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Future prospects of semiconductor materials for solar and photoelectrochemical cells. Solar to Fuel – Future Challenges and Solutions LBNL Workshop March 28 – 29, 2005. W. Walukiewicz Electronic Materials Program Materials Sciences Division Lawrence Berkeley National Laboratory.

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Future prospects of semiconductor materials for solar and photoelectrochemical cells

Solar to Fuel – Future Challenges and Solutions

LBNL Workshop March 28 – 29, 2005

W. WalukiewiczElectronic Materials ProgramMaterials Sciences Division

Lawrence Berkeley National Laboratory

This work was supported by the Director's Innovation Initiative, National Reconnaissance Office and by the Office of Science, U.S. Department of Energy under Contract No. DE-AC03-76SF00098.


Collaborators photoelectrochemical cells

J. Wu, K. M. Yu, W. Shan, J. W. Ager, J. Beeman, E. E. Haller, M. Scarpulla, O. Dubon, and J. Denlinger

Lawrence Berkeley National Laboratory,

University of California at Berkeley

W. Schaff and H. Lu, Cornell University

A. Ramdas and I. Miotkowski, Purdue University

P. Becla, Massachusetts Institute of Technology

Outline photoelectrochemical cells

  • High Efficiency Solar Cell Concepts

  • New semiconductors for multijunction solar cells

    • GaxIn1-xN alloys

  • Intermediate band solar cell materials

    • Highly mismatched alloys (HMAs)

    • II-Ox-VI1-x HMAs as intermediate band materials

  • Group III-nitrides for photoelectrochemical cells

  • Challenges and prospects

Solar cells ultimate efficiency limits
Solar Cells photoelectrochemical cellsUltimate Efficiency Limits

  • Intrinsic efficiency limit for a solar cell using a single semiconducting material is 31%.

    • Light with energy below the bandgap of the semiconductor will not be absorbed

    • The excess photon energy above the bandgap is lost in the form of heat.

    • Single crystal GaAs cell: 25.1% AM1.5, 1x

  • Multijunction (MJ) tandem cell

    • Maximum thermodynamically achievable efficiencies are increased to 50%, 56%, and 72% for stacks of 2, 3, and 36 junctions with appropriately optimized energy gaps

Eg1 > Eg2 > Eg3

Cell 1 (Eg1)

Cell 2 (Eg2)

Cell 3 (Eg3)

Multijunction solar cells
Multijunction Solar Cells photoelectrochemical cells

State-of-the art 3-junction GaInP/Ga(In)As/Ge solar cell: 36 % efficient

M. Yamaguchi et. al. – Space Power Workshop 2003

Direct bandgap tuning range of in 1 x ga x n potential material for mj cells
Direct bandgap tuning range of In photoelectrochemical cells1-xGaxNPotential material for MJ cells

  • The direct energy gap of In1-xGaxN covers most of the solar spectrum

  • Multijunction solar cell based on this single ternary could be very efficient

LBNL/Cornell work: J. Wu et al. APL 80, 3967 (2002)

Ingan is radiation hard electron proton and he irradiation
InGaN photoelectrochemical cellsis radiation hardelectron, proton, and He+ irradiation

Surface electron accumulation

E photoelectrochemical cellsFS


Eg = 0.7eV




Surface Electron Accumulation

  • Surface/interface native defects (dangling bonds) are similar to radiation-induced defects




  • High concentration of defects near surface – Fermi level pinning

P type doping of inn
P-type Doping of InN photoelectrochemical cells

In 1 x ga x n alloys as solar materials
In photoelectrochemical cells1-xGaxN alloys as solar materials

  • Significant progress in achieving p-type doping

  • Exceptional radiation hardness established

  • Surface electron accumulation in In-rich alloys

  • Quality of InN/GaInN interfaces

Multijunction vs multiband

junction3 photoelectrochemical cells




Multijunction vs. Multiband

  • Multi-band

  • Single junction (no lattice-mismatch)

  • N bands  N·(N-1)/2 gaps

    •  N·(N-1)/2 absorptions

  • Add one band  add N absorptions

  • Multi-junction

  • Single gap (two bands) each junction

  • N junctions  N absorptions

  • Efficiency~30-40%

Theoretical efficiency of intermediate band solar cells

CB photoelectrochemical cells








Theoretical efficiency of Intermediate band solar cells

  • Intermediate Band Solar Cells can be very efficient

    • Max. efficiency for a 3-band cell=63%

    • Max. efficiency for a 4-band cell=72%

    • In theory, better performance than any other ideal structure of similar complexity

      But NO multi-band materials realized to date

Luque et. al. PRL, 78, 5014 (1997)

Highly mismatched alloys for multiband cells
Highly Mismatched Alloys for Multiband Cells photoelectrochemical cells

  • Oxygen in II-VI compounds has the requisite electronegativity and atomic radius difference

    XO = 3.44; RO = 0.073 nm

    XS = 2.58; RS = 0.11nm

    XSe = 2.55; RSe = 0.12 nm

    XTe = 2.1; RTe = 0.14

  • Oxygen level in ZnTe is 0.24 eV below the CB edge

    • Can this be used to form an intermediate band?

