The influence of fluid droplet properties on the droplet-on-demand jetting of a Newtonian model fluid (water–isopropanol–ethylene glycol ternary system) has been studied. The composition of the fluid was adjusted to investigate how the Ohnesorge number ($\mathit{Oh}$) influences droplet formation (morphology and speed) by a microfabricated short-channel shear-mode piezoelectric transducer. The fluid space for satellite-free single droplet formation was indeed found to be bound by upper and lower $\mathit{Oh}$ limits, but these shift approximately linearly with the piezo pulse voltage amplitude ${V}_{o} $, which has a stronger influence on jetting characteristics than pulse length. Therefore the jettable fluid space can be depicted on a ${V}_{o} {{\ndash}}\mathit{Oh}$ diagram. Satellite-free droplets of the model fluid can be jetted over a wide $\mathit{Oh}$ range, at least 0.025 to 0.5 (corresponding to $Z= {\mathit{Oh}}^{\ensuremath{-} 1} $ of 40 to 2), by adjusting ${V}_{o} $ appropriately. Air drag was found to dominate droplet flight, as may be expected. This can be accurately modelled to yield droplet formation time, which turned out to be $20\text{{\ndash}} 30~\lrm{\ensuremath{\mu}} \mathrm{s} $ under a wide range of jetting conditions. The corresponding initial droplet speed was found to vary linearly with ${V}_{o} $, with a fluid-dependent threshold but a fluid-independent slope, and a minimum speed of about $2~\mathrm{m} ~{\mathrm{s} }^{\ensuremath{-} 1} $. This suggests the existence of iso-velocity lines that run substantially parallel to the lower jetting boundary in the ${V}_{o} {{\ndash}}\mathit{Oh}$ diagram.

Polymer light-emitting devices with a conducting polymer distributed Bragg reflector (DBR) are reported. Polymer DBRs were formed from alternate spin-coating of poly (3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulphonate) (PSS) as the low-index layer and a hole-transporting polyfluorene derivative as the high-index layer onto ITO deposited glass substrates. The polymer DBR with 5 periods showed reflectivity of 40% at the wavelength of 560nm. Onto this polymer DBR, a green-light-emitting polyfluorene was spin-coated to give the emissive layer and Ca/Al was deposited to form the cathode served another reflector. These LEDs operated successfully at low voltages and emitted light having a narrower spectrum (FWHM=47nm) than structures without the DBR (FWHM=102nm). The peak wavelengths varied corresponding to the thickness of the emissive layer, that is, the cavity length as expected from calculations. The luminance turn-on voltage of the LED with the polymer DBR is 2V, which is the same as LEDs without a DBR. The current was found to be limited by injection in the low field region and limited by space charge density at high fields. The current does not depend on the number of layers in the DBR in the low field region, and this implies that there are no effective energy barriers in the polymer DBR.