Supplementary Information ·

Supplementary Information

Cu:NiO as a hole-selective back contact to improve the photoelectrochemical performance of CuBi2O4 thin film photocathodes

Angang Song,ab Paul Plate,a Abdelkrim Chemseddine,a Fuxian Wang,a Fatwa F. Abdi,a Markus Wollgarten,c Roel van de Krol,ab and Sean P. Berglund a *

a. Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Hahn-Meitner-Platz 1, 14109 Berlin, Germany. b. Institute für Chemie, Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germanyc. Department Nanoscale Structures and Microscopic Analysis, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany* E-mail: [email protected]

0.4 0.6 0.8 1.0-5

-4

-3

-2

-1

0

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. RHE)

FTO/CuBi2O4 with H2O2 (backside) FTO/CuBi2O4 with H2O2 (frontside) FTO/Cu:NiO/CuBi2O4 with H2O2 (backside) FTO/Cu:NiO/CuBi2O4 with H2O2 (frontside)

Figure S1. LSV scans for CuBi2O4 films on FTO and FTO/Cu:NiO (7 nm) at a thicknesses of 260 nm measured in 0.3 M K2SO4 and 0.2 M phosphate buffer (pH 6.65) with H2O2 added as an electron scavenger under backside and frontside illumination.

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2019

0.4 0.6 0.8 1.0

-4

-3

-2

-1

0

Cur

rent

Den

sity

(mA

/cm

2 )


60 nm 150 nm 260 nm 380 nm 500nm

FTO/CuBi2O4 with H2O2 (a)

0.4 0.6 0.8 1.0-5

-4

-3

-2

-1

0

Cur

rent

Den

sity

(mA

/cm

2 )


150 nm 260 nm 380 nm

FTO/Cu:NiO/CuBi2O4 with H2O2 (b)

Figure S2. (a) LSV scans for FTO/CuBi2O4 and (b) FTO/Cu:NiO (7 nm)/CuBi2O4 films at different film thicknesses measured in 0.3 M K2SO4 and 0.2 M phosphate buffer (pH 6.65) with H2O2 added as an electron scavenger under backside illumination.

0.4 0.5 0.6 0.7 0.8-0.5

-0.4

-0.3

-0.2

-0.1

0.0

Cur

rent

Den

sity

(mA

/cm

2 )


FTO/Cu:NiO with Ar bubbling FTO/Cu:NiO with H2O2

Figure S3. LSV scans for Cu:NiO at a thicknesses of 34 nm measured in 0.3 M K2SO4 and 0.2 M phosphate buffer (pH 6.65) with Ar bubbling and with H2O2 under backside illumination.

0.4 0.6 0.8 1.0-5

-4

-3

-2

-1

0

Cur

rent

Den

sity

(mA

/cm

2 )


FTO/Cu:NiO/CuBi2O4 (7 nm) FTO/Cu:NiO/CuBi2O4 (34 nm) FTO/Cu:NiO/CuBi2O4 (102 nm)

Figure S4. LSV scans for CuBi2O4 films (~260 nm) based on different film thicknesses of Cu:NiO (7, 34, and 102 nm) measured in 0.3 M K2SO4 and 0.2 M phosphate buffer (pH 6.65) with H2O2 under backside illumination.

400 500 600 700 8000

10

20

30

40

50

IPC

E at

0.6

VR

HE (

%)

Wavelength (nm)

FTO/Cu:NiO/CuBi2O4 with H2O2

FTO/CuBi2O4 with H2O2

FTO/Cu:NiO/CuBi2O4 with Ar bubbling FTO/CuBi2O4 with Ar bubbling

400 500 600 700 8000

1x1018

2x1018

3x1018

4x1018

5x1018

Sola

r Pho

ton

Flux

(m-2s-1

nm-1)

Figure S5. IPCE spectra of CuBi2O4 on FTO (blue dot line), CuBi2O4 on Cu:NiO (red and green dot line). The measurements were performed in 0.3 M K2SO4 and 0.2 M phosphate buffer (pH 6.65) with H2O2 under backside illumination.

