©Fraunhofer ISE/Foto: Guido Kirsch

© Fraunhofer ISE FHG-SK: ISE-PUBLIC

ACCURATE MEASUREMENT OF BUSBARLESS SILICON SOLAR CELLS

Michael Rauer, Katinka Kordelos, Alexander Krieg, Jochen Hohl-Ebinger

Fraunhofer Institute for Solar Energy Systems ISE

8th Metallization Workshop

Constance, May 14, 2019

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

2

FHG-SK: ISE-PUBLIC

2017 2018 2020 2022 2025 20280

10

20

30

40

50

60

70

80

90

100

4 B

B3

BB 5

BB

≥ 6

BB

busb

arle

ss

World m

ark

et share

[%

]

Measurement of Busbarless Silicon Solar Cells Motivation

Busbarless solar cells become more and more important

Measurement of current-voltage (I-U) characteristics of busbarless cells

Contacting of grid fingers only

Different setups for measurement developed [2-6]

Challenging: Realization of 4-wire connection

”Voltages and currents shall be measured [..] using independent leads from the terminals” [7]

Separate, electrically isolated current and voltage contacts on fingers necessary

[1] ITRPV, Ninth Edition (2018). [2] G. Hahn, Patent, WO2014037382A1 (2013). [3] N. Bassi, Proc. 29th EUPVSEC, 1180 (2014). [4] S. Dauwe, 6th Workshop on Metallization (2016). [5] S. Raj, IEEE 7th WCPEC, 3294 (2018). [6] M. Osborne, Press release, PV-Tech, May 30th (2018). [7] IEC standard 60904-1.

data from [1]

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

Measurement of Busbarless Silicon Solar Cells Motivation

At CalLab PV Cells measurement unit constructed based on Pasan‘s GridTOUCH unit [1]

Conducting wires spanned over front side perpendicularly to grid finger orientation

(1) Front contact to solar cell

(2) Pressing of solar cell onto rear contact (additional vacuum suction)

Realization of active solar cell cooling

[1] N. Bassi, Proc. 29th EUPVSEC, 1180 (2014).

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

Measurement of Busbarless Silicon Solar Cells Motivation

Separate, electrically isolated current (I) and voltage (U) wires

Ideal 4-wire sensing: Distance dItoU = 0 between I and U wires

Sensing in reality : Distance dItoU ≠ 0 between I and U wires due to constructional restrictions

Aim of this work: Evaluation of “non-ideal” distance between I and U wires

Top view

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

5

FHG-SK: ISE-PUBLIC

I-U measurement of busbarless solar cells with different front grid resistivity Rgrid

Finger resistivity several orders of magnitudes higher than busbar resistivity

Experimental Investigation of Non-Ideal Sensing Current-Voltage Measurement of Busbarless Solar Cells

0 2 480

81

82

83

84

85

86

Measure

d fill

facto

r [%

]

Number of adjacent current wires disconnected

Standard

configuration

Front grid resistivity

40 m/cm

225 m/cm

2.6 mm

Front grid paste Rgrid [mΩ/cm]

High-temperature, conventional 6

Low-temperature, conventional 20-40

Low-temperature, advanced >200

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

Experimental Investigation of Non-Ideal Sensing Current-Voltage Measurement of Busbarless Solar Cells

0 2 480

81

82

83

84

85

86

Measure

d fill

facto

r [%

]

Number of adjacent current wires disconnected

Standard

configuration

Front grid resistivity

40 m/cm

225 m/cm22.5

23.0

23.5

24.0

Measure

d e

ffic

iency [%

]

I-U measurement of busbarless solar cells with different front grid resistivity Rgrid

Finger resistivity several orders of magnitudes higher than busbar resistivity

2.6 mm

Front grid paste Rgrid [mΩ/cm]

High-temperature, conventional 6

Low-temperature, conventional 20-40

Low-temperature, advanced >200

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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I-U measurement of busbarless solar cells with different front grid resistivity Rgrid

Disconnection of I wires next to U wires

Increase in distance dI-U between I and U wires

Experimental Investigation of Non-Ideal Sensing Current-Voltage Measurement of Busbarless Solar Cells

0 2 480

81

82

83

84

85

86

Measure

d fill

facto

r [%

]

