1

The Effect of Dopants on TiO2 Solar Cell Efficiency

Mini Project Presentation FS09

E. Buitrago Advisors: Dr. A. Teleki and A. Tricoli

Particle Technology Laboratory

Swiss Federal Institute of Technology (ETHZ)

2

Overview• Introduction

– Global energy problem– Solar Cell possibilities– Dye Sensitized Solar Cells– Narrowing the TiO2 bandgap: doping

• Experimental– FSP particle synthesis– Photocatalytic Experiments– Bandgap Calculations

• Results– Fe– Nb– Ru

• Conclusion• Outlook• Future Work

3

World Energy Use

http://www.flickr.com/photos/33264427@N06/3166865015/

, US, 2006

4

Solar Cell Possibilities

http://en.wikipedia.org/wiki/File:PVeff(rev110707)d.png

$$$$$$

$$

5

Dye Sensitized Solar Cell (DSSC) Schematic

I- / I-

3

Ru2+ → Ru3+ + e−

Anode (oxi): 3I−→I−3 +2e−

Cathode (red): I−3 +2e−→3I−

5

1. O’Regan et al. Nature . 1991.2. Nazeeruddin et al. J.Am.Chem.Soc. 2001.3. http://bouman.chem.georgetown.edu/S02/lect23/Solar_Spectrum.png\4. Grätzel et al. MRS Bulletin. 2005.

Cathode (+)Anode (-)

A.R.

U. A

bsor

banc

e1

2

3

Wide bandgap semiconductorEg = 3.2 eV ~ 385 nm (4)Visible light: 400-700 nm

1.8- 3.1 eV

6

Maximizing Visible Light Absorption: Dopants

Bandgap Method Concentration

1. TiO2 3.2 eV (anatase)

2. Fe-TiO2 2.2 eV FSP 30 mol%Solubility limit 5 mol%

3. Ru-TiO2 2.56 eV Ion exchange

0.018 mol%

4. Nb-TiO2 2.9 eV Sol gel 0.0017 mol%

1. Grätzel et al. MRS Bulletin. 2005.2.Teoh et al. Catalysis Today. 20073. Khan et al. Appl. Surf. Sci. 20094. Salvador et al. Solar Energy Materials. 1980 0.03, 0.3, 1 mol% dopant

7

FSP Particle Synthesis

• TiO2:• 0.5 M TTIP in

Xylene/Acetronile(3:1)• Dopant Precursors :

– Fe: Iron Acetylacetonate– Nb: Niobium 2-

Ethylhexaonate– Ru: Ruthenium (III)

Acetylacetonate• 5/5 Flame

Mädler et al. J. Aerosol Sci. 2002

d

8

200 300 400 500 6000.0

0.5

1.0

1.5

2.0

2.5

Abs

orba

nce

Wavelength (nm)

TiO2

Bandgap Calculations

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.40

1

2

3

4

(hv

)(1/2

)

Eg =hv [eV]

TiO2

Indirect Semiconductorhvα = const (hv-Eg)2

hv = energy of incident photon [eV]

α = absorption coefficient [cm-1]

α = A/lA = Absorbance

(measured with UV-vis)l = cuvette length

UV-vis Spectrometry

Singh et al. International Journal of Hydrogen Energy. 2008.

9TiO2 0.03 mol% Fe 0.3 mol% Fe 1 mol% Fe

2.90

2.95

3.00

3.05

3.10

3.15

3.20

Eg

(

Eg

12

14

16

18

20

22

24

Rut

ile w

t%

Rutile wt%

Fe-TiO2 Bandgap and Rutile %

Teoh et al.mol %

Eg [eV]

Rutile %

0 3.2 15

0.5 3.13 18

2 2.9 32

Teoh et al. Catalysis Today. 2007.0.03 mol% Fe 0.3 1

10

Photocatalytic Experiments with UV-Light

http://en.wikipedia.org/wiki/Methylene_blueHeight et al. Applied Catalysis B. 2005.

