fabrication-white-light-emitting-electrochemical-cells-with-stable

Fabrication of White Light-emitting Electrochemical Cells with Stable Emission from Exciplexes

The authors present an approach for fabricating stable white light emission from polymer light-emitting electrochemical cells (PLECs) having an active ...

JX Nippon Oil & Energy Corporation

The light harvesting efficiency of dye-sensitized photoelectrodes was enhanced by coupling a TiO(2) photonic crystal layer to a conventional film of TiO(2) nanoparticles. In addition to acting as a dielectric mirror, the inverse opal photonic crystal caused a significant change in dye absorbance which depended on the position of the stop band. Absorbance was suppressed at wavelengths shorter than the stop band maximum and was enhanced at longer wavelengths. This effect arises from the slow group velocity of light in the vicinity of the stop band, and the consequent localization of light intensity in the voids (to the blue) or in the dye-sensitized TiO(2) (to the red) portions of the photonic crystal. By coupling a photonic crystal to a film of TiO(2) nanoparticles, the short circuit photocurrent efficiency across the visible spectrum (400-750 nm) could be increased by about 26%, relative to an ordinary dye-sensitized nanocrystalline TiO(2) photoelectrode.

Standing wave enhancement of red absorbance and photocurrent in dye-sensitized titanium dioxide photoelectrodes coupled to photonic crystals.

Manipulation of light is in strong demand in information technologies. Among the wide range of linear and nonlinear optical devices that have been used, growing attention has been paid to photonic crystals that possess a periodic modulation of dielectric function. Among many photonic bandgap (PBG) structures, liquid crystals with periodic structures are very attractive as self-assembled photonic crystals, leading to optical devices such as dye lasers. Here we report a new hetero-PBG structure consisting of an anisotropic nematic layer sandwiched between two cholesteric liquid-crystal layers with different helical pitches. We optically visualized the dispersion relation of this structure, displaying the optical diode performance: that is, the non-reciprocal transmission of circular polarized light at the photonic-bandgap regions. Transmittance spectra with circularly polarized light also reveal the diode performance, which is well simulated in calculations that include an electro-tunable diode effect. Lasing action was also confirmed to show the diode effect with a particular directionality.

Electro-tunable optical diode based on photonic bandgap liquid-crystal heterojunctions.

Photoinduced circular anisotropy has been demonstrated in thin films of a main-chain polymeric liquid crystal (PLC) system doped with photochromic W-shaped molecules containing two azobenzene groups by irradiating with circularly polarized light (CPL). Reversible photoinduced circular dichroism (CD) was observed with sign relevant to the handedness of the CPL. The experimentally observed CD spectra were analyzed using the DeVoe polarizability model associated with the coupled oscillator method. We also propose a mechanism for the photoinduced circular anisotropy in our system; nucleating the W-shaped molecules with preferential twisted conformation by CPL irradiation, and triggering the local formation of a chiral structure in the W-shaped-molecule-doped main-chain PLC medium.

Photoinduced circular anisotropy in a photochromic W-shaped-molecule-doped polymeric liquid crystal film.

A cholesteric liquid crystal (CLC) is a self-assembled photonic crystal formed by rodlike molecules, including chiral molecules, that arrange themselves in a helical fashion. The CLC has a single photonic bandgap and an associated one-colour reflection band for circularly polarized light with the same handedness as the CLC helix (selective reflection). These optical characteristics, particularly the circular polarization of the reflected light, are attractive for applications in reflective colour displays without using a backlight, for use as polarizers or colour filters and for mirrorless lasing. Recently, we showed by numerical simulation that simultaneous multicolour reflection is possible by introducing fibonaccian phase defects. Here, we design and fabricate a CLC system consisting of thin isotropic films and of polymeric CLC films, and demonstrate experimentally simultaneous red, green and blue reflections (multiple photonic bandgaps) using the single-pitched polymeric CLC films. The experimental reflection spectra are well simulated by calculations. The presented system can extend applications of CLCs to a wide-band region and could give rise to new photonic devices, in which white or multicolour light is manipulated.

Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals.

Broadband cavity-mode lasing from dye-doped nematic liquid crystals sandwiched by broadband cholesteric liquid crystal bragg reflectors.

Color- and reflectance-tunable multiple reflectors assembled from three polymer films.

We fabricated novel hybrid structures composed of a dye-doped low-molecular-weight cholesteric liquid crystal sandwiched by multi-layered polymer cholesteric liquid crystal films and evaluated their lasing characteristics. Lasing was observed with an extremely reduced threshold (12 nJ/pulse) by a factor of 10 compared with that in a simple dye-doped low-molecular-weight cholesteric liquid crystal cell. Lasing characteristics experimentally obtained were discussed by comparing them with the simulated photonic density of states spectra.

Low threshold lasing from dye-doped cholesteric liquid crystal multi-layered structures.

We have demonstrated multicolored reflections showing various whitish colors from a bichiral liquid crystal (LC) film. The bichiral LC film was fabricated by using left-handed and right-handed polymeric cholesteric liquid crystal (CLC) films with two different helical pitches and an isotropic polymer film in between. Color temperatures of the multiple reflections are controlled from ~4000 K to ~10000 K by changing linear polarization directions of normally incident and reflected lights. This characteristic can extend practical applications of CLCs to illuminant devices.

Article	Year
Standing wave enhancement of red absorbance and photocurrent in dye-sensitized titanium dioxide photoelectrodes coupled to photonic crystals. Journal of the American Chemical Society\| PubMed ID: 12785864	2003
Electro-tunable optical diode based on photonic bandgap liquid-crystal heterojunctions. Nature materials\| PubMed ID: 15852019	2005
Photoinduced circular anisotropy in a photochromic W-shaped-molecule-doped polymeric liquid crystal film. Physical review. E, Statistical, nonlinear, and soft matter physics\| PubMed ID: 16605351	2006
Fabrication of a simultaneous red-green-blue reflector using single-pitched cholesteric liquid crystals. Nature materials\| PubMed ID: 17994028	2008
Broadband cavity-mode lasing from dye-doped nematic liquid crystals sandwiched by broadband cholesteric liquid crystal bragg reflectors. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 20432476	2010
Color- and reflectance-tunable multiple reflectors assembled from three polymer films. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 20496391	2010
Low threshold lasing from dye-doped cholesteric liquid crystal multi-layered structures. Optics express\| PubMed ID: 20588420	2010
Color-temperature tunable white reflector using bichiral liquid crystal films. Optics express\| PubMed ID: 21164984	2010
Polarization conversion in surface-plasmon-coupled emission from organic light-emitting diodes using spontaneously formed buckles. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 21341314	2011

Suzushi Nishimura

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