In the organic devices, an organic semiconductor is used as an active material and metal is generally used as electrodes, hence understanding metal-organic interface, from the point of view of electronic structure, in general and the energy level alignment (ELA), in particular, are of prime importance for the improvement of the charge injection at the metal-organic interfaces. Polar phthalocyanine (Pc), which has high thermal and air stability and can have finite dipole moment depending on its molecular orientation, encouraged many to study its molecular orientation dependent electronic structure. Despite large number of studies on polar Pc, the proper understanding of the electronic structure and interaction of chlorogalium-phthalocyanine (ClGaPc) molecules on or with Au substrate, in general and polycrystalline Au substrate, in particular (which better resembles the electrode structure compared to the crystalline Au substrate), are still lacking.
  Here the electronic structures and core-level spectra of ClGaPc molecules of different thicknesses (submonolayer to multilayer) adsorbed on a polycrystalline Au substrate and a highly oriented pyrolytic graphite (HOPG) substrate, before and after thermal annealing, were investigated using photoelectron spectroscopic techniques for better understanding the charge-transfer properties. The energy level diagrams (ELDs) of the ClGaPc thin films are found to evolve with film thickness, substrate nature, and thermal annealing. The interfacial dipole moment in the active Au substrate and the molecular dipole moment in the inactive HOPG substrate mainly dictate the ELD. Annealed monolayer films on both the substrates seem to adopt a similar well-ordered Cl-up orientated molecular organization, which is quite interesting, as it certainly indicates a substrate-nature-independent energy minimum configuration. The strong interaction of the active Au substrate gives rise to additional charge transfer and state transfer (of Ga) as evident from the formation of a former lowest unoccupied molecular orbital (F-LUMO) level in the highest occupied molecular orbital (HOMO) region and a low binding energy peak in the Ga 2p_3/2 core level. The presence of strong F-LUMO and molecular-dipole-related HOMOd levels in the predicted monolayer of well-ordered Cl-up oriented molecules on the Au and HOPG substrates, respectively, creates the optimum energy-level alignment (ELA) for both the systems, while the opposite shift of the vacuum levels in two different substrates makes the ionization potential (IP) for such a monolayer either minimum (on the Au substrate) or maximum (on the HOPG substrate), which is useful information for tuning the charge injection across the interface in organic semiconductorbased devices [published in ACS Appl. Mater. Interfaces 12, 45564 (2020)].
  Dinaphtho[2,3-b:20,30-f]thieno[3,2-b]thiophene (DNTT) is another molecule, which has very good air and thermal stability. An effective overlapping of its molecular orbitals along with a strong intermolecular interaction makes this material very promising for different device applications. Here the electronic structure and morphology of the DNTT/HOPG and DNTT/Au interfaces and their evolution with the film thickness and thermal annealing were studied using in situ ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) and AFM techniques to understand the correlation of the electronic structure with the molecular structure of DNTT. We observed distinct molecular electronic structures of the DNTT films on almost inert HOPG and comparably reactive Au surfaces, that possibly caused by the different molecular organization of the DNTT molecules in those thin films. The splitting of the HOMO level, that was also observed on Au(111) surface due to the densely packed phase of the molecules, is also observed here in the DNTT/Au film at the monolayer region. The XPS results confirm that the substrate-molecule interaction at the DNTT/Au interface is very weak compared to the intermolecular interaction. The packing or intermolecular interaction, however, weaken in subsequent layer, where desorption of DNTT molecules due to thermal annealing is evident from AFM and XPS results. Such structural change of the HOMO level of the DNTT thin films on polycrystalline Au with the change of thickness from monolayer to monolayer onwards was recognized for the first time, which has a strong effect on the charge transport properties of the DNTT based devices [published in Appl. Surf. Sci. 597, 153696 (2022)].