Upload
others
View
4
Download
0
Embed Size (px)
Citation preview
Woods Energy Seminar, 28 May 2008
Third Generation Photovoltaics
Photovoltaics Centre of Excellence supported by the Australian Research Council, the Global Climate and Energy Project and Toyota CRDL
Gavin Conibeer
Deputy DirectorARC Photovoltaics Centre of Excellence
University of New South Wales
Transforming the global energy mix: The exemplary path until 2050/ 2100appointed for a term of four years by the federal cabinet (Bundeskabinett)
Meeting the IPCC target of 60% reduction in GHG emission by 2050
The business case for early action60% reduction in GHG emission by 2050
Outline
• The importance of Photovoltaics• Three generations of Photovoltaics• The main losses in photovoltaic cells• Third Generation approaches
• Silicon nanostructure tandem cells• Band gap engineering – quantum confinement• Fabrication of materials / devices
• Hot Carrier cells• Contacts – energy filtering • Hot Carrier cooling – energy loss to phonons
• Summary
Booming Photovoltaics
Global PV market US$6.5 billion in 2006 → $16.4 billion in 2012
Market growth at 35%/yr for last 10 years, 60%+ in 2007Approx 1 million jobs in PV by 2020Approx 1 million jobs in RE by 2010
Driven by rebates/tariffs: Japan, Germany
Now other Euro. Countries and S Australia
USA: Power purchase agreements
Japan: market is stablewith reducing rebates
1988
1991
1994
1997
2000
2003
2006
0
1000
2000
3000
4000
5000
MW
p
USAEuropeJapanRest of WorldTotal
Annual capacity increase
2000
2001
2002
2003
2004
2005
2006
Solar Heating
Wind
Nuclear
Photovoltaics0
5
10
15
New
Cap
acity
, GW
.
Sources: Photon International, WNA, Sources: Photon International, WNA, WWEA, IEAWWEA, IEA
Google’s Mountain View campus
Learning curves
bulk-Si(~10%)
2003
US
$ /kW (~20%)
20000
10000
5000
2000
1000
500
2000.01 0.1 1.0 10.0 100.0
Gas turbines (USA)
1980
1963
1982
19871993
2001
Wind turbines
2002
Cumulative GW installed
1981
Photovoltaics
. more potential for learning
. lower cost at smaller volumes
20000
10000
5000
2000
1000
500
2000.01 0.1 1.0 10.0 100.0
2002
Cumulative GW installed
1981
Photovoltaics
bulk-Si(~10%)
2003
US
$/kW (~20%)
Thin-film PV
2nd Generation
2002
3rd Generation
1st Generation
100
80
60
20
0 100 200 300 400 500
US$0.50/W
US$1.00/W
US$3.50/W
Cost, US$/m2
US$0.10/W US$0.20/W
Present limit
Thermodynamiclimit
40
Effic
ienc
y,%
Photovoltaics: Three Generations
III
III
mc-Si
III-V tandem
c-Si
thin film
a-Si tandem
concentration
Efficiency Loss Mechanisms
Two major losses – 50%
Limiting efficiencies 1 sun Max concn.Single p-n junction: 31% 40.8%Multiple threshold: 68.2% 86.8%
qV
2. Lattice thermalisation
2
2
1. Sub bandgap losses
1
Energy
3
Also: 3. Junction loss
4
44. Contact loss
5
5
5. Recombination
Third generation options
100%
74%68%
54%49%44%39%31%
0%
58%
circulators
tandem (n )hot carrier
impurity PV & band, up-converterstandem (n = 3)thermal, thermoPV, thermionics
impact ionisationtandem (n = 2)down-converterssingle cell
tandem (n = 6)65%
Eg
Eh
Elintermediate level
Ef
Jl
Jh
JVC
CB
VB
Erela
x
h+
e-
e-e-
h+
E0,e
E2,h
E2,e
One photonMultiple electrons-
E0,h
Intrinsic Intrinsic radiativeradiative and Auger losses includedand Auger losses included
Free choiceor Si cell
Decreasing band gap
Sunlight
42.5%47.5%
50.5%
Number of cells1 2 3
10
20
30
40
00
Si bottomcellFreechoice
29%
33%
45%
AM1.
5GEf
ficie
ncy
Silicon based Tandem Cell
Silicon based Tandem Cell
SiO2
SRO
Substrate
Thin film Si cellEg = 1.1eV
2nm QD, Eg =1.7eV
Si QDs
defect or tunnel
junction
SiO2barriers
Engineer a wider band gap – Si QDsSo
lar
Cel
l 1
Sola
r C
ell 2
Sola
r C
ell 3
Decreasing band gapSo
lar
Cel
l 1
Sola
r C
ell 2
Sola
r C
ell 3
Decreasing band gap
SixSiOxSiOx ⋅⎟⎠⎞
⎜⎝⎛ −+→
21
2 2
SubstrateSubstrate
Anneal 1100°C – Si precipitation
Si nanostructure tandem cell
1.0
1.5
2.0
2.5
3.0
3.5
0 1 2 3 4 5 6 7Dot diameter [nm]
PL e
nerg
y [e
V]
Y. Kanemitsu et alH. Takagi et alS. Takeoka et alT. Y. Kim et alT. W. Kim et al Oxide (UNSW) Nitride (UNSW)
Si QDs in oxide/nitride
Si QDs in SiC
400 800 1200 1600 2000
a-Si
Nanocrystalline SiC
508 cm-1Si nanocrystal
As deposited
800 oC
1000 oC
1100 oC
Inte
nsity
(a.u
.)
