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Quantum Dot Solar Quantum Dot Solar Cells. Cells.
Tuning Photoresponse Tuning Photoresponse through through
Size and Shape Control Size and Shape Control of CdSeof CdSe--TiO2 TiO2 ArchitectureArchitectureKiarash KiantajKiarash Kiantaj
EEC235/Spring 2008EEC235/Spring 2008
IntroductionIntroduction
Sensitization of mesoscopic Tio2 Sensitization of mesoscopic Tio2 with dyes (11% efficiency)with dyes (11% efficiency)
Short band gap semi-conductors to Short band gap semi-conductors to transfer electrons to large band gap transfer electrons to large band gap semi-conductorssemi-conductors
Sensitizers: CdS, PbS, Bi2S3 CdSe, InP (short gap)
TiO2 , SnO2 ( large gap)
Short band gap semi-Short band gap semi-conductorsconductors
Harvesting visible light energy.Harvesting visible light energy. Electron injection under visible lightElectron injection under visible light Fast charge recombination Fast charge recombination low low
efficiency efficiency
Semiconductor Quantum Semiconductor Quantum dots dots
Visible light harvesting assemblies Visible light harvesting assemblies Size quantization Size quantization
Tune visible responseTune visible response Vary band energiesVary band energies
Open up ways utilize hot electrons Open up ways utilize hot electrons and multiple carriers with single and multiple carriers with single photon. photon.
Quantized CdSe Particles and Their Deposition on TiO2
Particulate Films and Nanotubes
Random versus Directed Electron Transport throughSupport Architectures, (a) TiO2 Particle and (b) TiO2 NanotubeFilms Modified with CdSe Quantum Dots
- Absorption spectra of 3.7, 3.0, 2.6, and 2.3 nm diameter CdSequantum dots in toluene.- Shift due to quantization
Scanning electron micrographs of (A) TiO2 particulate film caston OTE and (B, C, and D) TiO2 nanotubes prepared by electrochemicaletching of titanium foil. The side view (B), top view(C), and magnifiedview (D) illustrate the tubular morphology of the film
Deposition of QD on Tio2 films
40-50 nm particles ( diameter) 40-50 nm particles ( diameter) Electro chemical etching of Ti in fluorideElectro chemical etching of Ti in fluoride Tio2 Tio2
nanotubesnanotubes 80-90 nm ( diameter) , 8 um long 80-90 nm ( diameter) , 8 um long
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).
Growth detailsGrowth details
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 Absorption spectra
Absorption spectra of (a) 3.7, (b) 3.0, (c) 2.6, and (d) 2.3 nmdiameter CdSe quantum dots anchored on nanostructured TiO2 films (A)OTE/TiO2(NP)/CdSe (solid lines) and (B) (Ti/TiO2(NT)/CdSe (dashed lines).
•Peaks due to the 1S exciton transitions•Binding of CdSe to TiO2
Selectively harvest light Selectively harvest light CdSe maintains quantization CdSe maintains quantization
properties after bindingproperties after binding Absorbance = 0.7Absorbance = 0.7 more than 80% more than 80%
absorption of light below the onset.absorption of light below the onset. Uniform coverage of CdSe is similar Uniform coverage of CdSe is similar
to modified mesoscopic TiO2 with to modified mesoscopic TiO2 with sensitizing dyes. sensitizing dyes.
Photoelectrochemistry of TiO2 Films Modified with
CdSeQuantum Dots
Open circuit voltageOpen circuit voltage Short current circuit Short current circuit Open circuit voltage is Open circuit voltage is same for all. (650+-20 mV)same for all. (650+-20 mV) Injected electrons relax to Injected electrons relax to lowest conduction bandlowest conduction band conduction bandconduction band level of TiO2+ redox = 600 mVlevel of TiO2+ redox = 600 mV
Photocurrent response depends on Photocurrent response depends on particle size 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 photocurrentMaximum photocurrent 3.0 nm 3.0 nm CdSeCdSe
Two opposing effects:Two opposing effects:
1- decreasing size1- decreasing size shift of the shift of the conduction bad to more negative conduction bad to more negative potentialpotential driving force for charge driving force for charge injection injection
2- decreasing size2- decreasing size smaller response in smaller response in visible regionvisible region less photocurrent less photocurrent
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 spectraA) OTE/TiO2(NP)/CdSe and(B) (Ti/TiO2(NT)/CdSe electrodes
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 CellAssembled with Different-Sized CdSe Quantum Dots on a TiO2Nanotube Array
ConclusionConclusion
Size dependent charge injection ( Tio2-Size dependent charge injection ( Tio2-CdSe)CdSe)
Morphology dependence Morphology dependence Overall power efficiency of about 1% Overall power efficiency of about 1%
with 3nm with 3nm
CdSe QDCdSe QD Maximum IPCE value (45%) obtained
with CdSe/TiO2(NT) is greater than that of CdSe/TiO2(NP) (35%).