Ruidong Xia

Simultaneous optimization of charge-carrier mobility and optical gain in semiconducting polymer films.

The combination of efficient light emission and high charge-carrier mobility has thus far proved elusive for polymer semiconductors, with high mobility typically achieved by cofacial pi-electron system to pi-electron system interactions that quench exciton luminescence. We report a new strategy, comprising the introduction of a limited number of more effective hopping sites between otherwise relatively isolated, and thus highly luminescent, polyfluorene chains. Our approach results in polymer films with large mobility (mu approximately 3-6 x 10(-2) cm2 V-1 s-1) and simultaneously excellent light-emission characteristics. These materials are expected to be of interest for light-emitting transistors, light-emitting diode sources for optical communications and may offer renewed hope for electrically pumped laser action. In the last context, optically pumped distributed feedback lasers comprising one-dimensional etched silica grating structures coated with polymer have state-of-the-art excitation thresholds (as low as 30 W cm(-2) (0.1 nJ per pulse or 0.3 microJ cm-2) for 10 Hz, 12 ns, 390 nm excitation) and slope efficiencies (up to 11%).

Fluorene-based polymer gain media for solid-state laser emission across the full visible spectrum

We report a study of the optical gain properties of three polyfluorenes with chemically tuned emission characteristics that span 400–800 nm. We demonstrate low threshold light amplification in the blue, green, and red spectral ranges via amplified spontaneous emission (ASE) in optically pumped planar asymmetric waveguides. Gain and loss measurements at the peak ASE wavelengths show large net gains, 22⩽g⩽66 cm−1, and low losses, 15⩾α⩾3 cm−1. Our findings establish fluorene-based polymers as an attractive family of materials for use in tuneable solid-state lasers that emit at wavelengths across the whole visible spectrum.

Blue, surface-emitting, distributed feedback polyfluorene lasers

We report the fabrication of optically-pumped solid-state distributed feedback lasers utilizing two blue-light-emitting semiconducting polyfluorenes as gain media. The lasers were readily fabricated by solution deposition of thin polymer films on top of gratings etched into fused silica substrates. A compact Nd:YVO4 microchip laser was used as the pump source for the two polymers studied, and lasing was achieved at 455 and 465 nm. Low threshold energies, ⩾4 nJ per pulse, were obtained. The emission characteristics of the lasers are described along with the results of additional experiments that investigate in more detail the effect of the grating microstructure on polymer light emission.

Two-dimensional distributed feedback lasers using a broadband, red polyfluorene gain medium

We report the fabrication of widely tuneable (627–702nm) optically pumped two-dimensional distributed feedback polymer lasers that utilize a red-emission fluorene copolymer as the active gain medium. The lasers exhibit efficient, low threshold operation and emit highly directional output beams as a result of the enhanced two-dimensional photonic confinement provided by the employed resonator. Their emission and operating characteristics are described in detail. We demonstrate that the very wide spectral range (Δλ⩾75nm) over which these lasers can be systematically tuned is in very good agreement with theoretical predictions based on a simple waveguide model. In addition, we show that the lasers have long operating lifetimes τ1∕2⩾2×107 pulses and we discuss the impact that degradation has on the laser output characteristics.

Polyfluorene distributed feedback lasers operating in the green-yellow spectral region

We report solid-state, optically pumped poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), distributed feedback lasers that operate in the green-yellow spectral region, previously unaddressed with conjugated polymer gain media. The lasers were fabricated by spin coating F8BT (the gain medium) onto one-dimensional gratings patterned in silica substrates. The emission wavelength could be selected to lie within the range from 558 to 591 nm by controlling the F8BT film thickness and grating periodicity. The minimum lasing threshold of 6.5 nJ per pump pulse (10 ns, 10 Hz, 450 nm) was achieved for a 350 nm spatial period grating with a 180 nm thickness F8BT film. The corresponding emission wavelength was 573 nm and the laser slope efficiency was a relatively high 3%.

