Browsing by Author "Ramesh, Mohan"

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A cascade energy band structure enhances the carrier energy in organic vertical-type triodes
(2013-09-01)
Cheng, Shiau-Shin
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Ramesh, Mohan
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Chen, Guan-Yuan
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Fung, Chun-Lin
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Chen, Li-Ming
Organic vertical-type triodes (OVTs) based on the cascade energy band structure as emitter layer are studied. The electric characteristics were dramatically enhanced while incorporating the cascade energy under current driving and voltage driving modes. The improvement is attributed to that injection carriers can obtain higher energy through a stepwise energy level. When the device has a layered structure of F16CuPC (10 nm)/PTCDI (10 nm)/pentacene (100 nm) in emitter, it exhibits a common-base transport factor of 0.99 and a common-emitter current gain of 225 under current driving mode and exhibits a high current modulation-exceeding -520 mu A for a low collector voltage of -5 V and a base voltage of -5 V and the current on/off ratio of 10(3) under voltage driving mode. Furthermore, we realized first organic current mirror that exhibited out/in current ratio of 0.75 and output resistance of 10(5) Omega by using the OVTs. (C) 2013 Published by Elsevier B.V.
Enhancement of photovoltaic properties in supramolecular polymer networks featuring a solar cell main-chain polymer H-bonded with conjugated cross-linkers
(2012-03-09)
Patra, Dhananjaya
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Ramesh, Mohan
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Sahu, Duryodhan
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Padhy, Harihara
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Chu, Chih-Wei
Stille polymerization was employed to synthesize a low-band-gap (LOG) conjugated main-chain polymer PBTH consisting of bithiazole, dithieno[3,2-b:2',3'-d]pyrroles (DTP), and pendent melamine derivatives. Novel supramolecular polymer networks PBTH/C and PBTH/F were developed by mixing proper molar amounts of polymer PBTH (containing melamine pendants) to be hydrogen-bonded (H-bonded) with complementary uracil-based conjugated cross-linkers C and F (i.e., containing two symmetrical uracil moieties connected with carbazole and fluorene units through triple bonds). The formation of multiple H-bonds between polymer PBTH and cross-linkers C or F was confirmed by FT-IR measurements. In contrast to polymer PBTH, the supramolecular design with multiple H-bonds can enhance the photovoltaic properties of polymer solar cell (PSC) devices containing H-bonded polymer networks PBTH/C and PBTH/F by tuning their light harvesting capabilities, HOMO energy levels, and crystallinities. Initially, the power conversion efficiency (PCE) values of PSC devices containing supramolecular polymer networks PBTH/C and PBTH/F as electron donors and [6,6]-phenyl-C-71-butyric acid methyl ester (PC70BM) as an electron acceptor (polymer:PC70BM = 1:1 w/w) are found to be 0.97 and 0.68%, respectively, in contrast to 0.52% for polymer PBTH. The highest PCE value of 1.56% with a short-circuit current densities (J(sc)) value of 7.16 mA/cm(2), a open circuit voltages (V-oc) value of 0.60 V. and a fill factor (FF) of 0.36 was further optimized in the PSC device containing a supramolecular polymer network PBTH/C as polymer:PC70BM = 1:2 w/w. These results indicate that supramolecular design is an effective route towards better photovoltaic properties of V-oc, J(sc), and PCE values in polymer solar cells. (C) 2012 Elsevier Ltd. All rights reserved.
Natural polymers for disposable organic thin film transistors
(2018-03-01)
Rullyani, Cut
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Ramesh, Mohan
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Sung, Chao-Feng
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Lin, Hong-Cheu
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Chu, Chih-Wei
In this study, eco-friendly PTCDI-C8-based organic thin film transistors (OTFTs) were fabricated by incorporating polymers from renewable sources as gate dielectrics and substrates. Chitosan, a polysaccharide obtained from the deacetylation of chitin, and natural rubber (NR), extracted from natural rubber tree, have good dielectric properties, biocompatibility, and biodegradability. To examine the practical applicability of these materials, we fabricated OTFTs using bilayer PVPy/NR and chitosan/NR films as gate dielectric materials. NR not only improved the insulation properties of the dielectric film but also acted as a surface modifier. The fabricated OTFT incorporating chitosan/NR dielectric operated under a low driving voltage, with values of mu and V-th of 0.027 cm(2)V(-1)s(-1) and 0.83 V, respectively. It also displayed an electrical response in the presence of DNA at different concentrations. Because, chitosan possesses high transparency and good mechanical properties, it can be readily cast to form transparent flexible substrates. Accordingly, a flexible disposable OTFT device was fabricated using PVPy/NR as the dielectric material on chitosan as the substrate. The disposable device exhibited a carrier mobility of 0.011 cm(2)V(-1)s(-1), a value of V-th of 0.83 V, and an on/off ratio of 10(3), while being workable over 100 bending cycles.
