Ag-alloying effects in wide-bandgap CIGS solar cells

Kihwan Kim1, Christopher Thompson2, William N. Shafarman2, and Jae Ho Yun1 1

Photovoltaic Laboratory, Korea Institute of Energy Research 2 Institute of Energy Conversion, University of Delaware

Acknowledgements

Valuable discussions: SeJin Ahn, Jihye Gwak, SeungKyu Ahn, Ara Cho, Young-Joo Eo, Keeshik Shin, Jun-Sik Cho , Joo Hyung Park, Kyunghoon Yoon : KIER Device fabrication:

•

Juwan Park, Sol Lee, Taeyeon Kim, Janghun Choi

This work was supported by the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20138520011120)

Cu(InGa)Se2 (CIGS) Cell Structure

u Substrate / Mo / Cu(InGa)Se2 / CdS / ZnO / TCO / grid ITO/ZnO 200 nm CdS 40 nm Cu(InGa)Se2 1.5 – 3 µm Mo 0.2-1 µm substrate

sputtering chemical bath evaporation or precursor reaction sputtering glass/foil/web

Cu(InGa)Se2 (CIGS) Solar Cells

Cell efficiency = 20-22% Elemental evaporation using three-stage process (ZSW, EMPA, NREL) Rigid and Flexible substrate : EMPA >20%-efficient cell on polyimide Reaction of metal precursors: Solar frontier - From manufacturing line - Cd-free buffer layer (Zn-based buffer) Module performance Sub-module approaching 18% (Solar frontier) Pilot module exceeding 17% (Samsung SDI) Commercial modules with η =11-14% (up to 15%) A variety of options in process and substrate

Competitiveness to CIGS module

Low Cost Commercial

Module Target (18~20%) Utilize Lab Technology Base

Engineering/ Process Control

CIGS

Expanding Tech.Base

11%

13%

λ

λ

Commercial Product

Champion Modules with Variable Sizes from Manufacturing Line

Today

SOURCE : NREL

15%

λ

16%

λ Know-how On Pilot Line

80%

20%

25%

λ

λ

Lab Device

Challenging/doable Target

Future Generation * Efficiency loss from Cell to Module ratio : 0.1~0.2

Wide Bandgap CuInSe2 Alloys n

Theoretically better device performance

n

Improved module performance n

n

Lower current / higher voltage n

Wider cell interconnection

n

Thinner TCO thickness

Better performance at real electricity production (due to high T)

n

Road to develop tandem cells

Typical CIGS solar cells

VOC loss from wide bangap Cu(In,Ga)Se2-based cells Ga/(In+Ga)=0.3

VOC saturation

E-EV (eV)

1.5 1.2

Ga/(In+Ga)=0.8 Ec

Ec

0.8

0.8

0.2 0.0

0.2 Ev 0.0

Ev * Heath et al., APL 80 p.4540 (2002)

When Eg ≥ 1.4 eV ( High Ga content ), VOC saturation (not follow the increase of Eg) Eg ↑à raise conduction band minimum (CBM) à activation of mid-gap states Poor micro-structure of widegap CIGS

Structural/electronic defects

Highly efficient wide-bandgap cell

Bandgap control

Tandem solar cells

VOC loss from wide bangap CuInSe2-based cells 16

Ga/(In+Ga) = 0.18; Eg ~1.1 eV

0.75 0.70

12

Voc / V

EFF / %

14

10 8

0.60

6 35

0.55 0.75

30

0.70 F.F .

Ga/(In+Ga) = 0.27; Eg ~1.2 eV Jsc / mAcm-2

Ga

0.65

25

0.65 0.60

20

0.55

15 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Ga / (In+Ga) ratio

0.50 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Ga / (In+Ga) ratio

As Ga contents increases, finer grain structure Ga/(In+Ga) = 0.78; Eg ~ 1.5 eV

Eg ↑à VOC increases but saturates after Eg ≥ 1.4 eV

Solar cell efficiency vs. CIGS absorber bangap 20 18 Efficiency (%)

2

Current Density ( mA/cm )

30

20

1.4 eV 1.67 eV

10

14 12

NREL-SCHOTT NREL old in ref. [6] IEC in ref. {7] IEC-(Ag,Cu)(In,Ga)Se 2 in ref.[10] ZSW (current world record) in ref. [8] HZ-B in ref. [9] HZ-B CuInGaS 2 in ref. [11]

