insight into metal induced recombination losses and...

Insight into Metal Induced Recombination Losses

and Contact Resistance in Industrial Silicon Solar

Cells

Valentin D. Mihailetchi, Haifeng Chu, Radovan Kopecek

International Solar Energy Research Center e.V., Konstanz, Germany

V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018

Motivation

2

Metallization technologies in PV industry

World market share [%]

Data from: International Technology Roadmap for PV, 2018

screen printing

plating

PVD (evaporation/sputtering)

Challenges:

reduce contact recombination:

J0,met J0,pas

achieve low contact resistance (C)

Screen printing and firing-through of a

Ag paste


Introduction

3

Screen printing and firing-through metallization technology

Question: what causes the high J0,met?

- etching of passivation layer and Si emitter [1,2]

- spiking and/or Ag-crystallites formation [3-5]

Outline:

- Photoluminescence (PL) method to extract

J0,met: PL2J0met

- Experimental details

- Results for J0,met & C

- Validation on solar cells

[1] Koduvelikulathu et al., IEEE J. Photovoltaics, 5 (2015)

[2] Edler et al., Prog. Photovoltaics Res. App., 23 (2015)

[3] Ballif et al., Appl. Phys. Lett. vol. 82 (2003)

[4] Hoerteis et al., Adv. Funct. Mater., vol. 20 (2010)

[5] Kiefer et al., IEEE J. Photovoltaics, 6 (2016)


PL to J0,met analysis (PL2J0met)

4

Layout:

• five levels of metallization fractions

• QSSPC measurements for MF1:

implied VOC and J0,pas

Method:

1. Convert PL image to VOC[1-4]

2. Calculate J01 from VOC image

(1-diode model) [2]

3. Linear fit to obtain J0,met

MF1:

0%MF4:

7.6%

MF3:

5.3%

MF3:

5.3%

MF2:

2.6%

MF5:

10.2%

MF5:

10.2%

MF1:

0%

MF2:

2.6%

VOC image (PL2Voc)

[1] Glatthaar et al., J.Appl.Phys., 108 (2010);

[2] Shanmugam et al., Sol. Energy, 118 (2015);

[3] Trupke et al., Appl.Phys.Lett., 89 (2006);

[4] Shen et al., Sol. Energy Mater. Sol.Cells,109 (2013)


MF1:

0%MF4:

7.6%

MF3:

5.3%

MF3:

5.3%

MF2:

2.6%

MF5:

10.2%

MF5:

10.2%

MF1:

0%

MF2:

2.6%

VOC image (PL2Voc)

PL to J0,met analysis (PL2J0met)

5

0 2 4 6 8 10 12

80

120

160

200

240

280 Exp. data

linear fit

J0

1 (f

A/c

m2)

Metal fraction, MF (%)

Example fit:

J0,met = 1720 fA/cm2

R2 = 98.7%

slope = J0,met - J0,pas

[1] Glatthaar et al., J.Appl.Phys., 108 (2010);

[2] Shanmugam et al., Sol. Energy, 118 (2015);

[3] Trupke et al., Appl.Phys.Lett., 89 (2006);

[4] Shen et al., Sol. Energy Mater. Sol.Cells,109 (2013)


Experimental details

6

Sample structure and diffusion profiles

- symmetrically diffused planar or texture wafers

- SiO2/SiNx passivation stack [1]

- commercial firing-through Ag paste for both, n+ and p+

- different firing temperature profiles

- different SiNx thicknesses (on metal side)[1] Mihailetchi et al., IEEE J. Photovoltaics, 8 (2018)

0.0 0.1 0.2 0.3 0.4 0.5 0.61016

1017

1018

1019

1020

boron (140 sq)

phos. (150 sq)

Ca

rrie

r d

en

sity (

cm

-3)

depth (µm)

Ns ≈ 2×1019 cm-3


Experimental details

7

Contact formation [1-3] and firing profiles

[1] Fields et al., DOI: 10.1038/ncomms11143 (2016)

[2] Schubert G., PhD thesis Univ. of Konstanz, Germany (2006)

[3] Ballif et al., Appl. Phys. Lett. vol. 82 (2003)

1. T < 550 °C

- burning of organics

2. 550 °C <T < 700 °C

- SiNx etching

3. T > 700 °C

- Ag crystallites/nanocolloids

- Ohmic contact

0 10 20 30 40 50 60 70 80

300

400

500

600

700

800Profile (T

peak):

F1 (480 °C)

F2 (650 °C)

F3 (740 °C)

F4 (790 °C)

F5 (825 °C)

Tem

pe

ratu

re, T

(°C

)

Time, t (s)

Experimental firing profiles:


Results: C and J0,met

8

n+ (phosphorus) doped surface:

Results:

Simulation (Quokka3)

upper J0,met limit

(for Smet 107 cm/s)

Phase 1:

J0,met = J0,pas , C = n/a

1

10

100

1000

480 600 650 700 750 800 850

1020

500

1000

1500

2000

480 600 650 700 750 800 850

c=n/a

c=n/a

SiNx thickness:

38 nm

65 nm

91 nm

126 nm

c (

m

cm

2)

SiNx thickness:

40 nm

100 nm

150 nm

textureplanar

J0

,me

t(n

+) (

fA/c

m2)

Peak firing temperature, Tpeak

(°C)

Simulation Simulation

Phase 1 Phase 1



9


Results:


upper J0,met limit

(for Smet 107 cm/s)

Phase 1:


Phase 2:

J0,met J0,met (max.)

