Future prospects of semiconductor materials for solar and photoelectrochemical cells

1. 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.

2. Collaborators 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

3. Outline • 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

4. Solar 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)

5. Multijunction Solar Cells State-of-the art 3-junction GaInP/Ga(In)As/Ge solar cell: 36 % efficient M. Yamaguchi et. al. – Space Power Workshop 2003

6. Direct bandgap tuning range of In1-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)

7. InGaN is radiation hardelectron, proton, and He+ irradiation

8. EFS EF Eg = 0.7eV ~1eV Bulk Surface Surface Electron Accumulation • Surface/interface native defects (dangling bonds) are similar to radiation-induced defects CB EFS VB • High concentration of defects near surface – Fermi level pinning

9. P-type Doping of InN

10. In1-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

11. junction3 I junction2 junction1 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%

12. CB Eg Ec qV IB Ei 0 VB 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)

13. Highly Mismatched Alloys for Multiband 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

14. Zn1-yMnyOxTe1-x: Intermediate Band Material K. M. Yu et. al., Phys. Rev. Lett., 91, 246403 (2003)

15. Zn0.88Mn0.12O0.03Te0.97: Intermediate Band Semiconductor

16. Photovoltaic action

17. How efficient can they be?Multi-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

18. Intermediate band semiconductors Challenges 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

19. Photoelectrochemical cells for 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

20. J. A. Turner, Science 285, 687 (1999)

21. Photoelectrochemical H2 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)

22. Why is it hard to do? • 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)

23. What are the fundamental issues? • 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

24. Why use nitrides?Direct 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)

25. III-Nitrides – tuning the band edges • 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

26. PhotocurrentIn0.37Ga0.63N

27. Surface modification conceptsCatalysis 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 O2 H2O hn H+ Catalyst O Photoanode HO H H h + e e e To cathode

28. Fundamental and Practical Issues • 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.