Browsing by Author "Kekuda, Dhananjay"

Now showing 1 - 20 of 23

Annealing effect of polymer bulk heterojunction solar cells based on polyfluorene and fullerene blend
(2009-02-01)
Huang, Jen-Hsien
;
Yang, Chuan-Yi
;
Ho, Zhong-Yo
;
Kekuda, Dhananjay
;
Wu, Meng-Chyi
Control of blend morphology at the nanoscale and high charge mobility is essential for polymer photovoltaic devices in terms of their power conversion efficiencies (PCE). In the case of bulk heterojunctions solar cells, both blend morphology and charge mobility are influenced by thermal treatment. In this manuscript, we study the effects of annealing temperature on polymer PV devices with blends of poly[9,9'-dioctyl-fluorene-co-bithiophene] (F8T2) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). The morphological changes of blended films were observed upon thermal annealing temperature near and above glass transition temperature (130 degrees C). Such microstructural transformations resulted in modified charge transport pathways and therefore greately influenced the device performance. The highest PCE of 2.14% with an open-circuit voltage (V(OC)) of 0.99 V and a short-circuit current (J(SC)) of 4.24 mA/cm(2) was achieved by device annealing at 70 degrees C for 20 min. (C) 2008 Elsevier B.V. All rights reserved.
Controlled Growth of Nanofiber Network Hole Collection Layers with Pore Structure for Polymer-Fullerene Solar Cells
(2008-12-04)
Huang, Jen-Hsien
;
Ho, Zhong-Yo
;
Kekuda, Dhananjay
;
Chu, Chih-Wei
;
Ho, Kuo-Chuan
In this study, we compare a series of hole collector layers (HCLs) with pore structure fabricated via an electrochemical method to construct polymer-fullerene solar cells. The HCLs with a pore structure can offer a large interface to enhance hole collection; however, the series resistances are also increased by the relatively pore morphology. Photovoltaic device with the largest short circuit current (J(sc)) and efficiency is achieved using poly(3,4-propylenedioxythiophene) (PProDoT) as HCLs due to its highly porous structure and reasonable series resistance. By further optimizing the thickness of the HCLs in the solar cell, a power efficiency of 3.57% under simulated sun light is achieved.
Correlation between Exciton Lifetime Distribution and Morphology of Bulk Heterojunction Films after Solvent Annealing
(2010-05-20)
Huang, Jen-Hsien
;
Li, Kuang-Chieh
;
Chien, Fan-Ching
;
Hsiao, Yu-Sheng
;
Kekuda, Dhananjay
We have synthesized a low-bandgap polymer, bithiazole-based polymer (PCPDTTBT) containing cyclopentadithiophene and thiophene units, and studied its opticophysical properties and morphologies in bulk heterojunction films after annealing. We used confocal optical microscopy in conjunction with a fluorescence module to record exciton lifetime images within photoactive layers of PCPDTTBT and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). These images, which were consistent with those recorded using atomic force microscopy and transmission electron microscopy, revealed that phase separation of PCPDTTBT and PCBM occurred during slow solvent evaporation. This phase separation not only provided bicontinuous pathways for carrier transport to the respective electrodes hut also enhanced the degree of polymer chain stacking, thereby improving the absorption and balancing the electron and hole mobilities. We tested the performance of solar cells incorporating PCPDTTBT and various fullerene derivatives. Under AM 1.5 G illumination (100 mW cm(-2)), a PCPDTTBT/bisPCBM-based solar cell exhibited a power conversion efficiency of 3.8%, with a short circuit current of 7.3 mA cm(-2), an open circuit voltage of 0.88 V, and a fill factor of 59.1%.
