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Department of Chemistry. 235 Radiation Laboratory. Notre Dame IN 46556-0579. USA. Quantum Dot Solar Cells: Semiconductor Nanocrystals As Light ...
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Workshop on Nanoscience for Solar Energy Conversion 27 - 29 October 2008

Quantum Dot Solar Cells: Semiconductor Nanocrystals As Light Harvesters

Prashant KAMAT University of Notre Dame Department of Chemistry 235 Radiation Laboratory Notre Dame IN 46556-0579 USA

Meeting the Energy Demand

14 TW Challenge

Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters Prashant V. Kamat Dept Of Chemistry and Biochemistry and Radiation Laboratory Dept. of Chemical & Biomolecular Engineering University of Notre Dame, Notre Dame, Indiana 46556-0579 Support: US DOE (BES) http://www.nd.edu/~pkamat

After Oil Beyond Fossil Fuels

Farming Solar

Meeting the Energy Demand

L L I R D “

D Y B A B

” L L RI

www.solarsales.com.au

World Oil Production vs. Discovery Source: Dr. C.J. Campbell

Source:Wikipedia

Can we address the clean energy challenge with Nanotechnology?

Our Research Focus Photoinduced electron transfer in light harvesting systems Donor- Acceptor Semiconductor-Sensitizer Semiconductor-Semiconductor Semiconductor-Metal

e

Molecular linker AcceptorPt

TiO

2

Donor



e hν

e e

e

Ag

h

e h

h

CdSe

h

TiO2 ethanol products

e h

TiO2 e h



e

e

e

C60

C60

C60

Quantum Dot Solar Cells Tunable band edge Offers the possibility to harvest light energy over a wide range of visible-ir light with selectivity

Hot carrier injection from higher excited state (minimizing energy loss during thermalization of excited state)

Multiple carrier generation solar cells. Utilization of high energy photon to multiple electron-hole pairs

M et al IT O

m Aluminu

Glass

m Aluminu

Metal Metal Junction Junction Solar Solar Cell Cell

IT O

T

Semiconductor Nanocrystals

PE DO

Redox or Hole Transport Layer

Glass Active Polymer/ Semiconductor Layer

e

h

CdSe TiO2

Polymer Polymer Solar Solar Cell Cell

e OTE

h

h



Pt

Electrolyte

Quantum Quantum Dot Dot Sensitized Sensitized Solar Solar Cell Cell

1. Semiconductor/metal Interface ECB

e

EF EVB

h



CB



et

+

ht

+ hν

Red

VB

Ox

Because of smaller dimensions charge separation and transport issues in nanostructure films need to be tackled

Metal/PbSeNC/Metal Sandwich Photovoltaic Cell Aluminum

M et al IT O

PbSe NC film, deposited via layer-by-layer dip coating

Glass

Short-circuit photocurrent (>21 mA cm-2) by way of a Schottky junction EQE of 55-65% in the visible and up to 25% in the infrared region Power conversion efficiency of 2.1%.

Luther et al Nano Lett., Vol 8, 2008, 3488

Redox or Hole Transport Layer

Semiconductor Nanocrystals

IT O

PE DO

m Aluminu

T

2. Polymer –Semiconductor Nanocrystal Hybrids

Glass Active Polymer/ Semiconductor Layer

Adv. Mater, 2004, 16, 1009-1013

O

Hybrid Nanorod-Polymer Solar Cells IT

PE DO T

Aluminum

Glass Active Polymer/ Semiconductor Layer

SCIENCE VOL 295, 2002 2425

3. Quantum Dot Sensitized Solar Cell (QDSSC)

e



CdSe

TiO2

CB

e hν

e

CB

CdSe TiO2 VB VB

e

e

TiO2

e

h

e

e

e

e e

e

e h

e h

e

h

e h

CdSe

h

e e

Red

h

h

Ox

h



Photoelectrochemistry

Spectroscopy detector

e R O

hν pump



CB

probe

CB



et

ht

+

TiO2 VB

+ VB CdSe

Red Ox

GERISCHER H, LUBKE M A PARTICLE-SIZE EFFECT IN THE SENSITIZATION OF TIO2 ELECTRODES BY A CDS DEPOSIT JOURNAL OF ELECTROANALYTICAL CHEMISTRY 204 (1-2): 225-227 1986



detector

Photoexcitation of CdSe Quantum Dots CB

pump

probe

1P(e)-1P3/2(h)

