Four different perylene-based electron-acceptors having similar electron affinities, but different thermotropic phases are blended with nematic liquid crystalline electron-donors with a
fluorene-thiophene structure to form single layer photovoltaic devices. Best results are obtained when the nematic donor is mixed with an amorphous acceptor to give a supercooled nematic glass at room temperature. Atomic force microscopy operating in the phase contrast mode reveals phase separation on a nanometer scale with a broad distribution of domain sizes peaking at 26 nm. We correlate the
morphology of the different blends with the performance of the photovoltaic devices. Power conversion efficiencies up to 0.9 % are obtained with excitation at 470 nm.
Light-emitting reactive mesogens are promising materials for photolithographically processable organic light-emitting
diodes (OLEDs). We study the photopolymerization of a fluorene-based mesogen with methacrylate photoreactive
groups. We find that there is no significant change in the photoluminescence quantum efficiency when the mesogen is
photopolymerized in a glovebox with oxygen and water concentration at < 20 parts per million. There is significant
quenching of luminescence when the photopolymerization is carried out in a less controlled environment. A real-time
measurement is used to monitor changes in the FTIR spectrum of the compound during polymerization. No photo-initiator
is added to the mesogen suggesting that the photo-polymerization is self-initiated by the chromophore. We show
that the self-initiation does not proceed by fragmentation of the aromatic core and suggest an alternative mechanism.
Recently there has been renewed interest in the grating alignment of liquid crystals because of its application in bistable nematic displays. In this paper, gratin and photoinduced liquid crystal alignment techniques based on excimer laser exposure of thin polyimide films are discussed. Gratings are etched into the alignment film using a KrF laser illuminated through a phase mask. These give homogeneous liquid crystal alignment with the liquid crystal directors aligned along the grooves of the grating. The observed azimuthal anchoring strength is compared with that predicted using Berreman theory. No pretilt is observed because of the grating symmetry. When a polarized excimer laser beam is incident on the film with a fluence below that required for ablation, an anisotropy is created photochemically by selective depletion of the polymer chains. Exposure of the polyimide with elliptically polarized light at non-normal incidence gives pretilted alignment. Grating etching followed by photoinduced alignment can be used to obtain pretilted grating alignment with a pretilt angle of 3 degrees.
In this paper, a range of polymethacrylate derivatives of hydroxycoumarin are investigated as photoinduced alignment layers for liquid crystals, and their performance is compared with polyvinyl cinnamate. For al the coumarin- containing polymers, the liquid crystal alignment direction is parallel to the polarization direction of the incident UV light at low fluences. At a critical fluence threshold, a sharp change to perpendicular alignment is found. Molecular models and spectroscopy are used to explain this phenomenon as well as the fact that only perpendicular alignment is observed for polyvinyl cinnamate. The azimuthal anchoring energies of the alignment layers are measured and values greater than 6 by 10<SUP>-5</SUP> J m<SUP>-2</SUP> are found. The incorporation of a flexible spacer into the coumarin sidechain results in stronger anchoring at low fluences. Exposure of the coumarin-containing alignment layers at oblique incidence gives pretilted alignment with pretilt angles up to 5.1 degrees.
Gratings etched into polyimde thin layers are used to align nematic liquid crystals. The gratings are prepared by illumination of a 1.1 micrometer period phase mask with a KrF excimer laser at 248 nm. Fluences of 87 mJ cm<SUP>-2</SUP> and 128 mJ cm<SUP>-2</SUP> with one and two shots were used to ablate the gratings. Modelling of the fluence distribution behind the phase mask predicts a grating period equal to that of the phase mask and this is found experimentally. The amplitudes of the gratings are obtained from diffraction using a HeNe laser and are between 100 nm and 150 nm deep. The alignment layers are used in twisted nematic cells and the azimuthal anchoring energy is measured as a function of grating fabrication conditions. Anchoring energies of the order of 10<SUP>-5</SUP> J m<SUP>-2</SUP> are found in agreement with the Berreman theory.