Strain-Enhanced Coherent Exciton-Polaron States In 1d Phthalocyanine Crystalline Thin Films

Libin Liang, University of Vermont


Organic semiconductors are at the forefront of materials research, due to their desired electric and mechanical properties. They offer the unique opportunity to modify material properties during synthesis process, opening an avenue to the development of novel flexible and wearable electronic and photonic devices.Molecular excitons are of importance in organic semiconductor properties. While majority of research studies are centered on achieving good control of amorphous or polycrystalline thin film properties, the static disorder effect leads to poor device performance when compared to inorganic semiconductors with superior crystalline ordering. On the other hand, the macroscopic molecular long-range ordering can enhance device anisotropic properties, robust excitonic states and trigger coherent energy transfer region. Therefore, it requires further effort to fabricate thin films with macroscopic ordering, characterize the anisotropic properties of thin films and understand the effects of long-range intermolecular interactions on device performance. Materials in flexible devices will experience complex strain environment. There is a critical need for researchers to understand the effects of strain on thin films used in flexible electronics applications. It has been reported that lattice strain can enhance small-molecule organic semiconductor device performance by tuning the electron-phonon coupling. However, to fabricate novel flexible devices, it demands more research to explore the correlation between strain and bandgap transition, molecular vibration mode, exciton-phonon coupling, etc. For example, while the majority of strain studies focus on electric properties, strain-dependent optical and excitonic properties are less reported. Phthalocyanines (Pc’s) are one of the small-molecules that can be applied in the organic flexible electronics as alternative for certain traditional silicon-based semiconductor applications such as field-effect transistors and photovoltaic devices. They have great potential for the development of novel electronic and photonic devices due to their high mobilities and strong intermolecular interactions from the π-orbital overlap. Octabutoxy phthalocyanine (H2-OBPc) is a soluble derivative of the Pc which offers the solution-based thin film fabrication process. It provides a great platform to study the molecular exciton properties in 1D system. In this dissertation, H2-OBPc crystalline thin films with macroscopic grain size in millimeter scale are obtained with the pen-writing deposition technique. Orientations of transition dipole moment are resolved with the incident light orientation-dependent absorption measurement. Localized and coherent exciton-polarons are identified from temperature study of photoluminescence. Optical properties of H2-OBPc thin films at various strain are explored at room temperature. Strain-enhanced formation of delocalized exciton-polaron states is observed, and an 80% enhancement in photoluminescence intensity can be achieved with strain of 4.9%. Temperature studies were conducted to reveal the effect of strain on the excitonic properties. Results suggest strain can increase the relative strength between the intermolecular interaction and the activation energy, leading to the formation of coherent exciton-polaron states. This coherence can persist even with the presence of thermal fluctuations. These results grant H2-OBPc small molecules a promising application in the development of novel electronic devices.