0.5 1S(e)-1S3/2(h)

-0.04

0

0

440 nm 530 nm Time, ps

440 nm 530 nm

c

3x

d

0.5 1.0 Time, ps

400

a b c d e

450

Δ OD

ΔOD

ΔOD

CdSe

a b

-0.08

Absorbance

VB

0.2 ps 0.3 ps 0.4 ps 0.5 ps 1.1 ps

0.5

e 500

3x

550

Wavelength, nm

600

650

1.0

Charge Separation in TiO2/CdSe Transient Bleaching Recovery of 3 nm CdSe Quantum Dots Ex. 387 nm

B

CdSe-MPA

C

CdSe-MPA-TiO2

0.00

0.00

ΔA

ΔA

-0.04

1 ps 35 ps 400 ps 1500 ps

-0.08

450

500

550

Wavelength, nm

600

-0.04 1 ps 35 ps 400 ps 1500 ps

-0.08

650

450

500

550

Wavelength, nm

600

650

Modulation of the charge injection process by controlling the particle size?

CdSe e Slow Eg=1.9 eV h

TiO2

e CdSe 7.5 nm

CdSe e

Fast

Eg=2.4 eV h

TiO2 e

e TiO2

CdSe 2.4 nm

Photoexcitation of CdSe Quantum Dots CdSe +hν → CdSe (ep +hp) → CdSe (es + hs) (A) Absorbance

400

(B)

Wavelength, nm 500 600

700

CdSe quantum dots of size 2.4 nm to 7,5 nm were excited with 387 nm laser pulse (130 fs)

0.4 0.2 e

0.0

d

c

b

a

As the particle size decreases from 7.5 nm to 2.4 nm, the first (1S3/21Se) excitonic peak shifts from 645 nm (1.92 eV) to 509 nm (2.44 eV).

0.00

ΔA

-0.02

a e

-0.04

d CdSe a 7.5 nm (Eg 1.92 eV) b 4.6 nm (Eg 1.99 eV) c 3.5 nm (Eg 2.18 eV) d 2.7 nm (Eg 2.35 eV) e 2.4 nm (Eg 2.44 eV)

-0.06 400

Transient bleach corresponds to the first excitonic bleach

c b

500 600 Wavelength, nm

700 Robel, Kuno, Kamat, JACS 2007; 129, 4136

Electron transfer between CdSe and TiO2

0.8

a size 7.5 nm λ(1S)=645 nm

b

0.0

CdSe (es) + TiO2 → CdSe + TiO2(e)

Normalized bleach

Analysis of Bleaching Recovery

a

0.8

ΔA(t)=ΔA(0)×exp[-(t/τ)β] - where τ is the peak value of the characteristic lifetime

size 4.6 nm λ(1S)=605 nm

b

0.0 0.8

0.0

a size 3.5 nm λ(1S)=570 nm

b

0.8

a

size 2.7 nm 0.0 λ(1S)=527 nm

b

0.8

a size 2.4 nm λ(1S)=509 nm b

0.0 0 Robel, Kuno, Kamat, JACS 2007; 129(14) pp 4136 - 4137

500

1000

Time (ps)

1500

Size Dependent Quenching Phenomenon

Diameter

Eg

τCdSe

[nm]

[eV]

[ps]

7.5

1.92

2332

0.697

2281

0.755

9.58×106

4.6

1.99

7224

0.474

4961

0.446

6.3×107

3.5

2.18

4420

0.417

1117

0.475

6.7×108

2.7

2.35

6739

0.457

357

0.505

2.65×109

2.3

2.44

23119

0.51

83

0.493

1.2×1010

ΔA(t)=ΔA(0)×exp[-(t/τ)β]

βCdSe

τCdSe-TiO2

βCdSe-TiO2

[s-1]

[ps]

