Difluorinated cyclopentadithiophene derivatives for green emitters in organic light-emitting diodes: A theoretical investigation

Two types of difluorinated conjugated systems incorporating bridged cyclopentadithiophene, alternating fumaronitrile and 1,3,4-oxadiazole were designed and theoretically studied by density functional theory and its time-dependent extension with the B3LYP hybrid functional and the 6-311g(d,p) and 6-311++g(d,p) basis sets in tetrahydrofuran. The designed materials are denoted FCFO-1 (>C=O) and FCFO-2 (>C=C-(C N)(2). The electronic band gap ranges from 2.46 eV (FCFO-1) to 2.30 eV (FCFO-2). The difference in optical behaviors may originate from the effect of the bridging group on cyclopentadithiophene derivatives. The materials were examined as green emitters (519-564 nm). Organic light-emitting diodes based on these new fluorescent materials [indium tin oxide anode (100 nm)/N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (hole-injection layer) (150 nm)/FCFO-1 or FCFO-2 (50 nm)/Al cathode (100 nm)], were theoretically designed. The current-voltage characteristics were simulated, and the threshold voltages were estimated to be 3.25 V for FCFO-1 and 2.17 V for FCFO-2. Electric dipole moment calculations show the molecules possess large electric dipole moments (approximately 9.88 D for FCFO-1 and approximately 11 D for FCFO-2) and they thus exhibit excellent nonlinear optical properties.