Optical gain at 650 nm from a polymer waveguide with dye-doped cladding

Signal amplification at the polymer optical fiber low-loss window of 650 nm is reported in an SU8 rib waveguide coated with Rhodamine-640 doped poly(methyl methacrylate). A signal beam is end-fired into the facet of a 7×100μm waveguide and amplified by top pumping of the 2‐μm-thick cladding region with a pulsed pump source focused into a 9-mm-long stripe. A gain of 14dB and a minimum signal-to-noise ratio of around 2 dB are achieved in a 15-mm-long device with a low threshold pump intensity of 0.25μJ∕mm2, which is an order of magnitude lower than previously reported.

Deep-blue light emitting triazatruxene core/oligo-fluorene branch dendrimers for electroluminescence and optical gain applications

We report conjugated dendrimers that can be used to fabricate efficient (≤1.0 lm W−1 at ~100 cd m−2) deep-blue emission (1931 CIE (x, y) = (0.15, 0.10)) and high luminance (≤6000 cd m−2) organic light emitting diodes (OLEDs). Slab waveguide structures of these materials show low amplified spontaneous emission (ASE) thresholds (≥50 nJ for 390 nm, 10 ns, 10 Hz, pulsed excitation). In addition, we report OLEDs based on binary blends of the dendrimers with the green light emitting polymer poly(9, 9-dioctylfluorene-co-benzothiadiazole): these have low turn on voltages (≥2.2 V) and produce high efficiency (18.2 lm W−1 at ~100 cd m−2) green electroluminescence.

Blue polymer optical fiber amplifiers based on conjugated fluorene oligomers

We fabricated polymer optical fiber (POF) amplifiers operating between 440 and 480 nm, using POFs doped with a series of fluorene oligomers, including tri-, penta-(9,9-dioctylfluorene) and hepta-(9,9-dihexylfluorene). The gain properties of pure oligofluorene films demonstrate gain coefficients on the order of 250 dB/cm and amplified spontaneous emission thresholds between 1 and 8 μJ c m-2 , significantly lower than other fluorene gain media. The optical and morphological characteristics of PMMA thin films doped with the oligomers demonstrate that the oligomers are largely isolated within the PMMA. The optical and gain properties of POFs produced using an adapted preform-drawing technique and doped with the oligofluorenes provide gain values on the order of 0.07 dB for 2 mm of doped POF. The oligofluorenes are largely isolated within the POFs, paving the way for all optical gain-switching.

Well-Defined Star-Shaped Conjugated Macroelectrolytes as Efficient Electron-Collecting Interlayer for Inverted Polymer Solar Cells

A star-shaped monodisperse conjugated macroelectrolyte grafted with cationic side chains, TrNBr, was designed, synthesized, and utilized as efficient electron-collecting cathode interlayers for inverted polymer solar cells. A neutral one composed of identical star-shaped conjugated backbone, TrOH, was also investigated for comparison. The surface properties and the function as interfacial layers on modulating the work function of bottom electrode (indium tin oxide) were systematically studied. Both interfacial electron-selective materials show strongly thickness-dependent performance for inverted polymer solar cells, and the best performance could be achieved via optimizing the thickness with 2.4 nm of TrNBr and 8.7 nm of TrOH. Parallel investigations of optimized TrNBr and TrOH interlayer in inverted architecture with active blend layer of poly(3-hexylthiophene):indene-C60 bisadduct (P3HT:ICBA) demonstrated a remarkable power conversion efficiency (PCE) enhancement (PCE of 4.88% for TrNBr and 4.74% for TrOH) in comparison with those of conventional noninverted devices using Ca/Al cathodes (3.94%) and inverted devices with sol-gel ZnO buffer layer (4.21%). In addition, the inverted devices using the TrNBr and TrOH interlayer exhibited improved device stability in contrast to conventional noninverted devices using Ca/Al cathodes.

Rib waveguide dye-doped polymer amplifier with up to 26dB optical gain at 625nm

We report optical signal amplification in a solid-state dye-doped polymer with a rib waveguide structure. A 625nm pulsed signal and a collinear 575nm pump are coupled into a 1μm×120μm poly(methyl methacrylate) waveguide doped with 1% by weight Rhodamine 640 dye. Depending on the signal intensity, a maximum optical gain in the 21–26dB range is obtained from a 1.2-cm-long device, accompanied by a signal-to-noise ratio in the 9–16dB range.