Establishing design principles for efficient doping in organic semiconductors through the combination of advanced optical spectroscopy and transport measurements

Nontechnical description Organic semiconductors are ideal candidates for distributed sensors as they can be printed on any surface. This opens up possible applications such as flexible and wearable electronics. However, their low electrical conductivity does not meet the requirements of these technologies. Adding chemical impurities, a process known as doping, is a standard method to boost conductivity. Unfortunately, current doping methods introduce structural disorder and electronic traps that severely limit performance. The challenge is to enhance electrical transport without introducing such defects. This project combines advanced optical characterization with charge transport studies to understand doping at the nanoscale affects macroscopic charge transport. By bridging this gap, understanding developed in this project guides the development of advanced materials for next-generation flexible electronics. The educational component of this effort is highly integrated with the research. This includes training for participating students in materials science and modern electronics. The project also helps to revitalize local science education through modern, relatable physics demonstrations for K-12 students. These connect abstract scientific concepts with everyday technologies. Thereby, this project inspires future scientific talent. Technical description The technical objective of this research is to develop a comprehensive model of charge transport in chemically doped organic semiconductors by quantifying charge carrier interactions and identifying the factors that limit electrical conductivity. High conductivity requires maximizing both charge carrier concentration and carrier mobility, yet current doping methods suffer from low doping efficiency due to charge trapping, structural disorder, and strong Coulombic attraction between carriers and dopants. This project systematically investigates the relationship between molecular structure, energetic disorder, and vibrational landscapes. The research team utilizes a complementary approach that couples electrical characterization with advanced optical spectroscopy. The interface properties and density of electronic trap states are evaluated from organic field-effect transistor measurements, while bulk carrier mobility is extracted using space-charge limited current measurements. To resolve how molecular vibrations impact charge motion, the project employs impulsive vibrational spectroscopy to identify specific lattice vibrations responsible for charge localization. Nanoscale scanning near-field infrared microscopy is employed to map local structural order and chemical composition with sub-diffraction spatial resolution. The research team applies coherent two-dimensional electronic spectroscopy to measure the screening length and magnitude of carrier-induced electrostatic potentials in the ultra-high charge density regime, which is accessed by combining chemical and electrostatic doping. By resolving these fundamental microscopic interactions, this project establishes design principles and doping protocols to achieve highly conductive organic materials, directly supporting the goals of the Electronic and Photonic Materials program. 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.