Iron in crystalline silicon solar cells: fundamental properties, - - PowerPoint PPT Presentation

Iron in crystalline silicon solar cells: fundamental properties, detection techniques, and gettering

Daniel Macdonald, AnYao Liu, and Sieu Pheng Phang Research School of Engineering The Australian National University, Canberra

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• Origins of Fe in multicrystalline Si ingots
• Chemical states and recombination activity of Fe in silicon
• Measuring [Fei] by QSSPC and PL imaging
• Gettering of Fe during ingot growth and cell fabrication:

– Internal gettering – at GBs, dislocations and within grains – External gettering by P, Al and B diffusions

Outline

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• One of the most common

metal contaminants

• Total Fe concentration,

measured by NAA before and after P gettering

• Comes from the crucible,

not the feedstock

• Typically between

1012 -1015 cm-3

Origins of Fe in multicrystalline Si ingots

Macdonald et al. 29th IEEE PVSC New Orleans (2002)

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• Concentration increases

towards top - segregation

• Also increases at bottom –

solid-state diffusion from crucible

• Only a small fraction is

interstitial - around 1%

• Remainder is precipitated

(or substitutional)

• The dissolved fraction has a

much larger impact on lifetime

Origins of Fe in multicrystalline Si ingots

0.1 1 1011 1012 1013 1014 1015 1016 1017 keff = 0.65 B keff < 0.05 keff < 0.05 interstitial Fe Impurity concentration (cm-3) Fraction from top of ingot total Fe

Macdonald et al. J. Appl. Phys. 97 033523 (2005)

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Recombination activity of interstitial Fe in silicon

conduction band valence band FeB0/+ (EV+0.1 eV) FeB0/- (EC-0.26 eV) Fei

0/+ (EV+0.38 eV)

• Interstitial Fe (Fei) introduces a deep donor level
• Positively charged in p-type Si - mobile at RT -

forms pairs with ionised acceptors.

• Two FeB levels – acceptor and donor
• Pairs break under illumination – only Fei present

in working cells

Courtesy of J. Schmidt, ISFH

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Recombination activity of Fe in p-type silicon

• Different energy levels and capture cross sections - different lifetime

curves

• SRH modelling on left, QSSPC data on right (trapping restricts range)

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1 10 100 crossover point

1 Ω.cm p-type Si

FeB lifetime Carrier lifetime (µs) Excess carrier density ∆n (cm

• 3)

Fei lifetime

Macdonald et al. J. Appl. Phys. 95 1021 (2004)

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Detecting Fei using FeB pairing

• Manipulating the SRH equations

shows that:

• Zoth and Bergholz developed a

famous method based on SPV

• Extended to other methods (uW-

PCD and QSSPC)

• Very sensitive (similar to DLTS)
• Only works in p-Si
• Must avoid the crossover point

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10 100 1000

crossover point QSSPC SPV Fe

i lifetime

FeB lifetime Auger lifetime µW-PCD

Carrier lifetime (µs) Excess carrier concentration (cm

• 3)

p-Si, N

A=10 16cm

• 3, [Fe

i]=10 12cm

• 3

        − × =

FeB Fe i

i

C Fe τ τ 1 1 ] [

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Detecting Fei using FeB pairing

• Pre-factor C is a function of

doping and excess carrier density.

• In true low injection, C becomes

constant – ideal measurement region.

• Not accessible to QSSPC or

uW-PCD (trapping effects).

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• 1x10
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1x10

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2x10

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3x10

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4x10

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5x10

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6x10

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N

A= 3x10 16cm

• 3

1x10

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3x10

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1x10

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3x10

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1x10

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1/C (µs cm

• 3 )

Excess carrier density ∆n (cm

• 3)

        − × =

FeB Fe i

i

C Fe τ τ 1 1 ] [

Macdonald et al. J. Appl. Phys. 95 1021 (2004)

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Iron imaging with photoluminescence (PL)

• Band-to-band PL imaging - rapid and highly-resolved method for

low-injection lifetime imaging – no trapping effects.

