Kanazawa Institute of TechnologyOptoelectronic Device System Research & Development Center

Optoelectronic Device System R&D CenterKanazawa Institute of Technology

JAPAN

Present status and future prospects for super low-cost Cu2O-based solar cells

Toshihiro Miyata and Tadatsugu Minami

Materials Research and Technology 2018, Paris Feb. 19, 2018

1. Introduction

2. Fabrication procedure of heterojunction Cu2O solar cells

3. Photovoltaic properties for various n-type multicomponent oxide thin films/Cu2O heterojunction solar cells

4. Conclusion

Outline

・ Bandgap energy (Eg)≒2.1eV(direct-gap)

Background

Cuprous oxide (Cu2O)

Metal thin film/p-Cu2O Schottky barrier(SB) solar cellsn-type oxide semiconductor/p-Cu2O heterojunction solar cells

It is very difficult to achieve a high efficiency because of the difficulty of obtaining an n-Cu2O semiconductor. (Usually Cu2O have p-type conduction by intrinsic acceptor from Cu Vacancy.)

p-Cu2O

・ Non-toxic, low-cost

Long attracted much interest for application as an active layer in solar cells・ Theoretical energy conversion efficiency ≒ 20%

O

O

O Cu

CuVCu

h+p-Cu2O

Au

light

polycrystallineMetal thin film

electrode

n-type oxide semiconductor

膜厚が40-80[nm]の範囲において5[%]以上のηが得られ、膜厚が75[nm]において5.38[%]の高いηを実現できた。

Structure Eff.[%] Yare Ref.

SiO2/Cu/p-Cu2O 1.76 1982 1)

Al:ZnO(AZO)/p-Cu2O 1.20 2004 2)

Ga:ZnO(GZO)/p-Cu2O 1.52 2006 3)

MgF2/ITO/ZnO/p-Cu2O 2.01 2006 4)

FTO/ZnO/p-Cu2O 1.28 2007 5)

AZO/p-Cu2O(RTA treatment) 2.18 2011 6)

AZO/Sn-Pd/p-Cu2O 2.13 2011 6)

AZO/p-Cu2O(RTA treatment) 2.19 2011 6)

AZO/n-ZnO/p-Cu2O 4.12 2011 7)

AZO/n-Zn0.9Mg0.1O/p-Cu2O 4.29 2012 8)

AZO/n-Ga2O3/p-Cu2O 5.38 2012 9)

1)L.C.Olsen et al. Sol. Cells 7, 240(1982).

Bulk

5) Izaki, et al, J. Phys. D., 40, (2007) 3326.

2) T. Minami et al. Jpn.J.Phys., 43,pp-917-919 (2004).3) T. Minami et al. Thin Solid Films 528 (2013) 72.4) A. Mittiga, et al, J. Phys. D., 40, (2007) 3326.

Bulk Low damagedepositionmethods

Thinfilm

6) Y. Nishi et al. Thin Solid Films 520 (2012) 3819.7) Y. Nishi et al. Thin Solid Films 528 (2013) 72.8) T. Minami et al. ECS Transactions 8 (2012),

Honolulu, PRiME 2012 Photovoltaics for the 21st Century, in press.9) T. Minami et al, Appl. Phys. Express 6 (2013) 044101.

Structure and conversion efficency for Cu2O-based solar cells

An investigation of new n-type semiconductor thin film materials are very important for improvement of photo voltaic propartes of Cu2O heterojunction solar cells

★AZO/n-type semiconductor thin film /Cu2O heterojunction solar cells

4.130.609.690.71ZnO

Voc[V] Jsc[mA/cm2] FF η[%]

Ga2O3 0.80 9.99 0.67 5.38

n-typesemiconductor

1. Introduction

2. Fabrication procedure of heterojunction Cu2O solar cells

3. Photovoltaic properties for various n-type multicomponent oxide thin films/Cu2O heterojunction solar cells

4. Conclusion

Outline

Cu sheet (3×3[cm], thickness：0.2[mm])

Preparation method of p-type Cu2O sheet

Cu sheet p-type Cu2O sheet

1cm

14)

Resistivity ρ[Ωcm] 300-1000

Hall mobility μ[cm2/Vs] 100-120

hole concentrations

[cm-3]3-9×10 13

Air

Tem

pera

ture

[℃]

Time [min]30 60 60 120 120 60

1000

500 Ar Ar

0

14) Jian Li et al., J. Appl. Phys., 1021 (1991) 69.

