Session Index

In this study four emitters of blue light are synthesized by selecting pyrene, with its high photoluminescence quantum yield (PLQY), as the core group and variants of the electron-donating diphenylamine (DPA) as side groups. The four compounds have different numbers, sizes, and substitution positions of alkyl groups on the DPA. Each of the four compounds when doped in OLED devices shows a high current efficiency (CE) of over 7 cd A-1 and a high external quantum efficiency (EQE) of over 7.5%. In addition, the compounds yield electroluminescence (EL) spectra showing peaks with narrow full width at half maximum (FWHM) values of 37-40 nm and hence indicative of high color purity. Moreover, one compound shows a very high EQE of 9.25% and a very long lifetime, with an LT95 of 471 h. Other new blue emitters based on the core-side concept will be discussed.

 

Fabrication of GaN-based micro-LED display with 64 × 32 pixels was developed in this study. The variation of forward voltages was 2.7 % and 1.08 % in the column and row pixels, respectively. Uniform brightness and clear image were demonstrated by PWM source.

 

A low-temperature polycrystalline silicon pixel circuit using simultaneous-emission driving scheme is proposed for high-resolution active-matrix organic light-emitting diode displays. The design, which utilizes the emission voltage of the OLED from the previous frame to discharge current through driving TFT, is able to detect threshold voltage variations.

 

A gateless vertical channel device based on cylindrical nanopores was exploited for gas sensing. The device was optimized for detecting breath ammonia (ppm-regime) by proposing a high ammonia sensing material and by addressing sensing stability issue. A high correlation coefficient (r>0.93) between breath ammonia and blood urea nitrogen is achieved.

 

We experimentally demonstrate ultrafast flexoelectric liquid crystal displays in polymer stabilized cholesteric liquid crystals with uniform lying helix (PSULH) [1-3] and vertical standing helix (PSVSH) [4-5] modes. A method to control the flexoelectric properties of the CLC mixture by mixing nematic liquid crystal constituents is achieved by addition of a giant flexoelastic coefficient bimesogenic LC dimer into a low dielectric anisotropy commercial NLC mixture. High optical contrast is achieved with flexoelectrically driven in-plane or out-of-plane rotation of the cholesteric helical axis in a well-aligned PSULH or PSVSH texture [6-7]. The PSULH prototype sample reveals a sub-millisecond response time for switching between electric field on and off states, the change of polarity of applied electric field, and with high contrast ratio. A PSVSH device exhibits a response time of less than 0.7 ms, low hysteresis and color dispersion and 500:1 contrast ratio [8-9]. Detailed optical and electro-optical properties as well as computer simulations of flexoelectric-switching for flexoelectric mixtures with different flexoelectric coefficients will be presented. Our findings identified quality of ULH alignment and dark state of VSH, and material miscibility challenges, and will be useful for material design of future CLC mixtures for applications in field-sequential color displays and flexoelectro-optic elements. [1] J. S. Patel, & R. B. Meyer, Flexoelectric electro-optics of a cholesteric liquid crystal. Phys. Rev. Lett. 58, 1538–1540 (1987). [2] S. H. Kim, L.-C. Chien, & L. Komitov, Short pitch cholesteric electro-optical device stabilized by nonuniform polymer network. Appl. Phys. Lett. 86, 161118 (2005). [3] S. H. Kim, L. Shi, & L.-C. Chien, Fast flexoelectric switching in a cholesteric liquid crystal cell with surface-localized polymer network. J. Phys. D Appl. Phys. 42, 195102 (2009). [4] D.G. Gardiner, S.M. Morris, P.J.W. Hands, F. Castles, M.M. Qasim, W-S Kim, S.S.Choi, T.D. Wilkinson and H.J. Coles, Spontaneous induction of the uniform lying helix alignment in bimesogenic liquid crystals for the flexoelectro-optic effect, Appl. Phys. Lett. 100, 063501 (2012). [5] A. Lorenz, D. J. Gardiner, S. M. Morris, F. Castles, M. M. Qasim, S. S. Choi, W. S. Kim, H. J. Coles and T. D. Wilkinson, Electrical addressing of polymer stabilized hyper-twisted chiral nematic liquid crystals with interdigitated electrodes: Experiment and modelAppl. Phys. Lett., 104, 071102 (2014). [6] A. Varanytsia, & L.-C. Chien, Bimesogen-enhanced flexoelectro-optic behavior of polymer stabilized cholesteric liquid crystal. J. Appl. Phys. 119, 014502 (2016). [7] A. Varanytsia, & L.-C. Chien, Giant Flexoelectro-optic Effect with Liquid Crystal Dimer CB7CB, Sci. Rep. 7, 41333 (2017). [8] V. Joshi, K.-H. Chang, D.A. Paterson, J. M. D. Storey, C. T. Imrie and L.-C. Chien, Fast Flexoelectro-optic response of Bimesogen-doped Polymer Stabilized Cholesteric Liquid Crystals in Vertical Standing Helix mode, SID Digest (2017). [9] V. Joshi, K.-H. Chang, D.A. Paterson, J. M. D. Storey, C. T. Imrie and L.-C. Chien, Adv. Opt. Mater. 6, 1800013 (2018).