Effects of nanomorphological changes on the performance of solar cells with blends of poly[9,9 '-dioctyl-fluorene-co-bithiophene] and a soluble fullerene
(2009-01-14)
Huang, Jen-Hsien
;
Ho, Zhong-Yo
;
Kekuda, Dhananjay
;
Chang, Yung
;
Chu, Chih-Wei
Controlled nanophase segregation within the blended films of a conjugated polymer and a soluble fullerene has enabled us to form a continuous transfer pathway for the carriers, thereby increasing the photocurrent generation for polymer photovoltaic devices. Here, we study the effects of nanomorphological changes on the performance of polymer solar cells using blended films of poly[9,9'-dioctyl-fluorene-co-bithiophene] (F8T2) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Different weight ratios of the F8T2 and PCBM blends in various solvents were studied at different annealing temperatures. The morphology of the films seems to be a strong function of the processing conditions. The power conversion efficiency (PCE) of the photovoltaic devices has improved significantly from 0.34% to 2.14% under air mass 1.5 simulated solar illumination (100 mW cm(-2)), which could be attributed to the nanomorphological changes in the films.
Efficient bilayer polymer solar cells possessing planar mixed-heterojunction structures
(2010)
Huang, Jen-Hsien
;
Li, Kuang-Chieh
;
Kekuda, Dhananjay
;
Padhy, Hari Hara
;
Lin, Hong-Cheu
We have investigated the influence of thermal annealing on the performance of polymer bilayer solar cell devices incorporating poly{2,6-(4,4-bis[2-ethylhexyl]-4H-cyclopenta[2,1-b;3,4-b'] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)} (PCPDTTBT) as the donor and two kinds of fullerenes (C(60), C(70)) as acceptors. The higher absorption of C(70) increased the external quantum efficiency in the spectral range 400-600 nm. We observed morphological changes of the polymer films when the pre-annealing temperature was near the crystalline temperature (T(c), 207 degrees C). These nanostructural transformations resulted in a modified interfacial morphology of the donor phases and, therefore, greatly influenced the device performance. Post-annealing treatment reorganized the interface between the donor and acceptor phases, leading to better contact. The highest power conversion efficiency (2.85%) was obtained when we performed device pre- and post-annealing both at 200 degrees C for 30 min; the open-circuit voltage was 0.69 V and the short-circuit current was 8.42 mA cm(-2).
Efficient bulk heterojunction solar cells based on a low-bandgap polyfluorene copolymers and fullerene derivatives
(2009-09-01)
Huang, Jen-Hsien
;
Li, Kuang-Chieh
;
Wei, Hung-Yu
;
Chen, Po-Yen
;
Lin, Lu-Yin
A low-bandgap polymer (PF-PThCVPTZ) consisted of fluorene and phenothiazine was designed and synthesized. With the donor-acceptor segment, the partial charge transfer can be built in the polymer backbone leading to a wide absorbance. The absorption spectrum of PF-PThCVPTZ exhibits a peak at 510 nm and an absorption onset at 645 nm in the visible range. As blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor, narrow bandgap PF-PThCVPTZ as electron donor shows significant solar cell performance. Under AM 1.5 G, 100 mA/cm(2) illumination, a power conversion efficiency (PCE) of 1.85% was recorded, with a short circuit current (J(SC)) of 5.37 mA/cm(2), an open circuit voltage (V(OC)) of 0.80 V, and a fill factor (FF) of 43.0%. (C) 2009 Elsevier B.V. All rights reserved.
Electrochemical characterization of the solvent-enhanced conductivity of poly(3,4-ethylenedioxythiophene) and its application in polymer solar cells
(2009)
Huang, Jen-Hsien
;
Kekuda, Dhananjay
;
Chu, Chih-Wei
;
Ho, Kuo-Chuan
;
光電工程學系
The influence of solvent on the electrochemical property and conductivity of PEDOT has been investigated by electrochemical and physical characterizations. The PEDOT treated with different solvents reveals a different reversibility of ionic transport and cycling stability, which is associated with the conformational rearrangement from aggregated to linear polymer chains, as evidenced from atomic force microscopy (AFM). The conductivity of the PEDOT thin film can also be enhanced while the degree of linear polymer chains is higher. The application of highly conductive PEDOT thin film as a buffer layer in the polymer photovoltaic devices was realized. A PCE of 4.27% for photovoltaic devices based on P3HT-PCBM under simulated sun light is achieved by using a PEDOT thin film treated with DMSO.
