The Organic Led Surface: A Synchrotron Radiation Photoemission Study

Tris(8-hydroxyquinolato) aluminum(Alq3), a prototypical molecule for organic light-emitting devices, has been studied via synchrotron radiation photoemission to investigate (1) the surface electronic structure of the molecules at room temperature and at elevated temperatures, (2) adsorption onto the inorganicSi(001)-2×1surface, and (3) doping with the alkaline metalMg. For case (1), three chemical environments of carbon are resolved. Moreover, the shake-up satellite structures are detected in all theN1s,C1s,O1s, andAl2p core-level spectra, but with different magnitudes. Annealing allows for a charge redistribution withinAlq3itself. As to case (2), the organic molecules not only passivate the dangling bonds, but also rupture the dimer bonds. The wave function of the surface dangling bonds and of the pyridyl side of an 8-quinolinol ligand overlaps greatly so that charge is polarized towards, the organic adlayer. However, the polarization diminishes at greater coverage. With regard to case (3), theN1s core-level spectra appear as anMg-induced charge-transfer component with a binding energy lower than the original component. This new component grows gradually in intensity with increasing concentration of the dopant. Moreover,Mgalso affects theO1s core, as manifested by a component lying at a +1.09 eV higher binding energy than the original component. TheMg2p core-level spectra, although rather broad, exhibit a shift toward a lower binding energy with increasingMgvapor. Upon examining all these experimental results, we propose thatMgin the surfaceAlq3molecules forms clusters. Each cluster attaches to a pyridyl ring, affecting not only the nitrogen atom at that ring, but also the oxygen atom in the adjacent phenoxide ring. The depleted charge in the affected oxygen then flows about its adherent ligand and resides on the pyridyl ring at that ligand, resulting in a highAlq3anion state.