Browsing by Author "Keshtov, Mukhamed L."

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Cross-Linkable Hole-Transport Materials Improve the Device Performance of Perovskite Light-Emitting Diodes
(2016-10-12)
Huang, Chiung-Fu
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Keshtov, Mukhamed L.
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Chen, Fang-Chung
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光電工程學系
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Department of Photonics
Hybrid organic/inorganic perovskites are promising candidate materials for use in photovoltaic applications. More recently, they have also become highly attractive as active materials for other optoelectronic devices, including lasers, light-emitting diodes, and photodetectors. Nevertheless, difficulties in forming continuous and uniform films and the existence of a charge-injection barrier between the perovskite layer and the electrodes have hindered the development of high-performance perovskite light-emitting diodes (PeLEDs). In this study, a cross-linked hole-transport layer (HTL) is introduced to improve the hole-injection efficiency of PeLEDs. Furthermore, this layer simultaneously facilitates the formation of smooth perovskite layers, presumably because of the different surface energies. More interestingly, the HTL also exhibits strong solvent effects on the device performance. When the processing solvent for fabricating the HTLs is changed from chlorobenzene to N,N-dimethylformamide (DMF), the perovskite layer becomes more uniform and continuous, leading to better surface coverage and higher device efficiency, presumably because DMF has strong affinity toward the perovskite precursors. The approach presented herein could become a general method for decreasing the hole-injection barrier of PeLEDs and, eventually, lead to higher device performance.
Efficient solar cells based on a new polymer from fluorinated benzothiadiazole and alkylthienyl substituted thieno[2,3-f]benzofuran
(2015-05-01)
Cui, Ruili
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Zou, Yingping
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Xiao, Lu
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Hsu, Chain-Shu
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Keshtov, Mukhamed L.
A new conjugated D-A copolymer, TBFBT, containing a fluorinated benzothiazole electron-acceptor unit and an electron-donor segment of alkylthienyl substituted thieno[2,3-f]benzofuran, was synthesized using a Stille coupling reaction. The resulting copolymer was characterized by elemental analysis, GPC, TGA, UV-Vis absorption spectroscopy and cyclic voltammetry measurements. The copolymer was readily dissolved in common organic solvents, exhibited good film forming properties and displayed a broad absorption from 300 nm to 800 nm with a low optical bandgap of 1.56 eV. Cyclic voltammetry measurement gave HOMO and LUMO energy levels of -5.11 eV and -3.49 eV, respectively. Polymer solar cells based on TBFBT: PC61BM (1:1.5, w/w) demonstrated an initial power conversion efficiency (PCE) of 4.1% with a V-OC of 0.72 V and a J(SC) of 11.6 mA cm(-2). PSCs based on TBFBT:PC71BM (1:1.5, w/w, 3 vol% 1,8-diiodooctane as additive) were further optimized by using methanol. The optimized result exhibited a high PCE up to 6.1% with a high J(SC) of 14.4 mA cm(-2) and FF of 0.62, under the illumination of AM1.5G, 100 mWcm(-2). These investigations indicate that the new copolymer TBFBT is a promising donor material for PSCs and methanol treatment is a simple and effective way to improve PCE. (C) 2015 Elsevier Ltd. All rights reserved.
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.