Enhanced spectral response in polymer bulk heterojunction solar cells by using active materials with complementary spectra
(2010-01-01)
Huang, Jen-Hsien
;
Lee, Chuan-Pei
;
Ho, Zhong-Yo
;
Kekuda, Dhananjay
;
Chu, Chih-Wei
We have fabricated organic photovoltaic devices with blends of poly[9,9'-dioctyl-fluorene-co-bithiophene] (F8T2) and fullerene as an electron donor and electron acceptor, respectively. A significant improvement of the photovoltaic efficiency was found in device by using active materials with complementary spectra. The different weight ratios of composite film were also performed to correlate between morphology and the device performance. A power conversion efficiency (PCE) up to 2.0% with an open-circuit voltage of 0.85 V and a short-circuit current (J(SC)) of 5.3 mA/cm(2), was achieved by blending the F8T2 with [6,6]-phenyl-C71-butyric acid methyl ester (PC[70]BM). (C) 2009 Elsevier B.V. All rights reserved.
Fabrication of multilayer organic solar cells through a stamping technique
(2009)
Huang, Jen-Hsien
;
Ho, Zhong-Yo
;
Kuo, Tsung-Hsien
;
Kekuda, Dhananjay
;
Chu, Chih-Wei
In this article, we demonstrate a simple and reliable stamping technique for fabricating multi-layer solar cells. A poly(di-methylsilane) (PDMS) stamp is used for transferring the active layers onto the substrate. An intermediate solvent treatment is introduced to temporarily modify the PDMS surface; therefore, the polymer film can be uniformly formed on top of the PDMS surface. This method is involves non-contaminative and non-invasive processes, therefore it can avoid possible degradation or contamination of the polymer film and the PDMS stamp can be reused. Devices realized through this stamping technique both by direct and inverted structures exhibited power conversion efficiencies of 3.2 and 2.83% respectively.
Fine Tuning of HOMO Energy Levels for Low-Band-Gap Photovoltaic Copolymers Containing Cyclopentadithienopyrrole and Bithiazole Units
(2011-09-15)
Patra, Dhananjaya
;
Sahu, Duryodhan
;
Padhy, Harihara
;
Kekuda, Dhananjay
;
Chu, Chih-Wei
Five conjugated LBG polymers P1-P5 consisting of a DTP unit as an electron donor and various bithiazole units as electron acceptors are designed and synthesized for photovoltaic applications. The effects of the electron-deficient bithiazole derivatives on the thermal, optical, electrochemical, and photovoltaic properties are investigated. The molecular structures are confirmed and characterized. The resulting polymers have high thermal stabilities and broad absorption bands with low optical bandgaps. The hole and electron mobilities are calculated. PSC devices are fabricated utilizing the polymers as electron donors and PCBM as an electron acceptor. The PSC device with an active layer of P4:PCBM (1:1 by weight) exhibits the best power-conversion efficiency.
Flexible Fullerene Field-Effect Transistors Fabricated Through Solution Processing
(2009-12-18)
Sung, Chao-Feng
;
Kekuda, Dhananjay
;
Chu, Li Fen
;
Lee, Yuh-Zheng
;
Chen, Fang-Chung
C60-based thin-film transistors are fabricated through solution processing. On rigid indium tin oxide (ITO) glass, the transistors display electron mobilities as high as 0.21 cm(2) V(-1) s(-1) and a threshold voltage of 0.7V, only slightly lower than those of organic thin-film transistors prepared through vacuum deposition. On ITO-coated PET substrates, the mobilities in the flexible devices (see image) are approximately one order of magnitude lower than those of devices prepared on rigid glass substrates.
