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Semiconductor Materials and Tandem Cells

for Photoelectrochemical Hydrogen

Production: GaP 1-x N x and Cu(In,Ga)Se,S)

ICMR Symposium on Materials Issues in Hydrogen Production

and Storage

Santa Barbara, CA

August 24, 2006

John A. Turner

John_Turner@nrel.gov 303-275-4270


Sustainable Paths to Hydrogen

Solar Energy

Heat

Biomass

Mechanical Energy

Electricity

Conversion

Thermolysis

Electrolysis

Photolysis

Hydrogen


Direct Conversion Systems

Visible light has sufficient energy to split water

(H 2 O) into Hydrogen and Oxygen

Combination of a Light Harvesting System and a Water Splitting System

Semiconductor photoelectrolysis

Photobiological Systems

Homogeneous water splitting

Heterogeneous water splitting

Thermal cycles

(Sunlight and Water to Hydrogen

with No External Electron Flow)


Photoelectrochemical-Based Direct

Conversion Systems

• Combines a photovoltaic system

(light harvesting) and an

electrolyzer (water splitting) into a

single monolithic device.

– Electrolysis area approximates

that of the solar cell - the current

density is reduced.

– Efficiency can be 30% higher than

separated system.

• Balance of system costs reduced.

– Capital cost of electrolyzer

eliminated

• Semiconductor processing

reduced.


Current Density vs. Voltage for 2 Pt Electrodes of

Equal Area.

V,I

H 2

O 2

Water

Current Density, mA/cm 2

120

100

80

60

40

20

2M KOH

AM1.5 Solar Cell

current density

1.45V means ~85%

LHV electrolysis

efficiency

0

1.30 1.32 1.34 1.36 1.38 1.40 1.42 1.44 1.46

Voltage, V

O. Khaselev, A. Bansal and J. A. Turner, International Journal of

Hydrogen Energy, 26, p 127-132 (2001).


Chemical Reactions at a Semiconductor

Electrolyte Interphase

Electron energy

E c

Acceptor

Light (photons)

Donor

E v + e -

+

Solid (semiconductor)

Liquid (electrolyte)

P169-A302101


Band Edges of p- and n-Type

Semiconductors Immersed in Aqueous

Electrolytes to Form Liquid Junctions

E conduction

O 2

p-type

E valence

2H 2

O + 2e – = 2OH – + H

2

E conduction

H 2

n-type

H 2

O + 2h + = 2H + + 1/2 O 2

P158-A199107

E valence


Electron

Energy

Technical Challenges (the big three)

Material Characteristics for Photoelectrochemical

Hydrogen Production

H 2 O/H 2

‣Efficiency – the band gap

(E g ) must be at least 1.6-1.7

eV, but not over 2.2 eV, high

photon to electron

conversion.

E g

1.23 eV

1.6-1.7 eV

Counter

Electrode

‣Material Durability –

semiconductor must be

stable in aqueous solution

p-type

Semiconductor

H 2 O/O 2

‣Energetics – the band

edges must straddle H 2 O

redox potentials (Grand

Challenge)

i

All must be satisfied

simultaneously


Bandedge Energetic Considerations

T. Bak, J. Nowotny, M. Rekas, C.C. Sorrell, International Journal of Hydrogen Energy 27 (2002) 991– 1022


Gallium Indium Phosphide/Electrolyte System

Used to gain a fundamental understanding of

semiconductor/electrolyte junctions

(-)

E CB

Eg = 1.83 eV

OK

Energy

(+)

E VB

p-GaInP 2

E F

H 2

O/H 2

Band edges are

too negative

Band edges are

pH sensitive

H 2

O/O 2


Bandedge Energetic Considerations

GaInP

T. Bak, J. Nowotny, M. Rekas, C.C. Sorrell, International Journal of Hydrogen Energy 27 (2002) 991– 1022


Dual Photoelectrode Approach for

Bandedge Mismatch and Stability Issues

a

e

b

α -Fe

O

2 3

µA

p-GaInP

2

-2.0

-2.0

-1.0

e

CB

-1.0

hv

0.0

1.0

CB

e

E 0 (H /H O)

2 2

E 0 (H O/O )

2 2

h

VB

0.0

1.0

2.0

VB

2.0

h

Electrolyte

Lower cost Fe 2 O 3 electrode is a possibility, but while the system

splits water, the efficiency is very low.