- where τ is the peak value of the characteristic lifetime

ket

1/τ’ – 1/τ = ket

Size Dependent Electron transfer between CdSe and TiO2 ket= 107 s-1

ECB, V vs. NHE -0.9 -1.1 10

e

-1.3

CdSe 7.5 nm

TiO2

CdSe 2.4 nm

2.7 nm

9

10

3.5 nm

-1

ket, s

e

2.4 nm

10

ket= 1.2x1010 s-1

8

10

4.6 nm 7

10

7.5 nm

h 0.4

Eg=2.4 eV

Eg=1.92 eV

6

10

e

e

0.6 -ΔG ,eV

0.8

Robel, Kuno, Kamat, JACS 2007; 129 pp 4136 - 4137

CdSe 7.5 nm

TiO2

h CdSe 2.4 nm

Linking Q-CdSe to TiO2 particles a

e

b c

d

e

CdSe TiO2 hν -0.75

Electrode

nm 0 HOOC

OOC SH

SH

HOOC SH

SH

HOOC

OOC SH

OTE

Bifunctional linker molecule

TiO2

-0.75

OOC SH

2 μm

Chemically Modified TiO2 film

150 nm

OOC OOC

SSH

OOC

SH

OOC

OOC

S-

OOC

SSH

0

CdSe OTE/TiO2/CdSe Electrode

J. Am. Chem. Soc. 2006,128, 2385-2393

0

2 μm

Photoelectrochemical behavior of Q-CdSe-TiO2 films

Potentiostat WE

CE

Voltage vs. SCE, V

RE

TiO2

R O



Photocurrent, μA

0.2

e

0 0.0

-0.2 TiO2

-0.4

-0.6

-0.8

a

100

b 200

TiO2/CdSe

I-V characteristics of (a) OTE/TiO2 and (b) OTE/TiO2/MPA/CdSe films. Electrolyte 0.1 M Na2S. The filtered lights allowed excitation of TiO2 and CdSe films at wavelengths greater than 300 and 400 nm respectively

-1.0

Modification of TiO2 Films with Different Size CdSe Particles

2.6nm

3.7nm 3.7 nm 3.0 nm 2.6 nm 2.3 nm

0.6

Absorbance

2.3nm 3.0nm 3.7nm 2.6nm

3.0nm

0.4

0.2

0.0 300

3.7 nm 3.0 nm 2.6 nm 2.3 nm

0.9

Absorbance

2.3nm

0.6

0.3

0.0

400

500

600

Wavelength, nm

700

450

500

550

600

Wavelength, nm

650

700

Tuning the Photoresponse of Quantum Dot Solar Cells e IPCE or Ext. Quantum Eff. R



= (1240/λ) x (Isc/Iinc) x 100

O

E 50

3.7 nm 3.0 nm 2.6 nm 2.3 nm

(A)

IPCE (%)

40

e

h

h

e

CB

30

h

20

h

RO

10

VB

CdSe

TiO2

0 350

e

e

400

450

500

550

600

650

700

Wavelength (nm) J. Am. Chem. Soc., 130 (12), 4007 -4015, 2008

Photocurrent Response Efficiency of Charge Injection vs. Light absorption

e 2

Solar Flux

O



Current density, mA/cm

R

3.7 nm 3.0 nm 2.6 nm 2.3 nm

(A)

3

2

1

0 0

50

100

Time, sec

150

Can we employ the nanowire/nanorod architecture to improve the performance of quantum dot solar cells? CdSe CdSe

e

e

e

e

TiO2

OTE/TiO2 nanoparticles

Ti/TiO2 nanotubes

(a)

(b)

Recent advances Nanowire dye-sensitized solar cells LAW, GREENE, JOHNSON, SAYKALLY, YANG Nature Materials 4 , 455, 2005

Fast Electron Transport in Metal Organic Vapor Deposition Grown Dye-sensitized ZnO Nanorod Solar Cells Galoppini, Rochford, Chen, Saraf, Lu, Hagfeldt, and Boschloo J. Phys. Chem. B; 2006; 110 16159

Electron transport in solar cells with ZnO-nanorod electrodes was about 2 orders of magnitude faster (30μs) than ZnO-colloid electrodes

Mor, G. K. et al Use of highly-ordered TiO2 nanotube arrays in dye-sensitized solar cells.

Leschkies, K. S et al Photosensitization of ZnO nanowires with CdSe quantum dots for photovoltaic devices.

Nano Lett., 2006. 6, 215-218.

Nano Lett., 2007. 7, 1793-1798.

Martinson, A. B. F. et al., ZnO nanotube based dye-sensitized solar cells ZnO nanotube based dye-sensitized solar cells. Nano Lett., 2007. 7, 2183-2187.

B

A

5μm

200nm C

D

2μm

500nm

(B) (A)

2.3nm 3.0nm 3.7nm 2.6nm

2.3nm

3.0nm 2.6nm

3.7nm

(C) 2.3nm 3.0nm 3.7nm 2.6nm

(B) 3.7 nm CdSe Intensity (a.u.)

