Novel Photovoltaics based on Direct Interfacial Charge Transfer - - PowerPoint PPT Presentation

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Contents

• 1. Introduction
• 2. Purpose
• 3. Structure and formation

mechanism of TiO2-TCNX

• 4. Assignment of absorption
• f TiO2-TCNX
• 4. TiO2-TCNX based solar cells
• 5. Summary

Novel Photovoltaics based on Direct Interfacial Charge Transfer Transition from Surface-Bound Organic Compounds to Semiconductor

Research Center for Advanced Science and Technology, The University of Tokyo 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan

Hiroshi Segawa

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Development of solar cells based on a novel principle for efficient interfacial charge separation

(i) Dye-sensitized solar cells (DSSC) (ii) Organic thin film solar cells (OPV)

• a. Light absorption by dyes
• b. Charge separation from dyes

to carrier-transporting materials

• c. Carrier transport

A potential candidate for efficient and low-cost photovoltaic devices

Dye-sensitized solar cell Organic thin film solar cell

1) M. K. Nazeeruddin et al., J. Am. Chem. Soc. 2005, 127, 16835. 2) M. R. Reyes et al., Appl. Phys. Lett., 2005, 87, 083506.

I-/I3

• dyes

LUMO HOMO hν e- e- hν e- LUMO HOMO e- Donor Acceptor VB CB TiO2 e-

(General mechanism)

DSSC OPV Conversion efficiency ～11% 1) ～4.9% 2)

Load Load e- e- e- e-

• 1. Introduction: organic solar cells

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・TiO2 is colored by dicyano- methylene compounds (TCNX) ・The color is dependent on the kind of TCNX

TCNE TCNAQ TCNQ TiO2 Violet Orange Green TCNQ/CH3CN

Recent our work

: Novel chemical coloration of TiO2

Wide band gap semiconductor (Eg～3.2 eV) used for photocatalyst

Dicyanomethylene compounds Electron acceptor for organic charge-transfer complexes

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Absorbance (arbitrary units) 1000 800 600 400 200 Wavelength (nm) 1-Rdiff (arbitrary units)

Novel absorption appears in the visible to near IR region Appearance of novel absorption band

TiO2 TCNE TCNQ TCNAQ TiO2-TCNE TiO2-TCNQ

TiO2-TCNAQ

TiO2-TCNE TiO2-TCNAQ TiO2-TCNQ

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80 60 40 20 IPCE (%) 700 600 500 400 Wavelength (nm)

81% at 450 nm

TiO2-TCNQ

Efficient photon-to-current conversion

I- / I3

• redox

couple electrolyte (2M LiI, 0.025M I2 in CH3CN)

TiO2-TCNX is very promising for efficient photoelectric conversion

Incident photon-to-current conversion efficiency (IPCE) spectrum

Electrochemical solar cell

Pt FTO Efficient photoelectric conversion! Load e- e- Photoanode

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(i) Structure and formation mechanism of TiO2-TCNX surface complexes (ii) Assignment of the absorption band of TiO2-TCNX surface complexes (iii) Photoelectric conversion of TiO2-TCNX based solar cells Chemical control of photoelectric conversion

• a. Extension of π-conjugation of TCNX
• b. Chemical modification of conduction band of TiO2
• 2. Purpose

TCNE TCNAQ TCNQ Extension of π-conjugation

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Transmittance 3000 2000 Wavenumber (cm-1)

TiO2-TCNQ TCNQ C≡N stretching peaks FT-IR spectrum

3.Structure and formation mechanism of TiO2-TCNQ : Vibrational structure of TiO2-TCNQ

Transmittance 2400 2200 2000 Wavenumber (cm-1) 2253 2192 2129 2227

C≡N vibrational structure

No adsorption HCl-treated TiO2

Cl- Ti+

OH Ti

HCl-treated TiO2

• OH

1M HCl TiO2-TCNQ TCNQ

・Large structural change of TCNQ adsorbed on TiO2 ・TCNQ adsorption due to reactions with hydroxyl groups

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Nucleophilic addition of TCNQ with alcohol

NC CN CN NC H3CO NC CN CN NC

CH3OH + Nucleophilic addition

Methoxy-TCNQ adduct

Cα

H+

• H+

NC CN CN NC

Nucleophilic addition α

O Ti

• H+

H

+

NC CN CN NC O Ti

H+

？

Murata et al. (Bull. Chem. Soc. Jpn, 2008, 81, 331) σ bond between the O and C atoms (dO-C = 1.4Å)

