Morphology Control in Polymers for Enhanced Optoelectrical Properties Utilizing an External Electric Field

Abstract

Organic materials are being widely applied to next-generation optoelectronic devices today due to their outstanding electrical and optical properties. Efforts to enhance the optoelectronic performance of these organic materials have been carried out through various methods. Organic materials, which are mainly used in devices in the form of thin films, can maximize their inherent properties through morphology control either in solution before film formation or on the film itself. Methods such as applying shearing force or centrifugal force during film formation to induce molecular orientation, or applying light, magnetic field, or electric field right after film formation to apply an external field, are used. The method of applying an external field has the advantage of less restriction on the directional control of molecular orientation in three dimensions and can achieve uniform morphology on the surface and inside of the film, making it more advantageous compared to morphology control methods by film formation. In particular, the use of an external electric field to induce the alignment of the molecular dipole moment as a post-treatment method can also have the advantage of being applicable to any type of material, as all materials possess a dipole moment. However, the method of controlling the morphology of an organic film by applying an external electric field still lacks sufficient study on its mechanism and has not been widely applied as a process. Therefore, we have conducted research to improve the performance of organic optoelectronic devices by applying an external electric field to various organic thin films and have newly presented the mechanism of such a process. Chapter 1 provides a general introduction to organic materials (especially polymers), post-treatment using external electric field and organic electronic devices. Organic materials can have various bandgaps due to structural tunability. Typically, organic materials have wide bandgaps, resulting in insulating and dielectric properties. However, the conjugation in the backbone enables charge transporting properties, which can result in a reduction in bandgap as molecular weight increases, leading to the formation of semiconducting polymers. The morphology of these polymers can be manipulated by applying an external electric field in order to align the molecular dipole moment. Furthermore, the performance of organic optoelectronic devices, organic field-effect transistors (OFETs) and organic solar cells (OSCs) can be enhanced by this process. This chapter outlines the principles of organic optoelectronic devices. Chapter 2 demonstrates that the control of the crystal phase of PVDF-based high-k dielectric polymer utilized to fabricate OFETs results in an improvement in electrical performance by applying external electric field. The control of the crystal phase can be achieved through the application of an external electric field, which is the corona poling process. This process aligns the dipole moment of the high-k dielectric polymer. Consequently, the α-phase can be transformed into the β-phase, which is a polar crystal, thereby enhancing the dielectric polymer's capacitance. However, an increase in capacitance is accompanied by the creation of trap sites at the channel of charge carriers. To reduce the number of trap sites, a bi-layer system comprising a thin low-k dielectric layer was introduced between the high-k dielectric and the semiconducting layers. Finally, the electrical performance of OFETs was successfully enhanced by increasing the charge carrier amount in the channel. In Chapter 3, the morphology of the N-type semiconducting polymer P(NDI2OD-T2) was controlled in order to facilitate the transport of charge carriers. As a single component in the active layer of OFETs, the local dipole moment of the polymer was aligned after an appropriate intensity of external electric field treatment, resulting in enhanced backbone linearity and crystallinity of the polymer and enlarged crystals due to the internal dipole moment in the polymer crystals. Finally, the charge carrier mobility in the active layer of OFETs was enhanced as a consequence of the appropriate external electric field treatment. This chapter presents an analysis of the morphological factors that facilitate the transport of charge carriers. Continuously, in the active layer of OSCs, which is a bulk heterojunction system, P(NDI2OD-T2) and PTB7-Th as acceptor and donor materials, respectively, were pre-aggregated by polar solvent additive acetonitrile (ACN) in solution state, and the crystal of acceptor polymer was enlarged by external electric field treatment. The pre-aggregation process helped to maximize the effect of increasing the crystallinity of the acceptor polymer by applying an external electric field. Finally, the exciton generation site and charge transport pathway can be well secured by combining these processes, so that the performance of OSCs can be effectively improved.