Number of adjacent current wires disconnected

Standard

configuration

Front grid resistivity

40 m/cm

225 m/cm22.5

23.0

23.5

24.0

Measure

d e

ffic

iency [%

]

13.0 mm 7.8 mm 2.6 mm

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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Poss ible expectation:

Reducing number of current contacts leads to decrease in FF and η

Measurement:

Strong increase in measured FF and η to artificially high values

Increase the larger, the higher Rgrid

No effect on Isc and Voc

Position of I and U wires with very strong impact

Overestimation of FF and η by non-ideal sensing

Experimental Investigation of Non-Ideal Sensing Current-Voltage Measurement of Busbarless Solar Cells

0 2 480

81

82

83

84

85

86

Measure

d fill

facto

r [%

]

Number of adjacent current wires disconnected

Front grid resistivity

40 m/cm

225 m/cm

Standard

configuration

22.5

23.0

23.5

24.0

Measure

d e

ffic

iency [%

]

13.0 mm 7.8 mm 2.6 mm

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

Why does FF and η depend so severely on distance between I and U wires?

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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Theoretical Investigation of Non-Ideal Sensing Analytical Calculations

Analytical calculations of I-U curves [1,2]

Based on two-diode model approach for PERC-like solar cell

Series resistance of finger grid not yet considered in two-diode model

Current flow perpendicular to fingers only assumed

Increasing current between current wires

Voltage distribution in finger due to finite finger conductivity

Iterative calculation of voltage and current distribution

[1] J. Hohl-Ebinger, Proc. 23rd EUPVSEC, 2012 (2008). [2] C. Kruse, IEEE JPV 7.3, 747 (2017).

current contact

current wire

current current

current flow

current flow

front finger

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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Theoretical Investigation of Non-Ideal Sensing Calculation of Ideal Sensing

Ideal 4-wire sensing:

Voltage and current contact at same position

Contact arrangement similar to module interconnection

Reference configuration

voltage contact

front finger

voltage wire

[1] J. Hohl-Ebinger, Proc. 23rd EUPVSEC, 2012 (2008). [2] C. Kruse, IEEE JPV 7.3, 747 (2017).

current wire

current current

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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0 1 2 3 4 5568

570

572

574

576

578

580

582

ideal voltage

sensing

Vo

lta

ge

[m

V]

Distance from current wire [mm]

Rgrid

= 225 m/cm

Nwires

= 30

U = Umpp

Umpp

573.8 mV

Theoretical Investigation of Non-Ideal Sensing Calculation of Ideal Sensing

Ideal 4-wire sensing:

Voltage at voltage contact externally defined by voltage source

Voltage distribution in finger exemplarily for one voltage close to mpp

Measured voltage at contact similar, but higher voltage between contacts

grid resistance

neglected

573.8 mV Umeas

Advanced finger metallization

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

0 1 2 3 4 5

37.6

37.8

38.0

38.2

38.4R

grid = 225 m/cm

Nwires

= 30

U = Umpp

j no grid

meas = 37.88 mA/cm

2

Cu

rre

nt d

en

sity [m

A/c

m2]

Distance from current wire [mm]

j ideal

meas = 37.67 mA/cm

2

Theoretical Investigation of Non-Ideal Sensing Calculation of Ideal Sensing

Ideal 4-wire sensing:

Current distribution in finger

Lower current density between current contacts

Measured current density is average over distribution

Measured current density reduced

Current density

reduced

573.8 mV Umeas

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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Theoretical Investigation of Non-Ideal Sensing Calculation of Ideal Sensing

Ideal 4-wire sensing:

Calculation done for entire voltage range of forward I-U curve

0 100 200 300 400 500 600 7000

10

20

30

40

Nwires

= 30

Rgrid

= 225 m/cm

Cu

rre

nt d

en

sity [m

A/c

m2]

Voltage [mV]

Grid neglected

Ideal sensing

MPP zoom

reference

I-U curve

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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530 540 550 560 570 580 590 600 61030

35

40

Grid neglected

Ideal sensing

Nwires

= 30

Rgrid

= 225 m/cm

Curr

ent density [m

A/c

m2]

Voltage [mV]

Theoretical Investigation of Non-Ideal Sensing Calculation of Ideal Sensing

[1] B. Fischer, Proc. 16th EUPVSEC, 1365 (2000). [2] J. Greulich, Proc. 24th EUPVSEC, 2065 (2009).

