Organic semiconductors promise to enable new generations of low-cost, mechanically flexible, and wearable electronic devices. Their potential uses are only beginning to be realized on the commercial scale with the recent widespread adoption of organic light emitting diodes (OLED) in cell phone and tablet displays and televisions. Further, these materials are being explored for use in solar cells, electronic circuitry, sensors, and thermoelectric devices that are printed directly from solution. In many cases, organic semiconductors are chemically doped with molecules that introduce charge carriers to make the materials more electrically conductive, as is desirable for many applications. For example, chemical doping is used in OLED to increase brightness and power efficiency. Improving the performance and stability of doped organic semiconductors is critical to the further development of electronic devices in which they may be used, yet it is extremely difficult to predict how a chemical dopant will affect the electronic properties of an organic semiconductor. This research uses a tightly integrated experimental and theoretical approach to disentangle variables that influence the electronic properties of doped organic semiconductors, with the goal of enhancing the material performance and stability across multiple potential applications. In addition to accelerating the development of applications based on organic semiconductors, the proposal aims to promote interest, engagement, and participation in STEM disciplines through exposing high school students throughout the state of Kentucky to the exciting technologies enabled by organic semiconductors. In part, this goal involves a workshop where high school students make electrochromic devices, where a material changes color through electrochemical doping, and thermoelectric devices, where a material harvests heat and turns it into electricity, and learn about the power of computational chemistry.
The electronic and thermoelectric properties of doped organic semiconductors remain extremely difficult to predict due to the interplay of multiple variables and a lack of understanding of how each variable impacts the material properties. This research seeks to advance the state-of-the-art of doped organic semiconductors by refining models of their electronic structure and transport characteristics based on a highly integrated experimental and theoretical approach. The three primary research objectives are to determine the influence of the dopant size on the material electronic structure, establish critical connections between doped conjugated polymer energetics and morphology with dopant size and diffusion, and ascertain the influence of doped conjugated polymer electronic structure and morphology on the electrical conductivity and Seebeck coefficient of materials of interest for thermoelectric applications. The research approach involves application of ultraviolet and inverse photoelectron spectroscopy on model systems coupled with quantum-chemical calculations and molecular dynamics simulations to determine the influence of dopant size and polymer morphology on electronic structure. Furthermore, electrical conductivity and Seebeck measurements on electrochemical transistors combined with kinetic Monte Carlo simulations will uncover how energetics and morphology influence charge-carrier transport and thermoelectric performance. The overall objective of the research is to establish clear relationships between dopant molecular structure, organic semiconductor electronic structure, and doped organic semiconductor morphology, electrical conductivity, and the Seebeck coefficient. These findings are key to enable better predictive design of doped organic semiconductors with controlled electronic properties.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
