Browsing by Author "Tseng, Wei-Hsuan"

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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.
Origins of vertical phase separation in P3HT:PCBM mixed films
(2014-04-01)
Liu, I-Hsiu
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Chao, Yi-Ping
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Fang, Jian-Jhih
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Tseng, Wei-Hsuan
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Lan, Yu-Bing
The origins of vertical phase separation and their implication on the device efficiency of poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) based solar cells, with both regular and inverted structures, were investigated. We found that the light irradiation and the filtration processes during the device fabrications are two key steps that induce the vertical phase separation in the active layers. Upon light irradiation, the devices with inverted structures exhibit improved power conversion efficiency, whereas the regular devices show degradation. The inverted devices spun cast with filtered P3HT:PCBM solution also present a better improvement as compared to regular devices. X-ray and ultraviolet photoemission spectroscopies indicate that both illumination and filtration enhance the vertical phase separation of the blend film with additional PCBM segregated to the bottom interface. (C) 2014 The Japan Society of Applied Physics
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.