ARCHITECTURE-ELECTRONIC PROPERTY RELATIONS ACROSS MOLECULAR SEMICONDUCTOR INTERFACES
Reutt-Robey, Janice E
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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-C<sub>60</sub> and TiOPc-C<sub>70</sub>) 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 C<sub>60</sub> deposition, low-dimensional monolayer TiOPc-C<sub>60</sub> interfaces have been prepared on Ag(111) and characterized with STM/STS. Thermally stable honeycomb and metastable hexagonal TiOPc templates rearrange upon C<sub>60</sub> deposition to yield several binary film structures in the monolayer regime. These structures include phase-segregated TiOPc and C<sub>60</sub> domains and co-crystalline TiOPc<sub>(2)</sub>C<sub>60(1)</sub> 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 C<sub>60</sub> growth by forming nanophase-segregated TiOPc-C<sub>60</sub> on the scale of 10 nm. The variations of C<sub>60</sub> energy gap across the heterointerface created by depositing C<sub>60</sub> on hexagonal TiOPc are evaluated with STS. Energy level shift on TiOPc-C<sub>60</sub> co-crystal domain boundary is identified. This energy shift is correlated to an electron transport barrier from donor material (TiOPc) to acceptor (C<sub>60</sub>) in practical OPV cells. C<sub>70</sub>-TiOPc heterostructures are characterized and compared with those of C<sub>60</sub>. C<sub>70</sub> present a greater variety of molecular configurations and related properties than those of C<sub>60</sub>because of the ellipsoid shape with lower symmetry and higher dipole polarizability. C<sub>70</sub>deposited on TiOPc honeycomb phase shows completely different growth mode from that of C<sub>60</sub>. The TiOPc honeycomb structure, functionalized as a dipole buffer layer, plays a substantial role on sequential C<sub>70</sub> growth up to the fourth layer. Simple geometric effect and dipole-induced dipole interactions are considered to rationalize the intriguing C<sub>70</sub>a growth mode. The structural model for each layer is proposed. By employing fullerenes (C<sub>60</sub> or C<sub>70</sub>) 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.