Flexible Fullerene Field-Effect Transistors Fabricated Through Solution Processing (vol 21, pg 4845, 2009)
(2010-12-01)
Sung, Chao-Feng
;
Kekuda, Dhananjay
;
Chu, Li Fen
;
Lee, Yuh-Zheng
;
Chen, Fang-Chung
Hybrid TiO(x)/fluoropolymer bi-layer dielectrics for low-voltage complementary inverters
(2010-01-01)
Sung, Chao-Feng
;
Kekuda, Dhananjay
;
Chu, Li Fen
;
Chen, Fang-Chung
;
Cheng, Shiau-Shin
In this article, low temperature processed, reactively evaporated titanium oxide layers were utilized as gate dielectrics for achieving low voltage driven transistors and complementary inverters. The surface of the gate dielectric was modified by an ultra thin (similar to 30 nm) fluoropolymer Cytop (R) layer which partially helped to reduce the leakage in the dielectric films and also enhanced the organic transistor performance. The current investigation demonstrates the ability of these high capacitance bi-layer dielectrics (k similar to 20). The combined p-type and n-type field-effect transistors show similar saturation mobility similar to 0.3 cm(2)/Vs to achieve low voltage driven complementary circuits with output gain of 22. Low temperature processing of these dielectric layers make them easily integrated. (C) 2009 Elsevier B.V. All rights reserved.
Hybrid TiOx/fluoropolymer bi-layer dielectrics for low-voltage complementary inverters
(2010-01-01)
Sung, Chao-Feng
;
Kekuda, Dhananjay
;
Chu, Li Fen
;
Chen, Fang-Chung
;
Cheng, Shiau-Shin
In this article, low temperature processed, reactively evaporated titanium oxide layers were utilized as gate dielectrics for achieving low voltage driven transistors and complementary inverters. The surface of the gate dielectric was modified by an ultra thin (similar to 30 nm) fluoropolymer Cytop (R) layer which partially helped to reduce the leakage in the dielectric films and also enhanced the organic transistor performance. The current investigation demonstrates the ability of these high capacitance bi-layer dielectrics (k similar to 20). The combined p-type and n-type field-effect transistors show similar saturation mobility similar to 0.3 cm(2)/Vs to achieve low voltage driven complementary circuits with output gain of 22. Low temperature processing of these dielectric layers make them easily integrated. (C) 2009 Elsevier B.V. All rights reserved.
Label-free detection of DNA using novel organic-based electrolyte-insulator-semiconductor
(2010-08-15)
Lin, Tsung-Wu
;
Kekuda, Dhananjay
;
Chu, Chih-Wei
;
光電工程學系
;
Department of Photonics
In this study, we have constructed the first organic field effect sensor based on an electrolyte-insulator-semiconductor structure (OEIS) and applied this novel device to pH and DNA sensing. Variations in the insulator-electrolyte surface potential, which originate from either the change of the ionization states of the insulator surface groups or the binding of charged molecules to the insulator surface, modify the flat band voltage (V(FB)) of the OEIS sensor. The pH sensing experiments of OEIS sensor showed that the output signal linearly depended on pH solution in the range from pH 2 to pH 12, and an average sensitivity of 44.1 mV/pH was obtained. In the biosensing experiments, the absorption of positively charged poly-L-lysine on the insulator surface resulted in the reduction of the V(FB) value, whereas the subsequent binding of negatively charged single-stranded DNA probe (ssDNA) via electrostatic interaction increased the VFB value. Furthermore, the ssDNA-immobilized OEIS device was successfully used for the detection of DNA hybridization. The detection limit of complementary DNA was as low as 1 mu M, and the output signal of OEIS biosensor linearly increased with the logarithm of complementary DNA concentration in the range from 5 x 10(-5) to 10(-7) M. The easy and inexpensive fabrication of the OEIS device allows to be served as a potentially disposable and sensitive biosensor. (C) 2010 Elsevier B.V. All rights reserved.
Organic Base Modulation Triodes and Their Inverters on Flexible Substrates
(2009-05-11)
Cheng, Shiau-Shin
;
Chuang, You-Che
;
Kekuda, Dhananjay
;
Ou, Chun-Wei
;
Wu, Meng-Chyi
Organic base modulation triodes (OBMTs) are fabricated using two back-to-back Schottky diodes and deposited on a flexible substrate. With the advantage of the shorter channel, the OBMTs can be operated under lower voltages and faster frequencies. The inverter is fabricated using the OBMT connected with a resistor in series.