  • Synthesis

    • Very low solid solubility limits of O in II-VI compounds

    • Nonequilibrium synthesis required

Zn 1 y mn y o x te 1 x intermediate band material
Zn photoelectrochemical cells1-yMnyOxTe1-x: Intermediate Band Material

K. M. Yu et. al., Phys. Rev. Lett., 91, 246403 (2003)

Zn 0 88 mn 0 12 o 0 03 te 0 97 intermediate band semiconductor
Zn photoelectrochemical cells0.88Mn0.12O0.03Te0.97: Intermediate Band Semiconductor

Photovoltaic action
Photovoltaic action photoelectrochemical cells

How efficient can they be multi band znmnote alloys
How efficient can they be? photoelectrochemical cellsMulti-band ZnMnOTe alloys

  • The location and the width of the intermediate band in ZnMnOxTe1-x is determined by the O content, x

  • Can be used to maximize the solar cell efficiency

  • Calculations based on the detailed balance model predict maximum efficiency of more than 55% in alloys with 2% of O

Intermediate band semiconductors challenges an prospects
Intermediate band semiconductors photoelectrochemical cellsChallenges an prospects

  • Synthesis of suitable materials with scalable epitaxial techniques (MBE growth of ZnOxSe1-x achieved)

  • N-type doping of intermediate band with group VII donors (Cl, Br)

  • Control of surface properties of the PLM synthesized materials

  • Other highly mismatched alloys: GaPyNxAs1-x-y

  • Fundamentals

    • Nature of the intermediate band: localized vs. extended

    • Carrier relaxation processes


Photoelectrochemical cells photoelectrochemical cellsfor hydrogen generation

Joel W. Ager, Alexis T. Bell,* Miquel Salmeron, Wladek WalukiewiczElectronic Materials ProgramMaterials Sciences Division

Lawrence Berkeley National Laboratory

*Chemical Sciences Division

InN support:FY03 LDRD, FY04 Director's Innovation Initiative, National Reconnaissance Office


J. A. Turner, photoelectrochemical cellsScience 285, 687 (1999)

Photoelectrochemical h 2 generation
Photoelectrochemical H photoelectrochemical cells2 generation

1.       Absorption of light near the surface of the semiconductor creates electron-hole pairs.

2.       Holes (minority carriers) drift to the surface of the semiconductor (the photo anode) where they react with water to produce oxygen: 2h+ + H2O -> ½ O2 (g) + 2H+

3.       Electrons (majority carriers) are conducted to a metal electrode (typically Pt) where they combine with H+ ions in the electrolyte solution to make H2 :

2e- + 2H+ -> H2 (g)

4.       Transport of H+ from the anode to the cathode through the electrolyte completes the electrochemical circuit.

The overall reaction :2hn + H2O -> H2(g) + ½ O2 (g)

Why is it hard to do
Why is it hard to do? photoelectrochemical cells

  • Oxides

    • Stable but efficiency is low (large gap)

  • III-Vs

    • Efficiency is good but surfaces corrode

  • Approaches

    • Dye sensitization (lifetime issues)

    • Surface catalysis

  • No practical PEC H2 production demonstrated

    • Efficiency and lifetime

Adapted from M. Grätzel, Nature 414, 388 (2001)

What are the fundamental issues
What are the fundamental issues? photoelectrochemical cells

  • Band structure engineering

    • To match water redox potentials and achieve high solar efficiency

  • Fundamental understanding of the electrode/electrolyte interface

    • To accelerate water splitting reaction and reduce corrosion

Why use nitrides direct bandgap tuning range of ingan
Why use nitrides? photoelectrochemical cellsDirect bandgap tuning range of InGaN

  • The direct energy gap of In1-xGaxN covers most of the solar spectrum

  • Multijunction solar cell based on this single ternary could be very efficient

LBNL/Cornell work: J. Wu et al. APL 80, 3967 (2002)

Iii nitrides tuning the band edges
III-Nitrides – tuning the band edges photoelectrochemical cells

  • Their conduction and valence band edges straddle the H+/H2 and O2/H2O redox potentials.

  • They can be made with the optimal bandgap of ~2.0 eV

    • Experimentally determined by our group

  • They have superior corrosion resistance compared to other semiconductors of similar energy gaps.

InGaN: J. Wu et al. APL 80, 3967 (2002)

GaNAs: J. Wu et al., PRB

Photocurrent in 0 37 ga 0 63 n
Photocurrent photoelectrochemical cellsIn0.37Ga0.63N

Surface modification concepts catalysis and corrosion inhibition
Surface modification concepts photoelectrochemical cellsCatalysis and corrosion inhibition

  • Catalysts can facilitate the oxidation of water on the anode and reduction of protons on the cathode

  • Candidate materials

    • Anode – Pt, Pt/Ru alloys, RuO2, MoO3, ZrO2

    • Cathode – Porphyrins, phtalocyanins, ferrocenes

  • Corrosion can be inhibited by an oxide coating











h + e



To cathode

Fundamental and practical issues
Fundamental and Practical Issues photoelectrochemical cells

  • Synthesis of materials: MBE, MOCVD, PLM

  • Charge transport and doping

  • Evaluate photo cathode (p-type semiconductor surface) vs. photo anode (n-type semiconductor surface) designs

  • Measurements of band offsets

  • Fundamental studies in-situ and ex-situ of the electrolyte-semiconductor interface

  • Surface modification

  • Kinetic H2 production and corrosion rates.