400 500 600 700 8000.0

0.1

0.2

Pow

er D

ensi

ty (m

W/c

m2 )

Wavelength (nm)

Power density

Figure S6. Power spectra for back illumination IPCE measurements.

0 20 40 60 80 100 120-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Gas bubbles

Phot

ocur

rent

den

sity

at 0

.6 V

RH

E (m

A/c

m2 )

Time (min)

FTO/Cu:NiO/CuBi2O4 with H2O2

FTO/CuBi2O4 with H2O2

FTO/Cu:NiO/CuBi2O4 with Ar bubbling FTO/CuBi2O4 with Ar bubbling

dark

light

dark

light

dark

Figure S7. Constant potential measurement at 0.6 V vs RHE for 260 nm CuBi2O4 films based on bare FTO and 7 nm Cu:NiO in the dark and light (AM1.5 irradiation). Measurements were done in 0.3 M K2SO4 and 0.2 M phosphate buffer (pH 6.65) with Ar bubbling and with H2O2 under backside illumination.

400 500 600 700 8000

20

40

60

80

100

Abs

orpt

ance

(%)

Wavelength (nm)

FTO/Cu:NiO/CuBi2O4

FTO/CuBi2O4

FTO/Cu:NiO Bare FTO

Figure S8. Absorptance spectra for bare FTO, FTO/Cu:NiO (7nm) and FTO/CuBi2O4 and FTO/Cu:NiO (7nm)/CuBi2O4.

300 400 500 600 700 800 9000

20

40

60

80

100

Tran

sfle

ctan

ce (%

)

Wavelength (nm)

Glass Cu:NiO (7 nm) Cu:NiO (34 nm) Cu:NiO (102 nm)

(a)

1 2 3 40

1x1017

2x1017

3x1017

4x1017

5x1017

(b)

h2

m-2eV

2

Energy (eV)

Cu:NiO (7 nm) Cu:NiO (34 nm) Cu:NiO (102 nm)

3.75 eV

1 2 3 40.0

5.0x106

1.0x107

1.5x107

h2

m-2eV

2

Energy (eV)

Bare FTO

3.70 eV

(c)

Figure S9. (a) Transflectance spectra and (b) direct bandgap Tauc plots for Cu:NiO thin films with different thicknesses (7, 34, and 102 nm) deposited on glass substrates and (c) direct band gap Tauc plot of FTO substrate.

Figure S10. AFM images of (a) bare FTO, and (b) a 7 nm Cu:NiO layer, and (c) a 34 nm Cu:NiO layer deposited on FTO

Figure S11. Top-view SEM image of 260 nm CuBi2O4 deposited on FTO and corresponding EDX elemental mapping of (b) Cu (c) Bi, (d) O and (d) Sn.

Figure S12. Top-view SEM image of 7 nm Cu:NiO thin film deposited on FTO and corresponding EDX elemental mapping of (b) Cu (c) Bi, (d) O, and (e) Sn.

Figure S13. (a) Drawing of the laser cut pattern used for solid-state I-V measurements. (b) SEM image of an FTO substrate with a laser cut.

Figure S14. Schematics of the (a) FTO/glass/FTO, (b) FTO/CuBi2O4/FTO, (c) FTO/Cu:NiO/FTO, and (d) FTO/Cu:NiO/CuBi2O4/Cu:NiO/FTO samples used for solid-state I-V measurements. The CuBi2O4 and Cu:NiO thicknesses were 260 and 7 nm, respectively.

Figure S15. Schematic illustrating the band bending of back-to-back Schottky barriers (FTO/CuBi2O4/FTO sample) at (a) V = 0 V and (b) V ≠ 0 V.

