UV Photoelectron Spectroscopy Study of Various ZnO Structures and
Dye-Sensitized Solar Cells
Y.-F. Lai (賴易鋒)1, C.-P. Liu (劉全璞)1,2,
Y.-W. Yang (楊耀文)3, and L.-J. Fan (范良任)3
1
Department of Materials Science and Engineering, National Cheng Kung University,
Tainan, Taiwan
2
Center for Micro/Nano Science and Technology, National Cheng Kung University,
Tainan, Taiwan
3
National Synchrotron Radiation Research Center, Hsinchu, Taiwan
UV Photoelectron spectroscopy (UPS) is the most effective method to characterize valence
electronic structures of materials, which is crucial for important physical properties of
nanomaterials and devices. The nanomaterials under study include ZnO films, ZnO nanowalls and
ZnO nanotubes, whereas the dyes include N3 and Chlorophyll dyes with/without TiO2 and ZnO
films. In this work, we try to analyze the relative positions of valance electron energy, Fermi level,
conduction band minimum, work function of various ZnO structures, and highest occupied
molecular orbitals and lowest unoccupied molecular orbitals (HOMO/LUMO) of several dyes by
UPS along with an independent method for determining energy bandgap, such as
photoluminescence spectra and UV-Vis absorption spectra. About the results from ZnO, the
common features at 10.35, 8-5, and 4.4 eV correspond to Zn 3d, mixed Zn 4sp–O 2p, and O 2p
states, respectively. However, ZnO nanowalls and nanotubes have higher intensity ratio for the
hybrid bonding than ZnO films. Valence band structure is largely affected by the form of the
nanomaterials including the shift and broadening of the O 2p orbital, which may be arisen from
surface-related dangling bond states. As for the dyes, the LUMO level of N3 dye is higher than the
conduction band of TiO2 and ZnO, whereas the Chlorophyll dye is just opposite. This should be
responsible for the lower convergent efficiency of 0.205% of the green solar cells.