Supramolecular Assembly of H-Bonded Side-Chain Polymers Containing Conjugated Pyridyl H-Acceptor Pendants and Various Low-Band-Gap H-Donor Dyes Bearing Cyanoacrylic Acid Groups for Organic Solar Cell Applications
(2009-11-15)
Liang, Tzung-Chi
;
Chiang, I-Hung
;
Yang, Po-Jen
;
Kekuda, Dhananjay
;
Chu, Chih-Wei
Novel supramolecular side-chain polymers were constructed by complexation of proton acceptor (H-acceptor) polymers, i.e., side-chain conjugated polymers P1-P2 containing pyridyl pendants, with low-band-gap proton donor (H-donor) dyes S1-S4 (bearing terminal cyanoacrylic acids) in a proper molar ratio. Besides unique mesomorphic properties confirmed by DSC and XPD results, the H-bonds of supramolecular side-chain structures formed by pyridyl H-acceptors and cyanoacrylic acid H-donors were also confirmed by FTIR measurements. H-donor dyes S1-S4 in solid films exhibited broad absorption peaks located in the range of 471-490 nm with optical band-gaps of 1.99-2.14 eV. Furthermore, H-bonded polymer complexes P1/S1-P1/S4 and P2/S1-P2/S4 exhibited broad absorption peaks in the range of 440-462 nm with optical band-gaps of 2.11-2.25 eV. Under 100 mW/cm(2) of AM 1.5 white-light illumination, the bulk heterojunction polymer solar cell (PSC) devices containing an active layer of H-bonded polymer complexes P1/S1-P1/S4 and P2/S1-P2/S4 (as electron donors) mixed with [6,6]-phenyl C(61) butyric acid methyl ester (i.e., PCBM, as an electron acceptor) in the weight ratio of 1:1 were investigated. The PSC device containing H-bonded polymer complex P1/S3 mixed with PCBM (1:1 w/w) gave the best preliminary result with an overall power conversion efficiency (PCE) of 0.50%, a short-circuit current of 3.17 mA/cm(2), an open-circuit voltage of 0.47 V, and a fill factor of 34%. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5998-6013, 2009
Synthesis and application of H-Bonded cross-linking polymers containing a conjugated pyridyl H-Acceptor side-chain polymer and various carbazole-based H-Donor dyes bearing symmetrical cyanoacrylic acids for organic solar cells
(2010-12-10)
Sahu, Duryodhan
;
Padhy, Harihara
;
Patra, Dhananjaya
;
Kekuda, Dhananjay
;
Chu, Chih-Wei
A series of novel hydrogen-bonded (H-bonded) cross-linking polymers were generated by complexing various proton-donor (H-donor) solar cell dyes containing 3,6- and 2,7-functionalized electron-donating carbazole cores bearing symmetrical thiophene linkers and cyanoacrylic acid termini with a proton-acceptor (H-acceptor) side-chain homopolymer carrying pyridyl pendants (with 1/2 M ratio of H-donor/H-acceptor). The supramolecular H-bonded structures between H-donor dyes and the H-acceptor side-chain polymer were confirmed by FTIR measurements. The effects of the supramolecular architecture on optical, electrochemical, and organic photovoltaic (OPV) properties were investigated. From DFT (density functional theory) calculations, the optimized geometries of organic dyes reflected that the carbazole cores of H-donor dyes were coplanar with the conjugated thiophenes and cyanoacrylic acids, which is essential for strong conjugations across the donor-acceptor units in D1-D4 dyes. Under 100 mW/cm(2) of AM 1.5 white-light illumination, bulk heterojunction (BHJ) OPV cell devices containing an active layer of H-bonded polymers (PDETP/D1-D4) as an electron donor blended with [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM) as an electron acceptor in a weight ratio of 1:1 were explored. From the preliminary investigations, the OPV device containing 1:1 weight ratio of H-bonded polymer PDFTP/D2 and PCBM showed the best power conversion efficiency (PCE) value of 0.31% with a short-circuit current (J(sc)) of 1.9 mA/cm(2), an open-circuit voltage (V(oc)) of 0.55 V. and a fill factor (FF) of 29%, which has a higher PCE value than the corresponding H-donor D2 dye (PCE = 0.15%) or H-acceptor PDFTP homopolymer (PCE = 0.02%) blended with PCBM in 1:1 weight ratio. (C) 2010 Elsevier Ltd. All rights reserved.