Metal Oxides – A materials assessment

PEC devices must have the same internal photon-toelectron

conversion efficiency as PV devices.

• Transitions which impart color to the oxides typically

involve d and f levels and are generally forbidden

transitions = low absorption coefficients.

– The low absorption coefficients result in the incident light

penetrating deep into the material = 100’s um of material

• Carrier mobilities in oxides are very low

– Recombination of photogenerated carriers occurs before

they can reach the semiconductor/electrolyte interface.

• Even good single crystals of metal oxides have very

low energy conversion efficiencies due to inherently

short diffusion lengths.

However a success in this area, e.g. an oxide with good

to excellent conversion properties could revolutionize

the PV industry as well as make PEC near-term.


Approach: High Efficiency Materials and

Low-Cost Manufacturing

PEC devices must have the same internal photon-to-electron conversion

efficiency as commercial PV devices.

• III-V materials have the highest solar conversion efficiency of any

semiconductor material

– Large range of available bandgaps

– ….but

• Stability an issue – nitrides show promise for increased lifetime

• Band-edge mismatch with known materials – tandems an answer

• I-III-VI materials offer high photon conversion efficiency and

possible low-cost manufacturing

– Synthesis procedures for desired bandgap unknown

– ….but

• Stability in aqueous solution?

• Band-edge mismatch?

• Other thin-film materials with good characteristics

– SiC: low cost synthesis, stability

– SiN: emerging material


Approach: Materials Summary

The primary task is to synthesize the semiconducting material or the

semiconductor structure with the necessary properties. This involves

material research issues (material discovery), multi-layer design and

fabrication, and surface chemistry. Activities are divided into the task areas

below – focus areas in Green:

GaPN - NREL (high efficiency, stability)

CuInGa(Se,S) 2 - UNAM (Mexico), NREL (Low cost)

Silicon Nitride – NREL (protective coating and new material)

GaInP 2 - NREL (fundamental materials understanding)

Energetics

Band edge control

Catalysis

Surface studies


Gallium Indium Phosphide/Electrolyte System

Used to gain a fundamental understanding of

semiconductor/electrolyte junctions

(-)

E CB

Eg = 1.83 eV

OK

Energy

(+)

E VB

p-GaInP 2

E F

H 2

O/H 2

Band edges are

too negative

Band edges are

pH sensitive

H 2

O/O 2


Review: Comparison of p-GaInP 2 and PEC/PV device

0

Current Density, mA/cm 2

-40

-80

-120

3 M H 2

SO 4

, ~ 11.6 suns

1: p-GaInP 2

(Pt)/TJ/GaAs

2 : p-GaInP 2

(Pt)

2 1

-160

-1.5 -1.0 -0.5 0.0 0.5

Voltage versus Pt

The GaAs integrated PV cell compensates

for the energetic mismatch – but lifetime…


Photocurrent time profile for PEC/PV Water-Splitting device,

showing current decay due to corrosion.

O. Khaselev and J. A. Turner, Science, 280, pg 425 (1998).

Current Density, mA/cm 2

-40

-60

-80

-100

CD, mA/cm 2

-50

-100

0 5 10 15 20

Time, hours

We are looking for

materials with

inherently greater

stability.

-120

-140

Efficiency(%) = (120mA/cm 2 2

*1.23v/1190 mW/cm )*100

= 12.4%

0 500 1000 1500

Time, sec


Elemental Abundances in Earth’s Crust

Element

Concentration

(mg/kg)

Element

Concentration

(mg/kg)

Gallium

19

P

1050

Lead

14

Silicon

2.8 x 10 5

Boron

10

Indium

0.25

Lithium

20

Silver

0.075

Tungsten

1.25

Tin

2.3

Cobalt

25

Uranium

2.7

Gallium is found in bauxite (aluminum ore). There are 2

million kg of Ga in Arkansas bauxite deposits alone. Also

found in coal ash (up to 1.5%).


