Determination of the orbital lineup at reactive organic semiconductor interfaces using photoemission spectroscopy

Abstract

We determined the orbital lineup of the tris ͑8-hydroxyquinolinato͒ gallium (Gaq 3 )/Mg interface using combined x-ray and ultraviolet photoemission spectroscopy ͑XPS and UPS͒ measurements. The Gaq 3 /Mg system is a prototypical model structure for organic electron/low work function electrode transporting materials interfaces found in organic light emitting diodes ͑OLED͒. A Gaq 3 thin film was grown in 15 steps on a previously sputter-cleaned Mg substrate starting at a 1 Å nominal thickness up to a final thickness of 512 Å. Before, and in between the growth steps, the sample surface was characterized by XPS and UPS. The results indicate the formation of a reaction layer of about 12 Å thickness at the Mg interface, which resulted in a 0.96 V interface dipole potential. At Gaq 3 coverages higher than 256 Å, a strong charging shift occurred in the overlayer related UPS-emission lines, which was identified by measuring the high binding energy cutoff ͑secondary edge͒ of both the XP and UP spectra. The several magnitudes different x-ray and ultraviolet source photon intensities allow pinpointing charging shifts with high sensitivity. Due to the low work function of the reacted interface layer, the Gaq 3 electronic states are aligned at a binding energy below the substrate Fermi edge that exceeds the magnitude of the optical gap between the highest occupied and lowest unoccupied molecular orbitals ͑HOMO and LUMO͒. This allowed the conclusion that the ground state exciton binding energy of Gaq 3 needs to be larger than 0.43 eV. Based on these considerations, the lowest possible electron injection barrier matching the experimental data was estimated to be 0.15 eV.

Organic Electronic Devices and Their Functional Interfaces

“…For instance, one of the most prominent luminescent and electron-transporting materials, Alq 3 , reacts with Al and Mg to form organometallic complexes, [23,[81][82][83][84][85] which have new intragap states. Also, many other conjugated materials [86][87][88] react with Al and Mg, thus leading to a new distribution of occupied and unoccupied levels at such organic-metal interfaces. The description of details of the interaction depends very much on the actual material pairing, and cannot be readily generalized.…”

Thin-film metal oxides in organic semiconductor devices: their electronic structures, work functions and interfaces

“…[130][131][132][133][134][135][136] There are numerous examples from adsorption studies that demonstrate interfaces in which hybridization between adsorbate and substrate electronic states gives rise to distinct interface states. Work done between the 1970s and present day has provided many examples of such interfaces, involving small organic molecules on metals, 131,132,134,[136][137][138][139][140][141][142][143][144][145][146][147][148][149][150] noble gas atoms on metals, [151][152][153][154][155][156][157] small inorganic molecules (such as CO, [158][159][160][161][162][163][164] NO 165 and H 2 O 166,167 ) on metals and oxides, 160,161 and various organic semiconductors on metals and inorganic semiconductors.…”

Electronic structure of interfaces with conjugated organic materials

“…For instance, one of the most prominent luminescent and electron transporting materials, Alq 3 , reacts with Al and Mg to form organometallic complexes [25][26][27][28][29][30], which exhibit intra-gap states that pin E F . Also many other conjugated materials [31][32][33] react with Al and Mg, leading to a new distribution of occupied and unoccupied levels at such organic/metal interfaces. The description of details of the interaction depends very much on the actual material pairing, and cannot be readily generalized.…”