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 (VLs) 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)].
  The evolution of electronic structures and morphology of DNTT thin films on highly oriented pyrolytic graphite, due to the incorporation of a polar ClGaPc molecular layer at the interface, substitution of DNTT with alkyl side-chain-incorporated S-shaped DNTT (S-DNTT-C10), and thermal annealing, were investigated using photoelectron spectroscopic techniques to check the possibility of proper energy level alignment at the metal−organic interface, which is one of the key challenges to improve the charge transport in organic semiconductor-based devices. A significant modulation in the VL and a small downward shift in the HOMO level with an intermediate charge injection level (CIL) are evident in the DNTT thin film due to the incorporation of the ClGaPc layer at the interface. This is a clear signature of the molecular dipole layer (of well-ordered Cl-up orientated ClGaPc molecules)-induced realignment of the molecular energy levels of the DNTT thin films. On the other hand, a noticeable downward shift in the VL, a signature of an improvement in the bulk coverage, is evident in the S-DNTT-C10 film, which can be attributed to the presence of aliphatic hydrocarbons in the alkyl side-chain incorporated molecule. Furthermore, a double-peak-like HOMO level is evident in the S-DNTT-C10 film, which can be attributed to the two distinct orientations/arrangements of the molecules, one at the interface and other in the remaining part of the film. The formation of an intermediate CIL, through incorporation of a molecular dipole layer at the interface, is helpful to overcome the large hole injection barrier, while the enhancement of molecular coverage at the metal−organic interface and thereafter, through molecular engineering, is useful to increase the hole injection area, both of which are of immense importance in improving the device performances [published in J. Phys. Chem. C 127, 18176 (2023)].
  DNTT, being a highly π-extended heteroarene, is a promising active material for organic field-effect transistors (OFETs). However, the performance of such OFETs strongly depends on the structure and morphology (more specifically, molecular arrangement and wettability) of the active layer, especially near the active-layer/gate-dielectric interface, and thus, their improvement is of prime importance. Focusing on these aspects, a systematic investigation was carried out for low-thickness active layers of such a material by molecular engineering and dielectric surface energy (DSE) tuning. Although DSE modification creates a minor impact on the Volmer−Weber (V−W)-type growth mode of DNTT thin layers, it produces a subtle difference in the morphology of the islands, namely, more columnar (i.e., better out of-plane crystalline coherency) islands on the lower DSE substrate. Additionally, a strong 3D herringbone packing of DNTT molecules and a weak dielectric interfacial interaction lead to a dewetted island-like structure, restricting in-plane connectivity and hole mobility within layers of low thickness. On the other hand, DSE modification (from high to low) leads to a transition in the growth mode (from V−W to nearly Stranski−Krastanov type), a major change (improvement) in the morphology (wettability) of molecular-engineered S-DNTT-C10 thin layers, and a 3-fold improvement in the hole mobility, which is the maximum observed mobility for this molecule in such a low-thickness regime. Essentially, cofacial packing of S-DNTT-C10 molecules and the relatively strong interfacial interaction lead to wetted and better π-overlapped 2D layers, which improve the hole mobility and demonstrate that a synergistic approach of molecular engineering and DSE tuning is essential to improve the performance of DNTT-based OFETs, especially in the low-thickness regime [published in ACS Appl. Electron. Mater. 7, 9167 (2025)].