is it possible to enhance solar energy conversion ... · gainp/gaas/ge 32.0 % 1.5am spectrolab 2003...
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High-Efficiency Concepts for Photovoltaicsbased on Silicon Quantum Structures
Bert Stegemann, Thomas Lußky, Andreas Schöpke, Manfred Schmidt
Helmholtz Center Berlin for Materials and Energy GmbH* Department of Silicon Photovoltaics (SE1)
* formerly Hahn-Meitner-InstituteBert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Is it possible to enhance
solar energy conversion efficiencies by
utilization of quantum size effects ?
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Si Nanostructures for PV – Research at Helmholtz Center Berlin
Nano-Physicsexploring atomic scale matterbasic material research
Nano-Geometrynano-composite structuresnanostructured solar cells
Quantum-Technologyreduction of thermalizationutilization of quantum size effectsrealization of high-efficiency concepts
First BMBF joint project (2006 – 2008): „Band Structure Design: Charge Carrier Transport in Silicon-based quantum structures for high-efficiency solar cells“
New BMBF joint project (2009 – 2011) SINOVA:„Silicon-based nanostructured thin-film materials asinnovative functional elements in next generation solar cells“
20… ?
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Outline1. Introduction
- loss mechanisms- beyond Shockley Queisser- tandem cell concept
2. Si/SiO2 Multi Quantum Wells- quantum size effect- structure and photoconductivity
3. High Quality Si/SiO2 Single Quantum Wells- preparation- interface gap state spectroscopy- in situ interface passivation- photoelectrical properties
4. Conclusions and Outlook
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Loss mechanisms in photovoltaic energy conversion
• Aim of 3rd generation concepts: exceeding the Shockley Queisser limit (single junction efficiency)
transform excess energy intoelectrical rather than thermal energy
0
20
40
60
80
100
hν > Eg
%
usable energy
hν < Eg
excess energy
further losses
> 50 %
Shockley-Queisser limit: max. single bandgap efficiency: 32.7 %
Record cell: 24.7 %(mono-Si, „PERL“, UNSW 1994)
W. Shockley and H.J. Queisser, J. Appl. Phys. 32 (1961), 51 Zhao J, Wang A and Green M 1999 Prog. Photovolt. 7 471
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for PhotovoltaicsBeyond Shockley-Queisser : Approaches
• Energy bandgap engineering Tandem Cells• Hot Carrier Conversion
- Extraction, collection, and utilization of hot carriers- Impact ionization / multiple exciton generation
• Intermediate Band Solar Cell• Thermophotonic Solar Cells• Down-Conversion and Up-Conversion• …
See:
• M. Green: Third Generation Photovoltaics, Springer, 2003
• P. Würfel: Solarzellen der dritten Generation, Phys. J., 2003
• A. Marti / A. Luque: Next Generaton Photovoltaics, 2002
6
Quantum Structures
-
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Multijunction Solar Cells
wavelength / nm
spec
tral
irrad
ianc
e/ W
/m2 µ
m
• Multiple junctions (tandems or stacks)Jackson, E. D., Proc. Conf. on the Use of Solar Energy, Tuscon, AZ, (1955),122
solar cells with decreasing bandgap are stacked on each othereach of the cells converts photons from a certain energy rangetheoretical limits: N = 2: 42%, N = 3: 49%, …, N = ∞: 68%
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Multijunction Solar Cells
Considerations
• match the terrestrial solar spectrum
• match the current throughput
• control of band offsets
• matching of the lattice constants
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Multijunction Solar CellsRealizations
GaInP/GaAs/Ge 32.0 % 1.5AM Spectrolab 2003GaInP/GaInAs/GaAs 40.8 % 326 suns NREL 2008
a-Si/µc-Si 11.7 % 1.5AM Kaneka 2004a-Si/a-SiGe/a-SiGe 10.4 % 1.5AM USSC 1998
III-V based Multijunctions:
a-Si based Multijunctions:
☺ highest efficiencies, light weightcomplex, expensive, > 20 layers/ interfaces, need high concentration
☺ abundent material, large scale productionlow efficiencies
energy band gap engineering by making use of quantum size effects ?
