Defect-Engineered Silicon Heterojunction Solar Cells Session: Wafer Based

Faculty of Engineering School of Photovoltaic and Renewable Energy Engineering

Defect-Engineered Silicon Heterojunction Solar Cells Session: Wafer based

3rd International Workshop on SHJ Solar Cells 20th October 2020

Associate Professor Brett Hallam

Dr. Matthew Wright, Dr. Anastasia Soeriyadi, Dr. Brendan Wright, Chukwuka Madumelu, Bruno Vicari Stefani Key challenges for SHJ solar cells

• Wafer cost is ~8% higher for n-type than p-type Cz (2018 figures) • Updated cost to be presented in 20 minutes!

LONGi Solar, presentation at the 2018 Silicon PV Conference 2 Key challenges for SHJ solar cells

24.0

23.5

23.0

22.5

22.0 Efficiency (%) Efficiency 21.5

Cert. in Japan “Golden” Best Prod. Cert. “golden” Ave. Guarantee Best Best Ave. Ave. Tongwei Best Pilot. ave. Turnkey Prod. Prod. R&D MBB 244 cm2 Prod. prod. prod. Hanergy MB 24% Sunpreme INES MB Jinergy MB GS Hevel GS Sunpreme 24.32% 24% 23.9% 23.9% 23.5% 23% 23.3 22.8% 22.3%

http://taiyangnews.info/technology/latest-record-performances-of-hjt-solar-cells/ 3 Updated figure for MB from PV IndiaTech 2019. Updated number from Hevel and Tongwei at 3rd international SHJ workshop. Key challenges for SHJ solar cells

• Wafer cost is ~8% higher for n-type than p-type Cz (2018 figures) • Are we maximising the efficiency of SHJ cells? • Potential for reduced LCOE by increasing efficiency • Targeting >24% mass production (need to be 1-2% higher than PERC) • Consumption of rare materials • Ag (~2x PERC usage) and indium usage • Need to target <5 mg/W for a 3 TW PV market [1] • <30 mg Ag total for a SHJ cell (15 mg per side – 40 fingers/side at 40 μm wide x 20 μm high)

nd [1] Oral presentation by P. Verlinden, ”Manufacturing of the next generation of high-efficiency solar cells and modules”, 2 International workshop 4 on silicon heterojunction solar cells, Chengdu, Nov 11-12, 2019. Outline

• Progress with p-type SHJ solar cells • Defect engineering for n-type SHJ solar cells • LID in n-type SHJ solar cells

5 Outline

• Progress with p-type SHJ solar cells • Defect engineering for n-type SHJ solar cells • LID in n-type SHJ solar cells

6 Challenges for p-type SHJ solar cells

• Efficiency and cost potential for p-type SHJ cells • B-O light-induced degradation (LID) • Need for defect engineering steps – which steps are required? • Which of these steps are also beneficial for n-type SHJ cells? P-type N-type Removal of iron Gettering/thermal TD annihilation processing

Source of H for B-O Hydrogenation Passivation of oxygen passivation precipitates

SHJ solar cell fabrication

AHP for elimination Illuminated annealing/post Improved surface of B-O LID processing passivation/current transport

7 Cost comparison: p-type vs n-type wafers

• Monte Carlo simulations for cost comparison for n- N.L. Chang et al. Cell Reports Physical and p-type SHJ cells Science, p.100069. (2020) • $/W at the module level for standard SHJ

• p-type SHJ cells need to be within 0.4%abs of n- p-type is type with 8% n-type wafer premium cheaper • $0.03/cell increase in defect engineering cost ok for p-type to achieve the same efficiency as n-type • Illuminated annealing needs ~0.3% improvement n-type is if costing $0.02/cell cheaper

8 Promising results for p-type SHJ solar cells

• Peak efficiencies for full area bifacial p-type SHJ cells within 0.4%abs of n-type SHJ cells • High-resistivity p-type Cz wafers • Main efficiency loss is caused by lower FF

• Average efficiencies from p-type and n-type wafers are similar (± 0.1%abs) with similar FF • If these cells could be made stable, could be competitive with n-type Cz SHJ solar cells

Average efficiency of n-type SHJ batch

Descoeudres et al., Progress in Photovoltaics: Research and Applications 28, 569 – 577 (2020).

