In 1985, HP introduced the ThinkJet - the first low-cost, mass-produced thermal inkjet printer. Providing a reasonable
alternative to noisy dot matrix printers, ThinkJet set the stage for subsequent generations of HP thermal inkjet
technology (TIJ). With each new generation, HP TIJ products provided new standards for print quality, color, and an
unprecedented cost/performance ratio. Regarded as the first and most successful commercial MEMS technology, the
development of HP's TIJ printheads required multidisciplinary innovation in fluid dynamics, bulk and surface
micromachining, large-scale integration of electronics, packaging, and high volume MEMS manufacturing. HP's
current TIJ printhead products combine Pentium-class addressing circuitry, high voltage mixed-signal driver electronics,
dense electrical interconnects, and up to 3900 high-precision microfluidic devices - all on a single silicon chip. In this
paper, we will provide a brief history of HP's TIJ technology and discuss how the unique capabilities that were required
to advance the state-of-the-art of TIJ printheads are now providing a platform for the development of new MEMS
devices and systems.
Thermally stimulated currents (TSC) were measured in molecularly doped polymers consisting of the hole transport molecule p-diethylaminobenzaldehyde diphenyihydrazone (DEH) and the polymer binder bisphenol A polycarbonate (PC) at two different doping concentrations. The TSC spectrum, which consisted of a single, well resolved peak, was found to be dependent on the applied electric field, the heating rate and the dopant concentration. The peak maxima were located between 170K and 250K. The spectra were analyzed within the theoretical framework of Zielinski and Samoc which provided a procedure to extract the hopping activation energy for each concentration. The principle observations of this study are: (1) the TSC peak is unambiguously associated with charge transport, (2) the magnitude of the activation energies were found to be larger than values obtained from isothermal transient photocurrent measurements and (3) the activation energies obtained from analysis of the TSC spectra were found to be dependent on the doping concentration. This last observation is inconsistent with previous isothermal transient photocurrent measurements of doped polymer systems containing DEH.
The activation energy for charge transport in molecularly doped polymer films containing p-diethylaminobenzaldehyde diphenylhydrazone (DEH) is measured before and after systematic UV irradiation. UV exposure has been shown to induce a photochemical reaction of the DEH molecule which dramatically reduces the molecule's capability to transport charge. Presumably, the reduction in mobility is associated with the substantial increase (above 1 eV) in the ionization potential of the DEH molecule after photocyclization. The increase in ionization potential effectively removes the photochemically modified molecule from the primary transport manifold. In previous work, we have demonstrated that systematic UV irradiation of molecularly doped polymer films containing DEH provides a novel approach for diluting the dopant concentration and effectively increasing the intersite separation between 'active' dopant molecules in situ. Furthermore, ab initio calculations suggest that the dipole moment of the photoproduct is significantly lower than that of the unconverted DEH molecule. We exploit this photochemical process to prove the hopping activation energy, (Delta) , in DEH doped polycarbonate films parametric in UV irradiation time. The principal observation of this work is that despite a systematic and substantial elimination of active hopping sites and despite the systematic increase in the population of lower dipole moment neighbors, the hopping activation energy for the DEH/polycarbonate system remains constant.
Hole mobilities in p-diethylaminobenzaldehyde diphenylhydrazone (DEH) doped polycarbonate films are determined using the time-of-flight transient photocurrent technique. Measurements of hole transport parameters are determined over a range of electric fields before and after the samples are deliberately irradiated with UV light. UV irradiation of the hole transport molecule DEH results in the creation of a photoproduct, 1-phenyl-3-(4- diethylamino-1-phenyl)-1, 3-indazole with moderately high efficiency. Once formed, this photoproduct has been shown to act as a barrier to hole conduction. We exploit this photochemical reaction to examine the hole transport properties in a molecularly doped polymer system containing DEH doped polycarbonate. We propose that the increase in concentration of the photoproduct modifies the intrinsic order of the system and provides a unique probe to distinguish between the disorder formalism of Baessler and coworkers and models which propose polaron formation.
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