We demonstrate a novel approach to control β-phase content generated in poly(9,9-dioctylfluorene) (PFO) films. A very small amount of paraffin oil was used as the additive to the PFO solution in toluene. The β-phase fraction in the spin-coated PFO films can be modified from 0% to 20% simply by changing the volume percentage of paraffin oil in the mixed solution. Organic light emitting diodes (OLEDs) and amplified spontaneous emission (ASE) study confirmed low β-phase fraction promise better OLEDs device, while high β-phase fraction benefits ASE performance.

20F1/4F8BT (polystyrene equivalent Mw = 35,920, polydispersity index = 2.1) consisted of 20% 9,9-di(2-methyl)butyl (F1/4) , 30% 9,9-dioctyl substituted fluorine (F8) units and 50% benzothiadiazole (BT) units. The Suzuki arenebisboronic ester/dibromoarene coupling route was used. This method yields statistical copolymers for which there need not be a predetermined sequence of F8/BT and F1/4/BT units but instead where variable length 'blocks' (sequences of repeating F1/4/BT units or F8/BT units) could be presented in the backbone structure. PFO was purchased with typical polystyrene equivalent molecular weight Mw = 150,000-160,000.

One of the challenges toward electrically driven organic lasers is the huge optical loss associated with the contact of electrodes and organic gain medium in device. We demonstrated a significant reduction of the optical loss by using our newly developed conjugated polyelectrolytes (CPE) PPFN+Br− as interlayer between gain medium and electrode. The optically pumped amplified spontaneous emission (ASE) was observed at very low threshold for PFO as optical gain medium and up to 37 nm thick CPE as interlayer in device configuration, c.f., a 5.7-fold ASE threshold reduction from pump energy 150 μJ/cm2 for ITO/PFO to 26.3 μJ/cm2 for ITO/PPFN+Br−/PFO. Furthermore, ASE narrowing displayed at pump energy up to 61.8 μJ/cm2 for device ITO/PEDOT:PSS/PFO/PPFN+Br−/Ag, while no ASE was observed for the reference devices without CPE interlayer at pump energy up to 240 μJ/cm2. The optically pumped lasing operation has also been achieved at threshold up to 45 μJ/cm2 for one-dimensional distributed feedback laser fabricated on ITO etched grating in devices with CPE interlayer, demonstrating a promising device configuration for addressing the challenge of electrically driven organic lasers.