Chapter 4. High-performance flexible electroluminescent (EL) devices based on high-k nanodielectrics
4.2 Experimental Details
Bar-coating of cross-aligned AgNW flexible electrodes: For the aligned AgNWs arrays, the AgNW ink (Nanopyxis Corporation) was coated onto PET substrates treated with poly-L-lysine (PLL) adhesion layers by a Meyer-rod coating method, as reported in our previous study.42 In this process, the AgNW ink containing 0.15 wt% of AgNWs with an average length of 20 μm and diameter of 35 nm was injected into the empty space between the Meyer rod and the substrate, which resulted in the formation of a meniscus due to the capillary force. Subsequently, the meniscus dragging by the Meyer rod at a constant speed resulted in the formation of highly uniform and aligned AgNWs arrays along the coating direction. To fabricate cross-aligned AgNWs arrays, an additional bar-coating process was carried out on the aligned AgNW array in the perpendicular direction to the pre-aligned AgNW array.
Hydrothermal synthesis of La-doped BTO nanocuboids: BTO:La NCs (xLa: BaTiO3, x = 0, 0.5, and 1.0) were synthesized using a modified hydrothermal method previously reported in the literature.213, 214 The precursors used for the hydrothermal reaction include Ba hydroxide monohydrate [Ba2•(H2O), Sigma-Aldrich, 98%], Ti butoxide [Ti(O(CH)2CH3)4, Sigma-Aldrich, 97%], La nitrate hexahydrate [La(NO3)3•6H2O, Sigma Aldrich], and diethanolamine [NH(CH2CH2OH)2, DEA, Sigma- Aldrich]. First, 25 mmol of Ti butoxide was added to 10 mL of ethanol followed by the addition of 3.5 mL of ammonia at room temperature, resulting in the formation of TiO2•yH2O (Solution A). Another pre-solution (Solution B) was prepared by dissolving 37.5 mmol of Ba hydroxide and an appropriate amount of La(NO3)3•6H2O in 12.5 mL of DI water at 90 °C. Solution A was then mixed with Solution B to form a suspension. The DEA (2.5 mL) was then added to the solution to control the size and polydispersity of the nanoparticles. The final solution was transferred to a Teflon-lined stainless-steel autoclave and kept in an oven at 200 °C for 48 h for the hydrothermal reaction. Then, the autoclave was cooled down to room temperature and the particles were washed several times using ethanol and DI water. The final product was dried at 80 °C for 48 h.
Preparation of nanodielectrics ink: The dielectric ink was prepared by first mixing PDMS (base only) and BTO:La NCs followed by magnetic stirring for 30 min. The mixture was then subjected to successive bath sonication (4 h) and probe sonication (15 W, 5 min) to break down the large aggregates of BTO NCs. BTO/PDMS dispersions were prepared with a BTO content of up to 46%.
Later, the concentration was fixed at 26% due to the agglomeration observed at higher concentrations.
The dispersions with BTO were spin-coated on silicon or glass to fabricate parallel capacitors, which were used to characterize the dielectric constant of the BTO/PDMS films. The electroluminescent layer was prepared by mixing ZnS:Cu particles with PDMS (3:1 ratio) with a viscosity of ~10 Pas (Figure 4.1), which is suitable for the screen-printing process.215
132
Fabrication of flexible ACEL devices: For the fabrication of flexible ACEL devices, AgNWs were first Meyer-coated onto PET substrates, forming the bottom electrodes, followed by screen- printing of the mixture of BTO:La nanodielectrics/PDMS as the high-k dielectric layer onto the bottom electrodes. Subsequently, ZnS:Cu particles (SEM, PL, and XRD analysis data in Figure 4.2) mixed with liquid PDMS (ratio 3:1) prepolymer and BTO:La nanoparticles were then screen-printed on top of the high-k dielectric material layer. Finally, another layer of AgNWs on PET substrates was used to form the top electrodes. Thereafter, flexible ACEL devices were dried and encapsulated with thin PDMS layers to protect the devices.
Characterization: The Rsh values of the AgNW electrodes were measured using a four-point probe method (Keithley, 2400). Optical transmittance and absorption spectra of AgNW networks were measured by a UV–vis–NIR spectrophotometer (Jasco, V-670). The surface alignment structure of the AgNW arrays was examined using an optical microscope (PSM-1000, Olympus). The purity and crystalline phases of the BTO nanodielectrics were analyzed by powder XRD data, which were collected using a Bruker D8 Advanced diffractometer equipped with Cu Kα radiation source and a diffractometer monochromator that was operated at 40 kV and 40 mA. The surface morphology of the BTO powder was examined using a field-emission SEM (FE-SEM, Hitachi S4800) at an operating voltage of 10 kV. The specimens used for TEM (JEM 2100, JEOL) analysis were obtained by drying droplets of the as-prepared samples in an ethanol dispersion onto a carbon-coated copper grid. The chemical bonding and functional groups of the BTO powders were investigated using an X-ray photoelectron spectrometer (K-alpha, Thermo Fisher). The rheological properties of the BTO nanodielectric slurries were studied with a rheometer (Haake MARS3, Thermo Fisher). The dielectric data of the nanodielectrics were obtained using an impedance analyzer (BioLogic) equipped with a frequency response analyzer (FRA2 module), which has a current range from 10 nA to 10 mA with a resolution of 0.0003% and a potential resolution of 3 μV. The dielectric data was measured by supplying an unbiased voltage signal with a peak voltage of 0.5 V and a frequency range of 10−2 to 105 Hz with a resolution of 0.0003%. A function generator connected with a power amplifier was used to apply an alternating operation voltage to the flexible ACEL devices. Luminescence spectra of the flexible ACEL devices were recorded by a spectroradiometer (PR-655, Photo Research, Inc.).
133
Figure 4.1 (a) A photograph of ZnS:Cu/PDMS slurry in ambient light and (b) under a UV lamp. (c) Rheological behavior of slurry prepared with varying shear rate sweeps.
134
Figure 4.2 (a) SEM micrograph of ZnS:Cu particles with a mean particle size of μ = 26 μm and standard deviation of σ = 5.5 μm. The scale bar is 100 μm. (b) Photoluminescence mechanism in ZnS doped with Cu impurity. (c) A photograph of ZnS:Cu particles placed on a glass plate and in a glass vial (dispersed in methanol). (d) X-ray diffraction pattern of hexagonal ZnS matched with standard JCPDS card (JCPDS card no.: 79-2204). (e) Photoluminescence spectra of ZnS:Cu recorded at room temperature with an excitation wavelength of 400 nm, which resulted in a green emission spectrum with peak emission wavelength at 500 nm.
135