Browsing by Author "Wu, Chih-I"

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13% Efficiency Hybrid Organic/Silicon-Nanowire Heterojunction Solar Cell via Interface Engineering
(2013-12-01)
Yu, Peichen
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Tsai, Chia-Ying
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Chang, Jan-Kai
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Lai, Chih-Chung
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Chen, Po-Han
Interface carrier recombination currently hinders the performance of hybrid organic silicon heterojunction solar cells for high-efficiency low-cost photovoltaics. Here, we introduce an intermediate 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAP() layer into hybrid heterojunction solar cells based on silicon nanowires (SINWs) and conjugate polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS). The highest power conversion efficiency reaches a record 13.01%, which is largely ascribed to the modified organic surface morphology and suppressed saturation current that boost the open-circuit voltage and fill factor. We show that the insertion of TAP( increases the minority carrier lifetime because of an energy offset at the heterojunction interface. Furthermore, X-ray photoemission spectroscopy reveals that TAP( can effectively block the strong oxidation reaction occurring between PEDOT:PSS and silicon, which improves the device characteristics and assurances for reliability. These learnings point toward future directions for versatile interface engineering techniques for the attainment of highly efficient hybrid photovoltaics.
Band Alignment of 2D Transition Metal Dichalcogenide Heterojunctions
(2017-05-18)
Chiu, Ming-Hui
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Tseng, Wei-Hsuan
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Tang, Hao-Ling
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Chang, Yung-Huang
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Chen, Chang-Hsiao
It is critically important to characterize the band alignment in semiconductor heterojunctions (HJs) because it controls the electronic and optical properties. However, the well-known Anderson's model usually fails to predict the band alignment in bulk HJ systems due to the presence of charge transfer at the interfacial bonding. Atomically thin 2D transition metal dichalcogenide materials have attracted much attention recently since the ultrathin HJs and devices can be easily built and they are promising for future electronics. The vertical HJs based on 2D materials can be constructed via van der Waals stacking regardless of the lattice mismatch between two materials. Despite the defect-free characteristics of the junction interface, experimental evidence is still lacking on whether the simple Anderson rule can predict the band alignment of HJs. Here, the validity of Anderson's model is verified for the 2D heterojunction systems and the success of Anderson's model is attributed to the absence of dangling bonds (i.e., interface dipoles) at the van der Waal interface. The results from the work set a foundation allowing the use of powerful Anderson's rule to determine the band alignments of 2D HJs, which is beneficial to future electronic, photonic, and optoelectronic devices.
Bridging donor-acceptor energy offset using organic dopants as energy ladders to improve open-circuit voltages in bulk-heterojunction solar cells
(2014-12-01)
Chang, Jan-Kai
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Kuo, Ya-Ching
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Chen, Yu-Jen
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Lo, An-Lun
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Liu, I-Hsiu
A systematic way to improve the open circuit voltage of bulk-heterojunction (BHJ) organic photovoltaics is demonstrated by incorporating energy ladder material to form a ternary blend with cascade energy structures. We propose that doping organic molecules with appropriate energetic alignment to bridge the donor-acceptor energy offset can facilitate exciton dissociation and hence improve open-circuit voltage (V-oc) and power conversion efficiency. The cascade energy alignment structure is measured via ultraviolet photoemission spectroscopy. The Voc and power conversion efficiency of devices using the proposed schemes can be enhanced by 10% and 20%, respectively. The results provide a new path toward higher power conversion efficiency in BHJ systems by engineering the energy structures of the materials in the active layers. (C) 2014 Elsevier B.V. All rights reserved.
