1. Zinc Tetra Phenyl Porphyrin Chromophores February 2009

2. -Sunlight enters the structure, excites electron-hole pairs in dye. -Electron is transferred into TiO 2 conduction band, and hole is filled by I - ions (3I -  2e - + I 3 - ). -Electron travels through circuit, reduces I 3 - (2e - + I 3 -  3I - ) A Dye Sensitized Solar Cells (DSSCs): a potential low cost alternative to Si solar cells. Introduction

3. Zn-TetraPhenylPorphyrin ZnTPP Zinc TetraPhenylPorphyrin Zn-TPP exhibit a long-lived (>1ns)  * excited state Appropriate HOMO/LUMO levels position in energy. Functional groups have not much influence on the TPP electronic structure.

4. Energy levels alignment e- TiO 2 Electrolyte HOMO LUMO ZnTPP h e-

5. Ultra-violet Photoemission HOMO LUMO LUMO + 1 UV Inverse Photoemission HOMO LUMO LUMO + 1 Experimental setup Occupied and unoccupied states in the same UHV system

6. First results

7. Zn-based dyes These dye are believed to adsorb perpendicular to the substrate and might form clusters of parallel molecules. ZnTPP1 ZnTPP2

8. Zn-based dyes This dye is believed to adsorb flat on the substrate. ZnTPP3

9. ZnTPP1 on TiO 2

10. No apparent band shift in XPS ZnTPP1 on TiO 2

11. 3.4 eV 0.2 eV LUMO LUMO + 1 ZnTPP1 TiO 2 HOMO 2.6 eV 2.1 eV ZnTPP1 on TiO 2 Normalization established by comparison with ZnO

12. Electronic structure and molecular orientation of a Zn- tetra-phenyl porphyrin multilayer on Si(111) C. Castellarin Cudia et al. Compared to a multilayer DOS, same main characteristics with slightly different weight. Broadening of the principal features could be related to : the lack of resolution molecular interaction substrate influence photon energy 90 eV 40.8 eV

13. Electron Injection and Recombination in Dye Sensitized Nanocrystalline Titanium Dioxide Films: A Comparison of Ruthenium Bipyridyl and Porphyrin Sensitizer Dyes Yasuhiro Tachibana, Saif A. Haque, Ian P. Mercer, James R. Durrant and David R. Klug, J. Phys. Chem. B, Vol. 104, No. 6, 2000 2.0 eV TiO 2 (110) ZnTPP E c =0.6 eV E v =-3.2 eV E f =0 UPS-IPS HOMO LUMO Tetrachelate Porphyrin Chromophores for Metal Oxide Semiconductor Sensitization: Effect of the Spacer Length and Anchoring Group Position Jonathan Rochford, Dorothy Chu, Anders Hagfeldt, and Elena Galoppini JACS 129 (2007) 4655 650–700 nm Comparison with other methods

14. Reproducibility of results Variability between samples even when using the same dying procedure. Broadening of the principal features could be related to order on the surface.

15. ZnTPP1 on ZnO(11-20) TiO 2 (110) ZnO(11-20) Normalization to the substrate chosen to obtain similar molecular DOS after subtraction of the substrate contribution to the total DOS.

16. Comparison Zn-TPP derivatives ZnTPP1 ZnTPP2 ZnTPP3 Roughly the same features are found for the three dyes. In particular, HOMO at the same position in the three cases. Effect of adsorption mode difficult to interpret in terms of electronic structure. No obvious dependence. HOMO UPS He II

17. Comparison Zn-TPP derivatives Very different results from one dye to another. Need to be checked one more time. Effect of adsorption mode difficult to interpret in terms of electronic structure. No obvious dependence. ZnTPP1 ZnTPP2 ZnTPP3 IPS

18. Next steps Quantitative analysis of all the XPS data for coverage determination. Better understanding of the free ZnTPP electronic structure to anticipate the modifications of the DOS as a function of the adsorption geometry. Ab-initio electronic structure calculation (GAMESS) Literature?... Using the low cross section of Zn sp states in ZnO CB would allow a better characterization of the unoccupied molecular states. ZnTPP1

19. Next steps STM imaging of individual molecules on a pivalic acid passivated surface. Spectroscopic measurements (XPS-UPS-IPS) with prior pivalic acid passivation of the surface. 30 nm x 30 nm TiO 2 (110) Pivalate layer 30 nm x 30 nm

20. HOMOLUMO RR Ph Zn 3d DOS from calculated MO

21. HOMO HOMO+1 ZnP molecular orbitals Zn 3d dz2dz2 d zx d yz LUMO LUMO+1

22. ZnTPP molecular orbitals HOMO HOMO+1LUMO LUMO+1 LUMO+3

23. ZnTPP vs ZnP molecular orbitals ZnTPP ZnP

25. 15 nm x 15 nm 3 nm x 2 nm Pivalic acid covered surface 6 Å

26. ZnTPP3 covered surface 30 nm x 30 nm 15 Å 50 nm x 50 nm

27. Porphyrin as a model dye LUMOs HOMOs egeg a 2u a 1u Energy states Soret (B) band Q bands S0S0 S1S1 S2S2

28. Zn Tetra Phenyl Porphyrin derivatives ZnTPP1

29. HOMOs LUMOs ZnTPP vs ZnP molecular orbitals

30. Approaches for band alignment tuning 1/3 Molecule/Molecule interaction Adding spacer between them Changing the linker to the surface Adding a built-in dipole 6.5 Debeye dipole Effect on electron transfer?

31. Approaches for band alignment tuning 2/3 Modifying the ring electronic properties Ex: Electron withdrawing groups Fluorination

32. Approaches for band alignment tuning 2/3 Modifying the ring electronic properties Metal ion