Electrical properties ofpoly-crystal Cu2O sheet

1cm 1cm 1cm

Small grain size

Large grain size

Surface image of poly-crystal Cu2O sheet

Preparation methods of n-type multicomponent oxide thin film and AZO

Laser light source ArF Excimer laserWavelength [nm] 193

Repetition rate [Hz] 20

Pulse Laser Deposition (PLD)

Substrate RotatorLamp Heater

SubstrateView

PortPlume

Target

Lens

ArFExcimer Laser

O2 gas

p-Cu2O

Au electrode

AZO electrode

n-type Oxide thin film

1. Introduction

2. Fabrication procedure of heterojunction Cu2O solar cells

3. Photovoltaic properties for various n-type multicomponent oxide thin films/Cu2O heterojunction solar cells

4. Conclusion

Outline

Voltage V [V]0.10

2

4

6

8

12

Cur

rent

Den

sity

J[m

A/c

m2 ]

10

0.2 0.3 0.70.50.4 0.6

RT[℃] AZO/ZnO/Cu2O

0.8

150[℃]

300[℃]

AZO/ZnO/Cu2O

Deposition temperature dependence of J-V characteristics for AZO/ZnO/Cu2O solar cells

AZO/ZnO/Cu2O

-2 -1 0 1 2

103

10-210-110

0101102

Voltage V [V]

Cur

rent

Den

sity

J[m

A/c

m2 ]

10-3

0-4

Deposition Temperature[℃]

200

1

RT

:RT

200

temperatureFilm deposition

RT 200℃

2 nm2 nm

5 nm 5 nm

50nm 50nm

ZnOZnO

ZnOZnO

Cu2OCu2O

Cu2OCu2O

Carbon

ZnOZnO Cu2OCu2O

Carbon

EQE observed from AZO/ZnO/Cu2O solar cells fabricated with various deposition temperatures (R.T. And 200 [℃])

Deposition temperature dependence of the J-V characteristic obtained in AZO/ZnO/Cu2O heterojunction

solar cells measured under dark conditions.

Cross TEM images of AZO/ZnO/Cu2O heterojunction solar cells fabricated with various deposition temperatures

Objective

Sn Ti SiGe Group Ⅳ

GroupⅢ2 3

χφ

(GaIn)2O3

ZnGa2O4

(Ga:0.9)

Multicomponent Oxides as an n-type Semiconductor Layer

:3.2~4[eV]:4.1~5[eV]:4.9~5.2[eV]Eg

(Ga:0.975)Optimal Ga Content

2 32 3

GroupⅡ

CdO

MgO(ZnO)

χφ:4.6~4.8[eV]

:2.9~3.2[eV]

:1.95~2.1[eV]Eg

Cu2OWork function

Electron affinity

Energy gap

1

32η

[％]

0

4

0.05 0.15

5

Mg/(Mg+Zn) atomic ratio0.200.10

Optimal Mg Content: 0.09

Conversion efficiency:η

χ

φ

:3.5-3.7[eV]:4.54[eV]:3.6[eV]Eg

:2.5[eV]:8.8[eV]

:4.3~4.5[eV]:3.7~4.6[eV]:3.3~3.6[eV]

χφ

Eg

χ:2.5[eV]:7.8[eV]Eg

χ

Eg

Cu2O Heterojunction solar cells are prepared by using Ga2O3-based multicomponent oxide thin film as a n-type layer◎ Optimaization of the component