10 0

0.0

0.2

0.4

0.6

0.8

1.0

Volatage ( V ) Ga / III

16

Eg

VOC , V

JSC, mA/cm2

FF, %

Eff. %

0.59

1.40

0.74

23.7

70.0

12.3

1.00

1.67

0.67

17.2

67.0

8.5

1.2 1.4 Bandgap (eV)

1.6

Contreras, et.al, Proc. 37th IEEE PVSC (2011)

Ag alloying option for CuInSe2-based materials Tmelt vs. Eg

Composition vs. Eg

*W. N. Shafaraman et al., 35th IEEE PVSC (2010)

Ag alloying in wide Bandgap CuInSe2-based solar cells Ag alloying à melting point ↓ à improved crystallinity and reduced defect density Ag / (Ag + Cu) ratio > 0.5 à bandgap widening Improving efficiencies from wide bandgap cells (Eg = 1.4 eV and 1.6 eV)

Ag alloying effects in CIGS: Micro-structure ( Ga/III ~ 0.8)

(312)/(116)

(220)/(204)

Mo

Intensity (AU)

(a)

(112)

Co-evporated CIGS (or ACIGS) films

As Ag content increase :

1.00

Increased grain size (> 1μm ) and film thickness

0.75

Ag ion radius > Cu ion radius

0.57

Melting point lowering à enhanced recrystallization

0.21

Decreased preferred orientation

0.00

20

30

40

50 o

2q()

Increased ions’ mobilities 60

Ag alloying effects in CIGS: SIMS profiles ( Ga/III ~ 0.8) 5

CIGS

10

0.8

10

0.6

2

0.4

10

10

1

10

0

500

1000

1500

2000

2500

2

0.4

1

0.0 3000

10

0

500

1000

0.8

10

2

0.4

0.75

10

0

0

500

1000

1500

2000

2500

Sputtering Time (s)

3000

3500

Counts/s

0.6

10

1

2000

2500

3000

0.0 3500

Mo

CIGS

(d) CdS

1.0

Ga/(Ga+In)

TCO

4

Ga/(Ga+In) ratio

Counts/s

Ga/(Ga+In)

3

10

10

Cu

10

1500

5

1.0 Mo

10

TCO

0.2

0.21

0

Ag

4

0.6

Sputtering Time (s)

CIGS

CdS

1.0

0.8

10

10

5

(c)

Mo

Ag

3

Sputtering Time (s) 10

Ga/(Ga+In)

Cu

10

0.2

0 0

CIGS

CdS

4

Counts/s

Ga/(Ga+In)

Counts/s

(b) TCO

3

10

5

1.0

Cu

TCO

4

10

Mo

Ga/(Ga+In) ratio

CdS

0.8 Ag

3

0.6

2

0.4

10

10

1.00

1

0.2

10

0.0

10

0.2

Cu

0

0

500

1000

1500

2000

2500

3000

Ga/(Ga+In) ratio

(a)

Ga/(Ga+In) ratio

10

3500

0.0 4000

Sputtering Time (s)

Ag content ↑ : More homogenized Ga/(In+Ga) compositional profile à Enhanced ions’ mobilities by Ag alloying

Ag alloying effects in CIGS ( Eg ~ 1.6 eV ) : Device Characterizations

Ga/(Ga+In) ~ 0.8 1.0

5 0 -5

Normalized QE

2

Current density (mA/cm )

10

Ag/(Cu+Ag) ratio 0, Control 0.21 0.57 0.75 1.00

-10 -15 -20

0.8

0.6 Ag/(Cu+Ag) ratio 0, Control 0.21 0.57 0.75 1.00

0.4

0.2

-25 -30

0.0

0.2

0.4

0.6

0.8

1.0

0.0

300

400

500

600

700

800

900

1000

Wavelength (nm)

Voltage (V)

Ag/(Ag+Cu)

Ga/(Ga+In)

Eg,* eV

Eg,** eV

VOC, V

JSC, mA/cm2

FF, %

Eff., %

0

0.78

1.53

1.48

0.790

22.1

65.2

11.4

0.21

0.76

1.52

1.44

0.770

24.0

69.5

12.8

0.57

0.77

1.57

1.47

0.850

24.1

70.1

14.3

0.75

0.75

1.61

1.52

0.803

21.8

64.2

11.3

1.00

0.80

1.74

1.62

0.911

17.1

60.7

9.3

*: Nominal bandgap by Estimation from composition values **: Derived from EQE curves

