This CAREER project aims to understand the structure-function-property relationships in charged conjugated polymers for applications in plastic electronics. Conjugated polymers (CPs) are materials that have the optical and electrical properties of semiconductors with the mechanical properties of plastics. The project focus is on fundamental understanding of the optical and electronic properties of cationic and anionic conjugated polymers (conjugated polyelectrolytes) as a function of molecular structure, charge density (number of charge per repeat unit), type of counter ions, and material processing conditions. The advantage of conjugated polymers containing electrically charged functional groups is that they offer fine control of the polymer conformation in solution, and hence, the degree of electronic interaction of polymer chains in films (interchain interactions). These interchain interactions strongly influence optical and electronic properties and degradation rate such as photoluminescence quantum yield and lifetime, energy migration, and charge mobility in CP-based devices such as biosensors, light-emitting diodes (LEDs), solar cells, and field effect transistors (FETs), and so, the device efficiencies and operational lifetime. The approach is to control polymer conformation in solution by changing solvent, concentration, salt, functional group, charge density, and conjugated polymer backbone and film morphology by polymer conformation, annealing process, and various film fabrication methods. Light-scattering, steady-state and time-resolved spectroscopies, scanning probe techniques (Atomic Force Microscopy, Electrostatic Force Microscopy, and Conducting Atomic Force Microscopy), along with prototype device evaluation will be used to obtain a comprehensive understanding of polymer conformation, film morphology, optical and electronic properties, and charge transport at the nanoscale, and in bulk as a function of molecular structure and processing conditions. Specific goals include: 1) To understand and control charged conjugated polymer conformation in solution via molecular structure and processing conditions and how change in polymer conformation affects its photophysics. 2) To understand and control polymer photophysics and charge transport properties of polymer films as a function of molecular structure, charge density (number of charge per repeat unit), type of counter ions, and processing conditions. Non-Technical The broader impact of the project will be the link established between research and education at UCSB and the Santa Barbara community. Graduate and undergraduate students will be essential to carrying out the research. The research plan promotes teaching, training, and learning of graduate and undergraduate students in the field of organic semiconductors. The research is highly interdisciplinary; students will be exposed to a wide range of research experience in material design and synthesis, materials characterization, and device fabrication and evaluation that will provide breadth and flexibility for their future careers. They will develop knowledge in chemistry, physics, and materials science. Several graduate and undergraduate courses will be developed by the PI to strengthen and update new science in the science curriculum at UCSB. Through several outreach programs at UCSB, the PI will bring summer undergraduate students, college students, and high school teachers to her laboratory to participate in research activities. To increase the diversity and to promote children to go to college and major in science, the PI will participate in Science and Technology Day, an annual event that brings students and teachers from middle and high schools to UCSB to participate in science workshops and competitions.
Conjugated Polyelectrolytes (CPEs) are semiconducting plastics that have an electronically delocalized backbone and pendant groups bearing ionic groups as integral components of their molecular constitution; thus, they are soluble in polar solvents such as alcohols and water. At the beginning of this project, these materials were used mainly as optical components in biosensors and as electron injection layers in conjugated polymer light emitting diodes (PLEDs). Despite the potential applications, very little was known about their electronic properties such as charge transport, conductivity, and energy levels. Electronic charge transport studies by conventional methods including single-carrier diodes and field-effect transistors do not work due to the interference of ion conduction. Prior to our work, the charge transport study of CPEs was scarce in the literature. In fact, there was widespread belief that the positive or negative ions in these materials would trap the charge carriers, rendering the system non-conductive. This grant allowed us to investigate how chemical structures of CPEs may influence the optical and electronic properties, molecular packing, and charge transport. Various chemical structures were synthesized including the conjugated backbone, charge reversal (cationic versus anionic charges), the type of counterion, chemical structure of the charge group (carboxylic or sulfonic group), the number of charges per repeat unit, and the distance between the charged functional group and the main chain. To measure a single type of charge carrier mobility, we developed a short-pulsed bias technique (<100 ms) to suppress the ion motion, which typically occurs in a multisecond timeframe. Therefore, the charge carrier mobility can be measured for the first time. We use pulsed bias current-voltage measurements along with temperature-dependent studies to develop structure-charge mobility relationships in CPEs. Temperature and electric field-dependent studies provide the activation energy or charge carrier hopping barrier in CPE films. The activation energy for electron hopping in cationic and anion CPEs by doing temperature and electric field dependent charge transport studies as a function of charge reversal and type of counterion. Surprisingly, we found that simply replacing a cationic group by an anionic group while keeping the conjugated backbone the same can lead to three or four order of magnitudes different in electron mobility. These observations provide useful structural handles to tailor and manipulate charge carrier mobilities. We also investigated the impact of CPE electron mobility on the performance of PLEDs utilizing CPE electron injection layers. Without using a CPE electron injection layer, low work function metal cathode such as calcium or barium must be used to obtain ohmic contact and hence low device turn-on voltage. However, these metals are very reactive in air. Using a CPE charge injection layer, more stable aluminum or gold electrode can be used. The effect of thermal stability of cationic CPE films and PLEDs were also studied. Thermal annealing leads to a substantial drop in performance, which is due to loss of ionic functionality. This work is significant because cationic CPEs may not be suitable for long term applications of CPEs and it also provides feedback to synthetic teams to develop new CPEs that are more robust. In the past few years, CPEs have been incorporated in organic thin film transistors as charge injection layers and in organic solar cells as charge collection layers. Thus, the project impact more than one research area. Our results are important to the community because it helps synthetic groups to design the next generation of CPEs with better performance and more robust. Results from this grant were disseminated through a number of peer-reviewed scientific journals, presented at international conferences and incorporated in undergraduate and graduate level courses at UCSB. The project helped train two postdoc scholars, three graduate students, and three undergraduate students. Two graduate students now have a start-up company in Santa Barbara.
