Browsing by Author "Wu, Guan-Wei"

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Control of preferred orientation with slow crystallization for carbon-based mesoscopic perovskite solar cells attaining efficiency 15%
(2017-01-14)
Tsai, Cheng-Min
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Wu, Guan-Wei
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Narra, Sudhakar
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Chang, Hung-Ming
;
Mohanta, Nayantara
To produce mesoscopic carbon-electrode-based solar cells free of a hole-conduction layer, we developed a simple one-step drop-cast method (temperature 20 degrees C, relative humidity 50%) to grow uniform and compact perovskite nanocrystals via slow crystallization (SC). X-ray diffraction (XRD) results indicated that perovskite nanocrystals produced inside the mesoporous TiO2/Al2O3/C layers according to our SC approach with N-methyl-2-pyrrolidone (NMP) as a precursor solvent showed a preferred orientation at facet (004) with large crystalline grains, whereas those produced with other solvents or via thermal annealing (TA) methods showed greater intensity at facet (220) with small crystals. After the SC (NMP) procedure was completed, additional TA treatment would change the preferred orientation from (004) back to facet (220), which would break the connectivity and lose the uniformity of the crystals, causing the grains to become significantly smaller. Transient photoluminescence (PL) decay profiles of the SC devices made with four precursor solvents, NMP, g-butyrolactone (GBL), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), were recorded to show the trend of charge separation kinetics of the perovskite crystals inside the mesoporous films, which is consistent with the trend of the corresponding device performance showing the same order. The SC (NMP) device attained the best efficiency of power conversion (PCE), 15.0%, with average value (13.9 +/- 0.5%), which is much superior to those devices from either the traditional one-step TA (DMF) method (5.2 +/- 1.0%) or the traditional sequential TA method (10.1 +/- 0.7%). This work emphasizes the significance of the control of preferred orientation of perovskite nanocrystals using the SC approach, to obtain high-performance carbon-based mesoscopic solar cells with excellent reproducibility and stability.
Electro-thermally tunable reflective colors in a self-organized cholesteric helical superstructure
(2018-12-01)
Wu, Po-Chang
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Wu, Guan-Wei
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Timofeev, Ivan V.
;
Zyryanov, Victor Ya.
;
Lee, Wei
We propose to dynamically control the reflective color of a cholesteric liquid crystal (CLC) by electrically tuning the center wavelength (lambda(c)) of the bandgap. The CLC, sandwiched in a planar-aligned cell with indium-tin-oxide electrodes, possesses negative dielectric anisotropy and thermo-responsive spectral properties. The helix in the Grandjean planar state, which is subject to vertically applied voltage, should be undisturbed in that the long molecular axis is initially perpendicular to the direction of the electric field. Surprisingly, when the frequency of the applied voltage is higher than a critical value, lambda c of the CLC cell varies as a function of the voltage. The underlying mechanism is the voltage-induced temperature change through dielectric heating in the frequency regime of pseudo-dielectric relaxation, attributable to the significant equivalent resistance-capacitance circuit of the cell due to the use of electrode layers with finite conductance. The driving voltage enabling the tuning of lambda c in the entire visible spectrum is as low as 12 V-rms in a 5 pin thick cell at a frequency of 2 MHz. The proposed CLC cell, exhibiting a broad electrically tunable spectral range from the near-infrared to ultraviolet, holds great promise for developing tunable photonic devices such as multicolor reflectors, filters, and sensors. (C) 2018 Chinese Laser Press
Hot-Carrier Injection-Induced Disturb and Improvement Methods in 3D NAND Flash Memory
(2019-01-01)
Lin, Wei-Liang
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Tsai, Wen-Jer
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Cheng, C. C.
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Lu, Chun-Chang
;
Ku, S. H.
We investigate a hot-carrier injection- induced program disturb in a 3D NAND flash memory. As there exist specific coding patterns, a "down-coupling" region and a "pre-charge" regions are formed during program-verify and the following program phases, respectively, in the inhibit cell strings. A high heating field is built nearby the PGM wordline. Hot carriers may inject into the inhibit cells as Vpgm is applied. Soft ramp-down and pre-turn-on schemes are proposed to mitigate this disturb.