◎ Improving of band arrangement

Function n-type multicomponent oxide thin film Transparent electrode

Target (Ga2O3)X-(Al2O3)1-X AZO

Atmosphere O2 gas O2 gas

(Pressure[Pa]) (1.7[Pa]) (0.2 [Pa])Deposition temperature

[℃] RT RT

Film thickness [nm] 50 200

AZO and n-type Multicomponent Oxide Thin Film Deposition Conditions

p-Cu2OAu(back electrode)

AZO thin film

Multicomponent oxide thin film(50[nm])

Substrate RotatorLamp Heater

SubstrateView

PortPlume

Target

Lens

ArFExcimer Laser

O2 gas

Objective

(c) (Ga2O3)X-(Al2O3)1-X

2 3χ

φ

:3.5-3.7[eV]:4.54[eV]:3.6[eV]Eg

χφ

(GaIn)2O3

:3.2~4[eV]:4.1~5[eV]:4.9~5.2[eV]Eg

ZnGa2O4

2 3

2 3:2.5[eV]:8.8[eV]

χ

Eg

O2=1.7[Pa]

1.0

Voltage V [V]0

2

4

6

8

Cur

rent

Den

sity

J[m

A/c

m2 ]

10

0.2 0.80.4 0.6

12

1.0

0.975

Ga Content X [ratio]

0.9

0.8

0.1 0.3 0.5 0.7 0.9

0.95

Obtained FF η Jsc and Voc as functions of Ga content (X)for AZO/(Ga2O3)X-(Al2O3)1-X/Cu2O solar cells.

4

8

0.2

0.4

0.6

0.4

0.6

0.2

12

4.0

2.0

0

1.0

FFVo

c[V

]

Jsc

[mA

/cm

2 ]η

[％]

6.0

0.8

0

0 0

(Ga2O3)

(Ga2O3) (Ga2O3)

(Ga2O3)

η

FF Jsc

Voc

0.90 0.85 0.800.951.0 0.850.95 0.90 0.801.0

0.850.95 0.90 0.801.00.850.95 0.90 0.801.0

Ga Content X [ratio] Ga Content X [ratio]

Ga Content X [ratio] Ga Content X [ratio]

Obtained FF η Jsc and Voc as functions of Ga content (X)for AZO/(Ga2O3)X-(Al2O3)1-X/Cu2O solar cells.

Voltage [V]-2.0 -1.0 0 1.0

Cur

rent

[mA

]

0

2.0

-0.52.0

1.0

1.5

0.5

1.00.9750.950.90.8

Ga Content X [ratio]

100R

s[Ω

/cm

2 ] 200

Rs

0(Ga2O3)

0.850.95 0.90 0.801.0Ga Content X [ratio]

AZO/(GaXAl1-X)2O3/Cu2O

AZO/Ga2O3/Cu2O

Objective

Sn Ti SiGe Group Ⅳ

GroupⅢ2 3

χφ

(GaIn)2O3

ZnGa2O4

(Ga:0.9)

Multicomponent Oxides as an n-type Semiconductor Layer

:3.2~4[eV]:4.1~5[eV]:4.9~5.2[eV]Eg

(Ga:0.975)Optimal Ga Content

2 32 3

GroupⅡ

CdO

MgO(ZnO)

χφ:4.6~4.8[eV]

:2.9~3.2[eV]

:1.95~2.1[eV]Eg

Cu2OWork function

Electron affinity

Energy gap

1

32η

[％]

0

4

0.05 0.15

5

Mg/(Mg+Zn) atomic ratio0.200.10

Optimal Mg Content: 0.09

Conversion efficiency:η

χ

φ

:3.5-3.7[eV]:4.54[eV]:3.6[eV]Eg

:2.5[eV]:8.8[eV]

:4.3~4.5[eV]:3.7~4.6[eV]:3.3~3.6[eV]