Cell Area Cell type V J FF η Wafer type OC SC

(cm2) (mV) (mA/cm2) (%) (%) Efficiency [%] Efficiency Cz p-type 244 Bifacial 746 38.6 80.1 23.1 Cz n-type 244 Bifacial 742 38.5 82.4 23.5 non-G G1 G2 G3 Cz p-type 244 Monofacial 741 39.4 80.0 23.4 P-type SHJ (6-inch cells) Cz n-type 244 Monofacial 740 39.5 82.1 24.0

9 Elimination of B-O LID in large-area p-type SHJ cells

• Large-area SHJ solar cells fabricated with commercial-grade boron-doped (1.6 Ω.cm) p-type Cz silicon wafers

• 15%relative (3.1%abs) efficiency loss due to B-O LID • With addition of UNSW advanced hydrogen passivation (AHP) for B-O LID a stable efficiency of 22.0% (V of 736 mV) is achieved OC Vicari Stefani et al., Solar RRL, • LID reduced to < 2%rel (0.4%abs) and <1%rel on other cells 4(9), p.2000134 (2020). • This demonstrates the potential of using p-type Cz silicon to fabricate stable high-efficiency SHJ solar cells

2 Process JSC (mA/cm ) VOC (mV) FF (%) η (%) No AHP* 39.7 736 74.1 21.7 No AHP No AHP + LS* 38.8 661 71.9 18.5 With AHP AHP* 39.6 738 75.7 22.1 AHP + LS* 39.6 735 74.4 21.7 AHP + LS** 39.1 736 76.4 22.0

TM *Measured using GridTouch 10 ** Independently measured at SERIS Is prior-hydrogenation required for SHJ solar cells?

• Recent study by Sun et al. demonstrating effective passivation of B-O defects in SHJ solar cells without prior hydrogenation process

• Amount of hydrogen from a-Si comparable to that from SiNx:H films with high-temperature firing • Preliminary results suggest hydrogen sourced from the industrial SHJ fabrication sequence is sufficient to mitigate B-O LID • Only require AHP for treatment of B-O LID

C. Sun et al. Applied Physics Letters 115.25 (2019)

11 Progress with p-type SHJ solar cells - mini-module

• 4x stable cells connected in series to make a mini module

• VOC is preserved

• ISC loss is expected due to interconnection shading • Main efficiency loss came from FF • 2 cell mini-module for LID testing • 2 reference cells with no stabilization process (p-type, ref) • 2 stabilized cells (p-type, UNSW) less than 1% power drop

Cell No. Isc (A) VOC (mV) FF(%) Eff (%) Cell 1 9.46 714 73.83 20.44 Cell 2 9.49 712 73.76 20.42 Cell 3 9.47 707 74.03 20.34 Cell 4 9.44 728 74.13 20.93 Expected module 9.47 2.86 V 73.94 20.57 4-cells module 9.34 2.86 V 70.99 19.40

12 Future challenge for p-type SHJ cells

• Need to optimise cell process for p-type wafers • Lifetime at low injection limited by surface-related effects (not bulk) • >500 μs in low injection from commercial-grade boron-doped Cz • Changes in TCO layer properties to account for the front junction • Additional conductivity required on the front

D. Adachi et al. Appl. Phys. Lett. 107 , D. Chen et al. Progress in 233506 (2015) photovoltaics, pip.3230, 1-15 (2018)

P-type SHJ P-type SHJ N-type SHJ

13 Outline

• Progress with p-type SHJ solar cells • Defect engineering for n-type SHJ solar cells • LID in n-type SHJ solar cells