Hydrogen Evolution Current-Time Profile for p-GaN

Current density (A/cm 2 )

8.0x10 -5

7.0x10 -5

6.0x10 -5

5.0x10 -5

4.0x10 -5

3.0x10 -5

2.0x10 -5

9.0x10 -5 1 M KOH

pH 7 buffer

3 M H 2

SO 4

1.0x10 -5

0 1000 2000 3000 4000

Time (s)

S. Kocha, M. Peterson, D. J. Arent, J. M. Redwing, M. A. Tischler and J. A. Turner, “Electrochemical Investigation

of the Gallium Nitride-Aqueous Electrolyte Interface”, Journal of The Electrochemical Society, 1995,142, pg. L238-

L240.


Goal: Stable III-V nitride material

• GaN

– Capable of water-splitting and stable

– Band gap direct but too wide (~3.4 eV)

• GaP

– Band edges almost aligned

– Band gap indirect and too wide (~2.3

eV), but better

• GaP 1-x N x

– Addition of small amounts of N causes

GaP band gap to narrow (bowing) and

transition to become direct

– Nitrogen enhances stability

III-V Nitrides for PEC Water Splitting

Systems: GaP 1-x N x

Normalized Photocurrent 2 (AU)

GaP 1-x N x Epilayer Direct Band Gap

0.06% N

0.11% N

0.15% N

0.20% N

1.10% N

1.60% N

0

2 2.1 2.2 2.3 2.4

Photon Energy (eV)

Dr. Todd Deutsch


Goal: Stable III-V nitride material

• Control band gap energy

by varying nitrogen

composition,

• Direct band gap for all

nitrogen concentrations,

even at GaP .9994 N .0006

• The crossover from

indirect to direct occurs

significantly below the

theoretically predicted

0.26%

(F. Benkabou, J. P. Becker, M.

Certier, H. Aourag, Calculation of

Electronic and Optical Properties

of Zinc Blende GaP1-xNx.

Superlattices and Microstructures,

1998. 23(2): p. 453-465.)

Dr. Todd Deutsch

GaP 1-x N x Bandgap as a Function of

Nitrogen Concentration

Direct Band Gap (eV)

2.25

2.2

2.15

2.1

2.05

2

1.95

1.9

1.85

This Study

Yu et. al.

0 1 2 3 4

Percent Nitrogen

K.M. Yu, W. Shan, J. Wu, Nature of the fundamental band gap in

GaN x P 1-x alloys. Applied Physics Letters, 2000. 76(22): p. 3251-3253


Goal: Stable III-V nitride material

Corrosion

experiments on

GaP substrate

clearly show the

enhanced

corrosion

resistance from

nitrogen addition.

Dr. Todd Deutsch


Goal: Stable III-V nitride material

1.2

1.11

GaPN Stability Measured by Profilometry

and by ICP-MS of Solution

Average Etch Depth (µm)

1

0.8

0.6

0.4

0.2

0

0.56

0.36

0.07

GaInP2 GaP no Pt GaP GaPN

0.2%N

Epilayer Composition

0.25

GaPN

2.1%N

ppb Ga

3000

2500

2000

1500

1000

500

0

5.0

Dark

5mA/cm^2

130

AM 1.5

5mA/cm^2

540

AM 1.5

Open Circuit

2500

AM 1.5

5mA/cm^2

No Surfactant

2mm stylus scan

Conditions

5mm

Exposed/active

surface

Masked/native

surface

GaInP 2 ~ 1 µm

Nitrides ~ 0.1 µm

• Photocathode durability testing

– 5mA/cm 2 , AM 1.5, 24 hours, 3M H 2

SO 4

,

pulsed Pt treatment

– Ga content of solutions by ICP-MS

– Etching by profilometry

4mm

• Represents ~300 hrs of stability for 1 um layer

Dr. Todd Deutsch


Goal: Stable III-V nitride material

Band Edge Energetics

The band edges of these materials are too negative for

spontaneous water-splitting.

Potential (V vs. Ag/AgCl)

-1.5

-1

-0.5

0

0.5

1

GaP

0.06%N

0.11%N

0.10%N

0.15%N

0.17%N

0.18%N

0.19%N

1.1%N

1.6%N

GaAsPN

GaAsP

Hydrogen

Oxygen

Potential (V vs. Ag/AgCl)

0.2

0.24

0.28

0.32

0.36

0.4

0.44

0.48

GaP

0.06%N

0.11%N

0.19%N

1.6%N

GaAsPN

GaAsP

1.5

0 5 10 15

pH

0.52 1.5 2.5 3.5 4.5

pH


Approach: Two configurations

One for material study and one for possible high-efficiency

tandem cell.