χφ

Eg

χ:2.5[eV]:7.8[eV]Eg

χ

Eg

Cu2O Heterojunction solar cells are prepared by using ZnO-based multicomponent oxide thin film (Zn-Ge-O) as a n-type layer◎ Optimaization of the component

◎ Improving of band arrangement

J-V characteristics for AZO/ Zn1-XGeX-O /Cu2O:Na solar cells using various Ge content

0

2

4

6

8

10

0.2 0.4 0.6 0.8

Cur

rent

Den

sity

J[m

A/c

m2 ]

Voltage V [V]1.21.0

Ge Content [at.%]

0

30

62

12

(ZnO)

100 (GeO2)

0.5

1.0

2

4

8

12

η[％

]Js

c[m

A/c

m2 ]

0

0

4

6

1.5

Jsc:Current density

FF:Fill factor

η:Efficiency

Voc:Open circuit voltage

0

8

20 60 1000 40 80Ge Content [at.%](ZnO) (GeO2)

0.4

0.6

0

0.2

0.8

Voc

[V]

FF

Zn2GeO4

1.2[V]

6.67[%]

Ge=62[%]

40

0

Rs

[Ω/c

m2 ]

20 60 1000 40 80Ge Content [at.%](ZnO) (GeO2)

80

120Rs:Series resistance

Ge content dependence of photovoltaice properties for AZO/ Zn1-XGeX-O /Cu2O:Na solar cells

Dark J-V characteristics for AZO/ Zn1-XGeX-O /Cu2O:Na solar cells using various Ge content

GeO2 ZnO

Voltage V [V]-2 -1 0 1 2

Cur

rent

Den

sity

J[m

A/c

m2 ]

10-3

10-2

100

101

102

10-5

10-1

10-4

0

10062

30

Ge Content:[at.%]103

400 500 600Wavelength [nm]

EQ

E1.0

0.8

0.6

0.4

0.2

0350 450 550

0%30.2%

100%

Ge Content [at.%]

61.8%

300 650

EQE spectra for AZO/ Zn1-XGeX-O /Cu2O:Na solar cells using various Ge content

Ev

Ec

Eg=1.96[eV]Cu2O:Na

GeO2Eg=6.00[eV]

=0.87[eV]

Zn0.38Ge0.62OEg=5.00[eV]

=0.04[eV]Ec

Zn0.38Ge0.62O/Cu2OΔ

Zn0.70Ge0.30OEg=4.08[eV]

=0.66[eV]Ec

Zn0.70Ge0.30O/Cu2OΔ

ZnOEg=3.34[eV]

=1.03[eV]Ec

ZnO/Cu2OΔ

GeO2=0[at.%] Ge=30[at.%] Ge=62[at.%] Ge=100[at.%]

As the Ge composition increased, ΔEc decreased and ΔEc reached a minimum (about 0.04 [eV]) at a Ge composition of 62% at which the highest conversion efficiency was obtained

・

Reduction of ΔEc suppressed recombination at the interface・

The decrease of Jsc and FF over the Ge composition of 70 [%] or more is due to the formation of TYPE I junction・

Deposition technology Vacuum deposition methodDeposition material MgF2

Deposition temperature [℃] RT

Film thickness[nm] 75(Determined by simulation)

Atmosphere(pressure[Pa]) Vacuum(10-4)

Deposition technology and deposition conditions for MgF2

Deposition conditions for MgF2 thin film (anti reflection film)

0

20

40

60

80

100

300 350 400 450 500 550 600 650 700

反射

率/ %

波長 / nm

Reflectance properties of MgF 2 film

wavelength 300-600[nm]

Average reflectance reduction: about 8.4[%]

MgF2/AZO/n-(Ga0.975Al0.025)2O3/p-Cu2O:Na soler cells

material Film thickness[nm]MgF2 78AZO 200

n-(Ga0.975Al0.025)2O3 50 p-Cu2O ∞

with MgF2 antireflection film

no MgF2 anti reflection film

η

6.0

4.0

8.0

2.0

10.0

0

η[％

]