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Size QuantizationIncrease of bandgap by decrease of Si layer thickness
spatial carrier confinementquantization of wave functionsincrease of effective bandgap
c-Si
SiO2 3.2 eV
1.1 eV
4.7 eV
SiO2
bandgap tuningincreased absorptionindirect to direct conversion
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Silicon Based Tandem Cell
Si Quantum Well Solar Cell:⇒ Si/SiO2 superlattices with absorberstacks of different thickness and size-dependent bandgap energy
cf. M.A. Green: Third Generation Photovoltaics(Springer, Berlin, 2005).
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Outline1. Introduction
- loss mechanisms- beyond Shockley Queisser- tandem cell concept
2. Si/SiO2 Multi Quantum Wells- quantum size effect- structure and photoconductivity
3. High Quality Si/SiO2 Single Quantum Wells- preparation- interface gap state spectroscopy- in situ interface passivation- photoelectrical properties
4. Conclusions and Outlook
-
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Si/SiO2 Multi-QWs by R-PECVD
4 nm Si-QWs / 3 nm SiO2 Superlattice, RTA: 30‘ @ 1000°C
TEM cross-section
1.0 1.2 1.4 1.6 1.80.0
0.2
0.4
0.6
0.8
1.0
20 x 1 nm
10 x 2 nm
7 x 3 nm
PL in
tens
ity [a
.u.]
Photon energy [eV]
T = 75 K
5 x 4 nm
Photoluminescence
R. Rölver , B. Berghoff, D. Bätzner, B. Spangenberg, H. Kurz, M. Schmidt, B. Stegemann: Thin Solid Films 516, 6763 (2008)
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Lateral vs. vertical transport in QWs
14
Dark I-V characteristics of 20x 3 nm/3 nm Si/SiO2 MQWs
0 5 10 15 201E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
curr
ent d
ensi
ty [A
/cm
²]
voltage [V]
vertical
lateral
~3 orders of magnitude higher conductivity in the lateral configuration
R. Rölver , B. Berghoff, D. Bätzner, B. Spangenberg, H. Kurz, M. Schmidt, B. Stegemann: Thin Solid Films 516, 6763 (2008)
interface recombination
tunneling
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Photocurrent Measurements
⇒ Determination of quantum efficiencies:• Yint (hν) ~ η · μ · τeff• YextR (hν) ~ α (hν) , δn, δp = const
main onset of PC at bandgap of a-Si
a-Si
Spectral dependence of Yint
1 2 3 4 5
10-8
10-7
10-6
10-5
Intern-Yield-LateV.OPJ
Y int
.PC
hν / eV
10x MQW: 5nm Si / 5nm SiO2, (1000°C, 30s)
M. Schmidt, R. Rölver , B. Stegemann: in prep.
IphU
T
RΦ0 hν
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Crystallinity
Zacharias et. al. Phys. Rev. B, 62 (2000) 8391
low crystallinity: < 25%decreasing crystallinity with decreasing a-
Si layer thickness
in accordance with Zacharias Model:
„thin Si layers are more difficult to crystallize, due to increased interface binding energies“
data from: R.Roelver, PhD thesis RWTH Aachen
0 2 4 6
0
20
40
60
80
100
Ram
an c
ryst
allin
ity /
%
QW thickness
Si/SiO2-MQW - R-PECVD, RTA
/ nm
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Summing up: MQWs by R-PECVD ... so far
Photoconductivity:a-Si contribution dominates transportno quantum size effect
Photoluminescence:blue shift of PL signal due to quantum size effectoriginates from Si nano-crystals
Raman:crystalline fraction: < 25%nano-crystals embedded in a-Si matrix
complementaryprocesses !
need high-quality material
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Outline1. Introduction
- loss mechanisms- beyond Shockley Queisser- tandem cell concept
2. Si/SiO2 Multi Quantum Wells- quantum size effect- structure and photoconductivity
3. High Quality Si/SiO2 Single Quantum Wells- preparation- interface gap state spectroscopy- in situ interface passivation- photoelectrical properties
4. Conclusions and Outlook
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Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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ultrahigh vacuum preparation and analysis of thebuilding block of multiple-quantum wells and superlattices:
SiO2/Si/SiO2 single quantum well
B. Stegemann, A. Schoepke, M. Schmidt: J. Non-Cryst. Sol. 354 (2008) 2100
room temperature1 Å / sec.