• GaPN/tj/ p-GaP or

n-silicon substrate

– Undoped

– No low energy

transition expected

• GaPN/tj/ p/n silicon

substrate (tandem)

– Undoped

– Low energy

transition expected

1 µm

i-GaP.9806N.0194

1 µm p-GaP.9818N.0182

0.04 µm

GaP

n-Si substrate

0.04 µm GaP

n-Si

p-Si substrate

Ti/Pd/Al/Pd/AuOhmic contact

MF097

Al Ohmic contact

MF098


Goal: III-V Stable nitride III-V tandem nitride material water splitting cell

-

E

EF

VB

+

Ohmic Contact

p

Si

n

E

Tunnel Junction

GaP .98 N .02

E g ~2eV

e -

H 2

CB

O 2

e -

H + hν

h +


Goal: Stable III-V nitride material

Light-Driven Water Splitting

MF771-4 2-electrode J-V in acid 1 sun illumination

0.0005

0

-0.2 -0.1 0 0.1 0.2 0.3 0.4

Current Density (A/cm^2)

-0.0005

-0.001

-0.0015

Summary:

• GaPN/Si tandem can

photoelectrochemically split

water but very low efficiency.

• GaPN has enhanced corrosion

resistance

Dr. Todd Deutsch

-0.002

Applied Potential (V)


Goal: Stable III-V nitride material

GaInPN:Si – Two Tandem Cell

Configurations

1 µm

GaInPN

5.0E-02

4.5E-02

Ga .96

In .04

N .024

P .976

4.0E-02

Silicon

npc 2 (AU)

3.5E-02

3.0E-02

2.5E-02

2.0E-02

1.5E-02

1.0E-02

5.0E-03

E g

= 2.0 eV

1 µm

0.1 µm

GaInPN

0.0E+00

1.5 1.7 1.9 2.1 2.3 2.5 2.7

photon energy (eV)

Silicon

Zn-doped GaInPN

Jeff Head, Paul Vallett


I vs. V Plot

Ga .96 In .04 N .024 P .976

0.2

Characterization

PC Onset

-1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

0

-0.2

J (µA/cm 2 )

-0.4

-0.6

Dark

Light

Jeff Head, Paul Vallett

-0.8

-1

Potential (V vs. Ag/AgCl)

Cathodic PhotoCurrent

H 2 Evolution

p-type


Goal: Stable III-V nitride material

Stability by Profilometry

Ga .96 In .04 N .024 P .976 :

Etched: 0.13µm/24hrs

Ga .95 In .05 N .025 P .975 :

Etched: 0.14µm/24hrs

GaInP 2 :1-2µm/24hrs

Jeff Head, Paul Vallett


Goal: Stable III-V nitride material

Water Splitting

• Photon to chemical energy

conversion efficiency

– GaInPN: ~30%

–GaInP 2 : ~90%

• Zn-doped layer improves

efficiency

• Nitrogen decreases electronic

properties of material

Solar to Hydrogen Efficiency (%) .

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Zn -

GaInPN/Si

GaInPN/Si

Jeff Head, Paul Vallett


Goal: Evaluation of high efficiency thin-film CIS-based material

Electrodeposited CIGSS

• CIS material system: CuInSe 2

–CuGaS 2

– E g

range: 1.0 – 2.5eV

• CuGaSe 2

: E g

= ~1.7

• High PV efficiencies in this system

• Potential low cost, low energy deposition

• Two configurations:

– Single materials

– As a layer in a multijunction system

Relationship of bandgap to alloy composition. Gray

area represents compositions that produce

bandgaps in the range 1.7 – 2.1eV

Dr. Jennifer E. Leisch

Goal: Evaluation of high efficiency thin-film

material and thin-film based tandem cell


Goal: Thin-film based PEC tandem cell

Tandem Cell Configuration

CGS Grown by Thermal Evaporation

Maximum theoretical

water-splitting

efficiency = 28%

CGS: 1.3µm

ITO: 150nm

Dr. Jennifer E. Leisch


Goal: Evaluation of high efficiency thin-film CIS-based material

Results – Band Positions

Jennifer E. Leisch

Illuminated Open Circuit Potential Measurements

High intensity DC W lamp illumination


Goal: Thin-film based PEC tandem cell

Results – Band Energy Diagram

Energetics say:

H 2 O splitting, but

very low currents

Why?