O Methoxy C TCNQ

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2400 2200 2000 Wavenumber (cm

• 1)

Nucleophilic addition of TCNQ with TiO2 NC CN CN NC

NC CN CN NC O Ti

Nucleophilic addition α H+

O Ti

• H+

H

C≡N stretching modes

Transmittance 3000 2500 2000 1500 Wavenumber (cm

• 1)

TiO2-TCNQ Li(CH3O-TCNQ) C≡N伸縮 2253 2192 2129 2243 2178 2127 Li(CH3O-TCNQ) TiO2-TCNQ CH3O-TCNQ-

FT-IR spectra

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200 100 Yield (cps) 6.4 6.0 5.6 5.2 Energy (eV) Ip～5.8 eV

1.0 0.5 1-Rdiff 3.0 2.5 2.0 1.5 1.0 Energy (eV)

Interfacial charge-transfer transitions from surface-bound TCNQ to TiO2

Ionization potential Diffuse reflectance spectrum

• 4. Assignment of absorption of TiO2-TCNX

Interfacial charge-transfer transition TiO2 HOMO LUMO CB h+ VB Surface-bound TCNQ E (eV) 4.3 5.8 ΔE= 1.5eV (830nm)

ΔE agrees with the onset energy of the broad absorption approximately

7.5 e- TiO2 TiO2-TCNQ TiO2-TCNQ

NC CN CN NC O Tie-

Ip～5.8eV

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Yield (arb. units) 6.5 6.0 5.5 Photon energy (eV)

1000 800 600 400 Wavelength (nm) 1-Rdiff

・Longer wavelenght shift due to increase

• f HOMO energy of the surface-bound

TCNX with the extension of π-conjugation ・TCNX dependence suppports our assignment

(ii) π-conjugation effect

TiO2 HOMO Surface-bound TCNX E (eV) 4.3 7.5 5.6 5.8 ～6.0 TCNAQ TCNQ TCNE CB VB Interfacial charge-transfer transition

Diffuse reflectance spectra

Extension of π-conjugation

TiO2-TCNAQ TiO2-TCNQ TiO2-TCNE TiO2-TCNAQ Ionization potential TiO2-TCNQ TiO2-TCNE Ip Decrease Red shift

e-

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80 60 40 20 IPCE (%) 800 700 600 500 400 Wavelength (nm)

NC NC CN CN CN CN NC NC NC NC CN CN

TCNQ TCNAQ TCNE

・Photoelectric conversion occurs in the visible to near IR region. ・Near IR photoelectric conversion can be enhanced by π-conjugation extension → Control of spectral region by chemical modification of TCNX

TiO2 HOMO Surface-bound TCNX E (eV) 4.3 7.5 5.6 5.8 6.0 TCNAQ TCNQ TCNE CB VB Interfacial charge-transfer transition

• 5. TiO2-TCNX based solar cells

:π-conjugation effect on IPCE spectra

I-/I3

• e-

Novel electrochemical solar cell

Load

I- / I3

• 5.2

e-

Red shift

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60 40 20 IPCE (%) 800 700 600 500 400 Wavelength (nm) [Li+] 1M 2M 3M 4M

Red shift with [Li+]

Normalized IPCE 500 450 400 Wavelength (nm)

Effect of Li+ on IPCE spectra

Red shift with [Li+] TiO2 HOMO LUMO Surface-bound TCNQ E (eV) 4.3 5.8

Li+

CB e- Interfacial charge-transfe transition e- Spectral region is controllable by chemical band modification of TiO2 VB

I-/I3

• Novel electrochemical

solar cell

Load

Redmond et al. J. Phys. Chem. 1993, 97, 1426. Kelly et al. Langmuir, 1999, 15, 7047.

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Features of ICT-based solar cells

CB Dyes LUMO HOMO

e-

kinj Intra-molecular transition TiO2 CB TCNX LUMO HOMO

e-

ICT-based solar cells Conventional organic solar cells

TiO2

ICT is effective for efficient charge separation, in particular, advantageous in near IR photoelectric conversion.

Interfacial charge-transfer transition (ICT) Eonset=ECB-EHOMO +ΔE ΔE Eonset=ECB-EHOMO ΔE≥0.2～0.3eV

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Summary (i) Interfacial charge-transfer transitions from surface- bound TCNX to TiO2 enables wide light absorption (ii) Efficient photocurrent conversion occurs with IPCE exceeding 80%. (iii) Spectral region is controllable by chemical modifications of not only TCNX but also TiO2.

Novel organic solar cells based on interfacial charge-transfer transitions