1. Kiarash Kiantaj EEC235/Spring 2008
Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe-TiO2 Architecture
Kiarash Kiantaj
EEC235/Spring 2008

2. Introduction
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)

3. Short band gap semi-conductors
Harvesting visible light energy.
Electron injection under visible light
Fast charge recombination  low efficiency

4. Semiconductor Quantum dots
Visible light harvesting assemblies
Size quantization
Tune visible response
Vary band energies
Open up ways utilize hot electrons and multiple carriers with single photon.

5. 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

6. - Absorption spectra of 3.7, 3.0, 2.6, and 2.3 nm diameter CdSe
quantum dots in toluene.
- Shift due to quantization

7. 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

8. 40-50 nm particles ( diameter)
Photograph of 2.3, 2.6, 3.0, and 3.7 nm diameter CdSe quantum dots
in toluene,
anchored on TiO2 particulate films (OTE/TiO2(P)/CdSe,
(C) attached to TiO2 nanotube array (Ti/TiO2(NT)/CdSe).
40-50 nm particles ( diameter)
Electro chemical etching of Ti in fluoride Tio2 nanotubes
80-90 nm ( diameter) , 8 um long

9. Growth 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

10. 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).

11. 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.

12. Photoelectrochemistry of TiO2 Films Modified with CdSe Quantum Dots
Open circuit voltage
Short current circuit
Open circuit voltage is
same for all. ( mV)
Injected electrons relax to
lowest conduction band
conduction band
level of TiO2+ redox = 600 mV

13. 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).

14. 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

15. 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.

16. 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

17. 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.

18. Design of Rainbow Solar Cells
Artistic Impression of a Rainbow Solar Cell
Assembled with Different-Sized CdSe Quantum Dots on a TiO2
Nanotube Array

19. 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%).