Carbon-doped SnS2 nanostructure as a high-efficiency solar fuel catalyst under visible light
(2018-01-12)
Shown, Indrajit
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Samireddi, Satyanarayana
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Chang, Yu-Chung
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Putikam, Raghunath
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Chang, Po-Han
Photocatalytic formation of hydrocarbons using solar energy via artificial photosynthesis is a highly desirable renewable-energy source for replacing conventional fossil fuels. Using an L-cysteine-based hydrothermal process, here we synthesize a carbon-doped SnS2 (SnS2-C) metal dichalcogenide nanostructure, which exhibits a highly active and selective photocatalytic conversion of CO2 to hydrocarbons under visible-light. The interstitial carbon doping induced microstrain in the SnS2 lattice, resulting in different photophysical properties as compared with undoped SnS2. This SnS2-C photocatalyst significantly enhances the CO2 reduction activity under visible light, attaining a photochemical quantum efficiency of above 0.7%. The SnS2-C photocatalyst represents an important contribution towards high quantum efficiency artificial photosynthesis based on gas phase photocatalytic CO2 reduction under visible light, where the in situ carbon-doped SnS2 nanostructure improves the stability and the light harvesting and charge separation efficiency, and significantly enhances the photocatalytic activity.
Catalytically solid-phase self-organization of nanoporous SnS with optical depolarizability
(2016)
Cheng, Chih-Hsien
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Chi, Yu-Chieh
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Wu, Chung-Lun
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Lin, Chun-Jung
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Tsai, Ling-Hsuan
The catalytic solid-phase synthesis of self-organized nanoporous tin sulfide (SnS) with enhanced absorption, manipulative transmittance and depolarization features is demonstrated. Using an ultralow radio-frequency (RF) sputtering power, the variation of the orientation angle between the anodized aluminum oxide (AAO) membrane and the axis of the sputtered ion beam detunes the catalytically synthesized SnS from nanorod to nanoporous morphology, along the sidewall of the AAO membrane. The ultraslow catalytic sputtering synthesis on the AAO at the RF plasma power of 20 W and the orientation angle of 0 degrees regulates the porosity and integrality of nanoporous SnS, with average pore diameter of 80-150 nm. When transferring from planar to nanoporous structure, the phase composition changes from SnS to SnS2-Sn2S3, and the optical bandgap shrinks from 1.43 to 1.16 eV, due to the preferred crystalline orientation, which also contributes to an ultralow reflectance of <1% at 200-500 nm when both the transmittance and the surface scattering remain at their maxima. The absorption coefficient is enhanced by nearly one order of magnitude with its minimum of >5 x 10(4) cm(-1) at the wavelength between 200 and 700 nm, due to the red-shifting of the absorption spectrum to at least 100 nm. The catalytically self-organized nanoporous SnS causes strong haze and beam divergence of 20 degrees-30 degrees by depolarized nonlinear scattering at the surface, which favors the solar energy conversion with reduced surface reflection and enhanced photon scattering under preserved transmittance.
Growing GaN LEDs on amorphous SiC buffer with variable C/Si compositions
(2016-01-22)
Cheng, Chih-Hsien
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Tzou, An-Jye
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Chang, Jung-Hung
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Chi, Yu-Chieh
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Lin, Yung-Hsiang
The epitaxy of high-power gallium nitride (GaN) light-emitting diode (LED) on amorphous silicon carbide (a-SixC1-x) buffer is demonstrated. The a-SixC1-x buffers with different nonstoichiometric C/Si composition ratios are synthesized on SiO2/Si substrate by using a low-temperature plasma enhanced chemical vapor deposition. The GaN LEDs on different SixC1-x buffers exhibit different EL and C-V characteristics because of the extended strain induced interfacial defects. The EL power decays when increasing the Si content of SixC1-x buffer. The C-rich SixC1-x favors the GaN epitaxy and enables the strain relaxation to suppress the probability of Auger recombination. When the SixC1-x buffer changes from Si-rich to C-rich condition, the EL peak wavelengh shifts from 446 nm to 450 nm. Moreover, the uniform distribution contour of EL intensity spreads between the anode and the cathode because the traping density of the interfacial defect gradually reduces. In comparison with the GaN LED grown on Sirich SixC1-x buffer, the device deposited on C-rich SixC1-x buffer shows a lower turn-on voltage, a higher output power, an external quantum efficiency, and an efficiency droop of 2.48 V, 106 mW, 42.3%, and 7%, respectively.
