Synthesis of materials for organic light-emitting diodes in near IR region

Near-infrared light-emitting organic oligomers and polymers have shown great prospects in the development of efficient organic light emitting diodes (NIR OLEDs). Potential applications for NIR OLEDs are phototherapy devices, telecommunications, night-vision displays and defense and security electronics – among others. NIR light-emitting oligomers and polymers can be prepared from electron rich donor (D) and electron poor acceptor (A) alternating units. Further, the NIR emitting DAD segments can be chemically incorporated into host polymer backbones by copolymerization or physically blend with host polymers to form host-guest blend systems. In both cases, only a small amount of NIR emitting DAD segment is needed so that efficient energy transfer or charge transfer can occur.

Schematic structure and energy level diagram of typical OLED device are shown in figures 1 and 2. The OLED device is constructed on a glass substrate, which is first spin-coated with indium tin oxide (ITO) film forming a transparent cathode, followed by hole injection layer such as PEDOT:PSS. The hole injection layer helps the recombination of electron holes and electrons, and thus formation of the exciton. The NIR light-emitting polymer film is spin-coated onto the PEDOT:PSS layer. Finally, anodes of calcium and aluminum are evaporated on top of the polymer layer.

In this project, we focus on the synthesis of novel NIR emitting polymer materials containing low bandgap (Eg) DAD segments with strong electron acceptors and strong electron donors. Hence the HOMO and LUMO of the DAD segment fall between the HOMO and LUMO of the host polymer. When even a small amount DAD segment is incorporated into the host polymer backbone, e.g. poly-BDT-BTI shown in figure 3, the copolymer adapts LUMO level close to that of DAD but the HOMO remains close to that of the host. This can be attributed to the fact that the HOMO is spread over the polymer backbone, whereas the LUMO is located on the DAD sites. In OLED device the charges migrate through the polymer backbone onto the low energy DAD sites, leading into exciton formation on these sites and emission solely in the NIR region.

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Published: Tue 10 Feb 2015. Modified: Fri 06 Mar 2015