Solid-state polymer light-emitting electrochemical cells have been fabricated using thin films of blends of poly(1,4-phenylenevinylene) and poly(ethylene oxide) complexed with lithium trifluoromethanesulfonate. The cells contain three layers:  the polymer film (as the emissive layer) and indium-tin oxide and aluminum films as the two contact electrodes. When externally biased, the conjugated polymers are p-doped and n-doped on opposite sides of the polymer layer, and a light-emitting p-n junction is formed in between. The admixed polymer electrolyte provides the counterions and the ionic conductivity necessary for doping. The p-n junction is dynamic and reversible, with an internal built-in potential close to the band gap of the redox-active conjugated polymer (2.4 eV for PPV). Green light emitted from the p-n junction was observed with a turn-on voltage of about 2.4 V. The devices reached 8 cd/m(2) at 3 V and 100 cd/m(2) at 4 V, with an external quantum efficiency of 0.3-0.4% photons/electron. The response speed of these cells was around 1 s, depending on the diffusion of ions. Once the light-emitting junction had been formed, the subsequent operation had fast response (microsecond scale or faster) and was no longer diffusion-controlled.

Semiconducting (conjugated) polymers have several properties that are advantageous for photonic applications: they have high fluorescence efficiencies (>60 %), emit at wavelengths that span the entire visible spectrum, are mechanically flexible, and can be deposited as uniform thin films by casting from solution. Since the fabrication of the first polymer light-emitting diode (LED) in 1990, [1] there has been extensive research on polymer LEDs and many improvements have been made. [2±4] Single-color displays fabricated with arrays of polymer LEDs will soon be commercially available. Full color displays will require pure red, green, and blue emission.Obtaining pure emission colors from conjugated polymers or small organic molecules is difficult because their emission spectra typically have a full width at half maximum (FWHM) of 50±200 nm. Efficient, pure red-emitting polymer LEDs are particularly hard to make because the human eye is more sensitive to orange emission than red; if the spectrum falls even slightly in the orange, the perceived color is ªorangish redº. Red LEDs can be made by filtering out orange emission or by using polymers or dyes whose emission starts in the red and extends into the infrared, but these LEDs are inefficient because only part of their emission is useful. In contrast to organic chromophores, rare earth ions have very sharp emission spectra (FWHM < 4 nm). [5] In this paper we show that pure red emission can be achieved in polymer LEDs by transferring energy from blue-emitting conjugated polymers to europium complexes. Similar methods have been used to make red LEDs from small organic molecules. [6±10] We show that blends of Eu complexes in poly[2-(6¢-cyano-6¢-methyl-heptyloxy)-1,4-phenylene] (CN-PPP) have an emission spectral linewidth (FWHM) of only 3.5 nm, a photoluminescence (PL) efficiency of 27 %, and an electroluminescence (EL) efficiency of 1.1 %. These blends could be useful as a source of pure red light for full color displays or for photonic devices that require monochromatic light.To incorporate Eu 3+ into conjugated polymers, we synthesized a family of soluble Eu complexes with b-diketonate ligands, codissolved the complexes and polymers in a solvent, and cast films. We chose b-diketonate ligands because they sensitize Eu 3+ emission. [5,11] The sensitization process is as follows: the ligand absorbs energy, undergoes intersystem crossing into a triplet state, and then transfers its energy to the Eu 3+ ion. [12,13] Thus, the first design rule for making fluorescent Eu complexes is that the triplet level of the ligand must be higher in energy than the emissive level ( 5 D 0 ) of Eu 3+ . A second design rule is imposed by the need for energy transfer from the polymer to the rare earth complex. In order to transfer energy from a conjugated polymer to the ligands of a Eu complex by dipole coupling (Förster transfer), the emission spectrum of the polymer and the absorption spectrum of the ligand must overlap. [14,15] Since the ligands whose triplet level lies above the 5 D 0...

A novel series of semiconducting conjugated copolymers, derived from alkyl-substituted fluorene, 4,7-diselenophen-2‘-yl-2,1,3-benzothiadiazole (SeBT), and 4,7-diselenophen-2‘-yl-2,1,3-benzoselenadiazole (SeBSe), was synthesized by a palladium-catalyzed Suzuki coupling reaction with various feed ratios. The optical band gap of copolymers is very low, 1.87 eV for SeBT and 1.77 eV for SeBSe. The efficient fast energy transfer from fluorene segments to narrow-band-gap sites was observed. The emission of photoluminescence and electroluminescence is dominated by narrow-band-gap species and peaked at 670−790 nm, in the range from deep-red to near-infrared (NIR). The external electroluminescent (EL) quantum efficiencies reached 1.1% and 0.3% for devices from these two types of copolymers, respectively. Bulk−heterojunction polymer photovoltaic cells (PPVCs) made from composite thin film of the copolymer 9,9-dioctylfluorene and SeBT (PFO−SeBT) in blend with fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as an active layer show promising performances. The energy conversion efficiency (ECE) is up to 1% under AM1.5 solar simulator (78.2 mW/cm2). The spectral response is extended up to 675 and 750 nm for PPVCs from PFO−SeBT and PFO−SeBSe, respectively.

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