300°Cneutral atomic oxygenwith thermal impact
crystallization
1000°C anneal
Si/SiO2 Quantum Wells: UHV Preparation
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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• Si single QW 7SiO2 barrier (7 nm / 2 nm)• uniform layers• structurally abrupt interfaces• atomic resolution of the QW
• nano-crystalline structure• high crystallinity: ~80%
TEM Raman
Si/SiO2 Single Quantum Wells: Characterization
B. Stegemann, A. Schoepke, M. Schmidt: J. Non-Cryst. Sol. 354 (2008) 2100
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
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• photocurrent detectable in Si QWs
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Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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0.0 0.5 1.0
10-10
10-9
10-8
10-7
0.0 0.5 1.0
EV
EF
E - EV / eV
SiO2/Si(111) +energetic H plasma +1000°C annealing
Yin
t
SiO2/Si(111) +thermal H plasma +1000°C annealing
EV
EF
Si/SiO2 Interface Defect Passivation
Thermal impact:• decrease of midgap states
passivation of dangling bonds• no interface degradation
Energetic impact:• increase of interface states
generation of interface defectstates due to bond breaking
B. Stegemann, A. Schoepke, D. Sixtensson, B. Gorka, T. Lussky, M. Schmidt: Physica E (2008) doi:10.1016/j.physe.2008.08.012 Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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1 2 3 4 5 6
10-8
10-7
10-6
10-5
10-4
Y in
t, P
C
hν / eV
after hydrogen passivation without hydrogen passivation
a-Sic-Si
Photoconductivity of Si/SiO2 QWs: Hydrogen Passivation
26
Spectral dependence of Yint
• UHV H-plasma treatment improves Yint ~10× due to passivation of defect states (dangling bonds) at Si/SiO2 interfaces (and at intralayer grain boundaries)• db passivation results in increased carrier lifetimes and lower recombination velocity
B. Stegemann, D. Sixtensson, T. Lussky, A. Schoepke, I. Didschuns, B. Rech, M. Schmidt: Nanotechnology 19 (2008) 424020
µ · τeff ≈ 2×10-7 cm2V-1
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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Outline1. Introduction
- loss mechanisms- beyond Shockely Queisser- tandem cell concept
2. Si/SiO2 Multi Quantum Wells- quantum size effect- structure and photoconductivity
3. High Quality Si/SiO2 Single Quantum Wells- preparation- interface gap state spectroscopy- in situ interface passivation- photoelectrical properties
4. Conclusions and Outlook
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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• complete UHV cycle for producing high-qualitiy ultrathin-SiO2/Si interfaces
• key point: RF plasma oxidation and H passivation with nearly thermalimpact energies (< 1eV)
98 100 102 104 106
XPS
Si 2
p si
gnal
/ a.
u.
EB / eV
Si(100)
0.0 0.5 1.0
10-10
10-9
10-8
10-7
Yin
t
E-EV / eV
Si(111)7x7 SiO2/Si(111) SiO
2/Si(111):H thermal plasma (UHV)
SiO2/Si(111):H energetic plasma (CVD)EV
EF
• abrupt interfaces:
electronicallychemicallystructurally
preparation interfacepassivationgap state
spectroscopyinterface
passivationgap state
spectroscopy
Conclusions I
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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• successful preparation of
Si/SiO2 single/multi quantum wells (and quantum dots)
ultra-thin SiO2 layers
• correlation of structural
chemical and
electronic Si/SiO2 interface properties
• detection of lateral photocurrents in single QWs (and QD layers)
• determination of µ·τeff product ⇒ strong influence of Si/SiO2 interfaces
• improvement by passivation with hydrogen
SummaryConclusions II
Bert Stegemann 30.10.2008 Photovoltaics meets Microtechnology, Erfurt
Silicon Quantum Structures for Photovoltaics
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• B. Rech• D. Sixtensson, D. Patzek, • L. Korte, M. Schulz, B. Gorka, I. Didschuns, K. Jacob, A. Scheu• U. Bloeck, P. Schubert-Bischoff• Financial Support: BMBF 03SF0308
Acknowledgements
Project partners• IHT - RWTH Aachen (Prof. H. Kurz, Dr. B. Spangenberg)• IEF5-PV - FZ Jülich (Prof. J.H. Werner, Prof. U. Rau)• BTU Cottbus (Prof. M. Kittler)• ERC/GfE Jülich/Aachen (Prof. J. Mayer)• IFTO - FSU Jena (Prof. F. Bechstedt)• IZM-MLU Halle (Dr. H. Leipner)
Coorperations:• Center of Excellence, UNSW, Australia (Prof. M. Green)• Academy of Science, Czech Republic (Prof. Dr. J. Kocka)
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