Novel Green-Light-Emitting Polymers Based on Cyclopenta[def]phenanthrene

Abstract

New electroluminescent copolymers with BT units in the PCPP backbone, poly(4,4-bis(2-ethylhexyl)-4H-cyclopenta[def]phenanthrene)-co-poly(2,1,3-benzothiadiazole) (PCPPBTs), have been synthesized by the Suzuki coupling reaction. The BT units were introduced on the PCPP backbone to affect a red-shifted color change and to increase the electron affinities of the copolymers. The PCPPBTs exhibited absorption spectra with maximum peaks at 330−431 nm in THF solution and at 337−445 nm in the solid film state. In the PL spectra were two emission peaks at around 400 and 530 nm in solution and at 510−535 nm in thin film. The PL characteristics of these copolymers could possibly be influenced by two competing effects: Förster energy transfer and the intramolecular charge transfer. The HOMO energy levels of the copolymers were approximately −5.89 to −5.96 eV, and the LUMO energy levels were about −3.33 to −3.50 eV. The polymer LEDs (ITO/PEDOT/polymer/Ca:Al) of PCPPBTs showed emissions with maximum peaks at 508−528 nm. By randomly copolymerizing the BT into the PCPP, the PL efficiency was improved. Among all the devices fabricated, the best device was the one with PCPPBT10, which showed the highest luminous efficiency of 1.25 cd/A and the highest brightness of 1170 cd/m2. The present study suggests that the introduction of BT units in PCPP can enhance the device performance to result in stable PL and EL spectra with high current density and brightness.