• Allows low-injection Fe imaging - similar to original SPV technique
• Two PL images required, before and after breaking FeB pairs.

1011 1012 1013 1014 1015 1016 1017 1 10 100 1000

100% Fei : 0% FeB 75% : 25% 50% : 50% 25% : 75% 0% : 100% Auger lifetime crossover point QSSPC SPV PL imaging µW-PCD

Carrier lifetime (µs) Excess carrier concentration ∆n (cm

• 3)

p-Si, NA=1016cm-3, [Fei]+[FeB]=1012cm-3

Macdonald et al. J. Appl. Phys. 103, 073710 (2008)

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Recombination activity of Fe in n-type silicon

• Neutral charge state in n-type – less attractive for minority carriers

compared to p-type.

• Higher lifetime in n-type in low- to mid-injection.
• Possible incentive for using n-type substrates…

1012 1013 1014 1015 1016 1017 0.1 1 10 100 1000 10000 n-type FZ Si ND=2.4x1015cm-3 [Fei]=3.8x1012cm-3 p-type FZ Si NA=2.8x1015cm-3 [Fei]=3.4x1012cm-3 Recombination lifetime (µs) Excess carrier concentration (cm-3)

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0.01 0.1 1 10 100 1000 10000 n-type p-type

Effective lifetime τFe+intrinsic (µs) Interstitial Fe concentration [Fei ] (cm

• 3)

NA/D = 10

16 cm

• 3, 0.1 suns illumination

Macdonald and Geerligs, Appl. Phys. Lett. 85 4061 (2004)

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Fe images on mc-Si

• Wafer 20% from bottom
• f ingot
• High [Fei] (1013 cm-3)
• Internal gettering of Fe

during ingot cooling at GBs, dislocation clusters

Liu et al. Progress in PV, 19 649 (2011)

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Fe images on mc-Si

• Wafer from near very

bottom of ingot

• Moderate [Fei] (1012 cm-3)
• Small grains
• Fewer dislocation

clusters

• Lower [Fei] within grains

– presence of precipitation sites

Liu et al. Progress in PV, 19 649 (2011)

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Fe images on mc-Si

• Wafer from middle of

ingot

• Low [Fei] (1011 cm-3)
• Reduced gettering at

GBs (due to precipitation starting at lower T)

Liu et al. Progress in PV, 19 649 (2011)

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Internal gettering of Fe at GBs during ingot cooling

• Line-scans of PL images with resolution of 25 microns
• 1D diffusion/capture model – 2 free parameters – diffusion length of Fei LD(Fei) and

precipitation velocity P of the GB

• Have to take care of PL artifacts!

– Image smearing in the CCD camera - use point-spread function de-convolution – Carrier spreading in the sample – use low-lifetime – i.e. high [Fei] samples)

Liu and Macdonald, IEEE JPV 2, 479, (2012)

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Internal gettering of Fe at GBs during ingot cooling

• Same GB on different wafers reveals diffusion length of Fei LD(Fei) depends on initial [Fei]
• Lower [Fei] means that precipitation begins later during cooling
• Modelling ingot cooling time reveals data can only be explained if precipitation at GBs

commences after super-saturation ratio of about 50 is reached!

Liu and Macdonald, IEEE JPV 2, 479, (2012)

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Internal gettering of Fe at GBs during annealing

• Annealing at low temps can drive further precipitation at GBs, and within

grains.

• Higher temperatures tend to homogenise the dissolved Fe

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• At 600 ºC, [Fei] is far above

solubility limit:

• Strong super-saturation
• Drives precipitation at GBs,

and within grains

• At 800 ºC, [Fei] is approx equal to

the solubility limit:

• No precipitation
• At 900 ºC and above, [Fei] is

below solubility limit:

• No precipitation
• Homogenization by diffusion
• Dissolution of precipitates

also possible

Internal gettering of Fe at GBs during annealing

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Internal gettering of Fe at GBs during annealing

• 500 C annealing for various times
• 1D diffusion/capture model:
• Widening denuded zone
• Reduced intra-grain [Fei]

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• At fixed temp annealing, diffusion

length of Fe can be calculated from literature values.

