Threshold Reduction in Polymer Lasers Based on Poly(9,9-dioctylfluorene) with Statistical Binaphthyl Units

We introduce novel statistical copolymers of poly(9,9-dioctylfluorene), PFO, which contain various concentrations of 6,6′-(2,2′-octyloxy-1,1′-binaphthyl) spacer groups. We demonstrate that, owing to the large dihedral angle (> 60°) between neighboring naphthalene units, we could hinder the formation of the highly ordered β-phase in thin films of the copolymers. In low-temperature photoluminescence measurements, the typical signature of the PFO β-phase at 442 nm is no longer observed for copolymers with a binaphthyl concentration of about 12 %. Moreover, the optical properties of the copolymers resembled those of the glassy α-phase PFO. Second-order distributed feedback (DFB) lasers based on thin films of the homopolymer PFO showed a minimum lasing threshold of 11.7 μJ cm–2 (λmax = 452 nm, excitation at λ = 337 nm with 500 ps pulses). With increasing binaphthyl concentration in the copolymer backbone, the lasing threshold steadily decreased to 3 μJ cm–2 for a binaphthyl concentration of about 12 %. Therefore, our novel copolymers provide a vast improvement for PFO-based optoelectronics.