Mechanical flexibility of transparent PEDOT:PSS electrodes prepared by gravure printing for flexible organic solar cells

The mechanical flexibility of transparent poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films printed onto a flexible PET substrate using a gravure printing method was investigated using a lab-made bending test system. Gravure-printed PEDOT:PSS electrodes with a sheet resistance of 359 Ω/square and a transparency of 88.92% showed outstanding flexibility in several types of flexibility tests, including outer/inner bending, twisting and stretching. Notably, the PEDOT:PSS electrode had a constant resistance change (ΔR/R₀) within an outer and inner bending radius of 10 mm. In addition the stretched PEDOT:PSS electrode showed a fairly constant resistance change (ΔR/R₀) up to 4%, which is more stable than the resistance change of conventional amorphous ITO electrode. The twisting test revealed that the resistance of the PEDOT:PSS electrode began to increase at an angle of 36° due to delamination of the film from the PET substrate. Despite the high sheet resistance of the PEDOT:PSS electrode the flexible organic solar cells fabricated on the PEDOT:PSS electrode showed a power conversion efficiency of ∼2% (FF: 44.9%, V_o: 0.495 V and J_sc: 9.1 mA/cm²), indicating the possibility of using gravure printed PEDOT:PSS as a flexible and transparent electrode for printing-based flexible organic solar cells.     

π‐Conjugated Polymers Incorporating a Novel Planar Quinoid Building Block with Extended Delocalization and High Charge Carrier Mobility

Two novel conjugated polymers incorporating quinoidal thiophene are successfully synthesized. By combining 1D nuclear magnetic resonance (NMR) and 2D nuclear Overhauser effect spectroscopy analyses, the isomeric form of the major quinoid monomer is clearly identified as the asymmetric Z, E‐configuration. The quinoidal polymers are synthesized via Stille polymerization with thiophene or bithiophene. Both quinoidal polymers exhibit the low band gap of 1.45 eV and amphoteric redox behavior, indicating extended conjugation owing to the quinoidal backbone. These quinoidal polymers show ambipolar behaviors with high charge carrier mobilities when applied in organic field‐effect transistors. In addition, the radial alignment of polymer chains achieved by off‐center spin‐coating leads to further improvement of device performance, with poly(quinoidal thiophene–bithiophene) exhibiting a high hole mobility of 8.09 cm² V^?1 s^?1, which is the highest value among the quinoidal polymers up to now. Microstructural alteration via thermal annealing or off‐center spin‐coating is found to beneficially affect charge transport. The enhancement of crystallinity with strong π–π interactions and the nanofibrillar structure arising from planar well‐delocalized quinoid units is considered to be responsible for the high charge carrier mobility.     

Development of laser-induced breakdown spectroscopy (LIBS) with timed ablation to improve detection efficiency

A laser-induced breakdown spectrometer (LIBS) was developed for determining the elemental composition of individual airborne particles. The system employs two lasers focused on a narrow beam of particles. A continuous wave laser placed upstream scatters light from particles, while a pulse laser downstream ablates the particles. The scattered light from the upstream laser is used to trigger the downstream pulse laser, resulting in more accurate hitting of the particles than a free-firing laser system without the triggering signal (i.e., constant pulse laser firing). Various laboratory-generated aerosols (NaCl, MgCl₂, KCl, and CaCl₂) were used to evaluate the newly developed LIBS system. Particles were tightly focused into a center line with a sheath air focusing system using an optimum aerosol-to-sheath air velocity ratio. The locations of both the scattering laser and pulse laser beams were precisely controlled by a motorized X-Y stage controller. Data showed that for the LIBS with the triggering system, the hitting efficiency (%) of particles (200–600 nm) significantly increased (e.g., 350 nm particles had more than 26 times higher hitting efficiency at 1,000 particles/cm³), and much lower limits of particle size (～200 nm) and number concentration (<100 particles/cm³) were achieved compared to the free-firing laser condition. Additionally, the hitting rate (hits/min) significantly increased with the triggering system. Our results suggest that the LIBS with the triggering system can be useful for real-time detection of elements of particles existing at low number concentrations (e.g., atmospheric particles) and for the determination of the variation of elemental composition among particles.

