ARCHITECTURE-ELECTRONIC PROPERTY RELATIONS ACROSS MOLECULAR SEMICONDUCTOR INTERFACES
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Multicomponent organic films have increasing applications in photovoltaic technologies and other electronic devices. These applications depend strongly on the structural and electronic properties of the heterojunctions. This thesis reports a detailed investigation of these important aspects, such as structure control and structural-electronic correlation in molecular film heterojunctions, for two selected "donor-acceptor" model systems (TiOPc-C60 and TiOPc-C70) using STM/STS.
The UHV-STM studies were started on a single component system, TiOPc deposited on Ag(111). Along with increasing deposition flux, TiOPc selectively forms three distinct ordered monolayer structures, namely honeycomb phase, hexagonal phase, and a misfit dislocation triangular network. Localized electrostatic intermolecular interactions can be utilized to stabilize kinetically accessible structures and cause different phase structures formed on surface. Molecular packing models for these phases are proposed based on STM measurements.
By choosing different TiOPc monolayer phases as template for sequential C60 deposition, low-dimensional monolayer TiOPc-C60 interfaces have been prepared on Ag(111) and characterized with STM/STS. Thermally stable honeycomb and metastable hexagonal TiOPc templates rearrange upon C60 deposition to yield several binary film
structures in the monolayer regime. These structures include phase-segregated TiOPc and C60 domains and co-crystalline TiOPc(2)C60(1) honeycomb network formed through a dynamic process balanced by intermolecular and molecular-substrate interactions. The least stable TiOPc phase, the dislocation network, turns out to be the most robust template for sequential C60 growth by forming nanophase-segregated TiOPc-C60 on the scale of 10 nm.
The variations of C60 energy gap across the heterointerface created by depositing C60 on hexagonal TiOPc are evaluated with STS. Energy level shift on TiOPc-C60 co-crystal domain boundary is identified. This energy shift is correlated to an electron transport barrier from donor material (TiOPc) to acceptor (C60) in practical OPV cells.
C70-TiOPc heterostructures are characterized and compared with those of C60. C70 present a greater variety of molecular configurations and related properties than those of C60because of the ellipsoid shape with lower symmetry and higher dipole polarizability. C70deposited on TiOPc honeycomb phase shows completely different growth mode from that of C60. The TiOPc honeycomb structure, functionalized as a dipole buffer layer, plays a substantial role on sequential C70 growth up to the fourth layer. Simple geometric effect and dipole-induced dipole interactions are considered to rationalize the intriguing C70a growth mode. The structural model for each layer is proposed.
By employing fullerenes (C60 or C70) and TiOPc thin films as model system, I investigated the controlled formation of donor-acceptor molecular film architecture, measured the orientation and separation of donor-acceptor molecules along the domain boundaries, and correlated the structural information with the electronic structural information. These systematical works shed light on the optimization of molecular electronic devices from a fundamental microscopic perspective.