Electronic structure of starburst molecules and their interfaces with ITO studied by UV photoemission

The starburst molecules derived from triphenylamine have attracted much attention in relation to the application to organic electroluminescent devices since they have good thermal stability and hole-transporting nature due to their amorphous character. We performed UV photoemission spectroscopic (UPS) study of electronic structures of starburst molecules and their interfaces with indium tin oxide (ITO). The sample molecules studied were 1,3,5-tris(2- methylphenylphenylamino)benzene (o-MTDA), 4,4',4'-tris(3- methylpheyl phenylamino) triphey-lamine (m-MTDATA), 1,3,5- tris[4-(3-methylphenylphenyl amino)phenyl[benzene (m- MTDAPB), and 1,3,5-tris [N-(4- dipheylaminophenyl)phenylamino] benzene (p-DPA-TDAB). The observed ionization potentials were 5.4 plus or minus 0.1 eV, 5.0 plus or minus 0.1 eV, 5.45 plus or minus 0.05 eV, and 5.15 plus or minus 0.05 eV, for o-MTDA, m-MTDATA, m-MTDAPB, and p- DPA-TDAB, respectively. The whole valence region of UPS spectra were measured by using synchrotron radiation. The difference in bulk electronic structure among these molecules was discussed in comparison with MOPAC molecular orbital calculation. At ITO interfaces, the vacuum level shift was observed for these materials, indicating that the traditional model with an assumption of a common vacuum level at organic/metal interface is not valid even in the case of ITO electrode. The direction of the shifts was negative, i.e. the vacuum level of the starburst molecules is below that of the ITO electrode. The magnitude of the shift was dependent on the surface cleanliness of ITO substrate.