Studies On The Geometric Structure, Electronic Structure And Charge Transfer Dynamics Of Organic/inorganic Interfaces

Organic semiconductors have attracted widespread interests due to theirpromising applications in the field of organic electronics ranging from organic lightemitting diode (OLED), organic field effect transistor (OFET), organic photovoltaiccell (OPVC), to dye sensitized solar cell (DSSC). It has been widely acknowledgedthat interfaces between organic semiconductor molecules and their functionalsubstrates largely determine the device performance. Therefore, understanding theinterfacial electronic properties, especially charge transfer dynamics is crucial for bothlearning the device operation mechanism and optimizing the device performance.In this thesis, we systematically investigate the electronic structure, adsorptiongeometric structure and interfacial charge transfer dynamics, critical factors thatinfluence device performance, of organic semiconductor on inorganic substratesurfaces using synchrotron-based surface characterization and other surface analysistechniques.First, the PTCDA film was deposited on the Au(111)/mica surface using organicmolecular beam deposition (OMBD) method. The electronic structure and adsorptiongeometry were studied using synchrotron-based photoemission spectroscopy (PES),near-edge X-ray absorption structure spectroscopy (NEXAFS), X-ray diffraction(XRD) and atom force microscopy (AFM). It can be seen from the PES results thatthe Shockley-type surface state for fresh Au(111) near the Fermi level extinguishedimmediately after sub-monolayer of PTCDA was deposited onto the Au surfacewithout the emergence of interface hybrid state. This indicates that a charge transferprocess took place at the interface between PTCDA molecule and Au(111), but did notlead to strong chemical reaction. Angle dependent NEXAFS and ARUPS shown thatthe PTCDA overlayers are ordered with lying-down geometry on the Au(111)/micasurface at deposition rate of 2ML/min. According to the AFM images and theevolution of Au 4f and C 1s integral intensities with increasing film thickness, thetypical Stranski-Krastanov growth mode is proposed for PTCDA deposition onAu(111)/mica surface, that is layer-by-layer growth followed by island growth mode.The layer-by-layer thickness is 2ML. Then, dewetting transition occurred between the2D and 3D growth modes. However, PTCDA molecules are disorderly oriented onsubstrate surface with the deposition rate of ~0.5 ML/min. To understand the nature of interfacial interaction and the charge transferdynamics, the electronic structure, molecular orientation and charge transfer dynamicsof monolayer and multilayer PTCDA molecules on single crystal Au(111) werestudied using synchrotron-based PES, NEXAFS and core-hole clock spectroscopy.Orientationally ordered PTCDA thin films with flat lying molecular geometry wereobserved at both the monolayer and multilayer regimes as revealed by angulardependent NEXAFS at C K-edge and O K-edge. It was revealed that the LUMO ofPTCDA molecules is well below the Fermi level of substrate, so the electron transferfrom LUMO to substrate conduction band is energy forbidden at excited state. Theresonant structures for monolayer RPES spectra are almost the same as that formultilayer RPES spectra, at which no charge transfer is expected to be observed.Despite the lying-down geometry of molecules on gold, interfacial charge transfertimescale for higher LUMOs orbitals was found to be much longer than the core-holelifetime of 6 fs as revealed by the resonantly enhanced photoemission and Augerstructures. The slow charge transfer process for monolayer PTCDA on gold suggestsweak electronic coupling and predominant Van der Waals type interactions at thePTCDA/Au(111) interface, consistent with previous experimental findings.Consequently, although the gold substrate could potentially benefit the formation ofhighly-ordered PTCDA overlayers with weak interaction.To investigate the influence of electronic coupling and molecular orientation oninterfacial charge transfer dynamics, TiO2, which is one of the most active functionalsubstrate, is used as the substrate. The electronic structure, chemical interactions,molecular orientation and energy level alignment of PTCDA on reduced rutileTiO2(110) 1 1 surface were studied systematically using synchrotron-based PES,NEXAFS and core-hole clock spectroscopy. The evolution of PES spectra as afunction of PTCDA coverage indicates that PTCDA molecules are strongly coupledwith the TiO2surface Ti and O atoms: the anhydride terminal groups (O=C-O-C=O)of PTCDA molecules react with the TiO2surface through the ring opening from theC-O-C bonds, which are then covalently bonded to the five-fold coordinated Ti (Ti5f)atoms and the adjacent bridging oxygen (Ob) atoms on the TiO2surface by formingC-O-Ti bonds. In addition to the proposed reaction involving the anhydride groups,some C-atoms in the perylene core of PTCDA could also react with the TiO2substrate,forming a new species giving rise to new peak in C 1s PES spectra. The emergence ofan interfacial gap state observed at the lower binding energy side of the highest occupied molecular orbital (HOMO) of PTCDA corroborates the strong electroniccoupling between PTCDA molecules and TiO2substrate. The PTCDA molecules werefound to grow layer-by-layer on TiO2surface below 2 monolayers (ML) with a smalltilt angle due to strong molecule-substrate interactions and chemical reactions. Athigher coverage, island growth with lying-down geometry dominates due to reducedmolecule-substrate interactions. The intermolecular hydrogen bonding betweenPTCDA molecules stabilizes the planar geometry, similar to PTCDA on othersubstrates, such as Au(111). The transformation of thickness dependent molecularorientation from slightly tilting at 0.5 ML, disordered at 1 ML and lying-down at 28ML was observed by angular dependent NEXAFS. The energy level alignmentdiagram of the interface was plotted based on thickness dependent PES results, inwhich the LUMO of PTCDA is directly above the conduction band maximum (CBM)of TiO2. The resonant structures for RPES spectra within the first monolayer aredramatically different from that for multilayer RPES spectra, indicating the ultrafastcharge transfer dynamics at the PTCDA/TiO2interface. It was found that the ultrafastcharge transfer from dye molecules to TiO2substrate take place on the timescale of 10~ 75 fs due to strong electronic coupling and favorable energy level alignment withinthe first layer of PTCDA molecules. Moreover, the charge transfer timescale atPTCDA/TiO2(110) interface showed the expectant orientation dependence. Thechange of molecular orientation has a strong impact on the charge transfer dynamicsof the anhydride group because the charge transfer timescale for anhydride relativeresonance in submonolayer is much shorter than in monolayer, whereas the chargetransfer timescale associated with the perylene relative transition of submonolayerPTCDA molecules is comparable with that of smonolayer.The different charge transfer dynamics at PTCDA/Au(111) and PTCDA/TiO2interfaces indicate that the molecular coupling plays a dominant role in interfacialcharge transfer. Furthermore, for strongly coupling system, such as PTCDA/TiO2(110),molecular orientation has larger impact on the charge transfer dynamics than weaklycoupling system as PTCDA/Au(111).