Effects of Solution Processing Parameters on Nanostructure-Property Relations in Organic based Optoelectronic Devices

Abstract

Understanding the mechanism responsible for temperature dependent performances of emitting layers is essential for developing advanced phosphorescent organic light emitting diodes. We described the morphological evolution occurring in the PVK:Ir(ppy)3 binary blend films with respect to thermal annealing up to 300 ˚C by coupling atomic force microscopy and transmission electron microscopy. In particular, in–situ temperature dependent experimental characterization was performed to directly determine the overall sequence of morphological evolution occurring in the films. Thermally annealed device at 200 ˚C exhibits a noticeable enhancement in the performances, compared to the devices in as–processed state and to the devices annealed at 300 ˚C. Our approaches reveal that the Ir(ppy)3 molecules with needle–like structure in the as–processed state were aggregated and thus diffused into PVK without their morphological change at the temperature regime between 150 ˚C and 200 ˚C. Moreover, both network–like and droplet patterns existed in the devices annealed at 300 ˚C which was beyond glass temperature of PVK, leading to a profound increase in the surface roughness. The observed pattern formation is discussed in terms of viscoeleastic phase separation. Based on our experimental findings, we propose that the performances of devices are significantly controlled by the diffusion of dopant molecules and the morphological evolution of host materials in binary blend systems. Although the control of solution-processed emitting layers in organic-based optoelectronic devices enables highly efficient properties, a solution-based protocol for emitter fabrication is highly complex, and the link between the device performance and internal nanoscale features as well as three associated fabricating parameters (e.g., the employed solvents, annealing temperatures, and molecular concentration) need to be understood. Here, we investigated the influence of the solution-processing parameters on the nanostructure-property relationship in light-emitters that consist of iridium-complexes doped in polymer. The boiling points and evaporation rates of the selected solvents govern the nanomorphology of molecular aggregation in the as-processed state, and the aggregation is either needle-like, spherical or even a mixture of needles and spheres. Furthermore, a direct observation via in situ heating microscopy indicates that annealing of emitters containing a needle-type aggregation promotes the associated molecular transport, leading to a substantial reduction in the surface roughness. Consequently, a nearly threefold increase in the current efficiency of the device is induced. The processing-structure-property relation of Pd coated Ag core-shell nanowire (CS NW) film is also investigated. Pd@Ag CS NW film is one of the most promising candidates for flexible transparent electrodes due to good electrical conductivity, low haze, and chemically stability. To fabricate defect-free and homogeneous Pd coating layer on Ag surface the galvanic displacement reaction should be precisely controlled and characterized. We show that the thickness of Pd shell, uniformity of atomic distribution, and porosities of core material can be controllable by tuning concentration of reactants, reducing agent, and pH of aqueous solution. The morphological and compositional evolution of Pd@Ag NW was characterized by transmission electron microscopy (TEM) and atom probe tomography (APT). APT show a direct three dimensional quantitative evidence on formation of a conformal shell of Pd on the surface of the Ag NW. Our experimental finding show that the chemical stability, and electrical and optical properties of CS NW electrode are sensitive to radial and longitudinal inhomogeneites in palladium concentration which arise from nonuniform galvanic exchange. Our result confirmed that the chemical stability of Pd@Ag CS NW is greatly improved compared to the pristine Ag NW. The synthesized Pd@Ag CS NW applied as a constituent of transparent and stretchable electrode for optoelectronic device. These finding have important implications for the tuning of the shell coverage of individual NWs for transparent CS NW electrode used in the new evolution of flexible high-performance organic-based optoelectronic devices with high chemical stability.