Synthesis and electroluminescent properties of poly(p-phenylenevinylene)s with 3′,3′-diheptyl-3,4-propylenedioxythiophene pendant group for light-emitting diode applications

Using the Gilch polymerization method, we synthesized a new series of green electroluminescent polymer, poly[1,4-{2-(3,3'-diheptyl-3,4-propylenedioxythiophen-2-yl)}phenylenevinylene], poly(PDOT-PV), which is a series of fully conjugated poly(p-phenylenevinylene) derivatives with a propylenedioxythiophene (PDOT) moiety as a side-chain. We also synthesized copolymers, poly(PDOT-PV-co-m-SiPhPV), of poly(PDOT-PV) with poly[2-(3-dimethyldodecylsilyphenyl)-1,4-phenylenevinylene], poly(m-SiPhPV), segments. The resulting polymers were highly soluble in common organic solvents and could be easily spin-coated onto an indium-tin oxide coated glass substrate to obtain high quality optical thin films. The weight-average molecular weight (Mw) and polydispersity of poly(PDOT-PV) were 22.0 x 10(4) and 5.3, respectively, and those of poly(PDOT-PV-co-m-SiPhPV) were in the range of (23.2-36.7) x 10(4) and 5.0-5.8, respectively. The stability of the resulting polymers is adequate for the fabrication of devices, and they provide longevity to devices because they have high glass transition temperatures (Tg). We fabricated polymer light-emitting diodes (PLEDs) in ITO/PEDOT/light-emitting polymer/cathode configurations using either double-layer LiF/Al or triple-layer Alq(3)/LiF/Al cathode structures. For PLEDs containing poly(PDOT-PV) and poly(PDOT-PV-co-m-SiPhPV), the performance was highest using triple-layer cathodes. The turn-on voltages of PDOT-based light-emitting polymers were in the range of 6.0-9.0 V, and the maximum brightness and luminance efficiency were 5127 cd/m(2) at 18 V and 3.75 cd/A at 9 V.