Characterisation of bifacial cells

1. Characterisation of bifacial cells
Challenges and opportunities
A.R. Burgers
www.ecn.nl

2. Acknowledgements
n-type team at ECN
Ingrid Romijn Desislava Oosterling
Astrid Gutjahr John AnkerAstrid Gutjahr John Anker
Eric Kossen Kees Tool
Bart Geerligs Nicolas Guillevin
Anna Carr Nico van der Borg
2 25-4-2012

3. Features of the PANDA cell
• n-type material for good efficiency
- high carrier lifetimes
- Metal impurities, B-O complex
• Bifacial cell, on 6 inch Cz wafer.
• Junction at front side
• H-grid pattern on front- and rear side• H-grid pattern on front- and rear side
• phosphorous BSF
- lateral conductivity, good FF
- rear surface passivation
- better optics than Al BSF
• No aluminium BSF:
- no bowing of cells, suitable for thin wafers
3 25-4-2012
ARC
back contact coating
n-type Si wafer
n++ P-BSF
n++ B-emitter
front contact

4. Panda cells: bifacial potential
• Panda cells
- not applied in bifacial modules yet
- not optimized for bifacial performance
Front Side
Group Cell Isc (A) Voc (V) FF (-) eta (%)
4 25-4-2012
Group Cell Isc (A) Voc (V) FF (-) eta (%)
1 9 9.266 0.644 0.777 19.41
2 14 9.264 0.644 0.774 19.31
3 4 9.321 0.650 0.771 19.52
4 11 9.262 0.641 0.771 19.14
Rear side
Group Cell Isc (A) Voc (V) FF (-) eta (%)
1 9 8.052 0.640 0.785 16.93
2 14 8.059 0.640 0.780 16.82
3 4 8.169 0.645 0.778 17.16
4 11 8.111 0.638 0.779 16.86

5. front- and rear side efficiency distribution
• Efficiency distribution for rear side inherently wider than on front side
(mainly due to current variation)
• Impact on module level (increased mismatch)
- Cell selection of bifacial cells critical
- Efficiency distribution control more important for bifacial cells than
conventional cells
5 25-4-2012
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
7.0
8.90 8.95 9.00 9.05
Iscrear(A)
Isc front (A)
E204464
G1
G3
12.4
12.9
13.4
13.9
14.4
18.0 18.1 18.2 18.3 18.4 18.5
Effrear(%)
Eff front (%)
E204464
G1
G3

6. Panda cells: rear side IQE and current
• Rear side IQE nearly as good as front side
• Rear side IQE varies more than front side IQE
- Explains larger spread in cell parameters at rear
G3-22: 19.09%
G4-16: 19.43% (improved passivation)
6 25-4-2012

7. IQE rear: sources of variation
• Resistivity varied from 0.5 – 12 cm
• BSF profile and rear S always the same.
95
100
PC-1D rear IQE
7 25-4-2012
60
65
70
75
80
85
90
95
300 400 500 600 700 800 900 1000 1100
0.5 Ωcm 1 Ωcm
2 Ωcm 4 Ωcm
6 Ωcm 12 Ωcm

8. IQE rear: sources of variation
• Both resistivity and lifetime influence rear IQE level
• Influence of bulk lifetime and resistivity difficult to separate
95
100
influence of resistivity and bulk lifetime
90
95
100
influence of resistivity and bulk lifetime
8 25-4-2012
50
55
60
65
70
75
80
85
90
300 400 500 600 700 800 900 1000 1100 1200
IQE(%)
wavelength (nm)
6 cm
0.5 cm
1000µs
100µs
300µs
50
55
60
65
70
75
80
85
90
300 400 500 600 700 800 900 1000 1100 1200
IQE(%)
wavelength (nm)
6Ωcm, 3000 us
6Ωcm, 100 us
1Ωcm, 3000 us
front
rear

9. Doping and effective lifetime.
• R: recombination rate
• W: wafer thickness
• ∆(p): excess carrier concentration
• J0: dark current BSF
• Seff: effective SRV of BSF• Seff: effective SRV of BSF
Seff = J0 *Nd / (q * nie
2 )
Rtot = Rbulk + Rsurf = W*∆(p)/τbulk + Seff*∆(p)
• J0 of BSF doping independent
• Seff, and hence Rtot higher at higher base doping levels.
9 25-4-2012

