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

Novel Photovoltaics based on Direct Interfacial Charge Transfer Transition from Surface-Bound Organic Compounds to Semiconductor Hiroshi Segawa Research Center for Advanced Science and Technology, The University of Tokyo 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan Contents 1. Introduction 2. Purpose 3. Structure and formation mechanism of TiO 2 -TCNX 4. Assignment of absorption of TiO 2 -TCNX 4. TiO 2 -TCNX based solar cells 5. Summary 1

e - e - e - e - Load Load 1. Introduction: organic solar cells LUMO e - LUMO e - A potential candidate for efficient e - CB e - and low-cost photovoltaic devices h ν h ν HOMO (i) Dye-sensitized solar cells (DSSC) e - (ii) Organic thin film solar cells (OPV) I - /I 3 - HOMO VB (General mechanism) dyes Donor Acceptor TiO 2 a. Light absorption by dyes Dye-sensitized Organic thin film b. Charge separation from dyes solar cell solar cell to carrier-transporting materials DSSC OPV c. Carrier transport ～ 11% 1) ～ 4.9% 2) Conversion efficiency Development of solar cells based on a novel principle for efficient interfacial charge separation 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. 2

Recent our work : Novel chemical coloration of TiO 2 Dicyanomethylene compounds Electron acceptor for organic charge-transfer complexes TCNE Orange TCNQ/CH 3 CN TiO 2 TCNQ Wide band gap semiconductor Violet (Eg ～ 3.2 eV) used for photocatalyst ・ TiO 2 is colored by dicyano- methylene compounds (TCNX) Green TCNAQ ・ The color is dependent on the kind of TCNX 3

Appearance of novel absorption band TCNE Absorbance (arbitrary units) TiO 2 -TCNE 1- R diff (arbitrary units) TiO 2 TiO 2 -TCNE TiO 2 -TCNQ TCNQ TiO 2 -TCNQ TiO 2 -TCNAQ TCNAQ TiO 2 -TCNAQ 200 400 600 800 1000 Wavelength (nm) Novel absorption appears in the visible to near IR region 4

Efficient photon-to-current conversion Electrochemical solar cell 80 81% at 450 nm e - e - Load Photoanode 60 IPCE (%) 40 Efficient photoelectric conversion! TiO 2 -TCNQ 20 Pt 0 I - / I 3 - redox 400 500 600 700 FTO couple electrolyte Wavelength (nm) (2M LiI, 0.025M I 2 in CH 3 CN) Incident photon-to-current conversion efficiency (IPCE) spectrum TiO 2 -TCNX is very promising for efficient photoelectric conversion 5

2. Purpose (i) Structure and formation mechanism of TiO 2 -TCNX surface complexes (ii) Assignment of the absorption band of TiO 2 -TCNX surface complexes (iii) Photoelectric conversion of TiO 2 -TCNX based solar cells Chemical control of photoelectric conversion a. Extension of π -conjugation of TCNX b. Chemical modification of conduction band of TiO 2 TCNQ TCNE TCNAQ Extension of π -conjugation 6

3.Structure and formation mechanism of TiO 2 -TCNQ : Vibrational structure of TiO 2 -TCNQ C ≡ N vibrational structure FT-IR spectrum TCNQ Transmittance TiO 2 -TCNQ 2227 TCNQ Transmittance 2129 2253 2192 HCl-treated TiO 2 TiO 2 -TCNQ No adsorption C ≡ N 2400 2200 2000 Wavenumber (cm -1 ) stretching peaks OH 1M HCl Cl - Ti Ti + 3000 2000 - OH Wavenumber (cm -1 ) HCl-treated TiO 2 ・ Large structural change of TCNQ adsorbed on TiO 2 ・ TCNQ adsorption due to reactions with hydroxyl groups 7

Methoxy Nucleophilic addition of TCNQ with alcohol O C NC CN NC CN C α H 3 CO Nucleophilic addition TCNQ CH 3 OH + -H + H + Murata et al. ( Bull. Chem. Soc. CN CN NC Jpn , 2008, 81, 331) NC σ bond between Methoxy-TCNQ the O and C atoms adduct ( d O-C = 1.4 Å ) NC CN α NC H CN O Nucleophilic Ti addition O Ti -H + + ？ H + CN CN NC NC 8

