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%).