Browsing by Author "Sharma, Ganesh D."

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New D-A1-D-A2-Type Regular Terpolymers Containing Benzothiadiazole and Benzotrithiophene Acceptor Units for Photovoltaic Application
(2016-12-07)
Keshtov, Mukhamed L.
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Khokhlov, Alexei R.
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Kuklin, Serge A.
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Chen, Fang-Chung
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Koukaras, Emmanuel N.
Two novel regular terpolymers that are of D-A1-D-A2 type and contain benzothiadiazole and 2,5-dibromo-8-dodecanoylbenzo[1,2-b:3,4-b\':5,6-d \'\']trithiophene (P1) or 2,8-dibromo-5-dodecanoylbenzene[1,2-b:3,4-b\':5,6-d6-d \'\']trithiophene (P2) acceptor units with the same thiophene donor were synthesized through Stille coupling, and their optical and electrochemical properties were investigated. The highest occupied molecular orbital (HOMO) and lowest unoccupied (LUMO) molecular orbital energy levels of these terpolymers indicate that there is sufficient LUMO offset with PCBM for efficient exciton dissociation, and their deeper HOMO levels ensure the high open-circuit voltage for the resultant bulk heterojunction solar cells. Measurements on the solar cell devices also confirm that compared to those based on P2 the devices based on P1 possess a higher short-circuit photocurrent (J(sc)) as well as a higher fill factor (FF), which is attributed to the lower bandgap and higher hole mobility for P1, whereas the V-oc is higher for the devices that are based on P2, which may be a result of P2 having a lower HOMO energy level than P1. The optimized polymer solar cells fabricated using P1:PC71BM (DIO/CF) and P2:PC71BM (CF/DIO) for the active layers showed a PCE of 7.19% and 6.34%, respectively. Atomic force microscopy (AFM) images of PEPC71BM blend films show that they exhibit more suitable morphology with favorable interpenetrating networks, which favors high J(sc) and FF. Moreover, P1 exhibits a more crystalline nature than P2 that also favors the charge transport. This may be a result of better molecular packing, more distinct phase separation of the blended films, as well as a reduction of charge recombination.
New iridium-containing conjugated polymers for polymer solar cell applications
(2018-11-07)
Keshtov, M. L.
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Kuklin, S. A.
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Konstantinov, I. O.
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Chen, Fang-Chung
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Xie, Zhi-yuan
A series of novel donor-acceptor (D-A) copolymers P1-P5 with iridium-complexed moieties in their side chains have been synthesized on the basis of a new iridium-containing monomer. The results obtained show that P1-P5 have good thermal stability (317-347 degrees C) for photovoltaic applications. These copolymers absorb visible light in a broad spectral range up to 680 nm. The optical bandgaps of P1-P5 are in the range of 1.96-2.08 eV, respectively. The HOMO and LUMO energy levels of the polymers P1-P5 estimated from cyclic voltammetry measurements indicate that these copolymers are suitable as electron donors along with PC71BM as an electron acceptor for bulk heterojunction polymer solar cells. BHJ polymer solar cells were developed based on blend compositions (P1-P5):PC71BM. The values of J(sc), V-oc, and FF are in the range of 0.95-4.44 mA cm(-2), 0.67-0.69 V and 34.6-56.8%, and the power conversion efficiencies (PCE) are in the range of 0.22-1.74%, respectively, the highest value of 1.74% being for P3. Increase of the photovoltaic parameters was achieved with increasing iridium complex percentage in the polymers due to involvement of triplet effects. The improvement in the efficiency of the triplet-forming polymers P2 and P3 in comparison with P1 appears to be due to the formation of triplet excitons in comparison with singlet excitons in the polymer P1 which does not contain heavy metals. With further increase in the content of iridium complex fragments in the polymers, for example, up to 3 mol% for polymer P4, the efficiency falls to 1.23% and further decreased to 0.22% for P5.
