Nanocrystal Solar Cells

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Nanocrystal Solar Cells

• Solar Photovoltaics and Energy Systems EY-1.1

Laurent Balet
04-nov-2005
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Outline

• What are Nanocrystal Quantum Dots
• What are they useful for in Solar Cell
  technology
• The Hybrid Nanocrystal Polymer Solar Cells
• Air-stable all-inorganic Nanocrystal Solar Cells
• Discussion

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What are Nanocrystals?
Me2CdMe2Se In TOP
CdSe-TOPO/TOP
TOPO 320360ºC
TOP(O) trioctyl phosphine (oxide) Me2Cd
dimethylcadmium
1P(e)
2 10 nm 10 50 atoms
Murray et al., J. Am. Chem. Soc. 15, 8706 (1993)
1P(e)
1S(e)
1S(e)
Valence Band
Eg(8nm)
Eg(2nm)
Eg
Conduction Band
Hole states (quasicontinuum)
Bulk semiconductor
8 nm
2 nm
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What are Nanocrystals?
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Why use Nanocrystals?
Tunable bandgap
CdSe - 500nm
CdSe - 600nm
CdSe - 700nm
PbSe - 900nm
PbSe - 1100nm
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Why use Nanocrystals?
Use of hot photogenerated carriers higher
voltage or higher current?
R. D. Schaller, V. I. Klimov, PRL 92, 186601
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How to use those Nanocrystals?
QD used as sensitizer
QD-array as i-region of a p-i-n cell
QD dispersed in a blend of electron/hole
conducting polymer
A. J. Nozik, Physica E 14 (2002) 115
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Hybrid Nanorod-Polyemer Solar Cells
W. U. Huynh, J. J. Dittmer, A. P. Alivisatos,
Science 295, 2425 (2002)
90 by weight
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Hybrid Nanorod-Polyemer Solar Cells
W. U. Huynh, J. J. Dittmer, A. P. Alivisatos,
Science 295, 2425 (2002)
7x60 nm
7x60 nm
3x60 nm
7x30 nm
7 nm

• Polymer absorbs --gt 650 nm
• Other characteristic similar
• Charge transport through the length

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Hybrid Nanorod-Polyemer Solar Cells
W. U. Huynh, J. J. Dittmer, A. P. Alivisatos,
Science 295, 2425 (2002)
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Air-stable all-inorganic Solar Cell
I. Gur, N. A. Fromer, M. L. Geier, A. P.
Alivisatos, Science 310, 463 (2005)
Type II heterojunction (similar to organic D-A
model) serves to dissociate the strongly bound
excitons.
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Air-stable all-inorganic Solar Cell
I. Gur, N. A. Fromer, M. L. Geier, A. P.
Alivisatos, Science 310, 463 (2005)
AM1.5G
CdSe
CdTe
Dark
Isc 0.58 mA/cm2 Voc 0.41 V FF
IVmax/(IscVoc) 0.40
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Air-stable all-inorganic Solar Cell
I. Gur, N. A. Fromer, M. L. Geier, A. P.
Alivisatos, Science 310, 463 (2005)

• No free carriers in dark in CdSe arrays
• electrically insulating in dark
• CdSe (CdTe) films gt500GW/cm2 dark
• - 1 order of magnitude under AM1.5G
• Device - 3 orders of magnitude
• gtlimited number of untrapped carriers
• gt best described by rigid band
• gt carrier extraction by diffusion (typeII)

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Air-stable all-inorganic Solar Cell
I. Gur, N. A. Fromer, M. L. Geier, A. P.
Alivisatos, Science 310, 463 (2005)

• Role of charge transfer
• Separation of h and e- at interface increase
  efficiency
• Blend efficiency indicates no p-n type junction
  but D-A behavior

510-2mW/cm2
bilayer
blend
CdTe
CdSe
200nm thick layers, Similar OD across
spectrum Low intensity illumination
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Air-stable all-inorganic Solar Cell
I. Gur, N. A. Fromer, M. L. Geier, A. P.
Alivisatos, Science 310, 463 (2005)
CdTe
AM1.5G
Bilayer shows rectification Single material
device not Diode-like behavior doesnt come
from Schottky contacts or p-n but rather from the
intendend heterojunction (similar to organic D-A)
bilayer
CdSe
blend
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Air-stable all-inorganic Solar Cell
I. Gur, N. A. Fromer, M. L. Geier, A. P.
Alivisatos, Science 310, 463 (2005)
Sintering (saturated CdCl2 in MeOH) annealing
15 min at 400C
Isc 11.6 mA/cm2 Vos 0.4 V FF 0.45 Power
conversion efficiency (AM1.5G) 2.1
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Air-stable all-inorganic Solar Cell
I. Gur, N. A. Fromer, M. L. Geier, A. P.
Alivisatos, Science 310, 463 (2005)
Aging seems to improve device quality 13kh, open
circuit, ambiant Isc -1.3 Vos gt 10 FF
4.4 Overall 13.6 14h, short circuit,
AM1.5G 2 degradation in Photocurrent
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Discussion
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The end?