Disentangling Relationships among Dopant Structure, Dopant and Polymer Energetics, Thin-Film Morphology, and the Electrical Properties of Doped Conducting Polymer Films

Motivation and Expected Significance: Organic semiconductors (OSCs) are exciting materials for a wide number of electronics applications. With organic light emitting diodes (OLED) leading the way in commercialization, much promise exists for OSC in energy conversion technologies (e.g., photovoltaics and thermoelectrics), energy storage, sensing, and bioelectronics. An inherent materials advantage of OSCs is their mechanical flexibility and stretchability, which opens significant advantages in the realm of flexible and wearable electronics, an area of impact in the NSF’s 10 Big Ideas concerning the “future of work at the human-technology frontier.” Here, a complete wearable electronic device could be comprised of OSCs and organic-inorganic composites: For example, a sensor based on organic electrochemical transistors (OECTs) could be derived with driving electronics based on organic field effect transistors (OFETs), an OLED display, a power source that is either an organic thermoelectric (OTE) or an organic photovoltaic device, and a fully organic battery to provide power storage. Chemical doping, whereby small molecules are added to the OSC that can either accept (p-type doping) or donate (n-type doping) a charge carrier, is a key method of tuning OSC electronic properties. Chemical doping has been demonstrated to improve charge-carrier injection in OLEDs, passivate deep trap states in OFETs, tune the power factor in OTEs, and increase electrical conductivity. High electrical conductivity is particularly important in printed electronics, where conductive polymers may serve as electrodes, interconnects, or thermoelectric power sources. Despite the well-established importance of doping, it remains extremely difficult to predict how a given dopant will influence charge transport in an OSC. For OTEs, which have potential for generating electrical energy from waste heat, predicting the performance of a given OSC–dopant combination is even more difficult due to the influence of both the electrical conductivity and the Seebeck coefficient on device performance. Particularly difficult is predicting the effect of the dopant properties on the Seebeck coefficient, i.e., the electrical potential difference resulting from a temperature differential (ÄV/ÄT), which is even less understood than how the dopant influences the electrical conductivity. The challenge in determining how dopants regulate material properties arises from the massive number of compounding variables at play, including the doping efficiency, the influence of the dopant on film morphology, variations in OSC morphology from highly crystalline to completely amorphous, potential hybridization between dopant and OSC component molecular orbitals, dopant distribution in and diffusion through on OSC film, and the extent of coulomb binding energy between the ionized dopant and the polaron. An improved understanding of these variables will enable the more targeted design of OSC–dopant combinations for a given application and rapidly accelerate the pace of OSC development. Understanding how different chemical dopants influence the electronic properties of OSCs is a difficult task. As opposed to inorganic semiconductors, where a dopant atom directly contributes a free delocalized electron or hole into the conduction or valence band with little influence on the electronic structure and crystal structure, in OSC the addition of a dopant significantly perturbs the electronic structure and film morphology. Focusing on OSCs derived from ð-conjugated polymers, the addition of a charge-carrier leads to considerable electronic and nuclear relaxation along the polymer backbone, which leads to polaron or bipolaron states that lie within the bandgap of the neutral conjugated polymer. The energy of these states will be determined not only by the nuclear and electronic relaxations of the polymer backbone, but also by how the presence of the dopant perturbs the overall local and extended film morphologies. Furthermore, the introduced charge-carrier may not necessarily be “free”, as the spatial proximity of the ionized dopant and the low dielectric constant of the polymer may lead to a significant coulombic attraction between the polaron and ionized dopant. To address these questions and generate a better fundamental understanding of chemical doping in organic semiconductors, this proposal brings together the state-of-the-art photoelectron spectroscopies of the Graham group with the theoretical expertise of the Risko group. The proposed research starts with understanding the influence of the dopant size on the electronic structure of ð-conjugated polymers, followed by determining how the dopants are distributed within polymers of varying morphology, and finally relating electronic structure and morphology to electrical conductivity and the Seebeck coefficient.