Pseudo-dielectric relaxation-enabled electrothermal tuning of reflective color in a soft chiral photonic crystal
(2019-01-01)
Wu, Po-Chang
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Wu, Guan-Wei
;
Lee, Wei
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照明與能源光電研究所
;
影像與生醫光電研究所
Many published papers bear flaw information caused by misinterpreting the pseudo-dielectric relaxation in liquid crystal (LC) confined in thin cells. The pseudo-dielectric relaxation stems from the cell geometries or parameters, which should not be misrecognized as an inherent material property of the LC. In this work, we explored the pseudo-dielectric relaxation and demonstrated an unconventional tuning means to electrically manipulate the reflective color of a typical cholesteric LC cell based on the heating due to the pseudo-dielectric relaxation. A tuning range in the full-color spectrum can be easily achieved by a limited temperature change of merely a few degrees Celsius.
Single-pole Double-throw Switch Using Stacked-FET Configuration at Millimeter Wave Frequencies
(2018-01-01)
Mei, Peng-, I
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Wu, Guan-Wei
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Hsu, Heng-Shou
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Huang, Ting-Jui
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Tsao, Yi-Fan
In this paper, we present single-pole double-throw (SPDT) switch at Ka-band for phased array front-end applications. The proposed switch composed of three shunt arms and a quarter-wave length impedance transformer in series on each branch. The stacked-FET configuration was adopted to enhance the power handling capability. Implemented with 0.15um GaAs pHEMT technology by WIN Semiconductor, the overall chip size was 2 mm by 1 mm. The measured insertion loss was less than 3 dB with better than 25 dB isolation from 15 GHz to 35 GHz. The proposed configuration also featured an input 1 dB compression point of 26 dBm.
Solvent-extraction crystal growth for highly efficient carbon-based mesoscopic perovskite solar cells free of hole conductors
(2016)
Chan, Chien-Yi
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Wang, Yingying
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Wu, Guan-Wei
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Diau, Eric Wei-Guang
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應用化學系
We developed a simple drop-casting method via solvent extraction (SECG) to grow dense and uniform perovskite nanocrystals at room temperature for carbon-based mesoscopic solar cells free of an organic hole-transport layer. The CH3NH3PbI3/N-methyl-2-pyrrolidone (NMP) precursor solution (40%) was first dripped onto a substrate with film configuration TiO2/Al2O3/C and infiltrated at 70 degrees C for 10 min. The perovskite substrate was next immersed in a bath of diethyl ether at 25 degrees C for 30 min. Third, the solventextracted substrate was stored in a dry box (humidity 50%) at 25 degrees C for at least 100 h to complete the crystal growth. The device performance attained a power conversion efficiency (PCE) of 12.3%, which is significantly greater than that of DMF (6.3%) and NMP (8.3%) devices using traditional thermal annealing. The SECG device displayed a superior intrinsic enduring stability: the PCE exceeded 12% for 5000 h with a maximum value of 13.3% without light-soaking at 25 degrees C, but the performance degraded rapidly under one-sun irradiation without encapsulation. To understand the kinetics of charge transfer and defect relaxation for the devices under investigation, we recorded transient photoluminescence decays at an excitation wavelength of 635 nm and a probe wavelength of 770 nm.
Voltage-induced pseudo-dielectric heating and its application for color tuning in a thermally sensitive cholesteric liquid crystal
(1970-01-01)
Wu, Po-Chang
;
Wu, Guan-Wei
;
Yu, Chia-Hua
;
Lee, Wei
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照明與能源光電研究所
We previously proposed an electrical approach enabling the tuning of the center wavelength lambda(c) of the cholesteric liquid crystal (CLC) bandgap in the full-color visible spectrum based on the electro-thermal effect. The idea involved the design of a negative CLC with a thermally sensitive bandgap feature and the use of a sandwich-type cell with finite electrode conductivity, allowing the control of cell temperature by applied voltage via pseudo-dielectric heating. It has been demonstrated experimentally that the induced pseudo-dielectric heating is predominated by the pseudo-dielectric relaxation originating from the designated cell geometry. On the basis of this technique, key factors determining the tuning efficacy of the temperature and thus lambda(c) are primarily investigated in this study. Our results suggest that lowering the electrode resistivity and the specific heat conductivity of the cell can promote the maximum tunable temperature range. Expectedly, optimizing the electrode area, cell gap and dielectric permittivity of the CLC favors a decreased relaxation frequency and, in turn, reducing the voltage as well as the frequency required for lambda(c) tuning.