Derivation of Equation for Back-to-Back Schottky DiodesThe current densities across back-to-back Schottky barriers, J1 and J2 (A/cm2), are governed by the following equations.1-5

𝐽1 = 𝐴 ∗∗ 𝑇2exp ( ‒ 𝑞𝜙𝐵𝑒𝑓𝑓1

𝑘𝑇 ) [exp (𝑞𝑉𝑘𝑇) ‒ 1]

𝐽1 = 𝐽𝑠1[exp (𝑞𝑉𝑘𝑇) ‒ 1]

𝐽2 =‒ 𝐴 ∗∗ 𝑇2exp ( ‒ 𝑞𝜙𝐵𝑒𝑓𝑓2

𝑘𝑇 ) [exp (𝑞𝑉𝑘𝑇) ‒ 1]

𝐽2 = ‒ 𝐽𝑠2[exp ( ‒ 𝑞𝑉𝑘𝑇 ) ‒ 1]

A** is the reduced effective Richardson Constant (A cm-2 K-2), T is the temperature (K), q is the electronic charge (1.602 × 10-19 C), k is the Boltzmann constant (1.381 × 10-23 m2 kg s-2 K-1), Beff1 and Beff2 are the effective barrier heights (eV), V is the overall applied voltage (V), and V1 and V2 are resulting voltages at each back-to-back Schottky barrier (V).

From continuity and symmetry, the overall current density (J) and applied voltage (V) are related to the individual values as follows:

𝐽 = 𝐽1 = 𝐽2

𝑉 = 𝑉1 + 𝑉2

Substitution of variables can be used to solve for J.

𝑉1 = 𝑘𝑇𝑞

ln [1 +𝐽

𝐽𝑠1]

𝑉2 = ‒ 𝑘𝑇

𝑞ln [1 ‒

𝐽𝐽𝑠2

]

𝑉 = 𝑘𝑇𝑞

ln [1 +𝐽

𝐽𝑠1] ‒

𝑘𝑇𝑞

ln [1 ‒𝐽

𝐽𝑠2]

𝐽 =𝐽𝑠1 𝐽𝑠2exp ( 𝑞𝑉

2𝑘𝑇) ‒ 𝐽𝑠1 𝐽𝑠2exp ( ‒ 𝑞𝑉2𝑘𝑇 )

𝐽𝑠1exp ( 𝑞𝑉2𝑘𝑇) + 𝐽𝑠2exp ( ‒ 𝑞𝑉

2𝑘𝑇 )

𝐽 =𝐽𝑠1 𝐽𝑠2sinh ( 𝑞𝑉

2𝑘𝑇)𝐽𝑠1exp ( 𝑞𝑉

2𝑘𝑇) + 𝐽𝑠2exp ( ‒ 𝑞𝑉2𝑘𝑇 )

Under the assumption that the barrier height of each Schottky barrier, scales linearly with the applied voltage, the effective barrier heights can be defined in terms of the ideality factors, n1 and n2.

𝜙𝐵𝑒𝑓𝑓1 = 𝜙𝐵01 + 𝑉1(1 ‒1𝑛1

)𝜙𝐵𝑒𝑓𝑓2 = 𝜙𝐵02 ‒ 𝑉2(1 ‒

1𝑛2

)

B01 and B02 are the barrier heights at zero bias (eV).

𝐽 =

2 𝐴 ∗∗ 𝑇2exp ( ‒ 𝑞[𝜙𝐵01 + 𝑉1(1 ‒1𝑛1

)]𝑘𝑇 ) 𝐴 ∗∗ 𝑇2exp ( ‒ 𝑞[𝜙𝐵02 ‒ 𝑉2(1 ‒

1𝑛2

)]𝑘𝑇 ) sinh ( 𝑞𝑉

2𝑘𝑇)

𝐴 ∗∗ 𝑇2exp ( ‒ 𝑞[𝜙𝐵01 + 𝑉1(1 ‒1𝑛1

)]𝑘𝑇 )exp ( 𝑞𝑉

2𝑘𝑇) + 𝐴 ∗∗ 𝑇2exp ( ‒ 𝑞[𝜙𝐵02 ‒ 𝑉2(1 ‒1𝑛2

)]𝑘𝑇 )exp ( ‒ 𝑞𝑉

2𝑘𝑇 )

By the symmetry of the system the back-to-back Schottky diodes should have similar barrier heights at zero bias and ideality factors.

𝜙𝐵01 = 𝜙𝐵02 = 𝜙𝐵0

𝑛1 = 𝑛2 = 𝑛

Furthermore, if the channel resistance is low, then voltages at each Schottky barrier can be assumed to be equal.

