UCLA

Semiconducting polymers are an intriguing class of materials that have been attracting increasing attention over the years. As their name suggests, semiconducting polymers are used in similar fields as their inorganic counterparts, but they offer several advantages that make them particularly desirable. These polymers are solution-processable, flexible, and have intrinsically low thermal conductivity, all of which are important for applications such as wearable thermoelectric devices. The low thermal conductivity, in particular, contributes to the thermoelectric efficiency of these devices, making them a natural choice for such applications.However, like all materials in their developing stages, semiconducting polymers are not without drawbacks. In their pristine form, the use of semiconducting polymers in thermoelectric devices is limited by low electrical conductivity due to low intrinsic charge carrier density and mobility. To address these issues, we employ various dopants and doping methods to introduce carriers into semiconducting polymer thin films. Additionally, we developed a setup to rub-align semiconducting polymer thin films to study the effects of changing the molecular morphology on charge transport characteristics and doping. We characterize doped polymer films using techniques such as four-point probe conductivity, temperature-dependent conductivity, Seebeck coefficient measurements, Hall effect measurements, wide-angle X-ray scattering, and steady-state spectroscopy. The first part of this dissertation (Chapter 2) explores the effects of the ambient environment, particularly humidity, on semiconducting polymer films doped with a novel doping method recently reported in the literature called “anion-exchange.” The anion exchange doping method greatly enhances doping efficiency and also allows control of over the choice of counterion that accompanies the doped charge carrier. The counterion comes from an electrolyte solution, however, the electrolytes used are often made from hygroscopic salts. We show that these counterions can draw water into polymer films doped via anion exchange, which greatly reduces conductivity by acting as traps for carriers. The second part of the dissertation (Chapters 3 and 4) investigates the effect of rub-aligned polymer thin films on doping and charge transport. Charge transport in semiconducting polymer films is often limited by their semicrystalline nature, where poor mobility can be caused by structural defects like bends or kinks that create energetic barriers. One way to reduce such defects is through a novel “high-temperature rub-aligning” method to straighten and molecularly align the polymer chains. Our study showed that conductivity greatly improves with this method, however, we also found that literature reports of this improvement were exaggerated because the method to measure anisotropic conductivity is highly dependent on electrode geometry, and previous work did not take this into account. Additionally, rub-aligned films provide insights into the effect of different polymorphs on the doping process. We found that rub-aligning creates two polymorphs that happen to have face-on and edge-on structures. We found that face-on polymorph, whose structure is more similar to the final doped structure, has a lower barrier to doping than the edge-on polymorph, which requires a greater structural rearrangement to dope. For the final part of this dissertation (Chapter 5), we used a holistic approach to understanding the effect of different dopants, doping methods, and structure on charge transport in doped semiconducting polymer thin films. By taking advantage of temperature-dependent measurements and models based on the Boltzmann transport formalism, we demonstrated that the factor dominating charge transport in doped semiconducting polymer thin films is highly dependent on the type of dopants and the doping method. Our finding demonstrates that the relationship between the Seebeck coefficient and conductivity of doped P3HT films can be improved either by reducing Coulomb interactions or by adding additional charge transport pathways through doping the of amorphous regions. The latter is shown by an increase in the correlation length between domains.