Ag alloying effects in CIGS ( Eg ~ 1.4 eV ) : Device Characterizations Ga/(Ga+In) ~ 0.65

5

1.0

(a)

0 -5 -10 -15

Normalized QE

2

Current density (mA/cm )

10

Ag/(Ag+Cu) ratio 0, control 0.19 0.53 0.94

-20 -25

0.8

0.6

Ag/(Cu+Ag) ratio 0, Control 0.19 0.53 0.94

0.4

0.2

-30 -35

0.0

0.2

0.4

0.6

0.8

1.0

0.0

300

400

500

600

700

800

900 1000 1100

Wavelength (nm)

Voltage (V)

Ag/(Ag+Cu)

Ga/(Ga+In)

Eg,* eV

Eg,** eV

VOC, V

JSC, mA/cm2

FF, %

Eff., %

0

0.65

1.44

1.38

0.730

28.1

70.2

14.5

0.19

0.65

1.44

1.36

0.780

29.5

73.4

16.9

0.53

0.60

1.46

1.35

0.770

30.5

73.0

17.2

0.94

0.67

1.63

1.53

0.710

27.3

64.4

12.5

*: Nominal bandgap by Estimation from composition values **: Derived from EQE curves

Device performance improvement by Ag alloying CIGS Improved charge collection

100

e-

EQE

80 60

Ef Ag/(Cu+Ag) ratio 0, Control 0.21

40

TCO

n-CdS

20 0 300

h+

i-ZnO 400

500

600

700

800

900 1000 1100

Wavelength(nm)

Above 500 nm, electron-hole generated from the film bulk Thus, charge collection above 500 nm à sensitive to bulk properties Ag alloying à reduced structural / electronic defects Improved charge collection from bulk à improved performances

Mo

J-V Results of wide bandgap cells KIER’s recent achievements KIER’s previous achievements

5

20

(a)

18

0

JSC

FF

Eff

1.4 eV 0.780 29.5 -10 1.6 eV 0.807 21.3

73.4 65.8

16.9 11.3

-5

VOC

Efficiency (%)

Current density (mA/cm 2)

10

-15 1.6 eV

-20 -25

1.4 eV

0.0

0.2

0.4

14 12

NREL-SCHOTT NREL old in ref. [6] IEC in ref. {7] IEC-(Ag,Cu)(In,Ga)Se 2 in ref.[10] ZSW (current world record) in ref. [8] HZ-B in ref. [9] HZ-B CuInGaS 2 in ref. [11]

10

-30 -35

16

0.6

0.8

1.0

Voltage (V)

1.0

1.2 1.4 Bandgap (eV)

1.6

Ag-alloying of Wide gap CISe2–based solar cell - Device performance improvement:1.4 eV: 12.3%à16.9%, 1.6 eV: 8.5%à11.3% - may give an extra efficiency by High-Strain Point Glass (HSG) + high temperature process

18

18

16

16

14

14

12 10 8

Pmax.

Pmax.

Stability of Ag-alloyed CIGS solar cells

150 deg C with nitrogen CIGS ACIGS

0

20

40

12 10

60

80

8

200 deg C with nitrogen CIGS ACIGS

0

Time at temp

20

40

60

Time at temp

Ag-alloying of Wide gap CISe2–based solar cell - No significant degradation up to 200 oC - Comparable device stability with a conventional CIGS solar cell

80

Conclusions

Ag alloying in wide bandgap CIGS-based solar cells Ag alloying effects of CIGS films à enhanced grain size and compositional homogenization (due to faster ions’ mobilities) Ag alloying effects of CIGS solar cell à Improved charge collections above 500 nm Cell efficiencies: à 1.4 eV: 12.3% à 16.9% , 1.6 eV: 8.5% à 11.3%

Further Efforts

Module design/ Defect analyses

KIER P.I: Jaeho Yun

University of Delaware P.I: W. N. Shafarman

Highly efficient wide bandgap device

Support KIER device/module design and fabrication

Analyze device loss factors Establish advanced device characterization methods

Advance of Cell/module technology

Develop high-voltage CIGS modules Cell / module fabrication

Support KIER device characterization methods Develop optimized design for High-voltage CIGS module

Thank you for your attention