Insight into Metal Induced Recombination Losses and...
Transcript of 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
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
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)
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
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)
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
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)
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
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
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
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:
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
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
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
Results: C and J0,met
9
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
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)
Peak firing temperature, Tpeak
(°C)
Simulation Simulation
Phase 1 Phase 2 Phase 1 Phase 2
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
Results: C and J0,met
10
n+ (phosphorus) doped surface:
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 2 Phase 3 Phase 1 Phase 2 Phase 3 Results:
Simulation (Quokka3)
upper J0,met limit
(for Smet 107 cm/s)
Phase 1:
J0,met = J0,pas , C = n/a
Phase 2:
J0,met J0,met (max.)
C 100 mcm2
Phase 3:
J0,met J0,met (max.)
C ohmic contact
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
Results: C and J0,met
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)
Peak firing temperature, Tpeak
(°C)
Simulation Simulation
Results:
Simulation (Quokka3)
upper J0,met limit
(for Smet 107 cm/s)
Phase 1:
J0,met = J0,pas , C = n/a
Phase 2:
J0,met J0,met (max.)
C 100 mcm2
Phase 3:
J0,met J0,met (max.)
C ohmic contact
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
Results: C and J0,met
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)
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
Results: C and J0,met
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)
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 2018
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)
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 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)
V.D. Mihailetchi et al., 7th WCPEC, Waikoloa, June 13th, 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!