Hydroxynaphthyridine-Derived Group III Metal Chelates: Wide Band Gap and Deep Blue Analogues of Green Alq(3) (Tris(8-hydroxyquinolate)aluminum) and Their Versatile Applications for Organic Light-Emitting Diodes
(2009-01-21)
Liao, Szu-Hung
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Shiu, Jin-Ruei
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Liu, Shun-Wei
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Yeh, Shi-Jay
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Chen, Yu-Hung
A series of group III metal chelates have been synthesized and characterized for the versatile application of organic light-emitting diodes (OLEDs). These metal chelates are based on 4-hydroxy-1,5-naphthyridine derivates as chelating ligands, and they are the blue version analogues of well-known green fluorophore Alq(3) (tris(8-hydroxyquinolinato)aluminum). These chelating ligands and their metal chelates were easily prepared with an improved synthetic method, and they were facially purified by a sublimation process, which enables the materials to be readily available in bulk quantity and facilitates their usage in OLEDs. Unlike most currently known blue analogues of Alq(3) or other deep blue materials, metal chelates of 4-hydroxy-1,5-naphthyridine exhibit very deep blue fluorescence, wide band gap energy, high charge carrier mobility, and superior thermal stability. Using a vacuum-thermal-deposition process in the fabrication of OLEDs, we have successfully demonstrated that the application of these unusal hydroxynaphthyridine metal chelates can be very versatile and effective. First, we have solved or alleviated the problem of exciplex formation that took place between the hole-transporting layer and hydroxynaphthyridine metal chelates, of which OLED application has been prohibited to date. Second, these deep blue materials can play various roles in OILED application. They can be a highly efficient nondopant deep blue emitter: maximum external quantum efficiency (eta ext) of 4.2%; Commision Internationale de L'Eclairage x, y coordinates, CIE(x,y) = 0.15, 0.07. Compared with Alq(3), Bebqp(2) (beryllium bis(benzoquinolin-10-olate)), or TPBI (2,2',2 ''-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole), they are a good electron-transporting material: low HOMO energy level of 6.4-6.5 eV and not so high LUMO energy level of 3.0 3.3 eV. They can be ambipolar and possess a high electron mobility of 10(-4) cm(2)/V s at an electric field of 6.4 x 10(5) V/cm. They are a qualified wide band gap host material for efficient blue perylene (CIE(x,y) = 0.14, 0.17 and maximum (eta ext) 3.8%) or deep blue 9,10-diphenylanthracene (CIE(x,y) = 0.15, 0.06 and maximum (eta ext) 2.8%). For solid state lighting application, they are desirable as a host material for yellow dopant (rubrene) in achieving high efficiency ((eta ext) 4.3% and (eta P) 8.7 lm/W at an electroluminance of 100 cd/m(2) or (eta ext) 3.9% and (eta P) 5.1 lm/W at an electroluminance of 1000 cd/m(2)) white electroluminescence (CIE(x,y) = 0.30, 0.35).
KSCN-induced Interfacial Dipole in Black TiO2 for Enhanced Photocatalytic CO2 Reduction
(2019-07-17)
Fu, Fang-Yu
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Shown, Indrajit
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Li, Chia-Shuo
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Raghunath, Putikam
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Lin, Tsai-Yu
Tuning the electronic band structure of black titania to improve photocatalytic performance through conventional band engineering methods has been challenging because of the defect-induced charge carrier and trapping sites. In this study, KSCN-modified hydrogenated nickel nano cluster-modified black TiO2 (SCN-H-Ni-TiO2) exhibits enhanced photocatalytic CO2 reduction due to the interfacial dipole effect. Upon combining the experimental and theoretical simulation approach, the presence of an electrostatic interfacial dipole associated with chemisorption of SCN has dramatic effects on the photocatalyst band structure in SCN H-Ni-TiO2. An interfacial dipole possesses a more negative zeta potential shift of the isoelectric point from 5.20 to 3.20, which will accelerate the charge carrier separation and electron transfer process. Thiocyanate ion passivation on black TiO2 demonstrated an increased work function around 0.60 eV, which was induced by the interracial dipole effect. Overall, the SCN H-Ni-TiO2 photocatalyst showed an enhanced CO2 reduction to solar fuel yield by 2.80 times higher than H-Ni-TiO2 and retained around 88% product formation yield after 40 h.