Nucleophilic addition of TCNQ with TiO 2 C ≡ N stretching modes FT-IR spectra Li(CH 3 O-TCNQ) Li(CH 3 O-TCNQ) Transmittance 2127 2243 2178 CH 3 O-TCNQ - TiO 2 -TCNQ 2253 2129 TiO 2 -TCNQ C ≡ N 伸縮 2192 3000 2500 2000 1500 2400 2200 2000 -1 ) -1 ) Wavenumber (cm Wavenumber (cm NC CN α NC H CN O Nucleophilic Ti addition O Ti -H + H + CN CN NC NC 9

NC 4. Assignment of absorption of TiO 2 -TCNX CN O Ti e - Diffuse reflectance spectrum 1.0 E (eV) TiO 2 -TCNQ LUMO CN NC 1-R diff e - 0.5 CB 4.3 Interfacial charge-transfer Δ E = 1.5eV transition (830nm) 3.0 2.5 2.0 1.5 1.0 Energy (eV) h + 5.8 HOMO Ionization potential 200 7.5 Yield (cps) VB TiO 2 -TCNQ Surface-bound I p ～5.8 eV 100 I p ～ 5.8eV TiO 2 TCNQ TiO 2 Δ E agrees with the onset energy of the 0 broad absorption approximately 5.2 5.6 6.0 6.4 Energy (eV) Interfacial charge-transfer transitions from surface-bound TCNQ to TiO 2 10

(ii) π -conjugation effect Diffuse reflectance spectra E (eV) TiO 2 -TCNE Extension of TiO 2 -TCNQ e - π -conjugation 4.3 Interfacial CB charge-transfer TiO 2 -TCNAQ 1- R diff transition HOMO TCNAQ 5.6 Red shift TCNQ 5.8 TCNE ～ 6.0 400 600 800 1000 Wavelength (nm) VB 7.5 Yield (arb. units) Ionization potential TiO 2 -TCNAQ Surface-bound TiO 2 TCNX TiO 2 -TCNQ TiO 2 -TCNE ・ Longer wavelenght shift due to increase of HOMO energy of the surface-bound Decrease I p TCNX with the extension of π -conjugation ・ TCNX dependence suppports our assignment 5.5 6.0 6.5 Photon energy (eV) 11

Load 5. TiO 2 -TCNX based solar cells : π -conjugation effect on IPCE spectra I - /I 3 - 80 NC CN E (eV) Novel electrochemical NC CN solar cell TCNQ e - 60 NC 4.3 Interfacial CB CN charge-transfer IPCE (%) NC transition CN TCNAQ 5.2 e - 40 HOMO I - / I 3 - TCNAQ 5.6 TCNQ 5.8 20 NC CN TCNE 6.0 NC CN Red shift TCNE VB 7.5 0 400 500 600 700 800 Surface-bound Wavelength (nm) TiO 2 TCNX ・ 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 12

Load Effect of Li + on IPCE spectra I - /I 3 - Red shift with [Li + ] Normalized IPCE Novel electrochemical E (eV) solar cell 60 LUMO CB e - 4.3 Interfacial Li + e - charge-transfe IPCE (%) 40 transition 400 450 500 [Li + ] Wavelength (nm) 1M HOMO 2M 5.8 20 3M Red shift with [Li + ] 4M Surface-bound VB 0 TCNQ TiO 2 400 500 600 700 800 Wavelength (nm) Redmond et al. J. Phys. Chem. 1993, 97 , 1426. Kelly et al. Langmuir , 1999, 15 , 7047. Spectral region is controllable by chemical band modification of TiO 2 13

Features of ICT-based solar cells Conventional organic solar cells ICT-based solar cells LUMO LUMO k inj Δ E e - CB e - CB Intra-molecular Interfacial transition charge-transfer transition (ICT) E onset = E CB - E HOMO + Δ E E onset = E CB - E HOMO Δ E ≥ 0.2 ～ 0.3eV HOMO HOMO TiO 2 Dyes TiO 2 TCNX ICT is effective for efficient charge separation, in particular, advantageous in near IR photoelectric conversion. 14

Summary Novel organic solar cells based on interfacial charge-transfer transitions (i) Interfacial charge-transfer transitions from surface- bound TCNX to TiO 2 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 TiO 2 . 15