Speaker: Helen Huang

Department of Chemical Engineering

Rutgers University

Advisor: Prof. Tewodros (Teddy) Asefa

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

Introduction of Solar Cell

3

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

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

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

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

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

Photoelectrochemical Behavior of

Thin CdSe and Coupled

TiO2/CdSe Semiconductor Films

9 Gratzel et al., Nature 1991, 353, 737.

Hodes et al., Langmuir 1992, 8, 749.

Kamat et al., J. Phys. Chem. 1993, 97, 10769

hv

What Are Quantum Dots

(QDs) ?

http://www.photonics.com/Article.aspx?AID=29421

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

-

+

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

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

Configuration

15

QD sensitizer

Metal oxide—i.e. TiO2

Ph

oto

an

od

e

Electrolyte

TiO2 film: Porous material

Surface area

Absorb sensitizer

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

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

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

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

20

QD

Metal oxide—i.e. TiO2

Ph

oto

an

od

e

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

Solar Cell Sensitized with

Molecular Dipole-Modified

Quantum Dots

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

23 Arie Zaban et al., J. AM. CHEM. SOC. 2009, 131, 9876–9877

Table 1.

Figure Incident photon to current

efficiency (IPCE)

IPC

E[%

]

(c) Negative molecular dipoles QD energy level ↑

Electron injection↑

(b) Positive molecular dipoles QD energy level ↓

Electron injection↓

a) TiO2

Electrolyte

Acknowledgement

24

Tewodrows Asefa’s group

Q&A?

25