40 60 7050Ge Content [at.%]

Voc

1.0

1.5

0.5

0

80

Voc

[V]

7.53[%]

Oxygen pressure:4.0[Pa] Film thickness :50[nm]

Element temperature is constant at 25[℃]

Simulated sunlight (AM1.5G):100[mW/cm2]

Ge content :(Ge/(Ge+Zn)×100)[%]

light

Na doped p-Cu2O(p-Cu2O:Na)

ZnO：Al(AZO)

Zn1-XGeX-O

Ge content dependence of photovoltaic properties of MgF2/AZO/ Zn1-XGeX-O /Cu2O:Na solar cells

▲ : with MgF2 ○ : no MgF2

MgF2

η[％

]

η

6.0

2.0

8.0

Oxygen Pressure [Pa]

4.0

03.0 3.5 4.0 4.5 5.0

Voc

1.0

1.5

Voc

[V]

0.5

0

7.94[%]

Na doped p-Cu2O(p-Cu2O:Na)

ZnO：Al(AZO)

Zn1-XGeX-O

MgF2

MgF2/AZO/Zn0.38Ge0.62-O/Cu2O:Na太陽電池の光起電力特性の酸素ガス圧依存性

▲ : with MgF2 ○ : no MgF2

light

Oxygen pressure dependence of photovoltaic properties of MgF2/AZO/Zn0.38Ge0.62-O/Cu2O:Na solar cells

Film thickness :50[nm]

Element temperature is constant at 25[℃]

Simulated sunlight (AM1.5G):100[mW/cm2]

Ge content :62[%]

η

Jsc

Voc

FF4

8

0.2

0.4

0.6

6.0

4.0

800

1.0

FFVo

c[V

]

Jsc

[mA

/cm

2 ]η

[％]

10.0

40

0 0

0100

1.5

12

Film thickness[nm]

0.8

60

0.5

8040 10060 8040 10060

8040 10060

8.0

2.0

Film thickness[nm]

Film thickness[nm]

Film thickness[nm]

0

0 0

0

MgF2/AZO/Zn0.38Ge0.62-O/Cu2O:Na太陽電池の光起電力特性の酸素ガス圧依存性Film thickness dependence of photovoltaic properties of MgF2/AZO/Zn0.38Ge0.62-O/Cu2O:Na solar cells

8.23[%]

Oxygen pressure:4.0[Pa] Element temperature is constant at 25[℃]Simulated sunlight (AM1.5G):100[mW/cm2]Ge content :62[%]

MgF2/AZO/Zn0.38Ge0.62-O/Cu2O:Na太陽電池の光起電力特性の酸素ガス圧依存性Film thickness dependence of photovoltaic properties of MgF2/AZO/Zn0.38Ge0.62-O/Cu2O:Na solar cells

0

12

10

0.2 0.4 0.6 1.0

Cur

rent

Den

sity

J[m

A/c

m2 ]

Voltage V [V]0.8 1.2

2

4

6

8

8.23 0.6310.801.20η[%] Voc[V] Jsc[mA/cm2] FF

Oxygen pressure:4.0[Pa] Element temperature is constant at 25[℃]Simulated sunlight (AM1.5G):100[mW/cm2]Ge content :62[%]

Film thickness :57[nm]

Conclusion

◎AZO/n-type multicomponent oxide thin film/p-Cu2O solar cells

1) Photo voltaic properties are not only dependent on the component of metal element but also preparation conditions.

2) The efficiency was improved by using multicomponent oxide thin film as a n-type layer, such as Ga-Al-O and Zn-Ge-O.

3) The highest conversion efficiency of 8.23% could be achieved in a MgF2/AZO/ Zn0.38-Ge0.62-O/Cu2O:Na heterojunction solar cell fabricated using a 75-nm-thick Zn0.38-Ge0.62-O thin film and a Cu2O:Na sheet with a P of approximately 4×1015 cm-3.