10. Wafer resistivity, τbulk and performance
• Simulation: two parameters varied
- Wafer bulk resistivity (0.5 – 32 )
- Bulk lifetime (30 µs – 1 ms)
• Efficiency:
- Long τbulk: weak variation with Rwafer
- short τbulk: strong variation with Rwafer 35.2
35.4
35.6
35.8
36.0
36.2
36.4
36.6
36.8
37.0
37.2
37.4
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
Jsc(mAcm2)
Base resistivity (Ohm)
30 us
1000 us
10 25-4-2012
0.560
0.570
0.580
0.590
0.600
0.610
0.620
0.630
0.640
0.650
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
Voc(V)
Base resistivity (Ohm)
30 us
1000 us
Base resistivity (Ohm)
13
14
15
16
17
18
19
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
Eff(%)
R wafer (Ohm)
Eff 1000us
Eff 30us

11. Recombination currents at Jsc
0.5
1.0
1.5
2.0
2.5
Jrec(mAcm-2)
Recombination currents at Jsc 1000 us, bulk
1000 us,emitter
1000 us,BSF
1000 us, total
30 us, bulk
30 us,emitter
30 us,BSF
30 us, total
• 1000 µs: Jrec,emitter dominant (0.7mAcm-2)
• 1000 µs: Jrec increases with decreasing Rbulk (< 4 cm)
• 30 µs: Jrec,bulk dominant
• Lower Jrec and higher Jsc for higher resistivity
11 25-4-2012
0.0
0.4 4.0 40.0
Rbulk (Ohm)

12. Wafer resistivity, τbulk and performance
• low τbulk : strong dependence of FF on doping level
• High resistivity and low lifetime
→ bulk recombination dominant 84
→ bulk still in high injection
→ bulk recombination in high injection
leads to high (poor) ideality factor.
12 25-4-2012
70
72
74
76
78
80
82
84
0.0 10.0 20.0 30.0 40.0
FF(%)
R wafer (Ohm)
FF 1000us
FF 30us
Ps_FF 1000 us
Ps_FF 30 us

13. Response time of cells
• Cell performance governed by diffusion length.
• n-type material has lower mobility than p-type
• same diffusion length: n-type material longer lifetime.
13 25-4-2012
• same diffusion length: n-type material longer lifetime.
• Panda cells with higher Rbase, improved surface passivation
→ longer effective lifetime
→ slower response

14. Response time of cells
• Two cells (4.5 cm material)
- G3-22: 19.09%
- G4-16: 19.43% (improved passivation)
• Voltage sweep in two directions, different sweep times
14 25-4-2012

15. Capacitive effects
• 5 ms sweep time: severe effects
• Isc not affected.
15 25-4-2012

16. Capacitive effects: I-V parameters
9.10
9.11
9.12
9.13
9.14
9.15
9.16
9.17
9.18
9.19
9.20
9.21
5 10 15 20 25 30 35 40
Isc(A)
G3-22
G4-16
76.0
76.5
77.0
77.5
78.0
78.5
5 10 15 20 25 30 35 40
FF(%)
G3-22
G4-16
16 25-4-2012
5 10 15 20 25 30 35 40
Flash time (ms)
5 10 15 20 25 30 35 40
Flash time (ms)
0.632
0.634
0.636
0.638
0.640
0.642
0.644
5 10 15 20 25 30 35 40
Voc(V)
Flash time (ms)
G3-22
G4-16
18.5
18.6
18.7
18.8
18.9
19.0
19.1
19.2
19.3
5 10 15 20 25 30 35 40
Eff(%)
Flash time (ms)
G3-22
G4-16

17. Transmission through bifacial cells
• IQE: combination of EQE and reflection measurement.
IQE(λ) = EQE(λ) / (1 - R(λ))
• EQE measured on brass substrate
• Significant IR transmission through bifacial cell
17 25-4-2012

18. Transmission through bifacial cells
• Reflection must be measured with brass reflector
• Other implications
- In a conventional module
- Optical properties of materials behind cell important
- Choice between intimate mirror on cell, or transparent cell.
- Front side current in bifacial module will be lower than conventional
18 25-4-2012

19. Conclusions
• High efficiency bi-facial n-type cells measurements and analysis:
- Care required compared to e.g. p-type Al BSF cells
- Take into account response times of the cell
- especially at high resistivity and high eta.
- Consider optical transmission of cells
- Consider base resistivity
19 25-4-2012
• Rear side efficiency for front junction cells:
- Inherently wider efficiency distribution than front side
- Impact on cell selection for - and performance of bifacial modules.
• Panda cells:
- Excellent rear side efficiency potential
- Overall efficiency not very sensitive to resistivity