𝑉1 = 𝑉2 =𝑉2

𝐽 =2 𝐴 ∗∗ 𝑇2exp ( ‒ 2𝑞𝜙𝐵0

𝑘𝑇 )exp ( ‒ 𝑞(𝑉 ‒ 𝑉)2𝑘𝑇 )exp ( ‒ 𝑞(𝑉 ‒ 𝑉)

2𝑛𝑘𝑇 )sinh ( 𝑞𝑉2𝑘𝑇)

exp ( ‒ 𝑞𝜙𝐵0

𝑘𝑇 )[exp ( ‒ 𝑞𝑉2𝑘𝑇 )exp ( 𝑞𝑉

2𝑛𝑘𝑇)exp ( 𝑞𝑉2𝑘𝑇) + exp ( ‒ 𝑞𝑉

2𝑘𝑇 )exp ( ‒ 𝑞𝑉2𝑛𝑘𝑇)exp ( 𝑞𝑉

2𝑘𝑇)]

𝐽 =2 𝐴 ∗∗ 𝑇2exp ( ‒ 2𝑞𝜙𝐵0

𝑘𝑇 )sinh ( 𝑞𝑉2𝑘𝑇)

exp ( 𝑞𝑉2𝑛𝑘𝑇) + exp ( ‒ 𝑞𝑉

2𝑛𝑘𝑇)

𝐽 = 𝐴 ∗∗ 𝑇2exp ( ‒ 2𝑞𝜙𝐵0

𝑘𝑇 )sinh ( 𝑞𝑉2𝑘𝑇)

cosh ( 𝑞𝑉2𝑛𝑘𝑇)

References

1. E. H. Rhoderick, IEE Proceedings I - Solid-State and Electron Devices, 1982, 129, 1.2. S. Sze, Physics of semiconductor devices, 2007, 3.

3. X.-L. Tang, H.-W. Zhang, H. Su and Z.-Y. Zhong, Physica E: Low-dimensional Systems and Nanostructures, 2006, 31, 103-106.

4. T. Nagano, M. Tsutsui, R. Nouchi, N. Kawasaki, Y. Ohta, Y. Kubozono, N. Takahashi and A. Fujiwara, The Journal of Physical Chemistry C, 2007, 111, 7211-7217.

5. R. Nouchi, Journal of Applied Physics, 2014, 116, 184505.

-10 -8 -6 -4 -2 0 2 4 6 8 10-30

-20

-10

0

10

20

30

(a)

FTO/CuBi2O4/FTO, 214 oC J fit line (B0=0.35 eV, n=1.022) FTO/CuBi2O4/FTO, 311 oC J fit line (B0=0.36 eV, n=1.025)

Cur

rent

Den

sity

(A/c

m2 )

Voltage (V)

FTO/CuBi2O4/FTO, 25 oC J fit line (B0=0.38 eV, n=1.017) FTO/CuBi2O4/FTO, 90 oC J fit line (B0=0.30 eV, n=1.013)

-10 -8 -6 -4 -2 0 2 4 6 8 10-10

-5

0

5

10

FTO/Cu:NiO/FTO, 216 oC J fit line (B0=0.090 eV, n=1.021)

Cur

rent

Den

sity

(A/c

m2 )

Voltage (V)

FTO/Cu:NiO/FTO, 25 oC J fit line (B0=0.11 eV, n=1.010) FTO/Cu:NiO/FTO, 92 oC J fit line (B0=0.10 eV, n=1.014)

(b)

Figure S16. Current density vs. voltage (J-V) measurements at different temperatures for (a) FTO/CuBi2O4/FTO and (b) FTO/Cu:NiO/FTO samples with J fit lines included.

