Eray S. Aydil

Chemical Engineering and Materials Science Department

Acknowledgements: National Science Foundation NIRT ProgramMinnesota Initiative for Renewable Energy and the Environment (IREE)

ZnOZnO nanowire based solar cellsnanowire based solar cells

Prof. Uwe Kortshagen

Prof. David Norris

Kurtis LeschkiesJanice Boercker

Prof. C. Barry Carter

Dr. Joysurya Basu

Emil Enache-Pommer

Dr. DivakarRamachandran

StateState--ofof--thethe--art in solar cellsart in solar cells

United States Department of Energy Report on the Basic Energy Sciences Workshop on Solar Energy Utilization by N. S. Lewis et al. (2005) and from J. Crystal Growth 275, 292 (2005) by T. Surek.

Dye Sensitized Solar CellsDye Sensitized Solar Cells

O’Regan & Grätzel, Nature 353, 737 (1991).Grätzel, Nature 414, 338 (2001).

Nanocrystalline, mesoporous TiO2 photoelectrode on TCO.

TiO2 is photosensitized with a monolayer of dye.

Efficient light harvesting with large dyed surface area: ~ 1000 × flat film

Dye-sensitized nanowire solar cell

Wide band gap semiconductor nanowires with adsorbed dye monolayer

Light

Liquid electrolyte or hole conductor e-e-

Nanowires provide a direct path to the substrate for efficient charge collection.

Device optical absorption spectrum can be tuned through selection of QD material and size.

400 450 500 550 600 650Wavelength (nm)

Abs

orba

nce

(A.U

.)

Increasing size of QD

ħωEgQD

Electron states

Hole states

Colloidal QDs can generate multiple electron-hole pairs per absorbed photon.

(1) Baxter et al., Nanotechnology, 2006. (2) Law et al., Nature Mater., 2005. (3) Nozik, Physica E, 2003. (4) Schaller et al., Phys. Rev. Lett., 2005. (5) Ellingson et al., Nano Lett., 2005.

Emerging alternatives to Emerging alternatives to DSSCsDSSCs

How can we attach colloidal QDs onto the surface of the nanowires?

Can the QDs harvest light while attached to the nanowires?

Will the QDs transfer photogenerated electrons to the nanowires?

What factors limit the overall performance of these devices?

e-e-

Quantum dots attached to nanowires

Liquid electrolyteor hole conductor

Quantum dots attached to nanowires

Liquid electrolyteor hole conductor

CdSe quantum dots attached to ZnO nanowires

Light

0.05 M I2, 0.05 M LiI, 0.5 M tert-butylpyridinein short chain hydrocarbons

CdSe quantum dots attached to ZnO nanowires

0.05 M I2, 0.05 M LiI, 0.5 M tert-butylpyridinein short chain hydrocarbons

Light

Electrochemical Scale (V vs. NHE)

8.0 +3.5

3.0

4.0

5.0

6.0

7.0

-1.5

-0.5

+0.5

+1.5

+2.5

Solid State Scale (eV)

ZnO5 nm

CdSe QDs Redox

I- / I3-

-

-

)

-

-

)

ZnO

-3--

3-

QuantumQuantum--dotdot--sensitized solar cellssensitized solar cells

Low temperature Low temperature ZnOZnO nanowire growth nanowire growth from zinc nitrate and from zinc nitrate and MethenamineMethenamine solutionsolution

2 µm

0.025 M zinc nitrate, Zn(NO3)2

0.025 M methenamine

95 ºC

30 32 34 36 38 400

400

800

1200

1600

2000

Inte

nsity

2θ (degrees)

(000

2)

(101

0)

(101

1)

200 nm

XRD

Baxter, Walker, van Ommering, and Aydil, Nanotechnology 17, S304 (2006).

100 nm

[0001][1120]

Control of Control of ZnOZnO nanowire dimensionsnanowire dimensions4 cycles

1 µm

1 cycle

1 µm 2 µm

8 cycles

100 nm

0 2 4 6 80

2

4

6

8 684528 Aspect Ratio

Number of Growth Cycles

Leng

th (µ

m)

13

0

100

200

300

400

500

Diam

eter (nm)

CdSeCdSe quantum dotsquantum dots“One-pot synthesis” from CdO and tri-n-octylphosphine selenide (TOPSe) in hexadecylamine(HDA) with tri-n-octylphosphine oxide (TOPO),

QDs are single-crystals with diameters of 3–4 nm and surface-coordinated with TOPO and HDA.

