Transcript Quantum Dot Solar Cells. Tuning Photoresponse through Size

Quantum Dot Solar Cells.
Tuning Photoresponse
through
Size and Shape Control of
CdSe-TiO2 Architecture
Kiarash Kiantaj
EEC235/Spring 2008
Introduction
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Sensitization of mesoscopic Tio2 with dyes
(11% efficiency)
Short band gap semi-conductors to transfer
electrons to large band gap semi-conductors
Sensitizers: CdS, PbS, Bi2S3 CdSe, InP (short
gap)
TiO2 , SnO2 ( large gap)
Short band gap semi-conductors
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Harvesting visible light energy.
Electron injection under visible light
Fast charge recombination  low efficiency
Semiconductor Quantum dots
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Visible light harvesting assemblies
Size quantization
Tune visible response
 Vary band energies
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Open up ways utilize hot electrons and multiple
carriers with single photon.
Quantized CdSe Particles and Their
Deposition on TiO2
Particulate Films and Nanotubes
Random versus Directed Electron Transport through
Support Architectures, (a) TiO2 Particle and (b) TiO2 Nanotube
Films Modified with CdSe Quantum Dots
- Absorption spectra of 3.7, 3.0, 2.6, and 2.3 nm diameter CdSe
quantum dots in toluene.
- Shift due to quantization
Deposition of QD on Tio2 films
Scanning electron micrographs of (A) TiO2 particulate film cast
on OTE and (B, C, and D) TiO2 nanotubes prepared by electrochemical
etching of titanium foil. The side view (B), top view(C), and magnified
view (D) illustrate the tubular morphology of the film
Photograph of 2.3, 2.6, 3.0, and
3.7 nm diameter CdSe
quantum dots
(A) in toluene,
(B) anchored on TiO2
particulate films
(OTE/TiO2(P)/CdSe,
(C) attached to TiO2 nanotube
array (Ti/TiO2(NT)/CdSe).
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40-50 nm particles ( diameter)
Electro chemical etching of Ti in fluoride Tio2 nanotubes
80-90 nm ( diameter) , 8 um long
Growth details
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Constant absorption  monolayer CdSe
Linear increase in absorption with TiO2
thickness
CdSe quantum dots and TiO2 binding :
bifunctional linker molecules (HOOC-CH2-CH2-SH)
carboxylate and thiol functional groups
Absorption spectra
•Peaks due to the 1S exciton transitions
•Binding of CdSe to TiO2
Absorption spectra of (a) 3.7, (b) 3.0, (c) 2.6, and (d) 2.3 nm
diameter CdSe quantum dots anchored on nanostructured TiO2 films (A)
OTE/TiO2(NP)/CdSe (solid lines) and (B) (Ti/TiO2(NT)/CdSe (dashed lines).
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Selectively harvest light
CdSe maintains quantization properties after
binding
Absorbance = 0.7 more than 80% absorption
of light below the onset.
Uniform coverage of CdSe is similar to
modified mesoscopic TiO2 with sensitizing
dyes.
Photoelectrochemistry of TiO2
Films Modified with CdSe
Quantum Dots
Open circuit voltage
 Short current circuit
 Open circuit voltage is
same for all. (650+-20 mV)
 Injected electrons relax to
lowest conduction band
conduction band
level of TiO2+ redox = 600 mV
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Photocurrent response depends on particle size
Photocurrent response of (A) OTE/TiO2(NP)/CdSe and (B) (Ti/TiO2(NT)/CdSe
electrodes. Individual traces correspond to (a) 3.7, (b) 3.0, (c) 2.6,
and (d) 2.3 nm diameter CdSe quantum dots anchored on nanostructured TiO2
films (excitation >420 nm, 100 mW/cm2; electrolyte, 0.1 M Na2S solution).
Maximum photocurrent 3.0 nm CdSe
 Two opposing effects:
1- decreasing size shift of the conduction bad to
more negative potential driving force for
charge injection
2- decreasing size smaller response in visible
region less photocurrent
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I-V characteristics of (A) OTE/TiO2(NP)/CdSe and (B) (Ti/
TiO2(NT)/CdSe electrodes (excitation >420 nm; intensity 100 mW/cm2;
electrolyte, 0.1 M Na2S solution.)
Under the applied potential charge recombination is minimized.
Tuning the Photoelectrochemical Response
through Size
Quantization.
- incident photon to charge carrier efficiency
(IPCE)
Photocurrent action spectra
A) OTE/TiO2(NP)/CdSe and
(B) (Ti/TiO2(NT)/CdSe electrodes
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nanotube TiO2 films
generally absorb more light than nanoparticle TiO2 films, this
difference accounts for a no more than a 5% increase in overall
photons absorbed. Comparing this with a 10% improvement
in IPCE of the nanotube film over the nanoparticle film
demonstrates the measurable advantage of a nanotube
architecture
for facilitating electron transport in nanostructure-based
semiconductor films.
Design of Rainbow Solar Cells
Artistic Impression of a Rainbow Solar Cell
Assembled with Different-Sized CdSe Quantum Dots on a TiO2
Nanotube Array
Conclusion
Size dependent charge injection ( Tio2-CdSe)
 Morphology dependence
 Overall power efficiency of about 1% with 3nm
CdSe QD
 Maximum IPCE value (45%) obtained with
CdSe/TiO2(NT) is greater than that of
CdSe/TiO2(NP) (35%).
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