Monolayer MoSe2 Grown by Chemical Vapor Deposition for Fast Photodetection
(2014-08-01)
Chang, Yung-Huang
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Zhang, Wenjing
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Zhu, Yihan
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Han, Yu
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Pu, Jiang
Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful for guiding future designs in 2D material-based optoelectronic devices.
Monolayer MoSe2 Grown by Chemical Vapor Deposition for Fast Photodetection
(2014-08-01)
Chang, Yung-Huang
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Zhang, Wenjing
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Zhu, Yihan
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Han, Yu
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Pu, Jiang
Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful for guiding future designs in 2D material-based optoelectronic devices.
Ni-Nanocluster Modified Black TiO2 with Dual Active Sites for Selective Photocatalytic CO2 Reduction
(2018-01-11)
Billo, Tadesse
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Fu, Fang-Yu
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Raghunath, Putikam
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Shown, Indrajit
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Chen, Wei-Fu
One of the key challenges in artificial photosynthesis is to design a photocatalyst that can bind and activate the CO2 molecule with the smallest possible activation energy and produce selective hydrocarbon products. In this contribution, a combined experimental and computational study on Ni-nanocluster loaded black TiO2 (Ni/TiO2[Vo]) with built-in dual active sites for selective photocatalytic CO2 conversion is reported. The findings reveal that the synergistic effects of deliberately induced Ni nanoclusters and oxygen vacancies provide (1) energetically stable CO2 binding sites with the lowest activation energy (0.08 eV), (2) highly reactive sites, (3) a fast electron transfer pathway, and (4) enhanced light harvesting by lowering the bandgap. The Ni/TiO2[Vo] photocatalyst has demonstrated highly selective and enhanced photocatalytic activity of more than 18 times higher solar fuel production than the commercial TiO2 (P-25). An insight into the mechanisms of interfacial charge transfer and product formation is explored.
Rear interface engineering of hybrid organic-silicon nanowire solar cells via blade coating
(2016-01-25)
Lai, Yi-Chun
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Chang, Yu-Fan
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Tsai, Pei-Ting
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Chang, Jan-Kai
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Tseng, Wei-Hsuan
In this work, we investigate blade-coated organic interlayers at the rear surface of hybrid organic-silicon photovoltaics based on two small molecules: Tris(8-hydroxyquinolinato) aluminium (Alq(3)) and 1,3-bis(2-(4-tert- butylphenyl)-1,3,4-oxadiazol-5-yl) benzene (OXD-7). In particular, soluble Alq3 resulting in a uniform thin film with a root-mean-square roughness < 0.2nm is demonstrated for the first time. Both devices with the Alq3 and OXD-7 interlayers show notable enhancement in the open-circuit voltage and fill-factor, leading to a net efficiency increase by over 2% from the reference, up to 11.8% and 12.5% respectively. The capacitance-voltage characteristics confirm the role of the small-molecule interlayers resembling a thin interfacial oxide layer for the Al-Si Schottky barrier to enhance the built-in potential and facilitate charge transport. Moreover, the Alq3 interlayer in optimized devices exhibits isolated phases with a large surface roughness, in contrast to the OXD-7 which forms a continuous uniform thin film. The distinct morphological differences between the two interlayers further suggest different enhancement mechanisms and hence offer versatile functionalities to the advent of hybrid organic-silicon photovoltaics. (C)2016 Optical Society of America
Solution p-doped fluorescent polymers for enhanced charge transport of hybrid organic-silicon nanowire photovoltaics
(2016-07)
Tsai, Pei-Ting
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Li, Ming-Chin
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Lai, Yi-Chun
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Tseng, Wei-Hsuan
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Wu, Chih-I
The solution p-type doping of various conjugated polymers and small molecules via blending with tetrafluorotetracyano-quinodimethane (F4-TCNQ) is investigated to facilitate hole transport for hybrid organic-silicon-nanowire solar cells. Among all, the highly-fluorescent conjugated polymers such as poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) demonstrate superior transport characteristics, where both the device fill-factor and power conversion efficiency (PCE) are positively correlated with the doping concentration. The best PCE achieves 13.66% from the device with 30% p-doped PFO, compared to the reference at 12.50%. The enhancement is ascribed to the efficient interfacial charge-carrier recombination with minimal energy losses. A vertically graded doping profile is further revealed with F4-TCNQ molecules preferentially accumulated near the silicon surface. (C) 2016 Elsevier B.V. All rights reserved.
