Interfacial Engineering of Conjugated Polymers by Nanoconfinement for Optoelectronic Applications

Abstract

The conjugated polymers have gained enormous attention as next-generation semiconducting materials because of their tunable electrical and optical properties combined with plastic properties such as toughness, plasticity and elasticity. Therefore, various applications such as OTFT, OLED, OPV, photodetector, and sensor etc., were considered and widely studied. Among various application using conjugated polymers, OLED has already been commercialized and is about to become an alternative displays and light sources. Cheap, flexible and cost-effective solar cells by conjugated polymers are particularly prominent application to solve energy related problems. Less than 0.1 % of power conversion efficiencies in 1980s have been increased up to more than 12 % recently. There were several breakthroughs by developing of new process methods and new materials. New conjugated polymers such as low bandgap polymers which more efficiently absorb photons and transfer electrons were synthesized. In addition, bulk heterojunction was discovered which provide large interfaces for charge separation and pathways for efficient charge transfer. However, chemical instability as well as metastable nanostructures by bulk heterojunction which are intrinsic drawbacks of organic materials have made difficult for OPVs to be commercialized. Therefore, it required another breakthrough approach which can enhance both device performance and stability at the same time. Nanoconfinement could be effective approach to realize both efficient and stable optoelectronic devices. Nano-confined polymers have shown novel structural and dynamical properties which were not obtainable using conventional methods. Controlling the structural properties and dynamics by nanoconfinements could give us new approach to solve the problems. In this study, systematic studies of nanoconfinement effect on structural properties and dynamics of conjugated polymers will be shown. We applied nanoconfinements effects on organic solar cells to realize efficient and stable device performance based on those systematic studies. In the first Chapter, fundamentals on conjugated polymers, organic solar cells and nanoconfinement effects will be described to address the importance and effectiveness of our approach. In the second Chapter, nanoconfinement effects on crystalline structure of conjugated polymer were studied. Based on structural studies, organic semiconductor thin films with dramatically enhanced charge mobility along with superior oxidization resistivity were realized by exploiting nanoconfinement followed by solution-based doping. First, easy and mass-producible methods to fabricate nanostructures of conjugated polymers will be introduced. Parameters of conjugated polymer nanostructures such as size, geometry and composing materials were easily controlled using newly developed patterning methods. Based on those newly developed patterning methods, systematic studies on the effect of nanoconfinement on structural properties of conjugated polymers were conducted. We fabricated nanopillars, nanoholes, and nanocones of conducting polymer with different chemical structure, dimension, and crystallinity based on patterning with soft PFPE templates. GIWAXS measurements showed the changes in both chain orientation and crystallinity depending on the degree of confinement which are quite different from the bulk crystallinity of conducting polymers. More than 20 times higher population of crystallites having face-on orientation were obtained by nanoconfinement effect. In addition, they have shown more than two magnitudes higher charge mobility along vertical direction by increased crystallinity. Moreover, doping on those nanostructured films further increased the conductivity 400 ~ 500 times higher than that of bulk film. In the third chapter, two examples of nanoconfined geometry showing unusual dyanamics of molecules will be shown. Based on those dynamical studies, organic solar cells with increased device performance and stability were realized. First we used nanowire network structures in bulk heterojunction as confinement geometry. we demonstrate a novel strategy for the stabilization of nanomorphology of organic solar cells by inducing polymeric nanowire network structures. The physically interconnected network structures form robust electron donor domains and impose confinement which suppresses the aggregation of the electron acceptor, [6,6]-phenyl-C61-butylric acid methyl ester (PCBM). Organic solar cells having the nanowire network structures showed increased power conversion efficiencies and dramatically enhanced thermal stability compared to BHJ and non-network nanowire-based devices. Second geometry is quasi-OHJ fabricated by sequential process. For this study we used PTB7 and PCBM. PTB7 were first deposited on substrate. And on PTB7 films, PCBM dissolved in DCM/DIM were deposited by spin-casting. During the spin-coating process, some PCBM were diffused into PTB7 amorphorous phases forming interdigited structures at the interfaces. Because of such as interdigited structures, confined structures at the interfaces, they showed significantly enhanced device stabilities. Compared to device performance of BHJ were deteriorated within initial 10 hours, other nanoconfined devices showed stabilized performance more than 1,000 hours. In the last chapter, effect of nanoconfinement on both structural properties and dynamics was applied to organic solar cell to realize efficient and stable organic solar cells. P-type materials PCBM and PNDIT were deposited on top of 70 nm sized P3HT nanopillars to fabricate ordered heterojunction geometry. Strong face-on orientation with high crystallinity in P3HT nanopillars will provide effective pathways for hole transport along vertical direction. Their interdiffusion between donor and acceptor molecules were effectively controlled by changing annealing conditions. In addition, their dynamics were quantitatively analyzed by in-situ GISAXS experiments. They have shown different rate of diffusion and stabilized degree of diffusion. We could obtain highly enhanced device performance by decreasing domains sizes from the controlled interdiffusion of donor and acceptor molecules. Moreover, they endured harsh thermal conditions up to 200 ℃ for 4 hours by the stabilized interdiffusion due to nanoconfinement.