14 Defect engineering for n-type SHJ cells – pre-fabrication

• Low-injection lifetime improvement from ~4 ms to ~6 ms Lifetime measurement on • Improvement in iFF (0.7%) lifetime precursor (no metal) J iV • Cell results with cell efficiency enhancements (0.55%abs) 0e OC iFF (fA/cm2) (mV) • Potential cost reduction for n-type Cz silicon with defect Control 3.4 742 85.05 engineering (<$0.03/cell requirement for 0.4%abs increase) Gettered 3.1 742 85.74 • Chance to improve ingot utilisation/efficiency distribution 2.65 Ω.cm Pre-fabrication gettering/thermal processing

SHJ solar cell fabrication

Isc (A) VOC (mV) FF (%) Eff (%) Control 9.32 ± 0.049 735 ± 1.41 79.36 ± 1.02 22.25 ± 0.21 Gettered 9.42 ± 0.028 737.5 ± 0.71 80.16 ± 0.04 22.8 ± 0.05

15 Defect engineering for n-type SHJ cells – post-fabrication

• Demonstrated with multiple companies with enhancements in the range of 0.4 – 0.7%absolute • Works for PECVD and Cat-CVD SHJ solar cells • Best efficiencies achieved over 24% with similar level of increase

0.61% SHJ solar cell fabrication 0.42% 0.58% 0.70% 0.66%

0.37% Post-processing

16 Defect engineering for n-type SHJ cells – post-fabrication

• Multi-functional process for industrial silicon heterojunction solar cells

• Improvements in surface passivation quality and uniformity PLcounts Open circuit PL

image Frequency

PL counts

17 Defect engineering for n-type SHJ cells – post-fabrication

• Multi-functional process for industrial silicon heterojunction solar cells • Improvements in surface passivation quality and uniformity

• Improvements in current transport

( Ω Rs mapping

image .cm

) Frequency

2 Rs (Ω.cm )

18 Defect engineering for n-type SHJ cells – post-fabrication

• Multi-functional process for industrial silicon heterojunction solar cells • Improvements in surface passivation quality and uniformity • Improvements in current transport

• Significant improvements in VOC and FF, 0.7%abs efficiency enhancement

2 JSC (mA/cm ) VOC (mV) FF (%) Efficiency (%) As received 38.18 ± 0.05 740.85 ± 0.34 77.9 ± 0.82 22.03 ± 0.23 Processed 38.18 ± 0.05 745.13 ± 0.51 79.9 ± 0.76 22.73 ± 0.22 Change 0.00 ± 0.02 4.28 ± 0.48 2.0 ± 0.72 0.70 ± 0.21

19 Need for tailored optimisation

• Post-processing can be detrimental to cell performance • Identical process can have different impact on cells from different manufacturers

20 Need for tailored optimisation

• Post-processing can be detrimental to cell performance • Identical process can have different impact on cells from different manufacturers • Requires optimisation based on toolset and process used

• Loss of JSC appears to be related to changes in the ITO but is under investigation

• Scope for efficiency increase if JSC can be maintained

21 Outline

• Progress with p-type SHJ solar cells • Defect engineering for n-type SHJ solar cells • LID in n-type SHJ solar cells

22 SHJ field degradation data

• Degradation rates up 1% /year for n-type SHJ solar cells

• Reduced VOC due to increase in increased J0

D. Jordan et al., Progress in Photovoltaics: D. Jordan et al., IEEE Journal of Research and Applications, 24, (2016), 978 Photovoltaics, 8, (2018), 177

23 SHJ field degradation data

• Degradation rates up 1% /year for n-type SHJ solar cells

• Reduced VOC due to increase in increased J0 • Recent studies of modern modules display more stable in-field performance (<0.5% /year) • Latest results presented by Hevel yesterday 0.3 – 0.4%/year (same as Tier 1 PERC) Module type Nominal power, W Monitoring period Observed degradation Hevel n-type SHJ 60 300 – 325 9 – 30 months 0.44%/year avg mono-Si PERC GBs 60 Tier 1 305 12 months 0.8% 1st year mono-Si PERC GG 72 Tier 1 370 7 months 0.6% 1st half year