10 ppm Methylene Blue

Catalyst loading:0.3 kg/m3

8 WUV –lamp366 nm

500 550 600 650 700 7500

1

2

Abs

orban

ce

Wavelength (nm)

2 ppm 4 ppm 6 ppm 8 ppm 10ppm

UV –Vis 665 nm

11

UV-Photocatalytic Testing Fe-TiO2

0 10 20 30 40 50 60

0.4

0.5

0.6

0.7

0.8

0.9

1.0C

/C0

Time (min)

Control TiO

2

Fe 0.03 mol% Fe 0.3 mol% Fe 1 mol%

-0.5 kg(catalyst)/m3

-Hydrothermal doping-366 nm-100 ppm MB

12

TiO2 0.03 mol% Nb 0.3 mol% Nb 1 mol% Nb

2.90

2.95

3.00

3.05

3.10

3.15

3.20

Eg(

Eg

12

14

16

18

20

22

24

Rut

ile w

t%

Rutile wt%

Nb-TiO2 Bandgap and Rutile %

Nb2O5 Eg = 3.4 eV

Salvador et al. Solar Energy Materials. 1980.Teleki et al. Sensor. Actuator. B. 20070.03 mol% Nb 0.3 1

13

UV-Photocatalytic Testing Nb-TiO2

0 10 20 30 40 50 60

0.4

0.5

0.6

0.7

0.8

0.9

1.0

C/C

0

Time (min)

Control TiO

2

Nb 0.03 mol% Nb 0.3 mol% Nb 1 mol%

14

Nb-TiO2 Outperforms TiO2

0 10 20 30 40 50 60 70 80 90 100 110 1200.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Control TiO

2

Nb 0.3 mol%

C/C

0

Time (min)

15TiO2 0.03 mol% Ru 0.3 mol% Ru 1 mol% Ru

2.90

2.95

3.00

3.05

3.10

3.15

3.20

E(

Eg

12

14

16

18

20

22

24

Rut

ile w

t%

Rutile wt%

Ru-TiO2 Bandgap and Rutile %

Ru02 Eg = 2.4 eV

Gujar et al. Electrochemistry Communications. 2007.0.03 mol% Ru 0.3 1

16

UV-Photocatalytic Testing Ru-TiO2

0 10 20 30 40 50 60

0.4

0.5

0.6

0.7

0.8

0.9

1.0

C/C

0

Time (min)

Control TiO

2

Ru 0.03 mol% Ru 0.3 mol% Ru 1 mol%

17

Conclusion

• TiO2 Bandgap reduced by FSP with Nb and Fe.• Highest bandgap reduction Fe- 1 mol%.• Highest photocatalytic activity under UV light

for Nb-TiO2 at 0.3 mol% by (2.95 eV).

18

Outlook

• Investigation of photocatalytic activity under visible light for Fe.

-1 kg(catalyst)/m3

-FSP-λ > 400 nm-10 oxalic acid

= 5 mol%

Teoh et al. Catalysis Today. 2007.

19

Future Work

• Synthesis of DSSC with best catalyst.

20

Appendix

• Bandgap Calculations• Photocatalytic Process

21

Photocatalytic Process

• Photo-generation of electron/hole pair

• Formation of radicals (Ox)

• Radical oxidation of organic compound

TiO2 hv

e- + h+

h+ + H2O OH + H+

e- + O2 O2-

O2- + H+ HO2

TOC + Ox TOC (partially oxidized species) +

CO2 + H2O

Kim et al. Catalysis Letters. 2007

22

TiO2 0.03 mol% Fe 0.3 mol% Fe 1 mol% Fe3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Ave

rage

Partic

le D

iam

eter (n

m)

dBET

dXRD, A

dXRD, R

0.03 mol% Fe 0.3 1

Anatase

Rutile

20 30 40 50 60

1 mol% Fe

Inte

nsity

(a.u

.)

