Sander, M.; Jägermann, W.; Lewerenz, H. J.: Site-specific surface interaction of adsorbed H2O and halogens on CuInSe2 surfaces. The Journal of Physical Chemistry 96 (1992), p. 782-790
10.1021/j100181a048
Abstract:
The interaction of adsorbed H2O, I2, and Cl2 on cleaved CuInSe2 surfaces has been investigated in ultrahigh vacuum to model semiconductor/electrolyte interfaces. The semiconductor/adsorbate system has been characterized by UV and X-ray photoelectron spectroscopy (UPS, XPS), low-energy electron diffraction (LEED), low-energy ion scattering spectroscopy (LEISS), and surface photovoltage measurements (SPV). H2O is physisorbed at 80 K and desorbs completely at 300 K. It preferentially interacts with Cu surface sites at low coverages. The H2O dipole is oriented with the O pointing toward the surface which leads to a decrease of the work function by 0.9 eV. Semiconductor band bending or a surface photovoltage is not observed. At 80 K, I2 is dissociatively adsorbed at low coverages and molecularly adsorbed at high coverages. At 300 K, only the iodine adatoms remain on the surface. They are preferentially bonded to Cu leading to a monolayer of CuI moieties. The adsorbed I2 leads to a band bending of 0.5 eV, an increase of work function by 1.4 cV, and a surface photovoltage of 0.3 eV which corresponds to the theoretical expectation of ideal Schottky type behavior. At 80 K, Cl2 is also dissociatively adsorbed at low coverage and molecularly adsorbed at high coverages. The chlorine adatoms preferentially interact with Cu sites, increasing their oxidation state. The work function is increased by 0.6-0.8 eV depending on the coverage regime. Semiconductor band bending or a surface photovoltage is not observed, indicating Fermi level pinning. Annealing at 300 K leads to the formation of Cu+, InCl3, and Se0. The observed differences in reactivity are explained on the basis of thermodynamic calculations of bulk reactions. The results of the model experiments are discussed in relation to photo-electrochemical solar cells and their performance.