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.