• Very good agreement with fitted

values (500 ºC)

• A method to measure diffusivity?

Internal gettering of Fe at GBs during annealing

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• After homogenization on left, after 14 hour anneal at 500 °C on right.
• Precipitation rate varies from grain to grain.

Internal gettering of Fe within the grains

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• Precipitation rate varies despite initial Fe concentrations being similar

Internal gettering of Fe within the grains

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Final [Fei] with respect to intra grain dislocation density, after annealing at 500oC for 14.5 hours Microscope image of a defect etched mc-Si wafer

Internal gettering of Fe within the grains

• Less Fe precipitation in grains of low dislocation density
• Average distance between dislocations is less than Fe diffusion length
• Dislocations act as nucleation sites for Fe precipitation within grains

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External gettering of Fei by P, Al and B diffusions

• Fe-implanted, annealed, mono FZ p-Si, detected by QSSPC.

implant Fe anneal P, B or Al diffusion

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P , 8 5 C P , 8 C P , 7 5 C P , 8 5 + 6 5 C

• 0.01

0.1 1 10 100 Percentage of Fe remaining (%)

Phosphorus gettering of Fei

• P gettering removes between 90-99% of Fe – better at lower temp
• Adding a post-getter anneal improves gettering further – segregation ratio

improves

Phang and Macdonald, JAP 109, 073521 2011

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Phosphorus gettering of Fei

• Driving in P diffusion reduces gettering effectiveness
• Very heavily doped region (>1020 cm-3) required for best gettering

Phang and Macdonald, IEEE JPV accepted

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Aluminium gettering of Fei

• Al gettering removes more than 99.9%!
• However, typical BSF firing is too short for Al gettering to rear side…

P, 850 C P, 800 C P, 750 C P, 850 + 650 C Al, 850 C, 55 min Al, 750 C, 55 min Al, 750 C, 15 sec

• 0.01

0.1 1 10 100 Percentage of Fe remaining (%)

Phang and Macdonald, JAP 109, 073521 2011

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Boron gettering of Fei

• B gettering very effective if Boron Rich Layer (BRL) is present.
• However, BRL is oxidised in-situ to allow low Joe - re-injects Fe into base.
• Even a low temp anneal does not help much…

P, 850 C P, 800 C P, 750 C P, 850 + 650 C Al, 850 C, 55 min Al, 750 C, 55 min Al, 750 C, 15 sec B, 950 C +BRL B, 950 C no BRL B, 950+650 no BRL

0.01 0.1 1 10 100 Percentage of Fe remaining (%)

Phang et al., IEEE JPV 3, 261, 2013

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Boron gettering of Fei

• One solution is to remove the BRL at low temp by etch-back
• Preserves gettering
• Allows lower Joe

Phang et al., IEEE JPV 3, 261, 2013

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Boron gettering of Fei

• Another idea is to overlay a light phosphorus diffusion – ‘buried emitter’.
• Gives very good gettering

Phang and Macdonald, IEEE JPV accepted

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Conclusions

• Fe is common in mc-Si wafers, both dissolved and precipitated
• Dissolved iron more active in p-type than n-type due to charge state
• QSSPC allows very sensitive [Fei] measurements
• PL allows spatially resolved Fe imaging
• Gettering of dissolved Fe is critical for mc-Si cells
• Internal gettering at GBs, dislocations and in intra-grain regions
• Strong super-saturation required
• External gettering via P, B or Al diffusions can be very effective
• Al and B more effective than P
• However, B diffusions require the presence of a BRL

Acknowledgements: this work has been supported by the Australian Research Council (ARC).