4000 3500 3000 2500 2000 1500 10000

0.02

0.04

0.06

0.08

0.10

0.12

stretching

P=O stretchingN-H

x2

x10

Wavenumber (cm-1)

Abs

orba

nce

O=PTOPO:

H2NHDA:

CdSeCdSe quantum dot size controlled by reaction timequantum dot size controlled by reaction time

350 400 450 500 550 600 650

Wavelength (nm)

Abs

orba

nce

(AU

)

1.9 nm2.3 nm2.6 nm3.7 nm4.0 nm5.1 nm

350 400 450 500 550 600 650Wavelength (nm)

Abs

orba

nce

(AU

)

'As-synthesized' with TOPO ligands

Post surface exchange with MPA

Replace Alkyl ligands on Replace Alkyl ligands on CdSeCdSe with Xwith X--RR--YY

SCH2CH2COOHSCH 2CH 2C

OOH

SCH 2C

H 2CO

OH

SCH

2CH

2CO

OH

SCH2CH

2COOH

SCH2CH2COOH

SCH2CH2COOH SCH2CH2COOH

SCH

2CH

2CO

OH

SCH

2 CH

2 CO

OH

SCH2 CH

2 COOH

SCH2CH

2COOH

Hexane phase

Methanol phase

CH3 stretching from [N(CH3)4]+

3500 3000 2500 2000 1500 1000

0

0.002

0.004

0.006

0.008

0.010

COO–

asymmetric

CH3deformation

from [N(CH3)4]+COO– symmetric

x0.5 x0.25

Wavenumber (cm-1)

Abs

orba

nce

(A.U

.)

C=O from COOH

CdSeCdSe quantum dots adsorbed on quantum dots adsorbed on ZnOZnO NanowiresNanowires

Attaching quantum dots to Attaching quantum dots to ZnOZnO nanowiresnanowires

CdSeCdSe quantum dots on quantum dots on ZnOZnO nanowiresnanowires

0.14 nm

0.14 nm

Optical absorption of Optical absorption of CdSeCdSe QDsQDs on on ZnOZnO nanowiresnanowires

OOC-CH2CH2S-

OOC-CH2CH2S-

OOC-CH2CH2S-

QD optical absorption was preserved when QDs were attached to the nanowires.

400 500 6000

0.05

0.10

0.15

0.20

ColloidalCdSe QDs

CdSe QDson ZnO

ZnO

Wavelength (nm)

Film

Abs

orba

nce

(A.U

.)

CdSeCdSe quantum dot sensitized solar cellsquantum dot sensitized solar cells

Photovoltaic effect observed

0

40

80

120

160

0

0.5

1.0

1.5

350 400 450 500 550 6000

0.5

1.0

1.5

2.0

(b)

ZnO

CdSe QDs on ZnO

Phot

ocur

rent

(µA/

cm2 )

Abso

rban

ce

(a)

(c)

Wavelength (nm)

Irrad

ianc

e (W

m-2 n

m-1)

0

40

80

120

160

0

0.5

1.0

1.5

350 400 450 500 550 6000

0.5

1.0

1.5

2.0

(b)

ZnO

CdSe QDs on ZnO

Phot

ocur

rent

(µA/

cm2 )

Abso

rban

ce

(a)

(c)

Wavelength (nm)

Irrad

ianc

e (W

m-2 n

m-1)

0 0.2 0.4 0.6 0.8-3

-2-1

0

12

3

0 0.1 0.2 0.3 0.4

-0.10

-0.05

0

0.05

0.10

no O2 plasmawith O2 plasma

Potential (V)

Curr

ent D

ensi

ty (m

A/cm

2 )

ZnO

Electron transferred from QD to nanowire

CdSeCdSe quantum dot sensitized solar cellsquantum dot sensitized solar cells

43421IQE

LHEIPCE collinj ϕϕ ××=

injϕ

LHE

collϕ

450 500 550 6000

15

30

45

60

IPCE

LHE (Light harvesting efficiency)

Wavelength (nm)LH

E or

IPC

E (%

)

Internal quantum efficiency (IQE) as high as 45–58% between 500–600 nm.

Power conversion efficiency limited by the LHE and available nanowire surface area.

SummarySummary

CdSe quantum dots and ZnO nanowires form a new type of QDSSC.

Photogenerated electrons from CdSe transferred into ZnO.

Isc ≈ 2 mA/cm2, Voc ≈ 0.6 V, FF ~ 0.3 for typical QDSSCs. IQE as high as 58%.

Nanowire QDSSCs may be a promising solar cell design.

Replace liquid electrolyte.

Increase roughness factor of nanowires.