© 2017 American Association for Aerosol Research     

Characterization of deposition patterns produced by twin-nozzle electrospray

This paper presents the characteristics of two-nozzle electrospray deposition with a capillary–extractor–substrate configuration. A three-dimensional Lagrangian model was developed to simulate the spray evolution and deposition patterns. The droplet size distribution was not affected by the extractor–substrate voltage; however, spray evolution was found to be influenced considerably by the extractor–substrate voltage. Smaller particles are deposited densely in the outer region of the spray, whereas larger particles are deposited in the core region of the spray due to the size segregation effect. The normalized thickness in the intervening region of adjacent sprays is thinner than in the core region of sprays and gradually decreases as the extractor–substrate voltage increases. The normalized surface number density of deposited particles in the intervening region of adjacent sprays also decreased with the increasing extractor–substrate voltage, whereas the surface number density of them is larger than that in the core regions of sprays at an extractor–substrate voltage below 3 kV. The deposition patterns using a diluted PSL solution and spray visualization were in good agreement with the simulation results. It is suggested that the electrical self-dispersion effect and electrical repulsive interaction between adjacent sprays play an important role in determining the characteristics of the spray evolution and deposition patterns for multiplexed electrospray deposition.     

Experimental and numerical investigation of flexibility of ITO electrode for application in flexible electronic devices

For flexible electronic devices, the transparent conductive electrode (TCE) is the most important material for determining the flexibility of devices. Due to the brittleness of the indium tin oxide (ITO) electrode, several alternative TCEs have been developed. However, it is still difficult to successfully achieve quality as good as ITO. In this study, the flexibility of the ITO electrode was investigated with strain and fracture analysis. Effects of thickness of the ITO and the substrate on the flexibility of ITO were investigated by bending tests, numerical simulation and theoretical analysis. Flexibility of ITO electrode can be increased by reducing the thickness of ITO and substrate material. An ITO electrode with a substrate less than 50 μm could be bent to less than 4 mm without failure, and used in flexible electronics. Effects of different substrate materials on the flexibility of ITO were also investigated based on fracture analysis. We investigated the effects of PEDOT [poly(3,4-ethylenedioxythiophene)] as a buffer layer to improve flexibility. Higher flexibility of the ITO/PEDOT hybrid electrode compared to ITO was attributed to the PEDOT layer, which smoothened ITO surface and decreased the density of pinholes or voids of ITO, resulting in higher crack resistance.

     

Laser dissimilar welding of CoCrFeMnNi-high entropy alloy and duplex stainless steel

High entropy alloys(HEAs)have superior mechanical properties that have enabled them to be used as structural materials in nuclear and aerospace applications.As a dissimilar joint design is required for these applications,we created a dissimilar joint between CoCrFeMnNi-HEA and duplex stainless steel(DSS)through laser beam welding;a technique capable of producing a sound joint between the two materials.Microstructure examination using SEM/EBSD/XRD analysis revealed that the weld metal(WM)exhibits an FCC phase regardless of the postweld heat treatment(PWHT)temperature(800 and 1000℃)without forming detrimental intermetallic compounds or microsegregation.The heat-affected zone of the CoCrFeMnNi-HEA showed CrMn oxide inclusions while that of the DSS showed no inclusions.Moreover,a lower hardness was recorded by the WM compared to the base metal after welding.After PWHT,the hardness of the WM,CoCrFeMnNi-HEA,and DSS decreased with an increase in the PWHT temperature.However,the decrease in the hardness of the HEA was more significant than in the WM and DSS.The cause for this reduction in hardness was attributed to recrystallization and grain growth.In addition,a strength of 584 MPa with low ductility was recorded after welding.The obtained strength was lower than that of the BMs,but comparable to that of the welded CoCrFeMnNi-HEA.The application of PWHT resulted in over a 20％increment in ductility,with only a marginal reduction in strength.The deformation mechanism in the as-weld joint was mainly dominated by dislocation while that for the PWHT joint was twinning.We propose laser beam offset welding as a technique to improve the mechanical properties of the dissimilar joint,which will be the subject of future studies.