Photoexfoliation of two-dimensional materials through continuous UV irradiation
(2017-03-24)
Lin, Wai-Chen
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Chuang, Ming-Kai
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Keshtov, Muchamed L.
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Sharma, Ganesh D.
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Chen, Fang-Chung
This paper describes the preparation of fluorinated graphene nanosheets (FGNs) through photoexfoliation of fluorinated graphite (FG) in the liquid phase. We discovered that UV irradiation of FG dispersions in N-methyl-2-pyrolidone facilitated exfoliation to give FGNs. Transmission electron microscopy and atomic force microscopy revealed that the average thickness of the FGNs was approximately 3 nm; they were considerably thinner than the nanosheets prepared using a conventional sonication approach. Furthermore, when the FGNs were deposited uniformly onto substrates (through spin coating), they formed effective cathode interlayers for polymer solar cells (PSCs), the efficiency of which was 60% greater than that of PSCs containing FGNs prepared through ultrasonication.
Plasmonic effects of copper nanoparticles in polymer photovoltaic devices for outdoor and indoor applications
(2020-06-22)
Huang, Chien-Lun
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Kumar, Gautham
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Sharma, Ganesh D.
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Chen, Fang-Chung
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交大名義發表
The use of metal nanoparticles (NPs) that can trigger localized surface plasmon resonance (LSPR) is an effective method for improving the performance of organic photovoltaics (OPVs). Currently, most plasmonic NPs are based on noble metals, including gold and silver; their high cost limits their commercial applications in the cost-effective OPVs. Herein, copper (Cu) NPs, which are more abundant and cheaper, are adopted to fabricate OPVs. To avoid oxidation of Cu NPs, they are positioned at the cathode interface, so that their fabrication could be implemented in an inert environment. The resulting OPVs exhibited improved power conversion efficiencies (PCEs) under illumination at 1 sun, and the device enhancement could be attributed to the LSPR effects of Cu NPs. Further, their potential to enhance the performance of OPVs under indoor lighting conditions is evaluated. The enhancement factor of PCEs was higher, while the light source had a lower color temperature. It could be due to the fact that the main plasmonic band of the Cu NPs is localized in the red spectral range. The results reveal the consideration of matching between the LSPR spectral range and the emission spectra of the artificial light sources is very critical for indoor applications.
Polymer solar cells based low bandgap A1-D-A2-D terpolymer based on fluorinated thiadiazoloquinoxaline and benzothiadiazole acceptors with energy loss less than 0.5 eV
(2017-07-01)
Keshtov, M. L.
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Kuklin, S. A.
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Khokhlov, A. R.
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Osipov, S. N.
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Radychev, N. A.
We synthesized an ultra low bandgap terpolymer denoted as P containing fluorinated-fluorene attached thiadiazoloquinoxaline and benzothiadiazole acceptors and thiophene as donor in its backbone and investigated its optical and electrochemical properties. This terpolymer is used for as donor along with PC71BM as electron acceptor in solution processed polymer solar cells (PSCs). The P showed a shows strong absorption band from 650 nm to 1100 nm with an optical bandgap of 1.12 eV and highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of -5.25 eV and -3.87 eV, respectively. After the optimization of P to PC71BM weight ratio, the optimized weight ratio 1: 2 in chlorobenzene (CB) solution, the PSC showed overall power conversion efficiency of 4.10% (J(sc) of 10.96 mA/cm(2), V-oc of 0.68 V and FF of 0.55). After the solvent additive (3 v% DIO) followed by subsequent thermal annealing (SA-TA) the PCE has been increased up to 7.54% with Jsc of 16.12 mA/cm(2), Voc of 0.65 V and FF of 0.72. The increase in the PCE is related with the enhancement in the both Jsc and FF, attributed optimized nanoscale morphology of the active layer for both efficient exciton dissociation and charge transport towards the electrodes and balanced charge transport in the device, induced by the TSA treatment of the active layer. This is the highest PCE of PSCs with an energy loss about 0.47 eV with the low bandgap of 1.12 eV. (C) 2017 Elsevier B.V. All rights reserved.
