Synthesis and characterization of novel crosslinkable poly(propylenedioxythiophene) derivative as a buffer layer for organic light-emitting diode applications

A new crosslinkable polythiophene derivative, poly(hexynyl(2)-PDOT-co-heptyl(2)-PDOT), with propylenedioxythiophene moiety containing solubilizing and crosslinkable substitutents as a pendant group were synthesized by oxidative polymerization using FeCl3. The resulting copolymer was soluble in organic solvents and spun cast onto the indium tin oxide-coated glass substrate to create a uniform thin film. After thermal crosslinking and rinsing of the crosslinked thin film using chloroform, the pi-pi transition peak and the absorption edge appeared at almost the same wavelength as that of the untreated copolymer. We fabricated organic light-emitting diodes (OLEDs) in ITO/buffer layer/poly[2-(2'-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV)/LiF/Ca/Al configuration using either poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) or poly(hexynyl(2)-PDOT-co-heptyl(2)-PDOT) as buffer layers. Due to the intrinsic conductivity of poly(hexynyl(2)-PDOT-co-heptyl(2)-PDOT) vs. that of PEDOT:PSS caused decreased OLED performance. The turn-on voltage of the OLED using poly(hexynyl(2)-PDOT-co-heptyl(2)-PDOT) buffer layer was observed at 5 V. The maximum brightness and luminance efficiencies were 2,597 cd/m(2) at 12 V and 0.3 cd/A, respectively.