department of materials semiconductor and silicon photovoltaics group · 2018-08-12 · department...

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


Surface recombination in silicon


-+

-+

𝑅" = 𝑛𝑝 − 𝑛()*

𝑝 + 𝑝"𝜎-𝑁/𝑣/1

+ 𝑛 + 𝑛"𝜎2𝑁/𝑣/1

∝ 𝑁/× 𝑚𝑖𝑛𝑜𝑟𝑖𝑡𝑦

Shockley–Read–Hallrecombinationrate:


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)



The silicon-dielectric interface


𝑅 ∝ 𝑁/×𝑝;R. S. Bonilla and P. R. Wilshaw J. Appl. Phys. 121, 135301 (2017)


The silicon-dielectric interface• The ability of states to capture carriers


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


Surface recombination metrics


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)


Intrinsic vs Extrinsic passivation


𝑄(/ = 10""𝑒/𝑐𝑚*

Single interface mid-gap defect:

𝑆)?? =1Δ𝑛

𝑛;𝑝; − 𝑛()*𝑝; + 𝑛(𝑆-=

+ 𝑛; + 𝑛(𝑆2=

Δ𝑛 = 10"LcmOP

+


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


𝑹𝒔 ∝ 𝑫𝒊𝒕×𝒑𝒔𝑹𝒔 ∝ 𝑫𝒊𝒕×𝝈𝒑×𝒑𝒔


Potential of Extrinsic Passivation

• Externally added charge to the dielectric after deposition –e.g. Corona charge:


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




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=




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


State-of-the-art


A. Cuevas et al. IEEE PVSC 2015, 6pp


State-of-the-art


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


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?



Field Effect Passivation in cell performance (Quokka)


R.S. Bonilla et al. / Applied Surface Science 412 (2017) 657–667


But… Is this Charge Stable?

• Corona discharge ...



Ionic field effect passivation• Charge is introduced into dielectric films at high

temperature and then permanently quenched in place by cooling to room temperature



Ionic field effect passivation• Diffusion of Potassium ions into SiO2


Bonilla et al. Solid State Phenomena Vol. 242 (2016) pp 67-72


Diffusion + Drift of Potassium in SiO2



Long term stability of ion-charged SiO2


Diffusion Diffusion + Drift




Diffusion + Drift

Direct measurement of charge concentration using kelvin probe and capacitance-voltage




Exceeding 10k days ~ 30 Years


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 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


Shielded Hydrogen Passivation

[email protected]

• 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


SHP - Results

[email protected]

eCV measurements of active boronconcentration vs depth after 20 minexposure at 200 oC.


P Hamer, et al. Phys. Status Solidi RRL 11, 2017


Poisoning and Thicker Foils

[email protected]

eCV plots using leaves with andwithout “poisoning”.

eCV plots using a 100 nm thick“leaf” and a 10 µm thick Pd/Ag“foil”.


Bourret-Sicotte, et al. Phys. Status SolidiA 214, No. 7 (2017)


Industrial Application

[email protected]

• Potentialforin-lineprocessing• Quick,damage-freehydrogen

exposure• Potentialfor:

• lowtemperatureprocessing,

• passivationwithoutfiringdielectrics,

• passivationofcarrierselectivecontacts.


Bourret-Sicotte, et al. Phys. Status SolidiA 214, No. 7 (2017)


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.


Funded by:EP/M022196/1EP/M024911/1

In collaboration with:


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


-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

department of materials semiconductor and silicon photovoltaics group · 2018-08-12 · department...

Documents