101 102 103 104 105100

101

102

103

104

FTO/CuBi2O4

Real Imaginary

Impe

danc

e (O

hm)

Frequency (Hz)

(a)

100 101 102 103 104 105100

101

102

103

104

105

Impe

danc

e (O

hm)

Frequency (Hz)

Real Imaginary

FTO/Cu:NiO/CuBi2O4

(b)

100 101 102 103 104 105100

101

102

103

104

FTO/Cu:NiO (34 nm)

Impe

danc

e (O

hm)

Frequency (Hz)

Real Imaginary

(c)

100 101 102 103 104 105100

101

102

103

104

105

(d)

Impe

danc

e (O

hm)

Frequency (Hz)

Real Imaginary

Bare FTO

Figure S17. Electrochemical impedance spectroscopy (EIS) at 1.00 V vs. RHE for (a) CuBi2O4 deposited on FTO, (b) CuBi2O4 (260 nm) deposited on Cu:NiO (7 nm) on FTO, (c) Cu:NiO (34 nm) deposited on FTO, and (d) bare a FTO substrate. The measurements were performed in 0.3 M K2SO4 and 0.2 M phosphate buffer (pH 6.65). Note that above 1 kHz, the real part of the impedance is constant, whereas the imaginary part has a slope of -1. This implies that the system behaves as a resistance in series with a pure capacitance, which is a prerequisite for Mott-Schottky analysis.

0.6 0.8 1.0 1.2 1.40.0

5.0x1010

1.0x1011

1.5x1011

2.0x1011

2.5x1011

fb=1.17 VRHE

FTO/Cu:NiO/CuBi2O4


1/C

2 (F-2cm

4 )

5 kHz 7 kHz 9 kHz

(a)

NA= 3.4 x 1018 cm-3

0.2 0.4 0.6 0.80.0

5.0x1010

1.0x1011

1.5x1011

2.0x1011

Bare FTO

fb=0.305 VRHE

1/C

2 (1

/F2 c

m4 )


6 kHz

(b)

15 10 5 0

UPS

inte

nsity

(a.u

.)

Binding Energy (eV)

FTO

14.6 eV

(e)

Figure S18. Mott−Schottky plots obtained for CuBi2O4 films and bare FTO. (a) Mott−Schottky of CuBi2O4 films deposited on 7 nm Cu:NiO film (b) Mott−Schottky of bare FTO. UPS cutoff spectra measured without bias for (c) CuBi2O4 and (d) Cu:NiO thickness of 34 nm deposited on FTO substrates and UPS cutoff spectra measured with a 2 V bias for a bare FTO substrate. Mott−Schottky measurements were performed in 0.3 M K2SO4 and 0.2 M phosphate buffer (pH 6.65) at 5 kHz, 7 kHz, and 9 kHz.

0.0 5.0x104 1.0x105 1.5x105 2.0x1050.0

5.0x104

1.0x105

Z" (O

hm)

Z' (Ohm)

FTO/Cu:NiO/CuBi2O4

FTO/CuBi2O4

Backside illumination

Figure S19. Electrochemical impedance spectroscopy (EIS) performed under backside illumination at 1.1 V vs. RHE for CuBi2O4

films deposited on FTO (black circle line) and 7 nm Cu:NiO layer (red square line).

Table S1. Summarizing Energy Levels

Material Band Gap, Eg

(eV)

Flat-Band Potential, fb

(eV / V vs. RHE)

Work Function, WF

(eV / V vs. RHE)

Average, EF (fb & WF)

(eV / V vs. RHE)

n-type NA

p-type ND

(cm-3)

p-type

| EV - EF |

(eV)

Valence Band Offset

| EV - EF |

FTO 3.7 4.81 / 0.31 4.61 / 0.11 4.71 / 0.21 1 × 1021

Cu:NiO 3.75 5.21 / 0.71 5.11 / 0.61 5.16 / 0.66 4.6 × 1018 < 0.077 0.36

CuBi2O4 1.6 5.62 / 1.12 5.79 / 1.29 5.71 / 1.21 3.2 × 1018 < 0.077 0.19

Table S2. Sheet resistance of different thickness of Cu:NiO thin films deposited on glass substrates.

.

As Deposited Thickness

(nm)

Post Anneal Thickness

(nm)

Film Sheet Resistance

(/)

Film Resistivity

(m)

4.4 7 1.16 x 108 0.815

12 34 9.58 x 107 3.258

66 102 8.33 x 107 8.492

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Supplementary Information ·