Raman shift (cm-1)
20 30 40 50 60 70 80
Annealed at 1100 oC
Si (311)
Si (220)
β-SiC (111)
Si (111)
Inte
nsity
(a.u
.)2 Theta (deg.)
c-Si
SiC
0.9 eV
1.1 eV
0.5 eV
Si3N4
c-Si
1.9 eV
1.1 eV
2.3 eV
c-Si
SiO2
3.2 eV
1.1 eV
4.7 eV
Alternate matrices
Gaussian modelling of Si QD in SiC
Alternate QDsTin QDs in SiO2
Ge QDs in SiO2
Various material combinationsQuantum Dot / Matrix combinations and current status of investigations
-POSPOSn
--SPGe
SPODSPOEDSPOEDSi
SiCSi3N4SiO2
S = Simulation (ab-initio modelling - DFT)P = Physical (electron microscopy, X-ray difraction)O = Optical (photoluminescence, absorptance)E = Electronic (conductivity, conductivity with Temp.)D = Devices (Diodes, Cells)
Increasing conductivity
Decreasing processing temperature
Si QD
SiO2
SiC substrate
n-SiC wafer
Light
quartz
Si QD
SiO2
SiC pseudo-substrate
sputtered SiC
Light
quartz
p-Si QDSiO2
Homojunction– front back contact
sputtered SiC
n-Si QD
Light
Si substrate - problem – Can’t be sure absorption is not in Si Hence – transparent substrate or pseudo-substrate
Voc = 93 mVSi substrate
N+ - Si NC:SiC (100nm, Sb doping)
SiN (70 nm)
3mm
P-Si NC:SiC (200nm, B doping)(600 nm B-doping)emitter
absorberBarrier
(200 nm Sb-doping)
Open circuit Voltage = 83mV
Hot Carrier cellExtract hot carriers before they can thermalise:
small Eg
TH
Ef
Hot carrier distribution
• Need to slow carrier coolingRoss & Nozik, 1982
Würfel, 1995Green, 2003Würfel, 2005
Takeda et al, SOLMAT, 08
Eδ
Ef(n)
TA
qV=ΔµA
Ese- energy selective contact
h+ energy selective contact
Es
Ef(p)
TA
• Collect carriers over narrow range of energies
Ross & Nozik, JAP, 53 (1982) 3813Würfel, SOLMAT, 46 (1997) 43 1995Green, 3rd Gen PV (S-Verlag) 2003Würfel, PIP, 13 (2005) 277
Si QD
Dielectricmatrix
ResonantTransport
Resonant Tunneling Transport
Energy
Energy Selective Contact
Filter
ECEf
Ef
I
V
0
0.01
0.02
0.03
0.04
0 0.5 1 1.5
Gate voltage (V)
Ig(A
) Two different sites on the wafer
NDR at 300K - Repeatable
Hot Carrier cooling
Hot Optical phonon population“phonon bottleneck effect”
Slows further carrier cooling
Decay of Optical phonons to Acoustic is critical
Electrons carry most energy
Cool predominantly via small wave vector optical phonon
emission - timescale of psinelastic – energy relaxation
Optical phonons emitted
Energy
Optical phonon decay
Optical phonon decay
O → LA + LA (Anharmonicity or Klemens mechanism)
Compound – e.g. InN
Element– e.g. Si
Phon
on e
nerg
ies
(den
sity
of s
tate
s) 60
meV
30
0
Allowed phonon energies
E
Nō
Optical phonons(standing waves)
Acoustic phonons(heat in the lattice)
Some evidence for slowed carrier cooling in InN: Chen & Cartwright, APL, 83 (2003) 4984
And for longer phonon lifetimes in GaN, AlSb, InP – all of which have large phonon gaps
Phononic gaps in nanostructures
Linear force constant model: l = 4a1 + 4a2– mass ratio = 2; force constant ratio = 5
Nanostructure
Phon
on e
nerg
ies
(den
sity
of s
tate
s)
40
meV
20
0
Phonon propagation in nanostructure
Acoustic phonon reflected from zone edges → standing wave
Towards a complete cell•Fabrication of slowed cooling absorber
•Energy Selective Contacts
•Transport and Renormalisation of carrier energies
Summary•Relevance and growth of Photovoltaics•Three PV Generations•Main energy losses•Third Generation approaches•Si nanostructure tandem cells
•Band gap eng.•Range of QD materials•Early devices
•Hot Carrier cells•Energy filter contacts•Phonon bottleneck•Nanostructures - QD based cell
•Third generation multi-energy level devices•tend to involve QD nanostructures •enable tailoring of material properties
Research Staff: Martin Green, Richard Corkish, Gavin Conibeer, Dirk König, Eun-ChelCho, Tom Puzzer, Yidan Huang, Shujuan Huang, Dengyuan Song, Santosh Shrestha, Ivan Perez-Wufl, Supriya Pillai
PhD students: Chris Flynn, Jeana Hao, Sangwook Park, Lara Treiber, Yong So, Pasquale Aliberti, Yong So, Andy Hsieh, Bo Zhang, Rob Patterson, Binesh Puthen Veettil,
Visiting researchers: Fei Gao, Dong-Ho Kim, Ke Ma, Veronique Gevaerts, Martin Kirkengen, Martina Schmid
Third Generation Strand (2008)