Doping of conjugated polymer thin films for optoelectronic sensor and tunable plasmonic applications
Creators: name: Maddali, Hemanth
Publisher: name: No Publisher Supplied
Year: 2021

description: Doped polymer thin films have several applications in electronic, optoelectronic and thermoelectric devices. Often the electrical properties of doped conjugated polymer thin films are affected by their local physical and mechanical characteristics. However, investigations into the effects of doping on local domain properties have not been carried out. In chapter 2, we study the physical, mechanical and optical properties of electrochemically doped P3HT thin films at the nanoscale and establish a relation between doping level and the physical properties of P3HT thin films. Bulk crystallinity of both pristine and doped P3HT thin films characterized using grazing incidence wide angle X-ray scattering (GIWAXS) show a clear loss in crystallinity upon doping. Nanoscale crystalline and amorphous domains in the films visualized by atomic force microscopy (AFM) indicate that the crystalline domains are most affected by doping and have a higher degree of doping compared to amorphous domains. As a result, the crystalline domains exhibit superior electrical conductivity at a local level in CAFM measurements. This is further supported by Raman mapping of doped films. A direct relation is established between the physical, mechanical, and electrical properties of doped P3HT thin films based on the AFM data. The findings of chapter 2 show that higher dopant concentrations are found in crystalline domains compared to amorphous domains, which has not been demonstrated before, to the best of our knowledge. The study described in chapter 2 can be used to optimize the electrical properties of doped P3HT thin films for use in electronic and optoelectronic device applications.Polymer temperature sensors are important for applications in food packaging, air conditioning, wearable devices and biomedicine. However, the sensing range of these sensors is narrow, and the mode of sensing is restricted to either optical or electrical, which limits their implementation in practice. In chapter 3, dual-mode polymer-based temperature sensors are demonstrated with a wide sensing range based on a novel sensing mechanism that utilizes electrochemically doped (oxidized) regio-random poly(3-hexylthiophene) (RRa-P3HT). This design was based on the findings of chapter 2. When subjected to temperature, the electrochemically doped RRa-P3HT thin films dedope resulting in a visible color change from blue (the doped state) to yellow (the dedoped state; similar in color to the pristine film). Energy dispersive X-ray spectroscopy (EDS) and electron paramagnetic resonance (EPR) spectra show decreases in dopant concentrations with increase in the temperature that the doped films were subjected to, indicating a gradual thermal dedoping in the temperature range from 30 oC to 80 oC. Visible wavelength absorption spectra of doped films subjected to increasing temperatures depict both doped and dedoped peaks. The ratio of intensities of dedoped to doped peaks exhibits a linear trend between 30 oC and 80 oC that can be exploited for optical-mode thermal sensing. This temperature sensing range is the widest of any polymer-based temperature sensor reported to date. A unique aspect of this thermal sensor is that the thermally induced transition between doped and dedoped states for RRa-P3HT films can be translated into an electrical signal as doped films are electrically conducting. Two-point probe current measurements show an exponential decrease in the current with increase in temperature.Plasmonic materials are an excellent means for shaping light below the diffraction limit (i.e., at the nanoscale). Traditional plasmonic materials such as gold, silver, doped inorganic semiconductors are energy intensive to process and the development of plasmonic materials with the processing advantages of polymers would enable new applications. In chapter 4, we have proposed a method to tune the plasma frequency in the mid-infrared (mid-IR) wavelength region for doped CPs. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thin films were doped with a solution of para-toluenesulfonic acid (PTSA) in dimethylsulfoxide (DMSO) in order to improve the conductivity by two orders of magnitude. The conductivity was further enhanced to over 1000 S/cm by post-treatment with sulfuric acid. The higher conductivities were then fitted into the Drude model to calculate the dielectric constant. The calculations provide evidence for the tunability of plasma frequency with changing conductivity of the PEDOT:PSS thin films.