Strong optical nonlinearity of the nonstoichiometric silicon carbide
(2015-01-01)
Cheng, Chih-Hsien
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Wu, Chung-Lun
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Lin, Yung-Hsiang
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Yan, Wen-Long
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Shih, Min-Hsiung
Enhanced nonlinear refractive indices and absorption coefficients of nonstoichiometric SixC1-x with varying C/Si composition ratios from 0.51 to 1.83 grown by low-temperature plasma-enhanced chemical vapor deposition are demonstrated. When increasing the [CH4]/[CH4 + SiH4] fluence ratio from 70% to 92%, two Raman scattering signals at 795 and 970 cm(-1) contributed by transverse and longitudinal optical modes of Si-C bonds are simultaneously enhanced in stoichiometric and C-rich SixC1-x samples, whereas the Si-associated Raman peak at 520 cm(-1) decreases with increasing C/Si composition ratio. The C-rich SixC1-x film shows additional Raman scattering peaks at 1330 and 1580 cm(-1), which are individually contributed by the diamond-like C-C bonds and the G peak of the graphite-like C-C bonds, respectively. The nonlinear refractive index increases to 1 +/- 0.1 x 10(-11) cm(2) W-1 upon increasing the C/Si composition ratio up to 1.83, which is four orders of magnitude higher than that of bulk SiC. The increased C/Si composition ratio reduces the lattice constant and the effective mass of the SixC1-x film and enhances its nonlinear refractive index effectively. In comparison with the sp(3)-orbital C-C bond, the sp(2)-orbital C-C bond in SixC1-x films dominates the enhancement of nonlinear refractive index. By contrast, the nonlinear absorption coefficient changes sign and decreases magnitude when transferring the SixC1-x film from Si-rich to C-rich conditions. This sign reversal is primarily attributed to the conversion in the absorption mechanism from two-photon absorption to nonlinear saturable absorption. Unlike Si-Si bonds which favor two-photon absorption, the sp(2)-orbital C-C bonds with delocalized pi-electrons essentially enhance the saturable absorbance. Hence, a higher nonlinear refractive index and a higher saturable absorption coefficient can be obtained by enriching the sp(2)-orbital C-C bonds in C-rich SixC1-x films.
Two-dimensional dopant profiling by electrostatic force microscopy using carbon nanotube modified cantilevers
(2008-08-13)
Chin, Shu-Cheng
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Chang, Yuan-Chih
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Hsu, Chen-Chih
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Lin, Wei-Hsiang
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Wu, Chih-I
A two-dimensional (2D) dopant profiling technique is demonstrated in this work. We apply a unique cantilever probe in electrostatic force microscopy (EFM) modified by the attachment of a multiwalled carbon nanotube (MWNT). Furthermore, the tip apex of the MWNT was trimmed to the sharpness of a single-walled carbon nanotube (SWNT). This ultra-sharp MWNT tip helps us to resolve dopant features to within 10 nm in air, which approaches the resolution achieved by ultra-high vacuum scanning tunnelling microscopy (UHV STM). In this study, the CNT-probed EFM is used to profile 2D buried dopant distribution under a nano-scale device structure and shows the feasibility of device characterization for sub-45 nm complementary metal-oxide-semiconductor (CMOS) field-effect transistors.