Ideal 4-wire sensing:

Calculation done for entire voltage range of forward I-U curve

Grid resistance leads to “distributed series resistance” [1,2]

FF [%] advanced paste

FF [%] conventional paste

Grid neglected 80.48 80.48

Ideal sensing 80.05 80.41

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

Theoretical Investigation of Non-Ideal Sensing Calculation of Ideal Sensing

[1] B. Fischer, Proc. 16th EUPVSEC, 1365 (2000). [2] J. Greulich, Proc. 24th EUPVSEC, 2065 (2009).

Ideal 4-wire sensing:

Calculation done for entire voltage range of forward I-U curve

Grid resistance leads to “distributed series resistance” [1,2]

For ideal sensing reduction of FF compared to grid-free case

FF [%] advanced paste

FF [%] conventional paste

Grid neglected 80.48 80.48

Ideal sensing 80.05 80.41 530 540 550 560 570 580 590 600 610

30

35

40

Grid neglected

Ideal sensing

Nwires

= 30

Rgrid

= 40 m/cm

Curr

ent density [m

A/c

m2]

Voltage [mV]

Conventional paste

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

17

FHG-SK: ISE-PUBLIC

Theoretical Investigation of Non-Ideal Sensing Calculation of Non-Ideal Sensing

front finger

current wire

voltage contact

voltage wire

Non-ideal 4-wire sensing:

Voltage and current contact at different positions

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

0 1 2 3 4 5568

570

572

574

576

578

580

582

non-ideal

voltage sensing

ideal voltage

sensing

Vo

lta

ge

[m

V]

Distance from current wire [mm]

Rgrid

= 225 m/cm

Nwires

= 30

U = Umpp

Umpp

573.8 mV

Theoretical Investigation of Non-Ideal Sensing Calculation of Non-Ideal Sensing

Non-ideal 4-wire sensing:

Voltage and current contact at different positions

Voltage distribution different from ideal 4-wire sensing

Voltage in finger locally reduced compared to ideal sensing

573.8 mV Umeas

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

19

FHG-SK: ISE-PUBLIC

0 1 2 3 4 5

37.6

37.8

38.0

38.2

38.4R

grid = 225 m/cm

Nwires

= 30

U = Umpp

Cu

rre

nt d

en

sity [m

A/c

m2]

Distance from current wire [mm]

Theoretical Investigation of Non-Ideal Sensing Calculation of Non-Ideal Sensing

Non-ideal 4-wire sensing:

Current density distribution different from ideal 4-wire sensing

Current density in finger locally increased

non-ideal voltage

sensing

ideal voltage

sensing

573.8 mV Umeas

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

0 1 2 3 4 5

37.6

37.8

38.0

38.2

38.4R

grid = 225 m/cm

Nwires

= 30

U = Umpp

Cu

rre

nt d

en

sity [m

A/c

m2]

Distance from current wire [mm]

j non-ideal

meas = 37.98 mA/cm

2

j ideal

meas = 37.67 mA/cm

2

Theoretical Investigation of Non-Ideal Sensing Calculation of Non-Ideal Sensing

Non-ideal 4-wire sensing:

Current density distribution different from ideal 4-wire sensing

Current density in finger locally increased

Measured current density is average over distribution

Measured current density overrated compared to ideal 4-wire sensing

573.8 mV Umeas

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

0 100 200 300 400 500 600 7000

10

20

30

40

Nwires

= 30

Rgrid

= 225 m/cm

Cu

rre

nt d

en

sity [m

A/c

m2]

Voltage [mV]

Grid neglected

Ideal sensing

Non-ideal sensing

Theoretical Investigation of Non-Ideal Sensing Calculation of Non-Ideal Sensing

Non-ideal 4-wire sensing:

Calculation done for entire voltage range of forward I-U curve

MPP zoom

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

22

FHG-SK: ISE-PUBLIC

FF [%] advanced paste

FF [%] conventional paste

Ideal sensing 80.05 80.41

Non-ideal sensing 80.69 80.52

FF overestimation 0.64 0.11

Non-ideal 4-wire sensing:

