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Ge incorporated Cu2ZnSnSe4 thin-film solar cells
Shinho Kim, Kang Min Kim, Hitoshi Tampo, Hajime Shibata and Shigeru Niki
National Institute of Advanced Industrial Science and Technology (AIST)
Research Center for Photovoltaics (RCPV) Compound Semiconductor Thin Film Team
• In, Ga → Zn, Sn • High absorption
coefficient – α> 104 cm-1
• Using the earth abundant materials
• Production cost down
Introduction – Kesterite solar cells
Cu In Ga Se
Cu Zn Sn S/Se
I III VI III
I II VI IV
Cu2InxGa1-xSe4
Cu2ZnSnSe4
CZT(S)Se
1
Introduction – Band gap tuning of kesterite thin films
• The control of S/(S+Se) ratio is difficult
due to the high volatility of the anionic
components.
• Large VOC deficit (Eg/q-VOC) with S
incorporation 1
• CZTSe ≈ 0.577 mV → CZTSSe ≈ 0.647, (at
champion cells respectively)
• Ex) CIGSe ≈ 0.5
• Low FF 2
• Low VOC and high ideality factor (A)
• Secondary phase problems
Problems of S incorporation
2
Cu2ZnSn(SxSe)4 Cu2ZnSnSe4
1.50 eV 1.00
-0.15 0.00
~1.0 < Eg(CZT(SxSex-1)) < ~1.5 eV
Band gap tuning with S incorporation
1. A. Polizzotti et al., Energy & Environmental Science 6 (11), 3171-3182 (2013). 2. K. F. Tai et al., Advanced Energy Materials 6 (3), (2016)
3
• Tunable band-gap using cationic element → ~1.0
Cell Eff. (%) VOC (V)
JSC (mA/cm2)
FF (%)
Eg (eV) Eg/ q-VOC
CZTGSSe Perdue Univ. (2013) 1 9.40 0.460 31.9 63.8 1.19 0.730
CZTGSe AIST (2015) 2 10.03 0.543 29.5 62.7 1.19 0.647
CZTSe IREC(2015) 3 10.60 0.473 34.3 65.1 1.03 0.550
CZTGSe Univ. of Washington (2016) 4 11.00 0.583 33.6 55.9 1.30 0.717
I-V Results of Ge incorporated Cells
4
1. C. J. Hages et al., Progress in Photovoltaics: Research and Applications 23 (3), 376-384 (2013). 2. S. Kim et al., Solar Energy Materials and Solar Cells 144, 488-492 (2016). 3. S. Giraldo et al., Advanced Energy Materials 5 (21), (2015). 4. A. D. Collord and H. W. Hillhouse, Chemistry of Materials 28 (7), 2067-2073 (2016).
5
Experimental Procedure
SLG Mo
CZTGSe CdS
i-ZnO AZO
Al
As grown CZTGSe deposited by co-evaporation method. Composition Control
Annealing using two zone furnace Grain Growth
Co-evaporation Annealing
CZTGSe solar cell structure
Ge
SLG Mo
Heater 200 ℃
Sample Pellets
Internal gases flow
0.0 0.1 0.2 0.3 0.4 0.50
10
20
30
J (m
A/c
m2 )
V (V)
Eff. VOC JSC FF 12.322 0.527 32.157 0.727
400 600 800 1000 12000.0
0.2
0.4
0.6
0.8
1.0
EQ
E (%
)
Wavelength (nm)
0.9 1.0 1.1 1.2 1.30.0
0.2
0.4
0.6
0.8
1.0
[E ln
(1-E
QE
)]2
Energy (eV)
Eg=1.1eV
New efficiency of Ge incorporated kesterite solar cell
• The highest efficiency of Ge incorporated kesterite solar cell greater than 12%
6
7
Device parameters
Cell Eff. (%) VOC (V)
JSC (mA/cm2) FF
Rs (Ω·cm2)
Rsh (Ω·cm2) A
J0 (A/ cm2)
Eg (eV) Eg/q-VOC
CZTSSe IBM (2013) 12.60 0.513 35.2 0.698 0.72 621 1.45 7.0E-8 1.13 0.617
CZTGSe AIST (2015) 10.03 0.543 29.5 0.627 0.20 694 2.49 6.3E-6 1.19 0.647
CZTGSe AIST (2016) 12.32 0.527 32.2 0.727 0.36 1111 1.47 3.6E-8 1.11 0.583
• Highly improved fill factor over 0.7 • Reduced device parameters – A, Jo and VOC deficit → Improved junction quality and reduced carrier recombination in SCR
0.5 1.0 1.5 2.0 2.5 3.0
0
2
4
6
8
10
12 Efficiency FF
Zn/IVE
ffici
ency
(%)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
FF
Atomic ratio of CZTGSe thin films (EPMA)
• Efficiency shows similar tendency with FF. • Optimized surface conditions are observed at Cu/Zn=1.9 and Zn/IV=1.2.
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
2
4
6
8
10
12 Efficiency FF
Cu/Zn
Effic
ienc
y (%
)
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
FF
Surface composition ~ 120 nm
8
0 10 20 30 40 501
10
100
1000
10000
PL
Inte
nsity
(cou
nts)
Time (ns)
Decay curve Fit
τ2 = 5.6 ns
Lifetime measurement by TRPL Cell Eff. (%) Lifetime (ns)
CZTSSe IBM (2013) 12.60 6.7
CZTGSe AIST (2015) 10.03 2.5
CZTGSe AIST (2016) 12.32 5.6
0.9 1.0 1.1 1.2 1.3
PL In
tens
ity (a
rb. u
nits
)
Energy (eV)
Eg = 1.1 eVEgopt = 1.08 eV • Improved carrier life time
• PL peak is closed to the band edge position (≈0.03) – it may be beneficial effect in reducing VOC deficit
9
Summary
• We demonstrate new results of Ge incorporated kesterite
thin-film solar cell.
– High efficiency greater than 12%
– Large improvement in FF over 0.7
– Improved junction quality and reduced carrier recombination in SCR
– A, J0 and VOC deficit
– Increased carrier life time
10
Thank you for your attention!
スライド番号 1Introduction – Kesterite solar cellsスライド番号 3Ge incorporated CZTSe (CZTGSe)スライド番号 5Experimental Procedureスライド番号 7Device parametersスライド番号 9スライド番号 10スライド番号 11スライド番号 12
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