Electron‐Beam‐Induced Crosslinking of Electroluminescent Polymers for the Production of Multi‐Color Patterned Devices

Abstract

Organic light-emitting diodes are considered for various applications ranging from lighting to full-color emissive displays. In a very basic configuration such a diode is obtained by sandwiching an electroluminescent polymer between two electrodes.[1] Upon application of a voltage, light is generated as a result of the recombination of holes and electrons injected into the polymeric layer. In the case of low-molecular-weight organic molecules, various layers with various functions are used in order to obtain highly efficient devices.[2] These layers are chosen to have properties such as hole or electron transport, hole or electron blockage and high emission. The layers can be stacked by successive vapor deposition steps. As opposed to low-molecular-weight organic molecules, polymers layers can not be produced by physical vapor deposition. However, they can be easily processed from solutions. In order to obtain thin layers of polymers various techniques such as spin-coating, bar-coating, [3] and inkjet printing [4,5] have been suggested. However, the application of various layers on top of each other remains a problem as uncommon solvents need to be used in the production of successive layers. Furthermore, patterning such polymeric layers for the production of full-color displays is an issue. Currently inkjet printing is widely used in the production of full-color displays emitting red, blue, and green (RGB) light. [6] This technique is different than the technique of crosslinking which is used in established display technologies such as cathode ray tubes and liquid-crystal displays. In the case of liquid-crystal displays, [7] color filters are produced by successive steps of application of a colored layer, followed by crosslinking induced by illumination with ultraviolet (UV) light projected through a mask, and subsequent removal of the uncrosslinked regions by solvent-developing. This procedure is repeated for each color in order to obtain a color filter array. In the same way, full-color cathode ray tubes are also obtained by applying three different types of phosphor assisted by a polymeric binder which can also be crosslinked using UV light. In order to induce crosslinking by UV light it is necessary to have molecules with reactive groups such as acrylate, epoxy etc. Electroluminescent molecules with reactive groups have already been described. [8] However, these groups can have a negative effect on the luminescent properties of the polymer, and the photoinitiators which are needed for UV-initiated polymerization usually act as luminescence quenchers. Furthermore, UV radiation is known to induce photobleaching in such polymers. In this article, we will demonstrate the use of an alternative technique of electron-beam-induced crosslinking of two different types of electroluminescent poly(phenylene vinylene) (PPV). Electroluminescent polymers usually have flexible aliphatic sidegroups which are necessary to increase the solubility of the polymer in common organic solvents. When alkanes are subjected to high-ener...

UV‐Induced Modification of Conjugated Polymers Using Gaseous Organosilanes

“…Production of pixilated full color devices is possibly the biggest challenge in this field. Different methods such as electron beam2 or UV‐induced3 crosslinking for lithographic inscription of structures were developed. Nowadays, the preferred method for the production of multicolored PLEDs is inkjet printing 4.…”

Patterning of Conjugated Polymers for Organic Optoelectronic Devices

“…In the doped state, the conductivity of the e‐beam‐patterned polymer (measured by a nanoprobe) was found to be comparable to that of the unpatterned polymer (evaluated with a macroscopic probe), which indicated no major destruction to the conjugated system. In a similar manner, Hikmet et al121 patterned electroluminescent functional PPV by e‐beam‐induced crosslinking of its aliphatic side groups. It was found that e‐beam irradiation did not exert significant effects on the quantum efficiency of the electroluminescent device; therefore, this work has provided a possible method to build multicolor OLED devices.…”

Polyfluorene‐Based Blue‐Emitting Materials

“…To circumvent this problem, crosslinking techniques are believed to be a promising approach. [31][32][33][34] Initiated with ultraviolet light, heat, electron beams, etc., soluble polymers can be crosslinked into insoluble network structures. In order to utilize this technique, we introduced photocrosslinkable groups into the side chains at C-9 positions of fluorenes and synthesized blue-emitting hyperbranched PFs P10 and P11.…”