10000

Photoemission of CdSe/glass and CdSe/TNP

Intensity (a.u.)

1.2

2.6 nm diameter a. CdSe/glass b. CdSe/TiO2

3.7 nm diameter c. CdSe/glass d. CdSe/TiO2

CdSe/glass

1000 CdSe/TiO2(NP)

100

CdSe/TiO2(NT)

10 prompt

1 0

5

10

15

20

Time, ns

c

a

0.8

, ns CdSe/ glass

0.4 b ×3

0.0 500

550

d ×3

600

650

700

Wavelength, nm

Emission spectra of CdSe QDs (a, c) on glass and (b, d) chemically bound to TiO2 nanoparticle films at 2 different sizes of QDs (2.7 and 3.7 nm). Excitation was at 480 nm.

CdSe/ tnp CdSe/ tnt

ket, sec-1

2.6nm

4.1

3.7nm

7.9

2.6nm

0.4

2.5E+09

3.7nm

1.3

6.3E+08

2.6nm

0.4

2.2E+09

3.7nm

1.5

5.5E+08

25

Quantum Dot Solar Cells – Particle versus Tube Architecture (A) TiO2(NP)

CdSe Diameter 3.7 nm a. 3.0 nm b. 2.6 nm c. 2.3 nm d.

IPCE (%)

40 d

30

50

c

20

(B) TiO2 (NT)

40

IPCE (%)

50

b

CdSe Diameter 3.7 nm a. 3.0 nm b. 2.6 nm c. 2.3 nm d.

d c

30 b

20 a

a

10

10 0

0 350

400

450

500

550

600

650

700

350

400

450

500

550

600

650

700

Wavelength (nm)

Wavelength (nm) 200nm

500nm

ISC mA/cm2

VOC V

Pmax mW/cm2

FF

CdSe-TNP

1.64

0.591

0.25

0.26

CdSe-TNT

1.95

0.582

0.29

0.26

J. Am. Chem. Soc., 130 (12), 4007 -4015, 2008

Power conversion efficiency ~1%

Depositing CdS quantum dots on TiO2 nanotubes

Unsonicated TiO2 NT

0 dip

5 dip

10 dip

20 dip

100kX

500kX

Scale bars: Top 500nm, bottom 50nm

(TiO2)

Cd2+

(TiO2)Cd2+

Wash

S2-

1 dip cycle

(TiO2)CdS

Wash

Photocurrent Response of TiO2 (nanotube)CdS Films IPCE: CdS on Single TiO2 electrode 60

R

50



O Tubes 5 dip

30

10 dip 20 dip

20 10

0 350

400

450

500

550

600

Wavelngth (nm) Reflectence Absorbtion Spectra: CdS/P25/OTE 1 0.9 0.8 0.7 Absorbance

IP C E (% )

40

TiO2

0.6

5dip 0.5

10dip 15dip

0.4

20dip 0.3 0.2 0.1 0 350

400

450

500

Wavelength (nm)

550

600

Carbon nanostructures as conduits to transport charge carriers Advantages ¾ High surface area ¾ Good electronic conductivity, excellent chemical and electrochemical stability ¾ Good mechanical strength

Pt

Goal Effective utilization of carbon nanostructures for improving the performance of energy conversion devices - To develop electrode assembly with CNT supports - Improve the performance of light harvesting assemblies - Facilitate charge collection and transport in nanostructured assemblies

…..towards achieving ordered assemblies on electrode surface

c

SWCNT- TiO2 composite films • Mesoscopic TiO2 films are extensively used in Dye-Sensitized Solar Cells • A carbon nanotube support architecture can disperse the TiO2 particles and facilitate charge collection and charge transport within the film.

e e – hν

et

CB

– +

ht

+

VB

• The first step is to design the SWCNTTiO2 network and test the feasibility of the composite system in solar cells

Kongkanand, A.; Domínguez, R.M.; Kamat, P.V., Single Wall Carbon Nanotube Scaffolds for Photoelectrochemical Solar Cells. Capture and Transport of Photogenerated Electrons. Nano Lett., 2007. 7, 676-680. Vietmeyer, F.; Seger, B.; Kamat, P.V., Anchoring ZnO Particles on Functionalized Single Wall Carbon Nanotubes. Excited State Interactions and Charge Collection. Adv. Mater., 2007, 19: 2935-2940

Electrophoretic Deposition of SWCNT on Electrode Surfaces

Sonication for 1 min SWCNT+TOAB in THF

After Electrodeposition on OTE

SWCNT/TOAB in THF

OTE/SWCNT

0.8 B

0.6

OTE/SnO2/SWCNT

Abs 0.4

b OTE/SWCNT

200 nm

a

0.2 OTE/SnO2

0.0 400

c

600 800 1000 Wavelength, nm J. Am. Chem. Soc., 2004. 126 10757-10762.

