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

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1 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

2 2 Outline Origins of Fe in multicrystalline Si ingots Chemical states and recombination activity of Fe in silicon Measuring [Fe i ] 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

3 3 Origins of Fe in multicrystalline Si ingots 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 cm -3 Macdonald et al. 29 th IEEE PVSC New Orleans (2002)

4 4 Origins of Fe in multicrystalline Si ingots 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 Impurity concentration (cm -3 ) k eff = 0.65 k eff < 0.05 k eff < 0.05 total Fe B interstitial Fe Fraction from top of ingot Macdonald et al. J. Appl. Phys (2005)

5 5 Recombination activity of interstitial Fe in silicon Interstitial Fe (Fe i ) 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 Fe i present in working cells conduction band Courtesy of J. Schmidt, ISFH FeB 0/- (E C ev) Fe i 0/+ (E V ev) FeB 0/+ (E V +0.1 ev) valence band

6 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) 1 Ω.cm p-type Si 100 Carrier lifetime (µs) 10 1 Fe i lifetime FeB lifetime crossover point Excess carrier density n (cm -3 ) Macdonald et al. J. Appl. Phys (2004) 6

7 7 Detecting Fe i using FeB pairing Manipulating the SRH equations shows that: 1 [ Fei ] = C τ Fei 1 τ FeB 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 Carrier lifetime (µs) p-si, N A =10 16 cm -3, [Fe i ]=10 12 cm -3 Fe i lifetime FeB lifetime Auger lifetime SPV µw-pcd QSSPC crossover point Excess carrier concentration (cm -3 )

8 Detecting Fe i using FeB pairing 1 [ Fei ] = C τ Fei 1 τ FeB Pre-factor C is a function of doping and excess carrier density. In true low injection, C becomes constant ideal measurement region. 1/C (µs cm -3 ) 6x10-13 N A = 3x10 16 cm -3 5x x x x x10-13 Not accessible to QSSPC or -1x10-13 uw-pcd (trapping effects) Excess carrier density n (cm -3 ) 1x x x x x10 14 Macdonald et al. J. Appl. Phys (2004) 8

9 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. Carrier lifetime (µs) SPV 100% Fe i : 0% FeB 75% : 25% 50% : 50% 25% : 75% 0% : 100% PL imaging Auger lifetime QSSPC µw-pcd crossover point p-si, N A =10 16 cm -3, [Fe i ]+[FeB]=10 12 cm Excess carrier concentration n (cm -3 ) Macdonald et al. J. Appl. Phys. 103, (2008) 9

10 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 Recombination lifetime (µs) n-type FZ Si N D =2.4x10 15 cm -3 [Fe i ]=3.8x10 12 cm p-type FZ Si 1 N A =2.8x10 15 cm -3 [Fe i ]=3.4x10 12 cm Excess carrier concentration (cm -3 ) Effective lifetime τ Fe+intrinsic (µs) p-type n-type N A/D = cm -3, 0.1 suns illumination Interstitial Fe concentration [Fe i ] (cm -3 ) Macdonald and Geerligs, Appl. Phys. Lett (2004) 10

11 Fe images on mc-si Wafer 20% from bottom of ingot High [Fe i ] (10 13 cm -3 ) Internal gettering of Fe during ingot cooling at GBs, dislocation clusters Liu et al. Progress in PV, (2011) 11

12 Fe images on mc-si Wafer from near very bottom of ingot Moderate [Fe i ] (10 12 cm -3 ) Small grains Fewer dislocation clusters Lower [Fe i ] within grains presence of precipitation sites Liu et al. Progress in PV, (2011) 12

13 Fe images on mc-si Wafer from middle of ingot Low [Fe i ] (10 11 cm -3 ) Reduced gettering at GBs (due to precipitation starting at lower T) Liu et al. Progress in PV, (2011) 13

14 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 Fe i L D (Fe i ) 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 [Fe i ] samples) Liu and Macdonald, IEEE JPV 2, 479, (2012) 14

15 Internal gettering of Fe at GBs during ingot cooling Same GB on different wafers reveals diffusion length of Fe i L D (Fe i ) depends on initial [Fe i ] Lower [Fe i ] 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) 15

16 16 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

17 17 Internal gettering of Fe at GBs during annealing At 600 ºC, [Fe i ] is far above solubility limit: Strong super-saturation Drives precipitation at GBs, and within grains At 800 ºC, [Fe i ] is approx equal to the solubility limit: No precipitation At 900 ºC and above, [Fe i ] is below solubility limit: No precipitation Homogenization by diffusion Dissolution of precipitates also possible

18 18 Internal gettering of Fe at GBs during annealing 500 C annealing for various times 1D diffusion/capture model: Widening denuded zone Reduced intra-grain [Fe i ]

19 19 Internal gettering of Fe at GBs during annealing 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?

20 20 Internal gettering of Fe within the grains After homogenization on left, after 14 hour anneal at 500 C on right. Precipitation rate varies from grain to grain.

21 21 Internal gettering of Fe within the grains Precipitation rate varies despite initial Fe concentrations being similar

22 22 Internal gettering of Fe within the grains Final [Fe i ] with respect to intra grain dislocation density, after annealing at 500 o C for 14.5 hours Microscope image of a defect etched mc-si wafer 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

23 23 External gettering of Fe i by P, Al and B diffusions Fe-implanted, annealed, mono FZ p-si, detected by QSSPC. implant Fe anneal P, B or Al diffusion

24 Phosphorus gettering of Fe i P gettering removes between 90-99% of Fe better at lower temp Adding a post-getter anneal improves gettering further segregation ratio improves 100 Percentage of Fe remaining (%) P, 850 C P, 800 C P, 750 C P, C Phang and Macdonald, JAP 109,

25 Phosphorus gettering of Fe i Driving in P diffusion reduces gettering effectiveness Very heavily doped region (>10 20 cm -3 ) required for best gettering Phang and Macdonald, IEEE JPV accepted 25

26 Aluminium gettering of Fe i Al gettering removes more than 99.9%! However, typical BSF firing is too short for Al gettering to rear side 100 Percentage of Fe remaining (%) P, 850 C P, 800 C P, 750 C P, C Al, 850 C, 55 min Al, 750 C, 55 min Al, 750 C, 15 sec Phang and Macdonald, JAP 109,

27 Boron gettering of Fe i B gettering very effective if Boron Rich Layer (BRL) is present. However, BRL is oxidised in-situ to allow low J oe - re-injects Fe into base. Even a low temp anneal does not help much 100 Percentage of Fe remaining (%) P, 850 C P, 800 C P, 750 C P, 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, no BRL Phang et al., IEEE JPV 3, 261,

28 Boron gettering of Fe i One solution is to remove the BRL at low temp by etch-back Preserves gettering Allows lower J oe Phang et al., IEEE JPV 3, 261,

29 Boron gettering of Fe i Another idea is to overlay a light phosphorus diffusion buried emitter. Gives very good gettering Phang and Macdonald, IEEE JPV accepted 29

30 30 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 [Fe i ] 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).