- Poor ITO TJ

- Kinetics for H 2

- Recombination

- Band shifting

- Corrosion

Dr. Jennifer E. Leisch


Results – Water Splitting

Efficiency

Possible band edge alteration on anodic scan. . .

Jennifer E. Leisch

J sc (Cathodic Scan)

pH 6 RuHex, 1sun -5.7µA

Acid, ~ 20 Suns -3.4µA

Base, ~ 20 Suns -7µA

.01%


2mA/cm 2 , 3M H 2 SO 4 , 2hrs.

100mW/cm 2 W Illumination

.03mL/min-cm 2 H 2 Evolution

Results – Stability

The high temperature CGS

deposition process damages

the ITO layer.

Corroded

Electrodeposition may be a

better approach.

As-deposited

As-deposited

Dr. Jennifer E. Leisch


IV – Goal: Low-Temp CuGaSe 2 Deposition

CuGaSe 2 Electrodeposition

2+


0

Cu + 2e

⇔ Cu E = + 0. 342V

3+


0

Ga + 3e

⇔ Ga E = −0.

549V

HSeO e H Se HO E V

0

2 3

+ 4 − + 4 + ⇔ + 3

2

=+ 0.739

Lincot, D., et al., Chalcopyrite thin film solar cells by electrodeposition. Solar Energy, 2004. 77(6): p. 725-737.

Dr. Jennifer E. Leisch


IV – Goal: Low-Temp CuGaSe 2

Deposition

Film Microstructure

Dr. Jennifer E. Leisch

XRD

No Ga-containing

phases observed


IV – Goal: Low-Temp CuGaSe 2

Deposition

Film Composition by ICP-MS

(inductively coupled plasma-mass spectrometery)

• Films close to the proper

stoichiometry (1:1:2) were

electrodeposited (ambient T

and P)

• Gallium deposition can be

obtained via codeposition

mechanism

• Ga must be amorphous or of

very small structure

• Excess Se inhibits Ga

deposition

• Next step: Annealing

Dr. Jennifer E. Leisch


Gallium Indium Phosphide/Electrolyte System

Used to gain a fundamental understanding of

semiconductor/electrolyte junctions

(-)

E CB

Eg = 1.83 eV

OK

Energy

(+)

E VB

p-GaInP 2

E F

H 2

O/H 2

Band edges

are too

negative

Band edges

are pH

sensitive

H 2

O/O 2


Band Edge Engineering

The goal is to shift the band edges by surface

modification to provide the proper energetic overlap.

-

CB

Modification

Negative charges move the

bandedges negative.

E

+

VB

CB

VB

H 2 O/H 2

H 2 O/O 2

Positive charges move the

bandedges positive.

x


Metallo-Porphyrins

x

x

Octaethyl Porphyrin

~OEP~

Tetra(N-Methyl-4-Pyridyl)porphyrin

~TMPyP(4)~

Insoluble porphyrin dissolved in methlene chloride and electrode coated

by drop evaporation. Water soluble porphyrin applied by soaking the

electrode in a solution of the porphyrin.


Band Edge Engineering - GaInP 2

V FB

vs. pH for RuCl 3

+ RuOEP Treated GaInP 2

Electrode

VFB

-0.200

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

RuCl3 & RuOEP

Untreated

-2 0 2 4 6 8 10 12

The band edges here should

be able to split water…..but no

water splitting observed.

pH

H 2

O Oxidation

Potential

S. Warren and J. Turner, USDOE Journal of

Undergraduate Research, p 75 (2002)


Photoelectrochemical Water Splitting

Current Staff

Heli Wang

Todd Deutsch, Postdoc

Mark Reimann, Colorado School of Mines (PhD

Student)

Recent Past

Jennifer Leisch , Colorado School of Mines (PhD

2006 – now at Stanford)

Jeff Head, Colorado State University (SS- now at

ASU)

Paul Vallett, University of Vermont (SS)

Scott Warren, Whitman College (SS - now at

Cornell)

380 ppm

Others

Vladimir Aroutiounian, University of Yerevan,

Armenia (CRDF, ISTC)

Arturo Fernández, Centro de Investigación en

Energía-UNAM, Mexico

Funding: DOE Hydrogen Program

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