# Presentation: 266959

## Future prospects of semiconductor materials for solar and photoelectrochemical cells

**Future prospects of semiconductor materials for solar and photoelectrochemical cells**

**W. Walukiewicz**** **Electronic 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.

***Solar to Fuel – Future Challenges and Solutions***
***LBNL Workshop March 28 – 29, 2005***

## 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**

## Outline

**High Efficiency Solar Cell Concepts**
**New semiconductors for multijunction solar cells**
**Ga****x****In****1-x****N alloys**
**Intermediate band solar cell materials**
**Highly mismatched alloys (HMAs)**
**II-O****x****-VI****1-x**** HMAs as intermediate band materials**
**Group III-nitrides for photoelectrochemical cells**
**Challenges and prospects**

## Solar CellsUltimate 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**

**Cell 1 (E****g1****)**

**Cell 2 (E****g2****)**

**Cell 3 (E****g3****)**

**E****g1 ****> E****g2 ****> E****g3**

## 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

## Direct bandgap tuning range of In1-xGaxNPotential material for MJ cells

**The direct energy gap of In****1-x****Ga****x****N 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

##   Surface Electron Accumulation

**  ****Surface Electron Accumulation**

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

- VB

- CB

- High concentration of defects near surface – Fermi level pinning

- EFS

## P-type Doping of InN

## 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***

## Multijunction  vs. Multiband

**Multi-junction**
- Single gap (two bands) each junction
- N junctions  N absorptions
- Efficiency~30-40%

**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

## Theoretical efficiency of Intermediate band solar cells

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

**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***

## Highly Mismatched Alloys for Multiband Cells 

**Oxygen in II-VI compounds has the requisite electronegativity and atomic radius difference**
**X****O**** = 3.44;****	****R****O**** = 0.073 nm**
**X****S**** = 2.58;****	****R****S**** = 0.11nm**
**X****Se**** = 2.55;****	****R****Se**** = 0.12 nm**
**X****Te**** = 2.1;****	****R****Te**** = 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**

## Zn1-yMnyOxTe1-x: Intermediate Band Material

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

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

**Zn****0.88****Mn****0.12****O****0.03****Te****0.97****: Intermediate Band Semiconductor**

## Photovoltaic action

## 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

## Intermediate band semiconductors Challenges an prospects

**Synthesis of suitable materials with scalable epitaxial techniques *****(MBE growth of ZnO******x******Se******1-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: GaP****y****N****x****As****1-x-y**
**Fundamentals**
**Nature of the intermediate band: localized vs. extended**
**Carrier relaxation processes **

## Photoelectrochemical cells for hydrogen generation

**Photoelectrochemical cells ****for hydrogen generation**

**Joel W. Ager, Alexis T. Bell,* Miquel Salmeron, **_**Wladek Walukiewicz**_Electronic 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, *Science ***285**, 687 (1999)

## 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****+**** + H****2****O -> 1⁄2 O****2**** (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 H****2**** :**
**	****2e****-**** + 2H****+**** -> H****2**** (g)****	**
**4.       Transport of H****+**** from the anode to the cathode through the electrolyte completes the electrochemical circuit.  **
**The overall reaction :**** *****2h************ + H******2******O -> H******2******(g) + 1⁄2 O******2****** (g)****** ***

## 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 H******2****** production demonstrated***
***Efficiency ***_***and***_*** lifetime***

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

## 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**

## Why use nitrides?Direct bandgap tuning range of InGaN

**The direct energy gap of In****1-x****Ga****x****N 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

**Their conduction and valence band edges straddle the H****+****/H****2**** and O****2****/H****2****O 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In0.37Ga0.63N 

## Surface modification conceptsCatalysis and corrosion inhibition

**Surface modification concepts****Catalysis 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, RuO****2****, MoO****3****, ZrO****2**
**Cathode – Porphyrins, phtalocyanins, ferrocenes**
**Corrosion can be inhibited by an oxide coating**

**H****2****O**

**HO**

**O**

**H**

**H**

**e **

**H****+**

**h******

**h + e**

**e**

**Catalyst**

**Photoanode**

**O****2**

**To cathode**

## 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 H****2**** production and corrosion rates. **