A Carbon Fiber Paper (CFE) CFE

50 μm

5 μm

B TiO2 Deposition On CFE CFE/TiO2

50 μm

5 μm

50 μm

1 μm

50 μm

1 μm

C SWCNT –Deposition On CFE CFE/SWCNT

D SWCNT-TiO2 on CFE CFE/SWCNT/TiO2

Photocurrent Generation Photocurrent, μA/cm

2

CFE/TiO2 versus CFE/SWCNT –TiO2 60

a) SWCNT/TiO2 b) TiO2

a

40

b 20

0 0

20

40

60

Time, sec

UV light

The results are indicative of better charge collection and transport provided by the SWCNT -Network

IPCE, %

Higher IPCE (increase of factor ~2) was observed for mesoscopic CFE/SWCNT-TiO2 films

a

15

10

a) SWCNT/TiO2 at 0V b) SWCNT/TiO2 at SC c) TiO2 at 0V d) TiO2 at SC

b c

5

d

0 350

400

450

500

Wavelength, nm

Nano Lett., 2007. 7, 676-680

Dependence of TiO2/SWCNT Ratio on the Photocurrent Generation

2

Photocurrent (μA/cm )

60

a

50 40

b

30

1 μm

20 a) SWCNT/TiO2 b) TiO2

10 0

0

1

2

3

4 2

TiO2 loadings (mg/cm )

Increasing the TiO2 concentration results in enhanced photocurrent as they are dispersed on SWCNT network. At concentrations greater than 2 mg/cm2 the beneficial effect of SWCNT disappears. Under these conditions. TiO2 particles aggregate and the charge recombination dominates Nano Lett., 2007. 7, 676-680

Where do we go from here?

Capping CdSe with an Electron Acceptor Shell

Electrophoretic deposition of Cluster films

CdSe

1 μm

CdSe C60

1.2 μm

0

0 1 μm

0 5

4

IPCE (%)

C60

(a)

nC60

(b) (c)

CdSe CdSe/nC60

c

3

2

b

1 a 0 350

400

450

500

550

600

650

700

Wavelength (nm)

J. Am. Chem. Soc., 2008, 130, 8890–8891

Organized light harvesting assembly using carbon nanostructures hν

e h

1 μm

e

15 nm e h

e h

e

e

0

0 1 μm

0

Graphene-Semiconductor Nanocomposites

ACS Nano, 2008, 2, 1487-1491

hν h TiO TiO ‐GO 2 2

TiO2-GR

Graphene Oxide



0.6 μm

Reduced Graphene



e

Summary



Unique properties of quantum dots offer new opportunities to develop low-cost and high efficiency solar cells



1-D architectures are useful for designing next generation solar cells.



Opportunities exist for carbon nanostructures to facilitate capture and transport of electrons in nanostructure semiconductor based solar cells.

Kamat, P. V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion (Review) J. Phys. Chem. C, 2007. 111 2834 - 2860. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters (Centennial Feature) J. Phys. Chem. C 2008, 112, in press

Researchers/Collaborators Graduate students

Brian Seger (Chem. Eng.) David Baker (Chem. Eng.) Kevin Tvrdy (Chemistry) Clifton Harris (Chemistry) Matt Baker (Physics) Ian Lightcap (Chemistry) Philix Vietmeyer (Chemistry) Yanghai Yu (Chem. Eng.) Istvan Robel (Physics)

Collaborators Dr. K. G. Thomas (India) Prof. Fukuzumi (Osaka U.) Prof Ken Kuno (UND) Prof. K. Vinodgopal (IUN)

Post-Docs/Visiting Scientists

Jin Ho Bang

Undergraduate students

Pat Brown Chris Rodriguez David Riehm Rachel Staran

What will the future hold? Over the last twenty years, the per-kWh price of photovoltaics has dropped from about $500 to nearly $5; think of what the next twenty years will bring.

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