Committee Members:
- Prof. Natalie Stingelin, Advisor, MSE/ChBE
- Prof. Carlos Silva, CHEM/PHYS
- Prof. Shannon Yee, ME
- Prof. Rampi Ramprasad, MSE
- Prof. Joshua Kretchmer, CHEM
Experimental and Theoretical Approaches for Elucidating Doped Organic Semiconductors
Abstract:
The unique charge transport properties and the great optoelectronic characteristics of the next generation of organic semiconductors are opening new opportunities for both basic research and advanced applications. The multibillion-dollar organic light emitting diode (OLED) industry is the prime example how the unique combination of desirable properties of organic semiconductors, from their chemical versatility, processability, to their tunable performance, can be exploited. Yet surprisingly, many aspects about the intermolecular interactions, transport properties, and other relevant parameters of organic semiconductors such as solubility/processability, are still not fully understood. In this thesis, experimental methods and theoretical approaches are combined to unravel the factors that determine charge transport properties in organic semiconductors, and to provide new paradigms for understanding these materials when used in devices.
Starting with a model system based on poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene)] (PBTTT-C16) and molybdenum organometallic complexes, relevant structure-property relations are established for this binary, polymer-doping system. It is shown that eutectic vitrification can be utilized to induce molecularly intermixed blends which facilitates charge generation and increases electrical conductivity, while lowering the thermal transport. The generality of this approach is further investigated and applied to other polymer:dopant systems.
Charge transport properties are studied in detail next. Transient absorption spectroscopy data indicates the existence of a charge transfer complex between the charged and neutral species, i.e., a ground-state correlation between neutral and polaron states. This view is corroborated by non-adiabatic molecular dynamics simulations of neutral and charged BTTT oligomers and dimers.
A comparison between the hairy-rod-like PBTTT and flexible-chain polymers is subsequently provided, focusing on the electronic disorder landscape examined via the combination of 2D coherent spectroscopy and DFT simulations. PBTTT is found to feature dynamic electronic disorder in strong contrast to its flexible-chain counterpart, which displays static electronic disorder. The differences in photophysical behavior of polymers with different backbone rigidity is, thus, unraveled, providing fundamental understanding on polymeric semiconductors and their application.
Finally, a database of doped conductive polymers is constructed via the mining of literature and lab-generated data. Machine learning models are subsequently developed towards a fast-screening approach for candidate materials. Design guidelines for new conductive polymers are also proposed.
To conclude, this thesis contributes to building a knowledge platform that can offer new insights into charge transport properties in polymeric semiconductors, by creating a feedback loop between experiment, theory, and computation. The platform will be further utilized for development of quantitative design criteria towards new organic semiconductors, as well as for creation of a preliminary data-driven methodology for the organic electronics development.