When 1:00 PM - 3:00 PM May 20, 2013
Where Johnson Room A, Lurie Engineering Center
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Thermal Properties of Disordered Organic Solids and Interfaces involving Organics


Yansha Jin
Thesis Defense

Max Shtein, advisor

 

Research interest in energy conversion in organic materials has been growing steadily, driven in part by the potential advantages of light weight, mechanical flexibility, scalability and low-cost manufacturing capability. Among the unique properties of these materials is the strong coupling between charge carriers and phonons. Under this strong coupling, the organic molecules deform and rearrange in the presence of the charge carrier; the resulting carrier accompanied by local polarization of the solid is termed “Polaron”. The detailed physics of the polaronic behavior in organic materials is still relatively poorly understood, particularly at the interfaces between the organic semiconductor and the inorganic phase in energy conversion devices. Thus, there remain vast possibilities for future leaps in the development of new materials and device architectures.

This thesis is aimed to generate some of this needed knowledge, focusing on phonon dynamics and heat transport in van der Waals bonded organic thin films, especially at organic-inorganic interfaces.

Thermal Boundary Conductance (TBC) values are reported here for interfaces between several metals and small molecule organic semiconductors. Both experimental and simulation results suggest that for interfaces with large acoustic mismatch, the TBC is closely correlated to the bonding strength at the interface. Interfacial bonding between Copper Phthalocyanine (CuPc) and Silver (Ag) is van der Waals in nature, and the TBC value at this interface is 1~2 orders of magnitude lower than that of metal-inorganic dielectric interfaces. Therefore, the boundary effects cannot be neglected in the systematic thermal analysis of nanostructured organic optoelectronic/thermoelectric devices, and must be elucidated during material selection and design for these applications. One exceptional example demonstrated through simulations in this thesis is the 10-fold increase of thermoelectric figure of merit in organic-metal nanocomposites due to low values of TBC in the hybrid systems under consideration. The fundamentals of this low-conductance phenomenon are discussed in detail in this dissertation, with simulation results suggesting that it is the anharmonic nature of phonon transmission at weakly bonded interfaces, as well as a unique spatially non-uniform interfacial vibration at some (e.g. CuPc-Al) interfaces that can be at the root of the observed thermal transport behavior.