Blue-green Light-emitting Electrochemical Cells Research Articles

Green and blue–green light-emitting electrochemical cells based on cationic iridium complexes with 2-(4-ethyl-2-pyridyl)-1H-imidazole ancillary ligand

The ionic iridium complexes, [Ir(ppy)2(EP-Imid)]PF6 (Complex 1) and [Ir(dfppy)2(EP-Imid)]PF6 (Complex 2) are used as the light-emitting material for the fabrication of light-emitting electrochemical cells (LECs). These complexes have been synthesized, employing 2-(4-ethyl-2-pyridyl)-1H-imidazole (EP-Imid) as the ancillary ligand, 2-phenylpyridine (ppy) and 2-(2,4-difluorophenyl)pyridine (dfppy) as the cyclometalated ligands, which were characterized by various spectroscopic, photophysical and electrochemical methods. The photoluminescence (PL) emission spectra in acetonitrile solution show blue–green and blue light emission for Complexes 1 and 2 respectively. However, LECs incorporating these complexes resulted in green (522nm) light emission for Complex 1 with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.33, 0.56) and blue–green (500nm) light emission for Complex 2 with the CIE coordinates of (0.24, 0.44). Using Complex 1, a maximum luminance of 1191cdm−2 and current efficiency of 1.0cd A−1 are obtained while that of Complex 2 are 741cdm−2 and 0.88cd A−1 respectively.

Stable blue-green light-emitting electrochemical cells based on a cationic iridium complex with phenylpyrazole as the cyclometalated ligands

A novel blue-green light-emitting cationic iridium complex [Ir(ppz)2pzpy]PF6 (complex 1), where ppz is phenylpyrazole and pzpy is 2-(1-phenyl-1H-pyrazol-3-yl)pyridine, is developed. Its crystal, photophysical and electrochemical properties are characterized through experiments and quantum chemical calculations. The novel complex exhibits enlarged energy gap and blue-shifted emission spectrum at 77K compared to a reference complex, [Ir(ppy)2pzpy]PF6 (complex 2), with phenylpyridine (ppy) as the cyclometalated ligands. Moreover, light-emitting electrochemical cells (LECs) fabricated with complex 1 show both improved color purity with CIE (Commission Internationale de L’Eclairage) coordinates of (0.25, 0.39) and elongated lifespan (t1/2=218min, Etot=0.55mJ) compared to LECs based on complex 2.

Efficient sky-blue and blue-green light-emitting electrochemical cells based on cationic iridium complexes using 1,2,4-triazole-pyridine as the ancillary ligand with cyanogen group in alkyl chain

Sky-blue and blue-green light-emitting cationic iridium (III) complexes with triazole-pyridine as the ancillary ligand, with formula [Ir(dfppy) 2(trzpy-cn)](PF 6) ( C1), [Ir(mppy) 2(trzpy-cn)](PF 6) ( C2), [Ir(dfppy) 2(trzpy-hx)](PF 6) ( C3) and [Ir(mppy) 2(trzpy-hx)](PF 6) ( C4) were synthesized, where mppy, trzpy-cn, trzpy-hx are 2-methylphenylpyridine, 2,2-dimethyl-6-(3-methyl-5-(pyridin-2-yl)-4H-1,2,4-triazol-4-yl)hexanenitrile, and 2-(5-methyl-4-octyl-4H-1,2,4-triazol-3-yl)pyridine, respectively. C1/C3 and C2/C4 emit sky-blue and blue-green light peaked at 458 and 484 nm, respectively. Single layer light-emitting electrochemical cells (LECs) based on C1 and C2 show luminous efficiencies (LE) of 2.6 and 6.8 cd/A (corresponding to an external quantum efficiency (EQE) of 2.7 and 2.8%), with CIE coordinates of (0.21, 0.33) and (0.31, 0.53), respectively. As compared with the devices from C3 and C4, the device efficiencies based on C1 and C2 are found to be significantly enhanced by 10 times and 4 times, respectively, owing to the incorporation of cyanogen group in the alkyl chain. The results demonstrate that using the triazole-pyridine as ancillary ligand with a cyanogen group in the side alkyl chain are effective strategies to tune the emission into blue region and to improve the efficiency of the cationic iridium complex.

Enhanced stability of blue-green light-emitting electrochemical cells based on a cationic iridium complex with 2-(1-phenyl-1H-pyrazol-3-yl)pyridine as the ancillary ligand

A new cationic iridium complex has been developed with 2-(1-phenyl-1H-pyrazol-3-yl)pyridine as the ancillary ligand, which bears a pendant protective phenyl ring within the molecule; blue-green light-emitting electrochemical cells (LECs) based on the complex show dramatically enhanced stability compared to the LEC based on a similar complex without pendant phenyl rings.

Photo- and electroluminescence from hyperbranched phenylene vinylenes

We report photophysical and electroluminescence (EL) properties of three novel hyperbranched phenylene vinylenes (HPVs) with benzene as the core, 2,5-dimethyloxyl substituted phenylene vinylene as the connecting unit, and benzene, dimethylaniline or pyridine as the terminal group, respectively. The absorption maxima of the HPVs blue-shift from solution to the films due to their twisted molecular conformation. The emission spectra in the films red-shift compared to those in solution, which may originate from interchain excimer species. The thin films of the HPVs exhibit pretty high photoluminescence (PL) quantum yields around 50.5–87.8%. Blue–green light-emitting electrochemical cells (LECs) based on the HPVs have been demonstrated with turn-on voltages of 2.5–2.6 V. The ac impedance measurements indicate the operation of the LECs corresponds to an electrochemical doping model.

Polymer light-emitting electrochemical cell based on a novel block copolymer containing tri(ethylene oxide) spacers

We reported the optical properties of a novel block copolymer composed of chromophores, 1,5-di(3,5-dimethloxystyrylene)naphthylene, and spacers, tri(ethylene oxide). Intra- and interchain excitations due to aggregation are responsible for the changes in photoluminescence (PL) spectra when going from the solution to the film. Blue-green light-emitting electrochemical cells (LECs) based on this copolymer were demonstrated with the turn-on voltage of 2.5 V, the maximum electroluminescence (EL) efficiency of 4.2 cd/A, and the d.c. response time of less than 5 s, indicating strongly that an improved morphology of the active layer is formed because of the affinity of TEO with PEO. A.c. impedance measurements suggest these devices operate with an electrochemical doping model.

Efficient blue-green and white light-emitting electrochemical cells based on poly[9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl

Efficient blue-green polymer light-emitting electrochemical cells, based on poly[9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl] (BDOH-PF), have been demonstrated. BDOH-PF has a high photoluminescent quantum efficiency of 73% photon/photon in solid films. The polyether-type side groups of this polymer are built in to promote the ionic conductivity necessary for the operation of light-emitting electrochemical cells (LECs). Blue-green LECs using a blend of BDOH-PF and lithium triflate as the emissive layer, ITO and Al as the electrodes, achieve an external quantum efficiency of 4% photons/electron and a luminous efficiency of 12 lm/W at 3.1 V. The brightness reaches ∼200 cd/m2 at 3.1 V, and exceeds 1000 cd/m2 at 3.5 V. When additional poly(ethylene oxide) (PEO) is blended into the emissive layer, phase separation between PEO and BDOH-PF occurs, and the emission becomes white. These white emitting LECs reach an external quantum efficiency of 2.4% photons/electron and 400 cd/m2 brightness at a bias of 4 V.