Inkjet printing (IJP) technology is a popular technology for desktop publishing. Since some of the conducting (or conjugated) polymers are solution processable, IJP technology becomes an ideal method for printing polymer light-emitting diodes with high resolution. Unfortunately, the polymer film printed from an inkjet printer usually consists of pin-holes, and this intrinsic character makes it unsuitable for fabricating high quality polymer electronic devices, particularly for devices in the sandwich structure. In this letter, we submit a hybrid structure, which consists of an inkjet printed layer in conjunction with another uniform spin coated polymer layer, as an alternative to the regular inkjet printed structure. The uniform layer serves as a buffer layer to seal the pin-holes and the IJP layer is the layer consisting of the desired pattern, for example the red–green–blue dots for a multicolor display. To demonstrate, we applied this hybrid technology to fabricate efficient and large area polymer light-emitting logos. The use of this concept represents a whole new technology of fabricating polymer electronic devices with lateral patterning capability.
Organic light-emitting diodes (OLEDs) represent a novel approach for fabricating high quality, multicolor, electroluminescent displays. [1] Traditionally the fabrication of OLEDs has been achieved by the thermal sublimation of organic materials in an ultra-high vacuum environment onto transparent substrates, usually glass, coated with indium tin oxide (ITO). This is a rather time consuming process and the patterning of fine multicolor pixels is also quite complicated. The application of inkjet printing (IJP) technology to polymer electroluminescent devices has attracted considerable attention due to its low cost, large area processing, and multicolor addressing capabilities. [2±6] However, the IJP technology is afflicted with some serious problems such as uneven surface roughness and the intrinsic pinhole nature of the deposited material, which impede such applications. [2] Recently, these drawbacks have been overcome by hybrid inkjet printing technology (HIJP). [3±6] HIJP technology combines a pinhole-free polymer buffer layer and a regular inkjet-printed polymer or organic layer which allows the patterning of high quality polymer lightemitting devices. On proper selection of solvents the buffer layer can serve as an ink-absorbing layer and effectively fix the printed materials. The small diameter of the inkjet nozzles permits printing of minute amounts of materials such as dopants to give rise to displays with high resolution. Multicolor emission can be realized from an efficient energy transfer from an appropriate buffer layer (a semiconducting polymer layer) with a wide bandgap to the inkjet-printed materials (dopants) with smaller bandgaps than the buffer layer. Alternatively, the buffer layer can be a continuous polymer layer insoluble in the solvents used by the IJP droplets. In this case it seals the pinholes and can also function as the hole transport layer (HTL). The later case is ideal for the fabrication of organic LEDs since the most efficient OLEDs comprise a bilayer structure of a HTL and an emissive layer. [7] In this communication we present a successful demonstration of controllable patterning of red-green-blue (R-G-B) multicolor, organic, light-emitting pixels using this HIJP technique. In this demonstration the polymer buffer layer used is the blue-emission, semiconducting polymer, poly-9vinylcarbazole (PVK), prepared by the spin-casting technique. The inkjet-printed dopants are tris(4-methyl-8quinolinolato)Al(III) (Almq 3 ) [7] and DCM (4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran) which were inkjet-printed on the PVK buffer layer. Based on this principle, multicolor organic LEDs comprising bilayer structures of PVK/DCM (orange-red-emission) and PVK/Almq 3 (green-blue-emission) were fabricated with the blue-emission PVK buffer layer serving as the holetransport layer.The details of the device preparation are given in the experimental section. Due to the inability of commercially available inkjet printers to resist the corrosive action of regular solvents, methanol was ...
A hybrid inkjet printing (HIJP) technology, which combines a pin-hole free polymer buffer layer and an inkjet printed polymer layer, allows the patterning of high quality polymer light-emitting devices. In this letter, we present a successful demonstration of controllable patterning of dual-color polymer light-emitting pixels using this HIJP technique. In this demonstration, the polymer buffer layer is a wide band gap, blue emitting semiconducting polymer prepared by the spin-casting technique. The inkjet printed layer is a red-orange semiconducting polymer which was printed onto the buffer layer. When a proper solvent was selected, the printed polymer diffused into the buffer layer and efficient energy transfer took place generating a red-orange photoluminescence and electroluminescence from the inkjet printed sites. Based on this principle, blue and orange-red dual-color polymer light-emitting pixels were fabricated on the same substrate. The use of this concept represents an entirely new technology for fabricating polymer multicolor displays with high-resolution, lateral patterning capability.
Conjugated polymers are often treated as semiconductors with low doping concentrations. Unlike the traditional semiconductors which have a high density of surface states (mainly due to the dangling bonds), the nature of the metal/polymer interface, including barrier height and charge injection efficiency, is quite sensitive to the work function of the contact metal. In this article, we present evidence to show that the pinning of the surface Fermi level effect commonly observed at the silicon/metal interface can also be observed at the metal/polymer interface. It is achieved by controlling the doping level at the metal/polymer [poly(2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) or MEH-PPV] interface. ITO/MEH-PPV/Al devices doped with 2 Å of calcium on the cathode side of the interfacial layer have the same device performance as the ITO/MEH-PPV/Ca devices. The heavily n-doped region pins the surface energy level, hence the polymer interface at the cathode side is no longer sensitive to the work function of the overcoated metal. It is believed that either the midgap bipolaron energy states created by the dopants or the sharp band bending at the interface is responsible for facilitating the electron injection. On the other hand, a p-doped region at the anode side, obtained by using a thin layer of an acid at the interface, pins the surface energy level and makes the contact insensitive to the work function of the anode. Therefore, an efficient polymer light-emitting diode with the p-i-n structure has been demonstrated without the matching of the work function of the metal electrodes.
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