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