Novel metallo-dendrimers containing various Ru core ligands and dendritic thiophene arms for photovoltaic applications
(2014)
Satapathy, Rudrakanta
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Ramesh, Mohan
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Padhy, Harihara
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Chiang, I. -Hung
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Chu, Chih-Wei
Three series of supramolecular mono-(i.e., Ru1G1, Ru1G2 and Ru1G3), bis-(i.e., BTRu2G1, BTRu2G2 and BTRu2G3) and tris-(i.e., TPARu3G1, TPARu3G2 and TPARu3G3) Ru-based dendritic complexes were synthesized. Their photophysical and electrochemical properties were investigated. These metallo-dendritic complexes covered a broad absorption range of 250-750 nm with optical bandgaps of 1.51-1.86 eV. The energy levels of the metallo-dendrimers can be effectively adjusted not only by different generations of dendritic thiophene arms but also by their pi-conjugated core ligands bearing various electron donor (i.e., triphenylamine) and acceptor (i.e., benzothiadiazole) moieties. Due to the donor-acceptor effect, the bis-Ru-based dendrimers containing a benzothiadiazole electron-acceptor core ligand showed the highest power conversion efficiency (PCE) among these three series of metallo-dendrimers. The tris-Ru-based architecture with a triphenylamine electron-donor core ligand revealed moderate photovoltaic performance. Among the different generations (G1-G3) of dendrimers, the third generation (G3) possessed the highest PCE values in each series of Ru-based dendrimers. Hence, the third generation bis-\'Ru\'-based dendrimer BTRu2G3 blended with PC70BM (1 : 3 w/w) showed the highest PCE value of 0.77% (without the aid of additives or annealing), which is the highest efficiency among all bulk hetero-junction (BHJ) solar cells containing metallo-dendrimers reported to date.
Organic thin film transistors as selective sensing platforms for Hg2+ ions and the amino acid cysteine
(2013-04-15)
Ramesh, Mohan
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Lin, Hong-Cheu
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Chu, Chih-Wei
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材料科學與工程學系
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光電工程學系
In this short communication we report a sensor for divalent mercuric (Hg2+) ions that we constructed from a perylene bisimide (PBI)-based organic thin film transistor. We improved the performance of the n-channel device by positioning N,N'-dioctyl-3,4,9,10-perylenedicarboximide between the dielectric and the active layer (PSI), increasing the electron mobility (mu) from 0.002 to 0.25 +/- 0.04 cm(2) V-1 s(-1) and enhancing the on/off ratio (I-on/I-off) by two orders of magnitude (from 102 to 104). Based on a "thymine-Hg2+-thymine" mechanism and monitoring the shift in the threshold voltage (V-th), we used this transistor to discriminate Hg2+ ions from mixed ion solutions and it extended to different concentration Hg2+ solutions (from 50 to 350 mu M). By monitoring the shifts in drain current (I-DS) and V-th, we also used this bilayer device as a sensor for cysteine, a thiol-containing amino acid; the selective detection of cysteine was accompanied by a red shift in the fluorescence maximum of PBI, from 532 to 537 nm. (C) 2012 Elsevier B.V. All rights reserved.
Pyrene-SH functionalized OTFT for detection of Hg2+ ions in aquatic environments
(2019-06-01)
Rullyani, Cut
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Shellaiah, Muthaiah
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Ramesh, Mohan
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Lin, Hong-Cheu
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Chu, Chih-Wei
Mercury ion (Hg2+) sensor based on bottom gate top contact organic thin film transistor (OTFT) was fabricated. The OTFT channel area was functionalized with pyrene that contain thiol group, which has strong binding affinity toward Hg2+ ion. The OTFT sensor exhibited a charge mobility of 0.28 cm(2) V-1 s(-1), a threshold voltage of -22.3 V and on-to-off ratio 10(3). The sensor shows high selectivity to Hg2+ ion over other two valence metal ions. OTFT sensor exhibited high sensitivity to Hg2+ ion, indicated by increasing of drain current after exposed to different concentration of Hg2+ ion ranging from 1 mM to 0.01 mu M. Moreover, the OTFT sensor capability for practical application was also demonstrated by sensing the present of 25 mu M of Hg2+ ion in tap, drinking and seawater samples.