Calculation done for entire voltage range of forward I-U curve

Theoretical Investigation of Non-Ideal Sensing Calculation of Non-Ideal Sensing

530 540 550 560 570 580 590 600 61030

35

40

Grid neglected

Ideal sensing

Non-ideal sensing

Nwires

= 30

Rgrid

= 225 m/cm

Cu

rre

nt d

en

sity [m

A/c

m2]

Voltage [mV]

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

Non-ideal 4-wire sensing:

Calculation done for entire voltage range of forward I-U curve

FF [%] advanced paste

FF [%] conventional paste

Ideal sensing 80.05 80.41

Non-ideal sensing 80.69 80.52

FF overestimation 0.64 0.11

Theoretical Investigation of Non-Ideal Sensing Calculation of Non-Ideal Sensing

530 540 550 560 570 580 590 600 61030

35

40

Grid neglected

Ideal sensing

Non-ideal sensing

Nwires

= 30

Rgrid

= 40 m/cm

Cu

rre

nt d

en

sity [m

A/c

m2]

Voltage [mV]

Conventional paste

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

Non-ideal 4-wire sensing:

Calculation done for entire voltage range of forward I-U curve

η [%] advanced paste

η [%] conventional paste

Ideal sensing 21.62 21.72

Non-ideal sensing 21.79 21.75

η overestimation 0.17 0.03

Theoretical Investigation of Non-Ideal Sensing Calculation of Non-Ideal Sensing

530 540 550 560 570 580 590 600 61030

35

40

Grid neglected

Ideal sensing

Non-ideal sensing

Nwires

= 30

Rgrid

= 40 m/cm

Cu

rre

nt d

en

sity [m

A/c

m2]

Voltage [mV]

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

Non-ideal 4-wire sensing:

Calculation done for entire voltage range of forward I-U curve

For conventional metallization: Non-ideal sensing uncritical

For advanced metallization: Significant overestimation of FF and η

Overestimation increases Cell-to-Module (CTM) loss

Included in uncertainty budget as systematic uncertainty of FF, η and Pmpp

Theoretical Investigation of Non-Ideal Sensing Calculation of Non-Ideal Sensing

530 540 550 560 570 580 590 600 61030

35

40

Grid neglected

Ideal sensing

Non-ideal sensing

Nwires

= 30

Rgrid

= 40 m/cm

Cu

rre

nt d

en

sity [m

A/c

m2]

Voltage [mV]

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

26

FHG-SK: ISE-PUBLIC

How can the measurement unit be improved?

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

27

FHG-SK: ISE-PUBLIC

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

front finger

current wire

voltage contact

voltage wire

(1) Approaching voltage to current wire

Calculation of ΔFF for variation of voltage sense position

current contact

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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(1) Approaching voltage to current wire

Calculation of ΔFF for variation of voltage sense position

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 dI-I = 5.2 mm

Diffe

rence n

on-ideal to

ideal F

F [%

abs]

Voltage sense position [mm]

Rgrid

[m/cm]

225

dI-I / 2

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

(1) Approaching voltage to current wire

ΔFF decreases with square of distance from middle position

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 dI-I = 5.2 mm

Diffe

rence n

on-ideal to

ideal F

F [%

abs]

Voltage sense position [mm]

Rgrid

[m/cm]

225

dI-I / 4

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

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FHG-SK: ISE-PUBLIC

dI-I *3 / 4

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 dI-I = 5.2 mm

Diffe

rence n

on-ideal to

ideal F

F [%

abs]

Voltage sense position [mm]

Rgrid

[m/cm]

225

(1) Approaching voltage to current wire

ΔFF decreases with square of distance from middle position

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

31

FHG-SK: ISE-PUBLIC

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 dI-I = 5.2 mm

Diffe

rence n

on-ideal to

ideal F

F [%

abs]

Voltage sense position [mm]

Rgrid

[m/cm]

225

(1) Approaching voltage to current wire

ΔFF decreases with square of distance from middle position

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

32

FHG-SK: ISE-PUBLIC

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 dI-I = 5.2 mm

Diffe

rence n

on-ideal to

ideal F

F [%

abs]

Voltage sense position [mm]

Rgrid

[m/cm]

