Browsing by Author "Yang, Yaw-Wen"

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Anti-site defect effect on the electronic structure of a Bi2Te3 topological insulator
(2018-01-01)
Chuang, Pei-Yu
;
Su, Shu-Hsuan
;
Chong, Cheong-Wei
;
Chen, Yi-Fan
;
Chou, Yu-Heng
Tuning the Fermi level (EF) in Bi2Te3 topological-insulator (TI) films is demonstrated on controlling the temperature of growth with molecular-beam epitaxy (MBE). Angle-resolved photoemission spectra (ARPES) reveal that EF of Bi2Te3 thin films shifts systematically with the growth temperature (Tg). The key role that a Bi-on-Te(1) (BiTe1) antisite defect plays in the electronic structure is identified through extended X-ray-absorption fine-structure (EXAFS) spectra at the Bi L-3-edge. Calculations of electronic structure support the results of fitting the EXAFS, indicating that the variation of EF is due to the formation and suppression of BiTe1 that is tunable with the growth temperature. Our findings provide not only insight into the correlation between the defect structure and electronic properties but also a simple route to control the intrinsic topological surface states, which could be useful for applications in TIbased advanced electronic and spintronic devices.
Control of Crystal Structures and Optical Properties with Hybrid Formamidinium and 2-Hydroxyethylammonium Cations for Mesoscopic Carbon-Electrode Tin-Based Perovskite Solar Cells
(2018-09-01)
Tsai, Cheng-Min
;
Lin, Yu-Pei
;
Pola, Murali Krishna
;
Narra, Sudhakar
;
Jokar, Efat
Alcohol-based bifunctional ammonium cations, 2-hydroxyethylammonium (HEA(+)), HO(CH2)(2)NH3+, were introduced into formamidinium (FA(+)) tin-based perovskites (HEA(x)FA(1-x)Snl(3); x = 0-1) to absorb light in carbon-based mesoscopic solar cells. We found that HEAP cations play a key role to control the crystal structures, the lattice structures altered from orthorhombic (x = 0) to rhombohedral (x = 0.2-0.4) with greater symmetry. When x was increased to 0.6-1.0, tin and iodide vacancies were formed to generate 3D-vacant perovskites (HEA(x)FA(1-x)Sn(0.6)I(2.33), x >= 0.6) with a tetragonal structure. Tin-based perovskites in this series were fabricated into mesoporous solar cells using one-step drop-cast (DC), two-step solvent-extraction (SE), and SE + 3% ethyl-enediammonium diiodide (EDAI(2)) as an additive. After optimization of device performance with the SE + 3% EDAI2 approach, the HEA(0.4)FA(0.6)SnI(3) (HEAT = 40%) device gave the best photovoltaic performance with J(sc) = 18.52 mA cm(-2)) V-OC 371 mV, FF = 0.562, and overall efficiency 17 = 3.9% after the device was stored for a period of 340 h.
Face-on reorientation of pi-conjugated polymers in thin films by surface-segregated monolayers
(2020-04-07)
Wang, Wei-Chih
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Chen, Sheng-Yuan
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Yang, Yaw-Wen
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Hsu, Chain-Shu
;
Tajima, Keisuke
A new pi-conjugated polymer based on naphthalenediimide with a branched semifluoroalkyl side chain (FNDIT2) was synthesized and used to change the polymer orientations in thin films. X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy revealed that FNDIT2 spontaneously forms surface-segregated monolayers in blend films with other pi-conjugated polymers to adopt a preferred face-on orientation, with the pi-plane of the polymer aligned more parallel to the surface. Thermal annealing of the films induced further reorientation of the polymers inside the films closer to the face-on orientation, as shown by two-dimensional grazing-incidence wide-angle X-ray scattering. The induced face-on orientations in the films led to higher charge mobility in the vertical direction. This study provides a general and versatile strategy to control the molecular orientation in thin films and enhance the performance of thin-film organic electronics.
Formation of Stable Tin Perovskites Co-crystallized with Three Halides for Carbon-Based Mesoscopic Lead-Free Perovskite Solar Cells
(2017-10-23)
Tsai, Cheng-Min
;
Mohanta, Nayantara
;
Wang, Chi-Yung
;
Lin, Yu-Pei
;
Yang, Yaw-Wen
We synthesized and characterized methylammonium (MA) mixed tri-halide tin perovskites (MASnIBr(2-x)Cl(x)) for carbon-based mesoscopic solar cells free of lead and holetransporting layers. Varied SnCl2/SnBr2 ratios yielded tin perovskites with three halides (I, Br, and Cl) co-crystallized inside the tin-perovskite. When the SnCl2 proportion was >= 50% (x >= 1), phase separation occurred to give MAS-nI(3-y)Br(y) and MASnCl(3-z)Br(z) in the stoichiometric proportions of their precursors, confirmed by XRD. A device with MASnIBr(1.8)Cl(0.2) (SnCl2= 10%) showed the best photovoltaic performance: J(SC)= 14.0 mAcm(-2), V-OC= 380 mV, FF= 0.573, and PCE= 3.1%, and long-term stability. Electrochemical impedance spectra (EIS) show superior charge recombination and dielectric relaxation properties for the MASnIBr(1.8)Cl(0.2) cell. Transient PL decays showed the intrinsic problem of tinbased perovskites with average lifetimes less than 100 ps.
Role of Tin Chloride in Tin-Rich Mixed-Halide Perovskites Applied as Mesoscopic Solar Cells with a Carbon Counter Electrode
(2016-12)
Tsai, Cheng-Min
;
Wu, Hui-Ping
;
Chang, Sun-Tang
;
Huang, Chi-Feng
;
Wang, Chia-Hsin
We report the synthesis and characterization of alloyed Sn Pb methylammonium mixed-halide perovskites (CH3NH3SnyPb1-yI3-xClx) to extend light harvesting toward the near infrared region for carbon-based mesoscopic solar cells free of organic hole-transport layers. The proportions of Sn in perovskites are well controlled by mixing tin chloride (SnCl2) and lead iodide (Pbl(2)) in varied stoichiometric ratios (y = 0-1). SnCl2 plays a key role in modifying the lattice structure of the perovskite, showing anomalous optical and optoelectronic properties; upon increasing the concentration of SnC2, the variation of the band gap and band energy differed from those of the Snl(2) precursor. The CH3NH3SnyPb1-yI3-xClx devices showed enhanced photovoltaic performance upon increasing the proportion of SnCl2 until y = 0.75, consistent with the corresponding potential energy levels. The photovoltaic performance was further improved upon adding 30 mol % tin fluoride (SnF2) with device configuration FTO/TiO2/Al2O3/NiO/C, producing the best power conversion efficiency, 5.13%, with great reproducibility and intrinsic stability.