Solution-Processed Small-Molecule Bulk Heterojunction Ambipolar Transistors
(2014-04-01)
Cheng, Shiau-Shin
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Huang, Peng-Yi
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Ramesh, Mohan
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Chang, Hsiu-Chieh
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Chen, Li-Ming
Solution-processed small-molecule bulk heterojunction (BHJ) ambipolar organic thin-film transistors are fabricated based on a combination of [2-phenylbenzo[d,d \']thieno[3,2-b;4,5-b \']dithiophene (P-BTDT) : 2-(4-n-octylphenyl)benzo[d,(d) under bar \']thieno[3,2-b;4,5-b\']dithiophene (OP-BTDT)] and C-60. Treating high electrical performance vacuum-deposited P-BTDT organic semiconductors with a newly developed solution-processed organic semiconductor material, OP-BTDT, in an optimized ratio yields a solution-processed p-channel organic semiconductor blend with carrier mobility as high as 0.65 cm(2) V-1 s(-1). An optimized blending of P-BTDT:OP-BTDT with the n-channel semiconductor, C-60, results in a BHJ ambipolar transistor with balanced carrier mobilities for holes and electrons of 0.03 and 0.02 cm(2) V-1 s(-1), respectively. Furthermore, a complementary-like inverter composed of two ambipolar thin-film transistors is demonstrated, which achieves a gain of 115.
Star-shaped self-assembly of an organic thin film transistor sensor in the presence of Cu2+ and CN- ions
(2014-02-01)
Ramesh, Mohan
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You, Yuan-Ruei
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Shellaiah, Muthaiah
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Wu, Meng-Chyi
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Lin, Hong-Cheu
In this study we developed organic thin film transistors (OTFTs) for the sensing of metal ions and anions through the self-assembly of a pentacene/Schiff base pyrene derivative. Our bilayer OTFTs displayed attractive device parameters: an electron mobility (mu) of 0.12 cm(2) V-1 s(-1), a threshold voltage (Vth) of 22.20 V, and a five-orders-of-magnitude on/off ratio. This device was sensitive toward Cu2+ among 13 metal cations and toward CN among nine anions, as measured through changes in the values of Vth and I-off in the presence of Cu2+ cations and a change in the value of I-sat in the presence of CN anions. We observed selectivity toward both of these ions in mixed ion solutions, with sensitivity over different concentrations (from 20 to 350 mu M for Cu2+; from 100 to 350 mu M for CN ) as well as in sea water. The pyrene derivative self-assembled through pyrene-pyrene coordination in the presence of Cu2+ ions; the rods of the pyrene derivative broke into smaller pieces upon formation of benzoxazole rings in the presence of CN ions, as confirmed using atomic force microscopy and fourier transform attenuated total reflection spectroscopy. (C) 2013 Elsevier B.V. All rights reserved.
Synthesis and applications of a novel supramolecular polymer network with multiple H-bonded melamine pendants and uracil crosslinkers
(2012-03-01)
Patra, Dhananjaya
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Ramesh, Mohan
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Sahu, Duryodhan
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Padhy, Harihara
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Chu, Chih-Wei
A conjugated main-chain copolymer (PBT) consisting of bithiazole, dithieno[3,2-b:2',3'-d]pyrroles (DTP), and pendent melamine units was synthesized by Stille polymerization, which can be hydrogen-bonded (H-bonded) with proper molar amounts of bi-functional p-conjugated crosslinker F (i.e., two uracil motifs covalently attached to a fluorene core through triple bonds symmetrically) to develop a novel supramolecular polymer network (PBT/F). The effects of multiple H-bonds on light harvesting capabilities, HOMO levels, and photovoltaic properties of polymer PBT and H-bonded polymer network PBT/F are investigated. The formation of supramolecular polymer network (PBT/F) between PBT and F was confirmed by FTIR and XRD measurements. Because of the stronger light absorption, lower HOMO level, and higher crystallinity of H-bonded polymer network PBT/F, the solar cell device containing PBT/F showed better photovoltaic properties than that containing polymer PBT. The preliminary results show that the solar cell device containing 1:1 weight ratio of PBT/F and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) offers the best power conversion efficiency (PCE) value of 0.86% with a short-circuit current density (Jsc) of 4.97 mA/cm2, an open circuit voltage (Voc) of 0.55 V, and a fill factor (FF) of 31.5%. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012
Synthesis of Main-Chain Metallo-Copolymers Containing Donor and Acceptor Bis-Terpyridyl Ligands for Photovoltaic Applications
(2012-04-13)
Padhy, Harihara
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Ramesh, Mohan
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Patra, Dhananjaya
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Satapathy, Rudrakanta
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Pola, Murali Krishna
Two random (ZnII-based P1P2) and two alternating (RuII-based P3P4) metallo-copolymers containing bis-terpyridyl ligands with various central donor (i.e., fluorene or carbazole) and acceptor (i.e., benzothiadiazole) moieties were synthesized. The effects of electron donoracceptor interactions with metal (ZnII and RuII) ions on their thermal, optical, and electrochemical properties were investigated. Because of the strong ICT transitions between donor and acceptor ligands in both ZnII- and RuII-based metallo-coplymers and MLCT transitions in RuII-based metallo-coplymers, the absorption spectra covered a broad range of 260750 nm with the band gaps of 1.571.77 eV. In addition, the introduction of RuII-based metallo-coplymer P4 mixed with PC60BM as an active layer of the BHJ solar cell device exhibited the highest PCE value up to 0.90%.