Synthesis and characterization of thermally cross-linkable hole injection polymer based on poly(10-alkylphenothiazine) for polymer light-emitting diode

A new thermally cross-linkable hole injecting/transporting polymer, poly{[10-hexyl-10H-phenothiazine-3,7-diyl]-alt-[10-(6-hexyloxy-4-(2-methyl-acrylic acid benzyl ester))-10H-phenothiazine-3,7-diyl]} (PTMMA), was synthesized. From the HOMO energy level of thermally cured PTMMA (−4.95 eV), the polymer is a very good hole injection material. From the spectroscopic and electrochemical data, thermally cured PTMMA (c-PTMMA) film is very safe for the successive layer coating by the solution process. Bi-layer structured polymer light-emitting diode with c-PTMMA as a hole injection layer (HIL) and green-emitting copolymer (PF9B) as a emissive layer was fabricated successfully. The maximum efficiency and brightness of the device with c-PTMMA as a HIL (device II) are 0.34 cd/A and 854 cd/m2, respectively, which are superior to those of the device without c-PTMMA (device I, 0.039 cd/A and 228 cd/m2, respectively). Also, the turn-on electric field of device II (0.89 MV/cm) is much lower compared to that of device I (1.92 MV/cm). From the results, thermally cured PTMMA has not only good hole injection property but also good solvent resistant property for the successive layer coating.