon solar cell have a significant impact on efficiency, and as such,efforts to reduce reflection are necessary. In this work, a method to fabricate and passivate nanowire-pyramidhybrid structures formed on a silicon surface via wet chemical processing is presented. These high surface areastructures can be utilised on the front surface of back contact silicon solar cells to maximise light absorptiontherein. Hemispherical reflectivity under varying incident angles is measured to study the optical enhancementconferred by these structures. The significant reduction in reflectivity ( < 2%) under low incident angles ismaintained at high angles by the hybrid textured surface compared to surfaces textured with nanowires orpyramids alone. Finite Difference Time Domain simulations of these dual micro-nanoscale surfaces undervarying angles support the experimental results. In order to translate the optical benefit of these high surfacearea structures into improvements in device efficiency, they must also be well passivated. To this end, atomiclayer deposition of alumina is used to reduce surface recombination velocities of these ultra-black siliconsurfaces to below 30 cm/s. A decomposition of the passivation components is performed using capacitance-voltage and Kelvin Probe measurements. Finally, device simulations show power conversion efficienciesexceeding 21% are possible when using these ultra-black Si surfaces for the front surface of back contact siliconsolar cells.
1. Introduction
In order to improve the efficiency of silicon (Si) solar cells, the frontsurface can be designed with antireflective (AR) structures thatenhance carrier generation in the cell. Current approaches to reducingtop surface reflectance for monocrystalline silicon cells use thin filmcoated micron-scale pyramidal structures formed from alkaline solu-tions that etch preferentially along certain Si crystal planes. For solarcells made from multicrystalline silicon wafers, where the randomnature of the grain orientations precludes the formation of pyramids,acid texturing is instead used. Whilst these methods are industrystandards, new approaches have emerged to improve on opticalabsorption by utilizing nanoscale texturing. Examples include nano-textured surfaces formed via laser ablation [1,2], reactive ion etching[3,4], and wet chemical etching [5–9]. Several solar cells with nanowireAR structures have recently been reported with high power conversionefficiencies. Savin et al. created a nanowire solar cell using deepreactive ion etching (DRIE) with an efficiency of 22.1% [10]. Thisapproach to texturing has also grown in interest recently due toemergence of diamond-wire sawing which complicates traditionalmethods for texturing of multicrystalline silicon [11]. Ingenito et al.[12] demonstrated the use of dual-textured black Si (nano-cones on
micro-pyramids), fabricated also via plasma etching, on back contactsolar cells, achieving power conversion efficiencies up to 19.1%.Recently, dual-textured black Si surfaces created using solution-basedprocesses to form nanowire-pyramid hybrid structures have beenshown to exhibit average reflectance < 2% for light incident perpendi-cular to the surface [13]. The simplicity of producing this ultra-black Simakes it promising for photovoltaic applications.
The top-down fabrication of silicon nanowires using solutionprocessing has risen in popularity as a cost effective and practicalway of producing high aspect ratio nanowires with near-verticalsidewalls, compared to lithography and etching methods [14]. Inparticular, the metal assisted chemical etch (MACE) process, wherebysilicon nanowires are formed using a noble metal and hydrofluoric acid(HF) has attracted much attention for several reasons. Firstly, it is asimple and cost-effective method of etching various Si nanowires withcontrollable parameters. All of the processes can be done in chemicalfacilities without the need for expensive and complicated equipment.Secondly, MACE is much more flexible and can be used to make highsurface-to-volume ratio structures with various cross-sectional shapesin comparison to vapour-liquid-solid (VLS) based methods that canonly be used to grow wires with circular cross sections [15]. Sinanowires fabricated by MACE generally possess high crystalline
http://dx.doi.org/10.1016/j.solmat.2016.10.044Received 4 July 2016; Received in revised form 10 October 2016; Accepted 26 October 2016
⁎ Corresponding author.
Solar Energy Materials & Solar Cells 160 (2017) 444–453
0927-0248/ © 2016 Published by Elsevier B.V.
crossmark
Extrinsic surface passivation [email protected] 31
-Fundamentals and new concepts of silicon surface passivation-Novel thin film dielectrics and deposition technology-Modelling and characterisation methods of surface charge dynamics-Review papers on surface passivation using AlOx, SiOx and SiNx-Implementation of passivation technology into solar cell manufacturing-Passivated hole and electron selective contacts