Characterisation of bifacial cellsChallenges and opportunities

A.R. Burgers

www.ecn.nl

Acknowledgements

n-type team at ECN

Ingrid Romijn Desislava OosterlingAstrid Gutjahr John AnkerAstrid Gutjahr John AnkerEric Kossen Kees ToolBart Geerligs Nicolas GuillevinAnna Carr Nico van der Borg

2 25-4-2012

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

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

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 9.05Is

c re

ar

(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

Eff

re

ar

(%)

Eff front (%)

E204464

G1

G3

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

IQE rear: sources of variation

• Resistivity varied from 0.5 – 12 Ωcm• BSF profile and rear S always the same.

95

100PC-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

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 Ωcm

0.5 Ωcm

1000µs

100µs300µs

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

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 * nie2 )

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

Wafer resistivity, τbulk and performance

• Simulation: two parameters varied- Wafer bulk resistivity (0.5 – 32 Ω)- 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

(mA

cm2

)

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

Vo

c (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

Recombination currents at Jsc

0.5

1.0

1.5

2.0

2.5

Jre

c (m

Acm

-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 Ωcm)• 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)

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

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

Response time of cells

• Two cells (4.5 Ωcm material)- G3-22: 19.09%- G4-16: 19.43% (improved passivation)

• Voltage sweep in two directions, different sweep times

14 25-4-2012

Capacitive effects• 5 ms sweep time: severe effects• Isc not affected.

15 25-4-2012

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

Vo

c (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

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

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

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