C 100 mcm2

1

10

100

1000

480 600 650 700 750 800 850

1020

500

1000

1500

2000

480 600 650 700 750 800 850

c=n/a

c=n/a

SiNx thickness:

38 nm

65 nm

91 nm

126 nm

c (

m

cm

2)

SiNx thickness:

40 nm

100 nm

150 nm

textureplanar

J0

,me

t(n

+) (

fA/c

m2)


(°C)


Phase 1 Phase 2 Phase 1 Phase 2



10


1

10

100

1000

480 600 650 700 750 800 850

1020

500

1000

1500

2000

480 600 650 700 750 800 850

c=n/a

c=n/a

SiNx thickness:

38 nm

65 nm

91 nm

126 nm

c (

m

cm

2)

SiNx thickness:

40 nm

100 nm

150 nm

textureplanar

J0

,me

t(n

+) (

fA/c

m2)


(°C)


Phase 1 Phase 2 Phase 3 Phase 1 Phase 2 Phase 3 Results:


upper J0,met limit

(for Smet 107 cm/s)

Phase 1:


Phase 2:


C 100 mcm2

Phase 3:


C ohmic contact



11

p+ (boron) doped surface:

Phase 1 Phase 2 Phase 3 Phase 1 Phase 2 Phase 3

1

10

100

1000

480 600 650 700 750 800 850

10

1000

2000

300040005000

480 600 650 700 750 800 850

c=n/a

c=n/a

SiNx thickness:

38 nm

65 nm

91 nm

126 nm

c (

m

cm

2)

SiNx thickness:

40 nm

100 nm

150 nm

textureplanar

J0

,me

t(p

+) (

fA/c

m2)


(°C)


Results:


upper J0,met limit

(for Smet 107 cm/s)

Phase 1:


Phase 2:


C 100 mcm2

Phase 3:


C ohmic contact



12

At optimum firing temperature, Tpeak = 790 °C:

Results:

1

10

100

20 40 60 80 100 120 140 160

400

800

1200

1600

2000

2400

2800

3200

3600

4000

Boron (planar)

Boron (texture)

Phosphorus (planar)

Phosphorus (texture)

c (

m

cm

2)

J0

,me

t (fA

/cm

2)

SiNx thickness, d

SiNx (nm)

Tpeak

= 790 °C

SiNx J0,met

SiNx C constant (texture)

J0,met (p+) > J0,met (n+)

Thicker SiNx improves Voc (and)



13

At optimum firing temperature, Tpeak = 790 °C:

SiNx = 38 nm SiNx = 126 nm

1

10

100

20 40 60 80 100 120 140 160

400

800

1200

1600

2000

2400

2800

3200

3600

4000

Boron (planar)

Boron (texture)

Phosphorus (planar)

Phosphorus (texture)

c (

m

cm

2)

J0

,me

t (fA

/cm

2)

SiNx thickness, d

SiNx (nm)

Tpeak

= 790 °C

SiNxSiNx

contact

area

40 60 80 100 120 140

10

20

30

40

50

60 Boron

Phosphorus

co

nta

ct S

iNx c

ove

rag

e (

%)

SiNx thickness, d

SiNx (nm)


Validation on solar cells

14

ZEBRA IBC: cell structure and characteristics [1]

• BBr3 and POCl3 diffusions

• SiO2/SiNx passivation stack [2]

• firing through Ag paste for both p+ and n+

[1] Galbiati et al., 7th WCPEC, Waikoloa (2018)

[2] Mihailetchi et al., IEEE J. Photovoltaics, 8 (2018)


-40 -20 0 20 40

674

676

678

680

682

684

686

Exp. data

Quokka3 simulation [1]

Vo

c (m

V)

relative SiNx thickness (nm)

Ref.

Validation on solar cells

15

VOC improvement and best cell results

SiNx VOC

FF constant

JSC

[mA/cm2]

VOC

[mV]

FF

[%]

Efficiency

[%]

41.7 684 81.4 23.2

Best IBC (ZEBRA) cell [2]:

[1] A. Fell, IEEE Trans. Electron Devices, 60 (2013)

[2] Galbiati et al., 7th WCPEC, Waikoloa (2018)


Conclusion

Causes for high J0,met on p+ & n+ Si:

16

Main: etching of SiNx & Si

Minor: Ag crystallites (contact formation)

SiNx J0,met , C constant

C and J0,met: not necessarily correlated

Thank you for your attention!

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