2

TiO2

0.03 mol% Fe

0.3 mol% Fe

14.3 wt%

14.3 wt%

17.1 wt%

Rutile = 23.1 wt%

Fe-TiO2

Fe3+ ionic radius: 0.55 Å Ti4+ ionic radius is: 0.67 Å Wikipedia

23

TiO2 0.03 mol% Nb 0.3 mol% Nb 1 mol% Nb3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20 d

BET

dXRD, A

dXRD, R

Ave

rage

Partic

le D

iam

eter (n

m)

0.03 mol% Nb 0.3 1

Anatase

Rutile

20 30 40 50 60

Inte

nsity

(a.u

.)

2

1 mol% Nb

TiO2

0.03 mol% Nb

0.3 mol% Nb

14.3 wt%

18.0 wt%

17.1 wt%

Rutile = 13.71 wt%

Nb-TiO2

Nb5+ ionic radius: 0.64 Å Ti4+ ionic radius is: 0.68 Å Wikipedia

24

TiO2 0.03 mol% Ru 0.3 mol% Ru 1 mol% Ru3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20 d

BET

dXRD, A

dXRD, R

Ave

rage

Partic

le D

iam

eter (n

m)

0.03 mol% Ru 0.3 1

Anatase

Rutile

20 30 40 50 60

14.3 wt%

13.2 wt%

14.3 wt%

Rutile = 13.0 wt%

Inte

nsity

(a.

u.)

2

1 mol% Ru

TiO2

0.03 mol% Ru

0.3 mol% Ru

Ru-TiO2

Fe3+ ionic radius: 0.57.5 Å Ti4+ ionic radius is: 0.67 Å Wikipedia

250 10 20 30 40 50 60

0.4

0.5

0.6

0.7

0.8

0.9

1.0

C/C

0

Time (min)

Control TiO

2

Fe 0.03 mol% Fe 0.3 mol% Fe 1 mol%

Photocatalytic Testing with MB

Li et al. J. Hazardous Materias. 2008

-0.5 kg(catalyst)/m3

-Hydrothermal doping-366 nm-100 ppm MB

-1 kg(catalyst)/m3

-FSP-λ > 400 nm-10 oxalic acid

= 5 mol%

Teoh et al. Catalysis Today. 2007.

-0.5 kg(catalyst)/m3

-Impregnation methodn: Fe(NO3)3•9H2Oa: Iron acetylacetonate complex-300-400 nm-5 ppm oxalic acid

Navío et al. Journal of Molecular Catalysis A. 1996.

-1 kg(catalyst)/m3

-Impregnation methodn: Fe(NO3)3•9H2Oa: Iron acetylacetonate complex-λ > 400 nm-5 ppm oxalic acid

0.004 mol%

0.04 mol%

26

27

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.40

1

2

3

4

(hv

)(1/2

)

Eg =hv [eV]

TiO2

Bandgap Calculations

Indirect Semiconductorhvα = const (hv-Eg)2

hv = energy of incident photon [eV]

α = absorption coefficient [cm-1]

α = A/lA = Absorbance

(measured with UV-vis)l = cuvette length

28

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.40.0

0.5

1.0

1.5

2.0

2.5

3.0

(

hv)(1

/2)

Eg =hv [eV]

TiO2

1 mol% Fe 0.3 mol% Fe 0.03 mol% Fe

Bandgap Calculations Fe

29

Bandgap Calculations Nb

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.40.0

0.5

1.0

1.5

2.0

2.5

3.0 TiO

2

1 mol% Nb 0.3 mol% Nb 0.03 mol% Nb

(

hv)(1

/2)

Eg =hv [eV]

30

Bandgap Calculations Ru

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.40.0

0.5

1.0

1.5

2.0

2.5

3.0

(hv

)(1/2

)

Eg =hv [eV]

TiO2

0.03 mol% Ru 0.3 mol% Ru 1 mol% Ru

31

TiO2 BondOrbitals

Ti d + (O2p)

O2P + ( Ti d)

Ti d Eg = 3.2 eV

O2p

Energy

Conduction Band

Valence Band