101%

% 100% 99%

mpp,initial 98%

P /

97% Damp Heat: DH 6800 mpp

P 96% <1% power drop 95% 0 2000 4000 6000 Damp heat time (hours)

D. Andronikov, 2nd SHJ workshop, Chengdu, China, (2019).

24 Illuminated annealing to improve n-type SHJ solar cell efficiencies

• Reports of ~0.2 – 0.3%abs efficiency improvement of n-type SHJ cells using illuminated annealing

• Validation studies on commercial n-type SHJ cells show performance improvement (~0.1%abs efficiency) after light-soaking (1-sun) at 75 °C for 20 hours

• Improvement arising primarily from increase in VOC

B.Wright et al., Solar E. Kobiyashi et al. SOLMAT RRL, (2020), 2000214. 173, 43-49 (2017).

25 Temperature-dependence of response to illuminated annealing

• Light-soaking (1-sun) at elevated temperatures (>75 °C) shows acceleration of improvement • High annealing temperature can result in initial degradation phase prior to recovery and net improvement

• Up to 1%abs efficiency loss due to light-induced degradation observed (<5 mins at 1-sun, 160 °C) • In some cases efficiency does not fully recover C. Madumelu et al., SOLMAT, • Extent and rate of degradation increases with temperature 218, (2020), 110752.

Dark: 160 °C Light: 1-sun, 160 °C

B.Wright et al., Solar RRL, (2020), 2000214

26 Surface passivation and light-induced degradation mechanism

• Change in J01 accounts for the majority of performance changes (likely surface passivation) • Underlying mechanism is uncertain, however defect migration within a-Si interlayers is likely responsible • Highly variable degree of LID extent from cell-to-cell within a batch, and across cell manufactures • Not all industrial SHJ solar cells tested exhibit LID behaviour, further investigation required to understand why

C. Madumelu et al., Solar Energy Materials & Solar Cells, 218, (2020), 110752

27 Preliminary degradation results using current injection

• Current injection also shown to cause degradation at 75 – 100 °C • Constant voltage technique used to study LeTID suggested by Kwapil et al. [1]

• Increased current indicated degradation of VOC

100 °C

Constant voltage: 0.5V Condition: Forward bias Increasing current as a

result of increasing J0

75 °C

K.Wolfram,et.al. "Kinetics of carrier-induced degradation at elevated temperature in multicrystalline silicon solar cells." Solar Energy Materials and Solar Cells 173 (2017): 80-84. Temperature dependent IV measurements of LID in n-type SHJ solar cells

• Degradation/recovery at 200 °C Reference Dark anneal (20 s, 200 °C) • Loss of performance similar at 75 °C as at 25 °C Degraded (5 s, 200°C) • Slight loss of performance can remain at 75 °C Degraded + recovered (30 s, 200°C)

29 Summary and Outlook

• P-type SHJ cells

• 8% n-type wafer premium requires p-type SHJ cell efficiency within 0.4%abs of n-type • B-O LID can be mitigated in SHJ solar cells only requires AHP • N-type SHJ cells

• Pre-fabrication process can improve n-type SHJ solar cell efficiency by 0.55%abs

• Post-fabrication process can improv n-typ SHJ cell efficiency by 0.4 – 0.7%abs • Dependent on cell process, identical process on other industrial cells can be detrimental – likely related to TCO • LID in n-type SHJ cells • LID and current-induced degradation observed in n-type SHJ cells (down to 75 °C) • Likely related to defect migration in a-Si • Highly variable, not observed in all cells • Needs to be considered if applying illuminated annealing

WeChat: BrettHallamUNSW [email protected]