Small molecule based N-phenyl carbazole substituted diketopyrrolopyrroles as donors for solution-processed bulk heterojunction organic solar cells
(2016-09-07)
Patil, Yuvraj
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Misra, Rajneesh
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Chen, F. C.
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Sharma, Ganesh D.
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光電工程學系
We report two acetylene-bridged small molecules DPP5 and DPP6 with low HOMO-LUMO gaps as donors along with PC71BM as an acceptor for the fabrication of solution-processed bulk heterojunction solar cells. After the optimization, i.e. weight ratio of donor to acceptor and surface treatment of the active layer, we achieved overall power conversion efficiencies up to 4.65% (J(SC) = 8.19mA cm(-2), V-OC = 0.98 V and FF = 0.58) and 5.73% (J(SC) = 9.58 mA cm(-2), V-OC = 0.98 V and FF = 0.61), for DPP5:PC71BM and DPP6:PC71BM respectively, which are superior to those for the devices based on as-cast active layers. The significant change in the power conversion efficiency is attributed to the improvement in nanoscale morphology, balanced charge transport and charge collection efficiency, induced through the surface treatment.
Symmetrical and unsymmetrical triphenylamine based diketopyrrolopyrroles and their use as donors for solution processed bulk heterojunction organic solar cells
(2016-01-01)
Patil, Yuvraj
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Misra, Rajneesh
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Chen, F. C.
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Keshtov, M. L.
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Sharma, Ganesh D.
Two small molecules DPP3 (D-pi-A) and DPP4 (D-pi-A-pi-D) with triphenylamine (TPA) donors and diketopyrrolopyrrole (DPP) acceptors linked with ethyne linkers were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. Their photonic, electronic, thermal and computational properties were investigated. The red shift in the electronic absorption spectra of DPP4 as compared to DPP3 is related to extended conjugation and increased donor-acceptor interaction. We have used DPP3 and DPP4 as electron donors along with PC71BM as an electron acceptor for solution processed bulk heterojunction organic solar cells. The solar cells prepared from DPP3: PC71BM and DPP4: PC71BM (1 : 2) processed from chloroform (CF) exhibit a power conversion efficiency (PCE) of 2.23% (J(sc) = 6.74 mA cm(-2), V-oc = 0.92 V and FF = 0.36) and 3.05% (J(sc) = 8.26 mA cm(-2), V-oc = 0.88 V and FF = 0.42), respectively. The higher PCE of the device with DPP4 compared to DPP3 was demonstrated as to the higher hole mobility and broader IPCE spectra. The devices based on DPP3: PC71BM and DPP4: PC71BM processed with solvent additive (1 v% DIO, 1,8-diiodooctane) showed PCE values of 4.06% and 5.31%, respectively. The device optimization results from the improvement of the balanced charge transport and better nanoscale morphology induced by the solvent additive.
Toward High-Performance Polymer Photovoltaic Devices for Low-Power Indoor Applications
(2017-12-01)
Yang, Shun-Shing
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Hsieh, Zong-Chun
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Keshtov, Muchamed L.
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Sharma, Ganesh D.
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Chen, Fang-Chung
This article describes the performance of organic photovoltaic (OPV) devices, incorporating three different polymer/fullerene derivative blends, under low-level lighting conditions. The devices exhibit much higher power conversion efficiencies (PCEs) under indoor lighting conditions than they do under sunlight. The best-performing device is capable of delivering a power output of 22.57Wcm(-2), corresponding to a PCE of 13.76%, under illumination with indoor lighting conditions at 500 lux. Increasing the open-circuit voltage (V-oc) of the OPV devices is the most critical factor for achieving high device performance for low-power indoor applications. Therefore, the device power output will be maximized if we could obtain a larger energy difference between the highest occupied molecular orbital of the polymer donor and the lowest unoccupied molecular orbital of the electron acceptor, thereby ensuring a high value of V-oc.