This document discusses quantum dot sensitized solar cells (QDSSCs). It begins with an introduction to solar cells in general, explaining how they work by converting light energy to electricity. Quantum dots (QDs) are then introduced as a potential sensitizer material that could make solar cells cheaper than traditional dye-sensitized solar cells. The document outlines the advantages of using QDs, such as their tunable bandgaps and potential for multiple exciton generation. Finally, it describes recent work modifying the energy levels of CdS QDs used to sensitize TiO2 through the addition of molecular dipoles, demonstrating how this can increase electron injection and incident photon to current efficiency in QDSSCs.

1. Speaker: Helen Huang
Department of Chemical Engineering
Rutgers University
Advisor: Prof. Tewodros (Teddy) Asefa

2. Outline
1. Introduction of solar cell
2. Quantum dot sensitized solar cell
a. Introduction of quantum dots
b. Quantum dots as solar cell sensitizer
3. Solar cell sensitized with molecular dipole-
modified quantum dots
2

3. Introduction of Solar Cell
3

4. Derivation of Solar Cell
• Need for renewable energy
– Energy crisis and environmental
reasons
• Solar cell
– Harness the energy of sun
– Convert energy of light into
electricity
• The world's first photovoltaic cell
– A. E. Becquerel, in 1839
4
http://environmentalgeography.wordpress.com/2012/10/08/appro
aches-to-the-environment/
http://www.ief-energy.org/wp-content/uploads/2013/06/solarcal.jpg
Fig 1. Environmental pollution
Fig 2. Solar cell

5. Mechanism of Solar Cell (Photovoltaic Cell)
• Convert energy of light into
electricity
1. Absorb light and generate
electron-hole pairs (excitons)
2. The separation of and
charge carriers
3. Extraction of charge carriers to an
external circuit
5
Ref: http://www.mrsolar.com/images/solar-panel-diagram.gif
http://en.wikipedia.org/wiki/Solar_cell
- +
Fig 3. Mechanism of solar cell

6. Types of Solar Cell Materials
1. Crystalline silicon based solar cell
– Expensive to manufacturing
2. Thin film solar cell
– Silicon thin films
– Cadmium telluride
– Copper-indium-gallium selenide
6
http://en.wikipedia.org/wiki/Solar_cell
http://www.sunconnect.com.au/solar-panel-info/types-of-solar-panels/
Fig 4. Polycrystalline solar cell
Fig 5. Monocrystalline solar cell
Fig 6. Thin film solar cell

7. Types of Solar Cell Material—Contd.
Made via chemical solution process
1. Dye sensitized solar cell
– costly ruthenium (dye), platinum (catalyst)
and organic solvent in the synthesis
2. Quantum dot sensitized solar cell
7
http://inhabitat.com/canadian-researchers-move-closer-to-affordable-
efficient-solar-power/
http://upload.wikimedia.org/wikipedia/commons/5/5a/Sargent_Group_
quantum_dot_solar_cell.jpg

8. Quantum Dot Sensitized
Solar Cell (QDSSC)
http://www3.nd.edu/~kamatlab/research_solarCells.html
http://www.theochem.kth.se/
Zaban et al., Nano Lett., 2013, 13, 4456

9. Photoelectrochemical Behavior of
Thin CdSe and Coupled
TiO2/CdSe Semiconductor Films
9Gratzel et al., Nature 1991, 353, 737.
Hodes et al., Langmuir 1992, 8, 749.
Kamat et al., J. Phys. Chem. 1993, 97, 10769
hv

10. What Are Quantum Dots
(QDs) ?
http://www.photonics.com/Article.aspx?AID=29421

11. Properties of semiconductors：
1. Exciton
 An electron-hole pair forming
when a photon is absorbed by
a semiconductor
VB e-
CB
11
Semiconductor Nanocrystals
(Quantum Dots)
Band gap
Nanocrystal Bulk
semiconductor
-
+

12. Fig.1 The correlation of density of states and energy in different nanomaterials
12
Quantum Dots—Nanomaterials
Definition:
A semiconductor whose excitons are confined in all three spatial
dimensions

13.  Bulk solid:
Quasicontinuous energy
 Nanocrystal:
Quantized band states
• Charge carrier
– Difference among energy level
Size-Quantization Effect
Size↓ ，VB and CB become more
discrete
13
CdSe QDs in different sizes:
2.2, 2.6, 3.2, 4.3 and 5.5 nm
Valence
Band
Conduction
Band

14. Configuration
15
QD sensitizer
Metal oxide—i.e. TiO2
Photoanode
Electrolyte
TiO2 film: Porous material
 Surface area
 Absorb sensitizer

15. QD Synthesis
16
Deposition of QDs onto TiO2 electrodes
Scheme 1: Sensitize the TiO2 film with CdS QDs
Cd(ClO4)2 Na2S
Rinse CdS QDs
on TiO2
Step1 Step2
Park et al. Scientific Report, 2013, 3, 1050
Arie Zaban et al., J. AM. CHEM. SOC. 2009, 131, 9876
TiO2

16. Quantum Dot Sensitized Solar Cells (QDSSC)—
Mechanism
17
http://www.geog.ucsb.edu/img/news/2010/Dye_Sensitized_Solar_Cell_Scheme.png
e
e
e
e
e
e
e
e
e

17. Advantages of QDs as Solar Cell Sensitizer
18
CdSe QDs in different sizes:
2.2, 2.6, 3.2, 4.3 and 5.5 nm
1) Broad excitation spectra and
large absorption coefficients
2) Size-dependent tunable
energy gaps
A.J. Nozik, Physica E, 2002, 14, 115

18. Advantages of QDs as
Solar Cell Sensitizer--
Contd.
19
3) Multiple exciton generation1
 One photon yields more
than one electron-hole
pairs
More electricity
4) Manufacturing through
chemical solution process
 Lower cost
 Relative ease of
preparation
1. D. Timmerman, Nature Photonics, 2008, 2, 105
A.J. Nozik, Physica E, 2002, 14, 115

19. 20
QD
Metal oxide—i.e. TiO2
Photoanode
Electrolyte
Approaches to Improve Solar Cell Efficiency
1. Metal oxide material
I.e. TiO2, ZnO
2. QD sensitizer
I.e. CdTe, CdS
3. Electrolyte
4. Counter electrode

20. Solar Cell Sensitized with
Molecular Dipole-Modified
Quantum Dots
21

21. 22
CdS QDs
on TiO2
Energy Level Alignment in CdS QDSSCs Using
Molecular Dipoles Arie Zaban et al., J. AM. CHEM. SOC. 2009, 131, 9876–9877

22. 23
Arie Zaban et al., J. AM. CHEM. SOC. 2009, 131, 9876
Table 1.
Figure Incident photon to current
efficiency (IPCE)
IPCE[%]
(c) Negative molecular dipoles
QD energy level ↑
Electron injection↑
(b) Positive molecular dipoles
QD energy level ↓
Electron injection↓
a) TiO2
Electrolyte

23. Acknowledgement
24
Tewodrows Asefa’s group

24. Q&A?
25