10

30

45

100

225

300

(1) Approaching voltage to current wire

ΔFF decreases with square of distance from middle position

ΔFF increases for increase of front grid resistivity

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

33

FHG-SK: ISE-PUBLIC

(1) Approaching voltage to current wire

ΔFF decreases with square of distance from middle position

ΔFF increases for increase of front grid resistivity

Improvement of measurement unit

Strong reduction of distance between I and U wires necessary

ΔFF > 0.2%rel for minimal distance (= wire diameter)

Approach not effective

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 dI-I = 5.2 mm

Diffe

rence n

on-ideal to

ideal F

F [%

abs]

Voltage sense position [mm]

Rgrid

[m/cm]

10

30

45

100

225

300

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

Minimal distance

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

34

FHG-SK: ISE-PUBLIC

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

front finger

current wire

voltage wire

(2) Approaching of current wires

Calculation of difference ΔFF between ideal and non-ideal voltage sensing

current contact

voltage contact

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

35

FHG-SK: ISE-PUBLIC

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Rgrid

[m/cm]

225

Voltage contact in

center: dU-I

= 0.5 dI-I

Diffe

ren

ce

no

n-id

ea

l to

id

ea

l F

F [%

abs]

Distance between current wires dI-I [mm]

dI-I(0)

(2) Approaching of current wires

Calculation of difference ΔFF between ideal and non-ideal voltage sensing

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

36

FHG-SK: ISE-PUBLIC

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Rgrid

[m/cm]

225

Voltage contact in

center: dU-I

= 0.5 dI-I

Diffe

rence n

on-ideal to

ideal F

F [%

abs]

Distance between current wires dI-I [mm]

dI-I(0) / 2

(2) Approaching of current wires

ΔFF decreases strongly with distance between current wires

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

37

FHG-SK: ISE-PUBLIC

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Rgrid

[m/cm]

225

Voltage contact in

center: dU-I

= 0.5 dI-I

Diffe

rence n

on-ideal to

ideal F

F [%

abs]

Distance between current wires dI-I [mm]

(2) Approaching of current wires

ΔFF decreases strongly with distance between current wires

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

38

FHG-SK: ISE-PUBLIC

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Rgrid

[m/cm]

10

30

45

100

225

300

Voltage contact in

center: dU-I

= 0.5 dI-I

Diffe

rence n

on-ideal to

ideal F

F [%

abs]

Distance between current wires dI-I [mm]

(2) Approaching of current wires

ΔFF decreases strongly with distance between current wires

ΔFF increases for increase of front grid resistivity

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

39

FHG-SK: ISE-PUBLIC

Theoretical Investigation of Non-Ideal Sensing Approaches for Improving Contacting Unit

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Rgrid

[m/cm]

10

30

45

100

225

300

Voltage contact in

center: dU-I

= 0.5 dI-I

Diffe

rence n

on-ideal to

ideal F

F [%

abs]

Distance between current wires dI-I [mm]

(2) Approaching of current wires

ΔFF decreases strongly with distance between current wires

ΔFF increases for increase of front grid resistivity

Improvement of measurement unit

Already small reduction of distance with significant effect

ΔFF < 0.1%rel for moderate distance of 2 mm

Approach promis ing, but implies asymmetric arrangement of I wires

Required distance

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

40

FHG-SK: ISE-PUBLIC

Summary

Accurate measurement of busbarless solar cells in 4-wire connection challenging

Resistivity of fingers much higher than resistivity of busbars

Investigation of influence of distance between U and I wires by experiments and calculations

Voltage distribution in front finger can affect measured fill factor and efficiency

Conventional metallization: Non-ideal sensing uncritical

Advanced metallization: Significant overrating of FF and η for non-ideal sensing

Approaches for improvement of contacting unit

Reduction of distance between voltage and current wires not effective

Reduction of distance between current wires promising

R 23 G 156 B 125

R 242 G 148 B 0

R 31 G 130 B 192

R 226 G 0 B 26

R 177 G 200 B 0

R 254 G 239 B 214

R 225 G 227 B 227

© Fraunhofer ISE

41

FHG-SK: ISE-PUBLIC

Thank you very much for your attention!

Fraunhofer Institute for Solar Energy Systems ISE

Michael Rauer

www.ise.fraunhofer.de

michael.rauer@ise.fraunhofer.de