Synthesis and Applications of 2,7-Carbazole-Based Conjugated Main-Chain Copolymers Containing Electron Deficient Bithiazole Units for Organic Solar Cells
(2010-12-01)
Patra, Dhananjaya
;
Sahu, Duryodhan
;
Padhy, Harihara
;
Kekuda, Dhananjay
;
Chu, Chih-Wei
A series of low-band-gap (LBG) donor-accepor conjugated main-chain copolymers (P1-P4) containing planar 2,7-carbazole as electron donors and bithiazole units (4,4'-dihexyl-2,2'-bithiazole and 4,4'-dihexyl-5,5'-di(thiophen-2-yl)-2,2'-bithiazole) as electron acceptors were synthesized and studied for the applications in bulk heterojunction (BHJ) solar cells. The effects of electron deficient bithiazole units on the thermal, optical, electrochemical, and photovoltaic (PV) properties of these LBG copolymers were investigated. Absorption spectra revealed that polymers P1-P4 exhibited broad absorption bands in UV and visible regions from 300 to 600 nm with optical band gaps in the range of 1.93-1.99 eV, which overlapped with the major region of the solar emission spectrum. Moreover, carbazole-based polymers P1-P4 showed low values of the highest occupied molecular orbital (HOMO) levels, which provided good air stability and high open circuit voltages (V(oc)) in the PV applications. The BHJ PV devices were fabricated using polymers P1-P4 as electron donors and (6,6)-phenyl-C(61)-butyric acid methyl ester (PC(61)BM) or (6,6)-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) as electron acceptors in different weight ratios. The PV device bearing an active layer of polymer blend P4:PC(71)BM (1:1.5 w/w) showed the best power conversion efficiency value of 1.01% with a short circuit current density (J(sc)) of 4.83 mA/cm(2), a fill factor (FF) of 35%, and V(oc) - 0.60 V under 100 mW/cm(2) of AM 1.5 white-light illumination. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5479-5489, 2010
Synthesis and Applications of Low-Bandgap Conjugated Polymers Containing Phenothiazine Donor and Various Benzodiazole Acceptors for Polymer Solar Cells
(2010-11-01)
Padhy, Harihara
;
Huang, Jen-Hsien
;
Sahu, Duryodhan
;
Patra, Dhananjaya
;
Kekuda, Dhananjay
A series of soluble donor-acceptor conjugated polymers comprising of phenothiazine donor and various benzodiazole acceptors (i.e., benzothiadiazole, benzoselenodiazole, and benzoxadiazole) sandwiched between hexyl-thiophene linkers were designed, synthesized, and used for the fabrication of polymer solar cells (PSC). The effects of the benzodiazole acceptors on the thermal, optical, electrochemical, and photovoltaic properties of these low-bandgap (LBG) polymers were investigated. These LBG polymers possessed large molecular weight (M(n)) in the range of 3.85-5.13 x 10(4) with high thermal decomposition temperatures, which demonstrated broad absorption in the region of 300-750 nm with optical bandgaps of 1.80-1.93 eV. Both the HOMO energy level (-5.38 to -5.47 eV) and LUMO energy level (-3.47 to -3.60 eV) of the LBG polymers were within the desirable range of ideal energy level. Under 100 mW/cm(2) of AM 1.5 white-light illumination, bulk heterojunction PSC devices containing an active layer of electron donor polymers mixed with electron acceptor [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(61)BM) or [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) in different weight ratios were investigated. The best performance of the PSC device was obtained by using polymer PP6DHTBT as an electron donor and PC(71)BM as an acceptor in the weight ratio of 1:4, and a power conversion efficiency value of 1.20%, an open-circuit voltage (V(oc)) value of 0.75 V, a short-circuit current (J(sc)) value of 4.60 mA/cm(2), and a fill factor (FF) value of 35.0% were achieved. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4823-4834, 2010