Imperial College London

DrHyojungCha

Faculty of Natural SciencesDepartment of Chemistry

Research Associate
 
 
 
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h.cha

 
 
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G22Molecular Sciences Research HubWhite City Campus

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Summary

 

Publications

Publication Type
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52 results found

Kosco J, Bidwell M, Cha H, Martin T, Howells CT, Sachs M, Anjum DH, Gonzalez Lopez S, Zou L, Wadsworth A, Zhang W, Zhang L, Tellam J, Sougrat R, Laquai F, DeLongchamp DM, Durrant JR, McCulloch Iet al., 2020, Enhanced photocatalytic hydrogen evolution from organic semiconductor heterojunction nanoparticles, NATURE MATERIALS, ISSN: 1476-1122

Journal article

Wu J, Luke J, Lee HKH, Tuladhar PS, Cha H, Jang S-Y, Tsoi WC, Heeney M, Kang H, Lee K, Kirchartz T, Kim J-S, Durrant JRet al., 2019, Tail state limited photocurrent collection of thick photoactive layers in organic solar cells, Nature Communications, Vol: 10, ISSN: 2041-1723

Weanalyseorganic solar cells with four differentphotoactive blends exhibiting differing dependencies ofshort-circuit current upon photoactive layer thickness.These blends and devices are analysedbytransient optoelectronic techniques ofcarrier kinetics and densities, airphotoemission spectroscopyof material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We concludethat,for the device series studied, the photocurrent losswith thick active layersis primarilyassociatedwith the accumulation of photo-generated charge carriers in intra-bandgap tail states.This charge accumulation screens the device internal electricalfield, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail statedistribution anddensity and increase the maximal photoactive thickness forefficient operation. Ourwork suggests that selectingorganic photoactive layerswith a narrow distribution of tail states isa keyrequirement for the fabrication of efficient, high photocurrent, thick organic solar cells.

Journal article

Sung MJ, Hong J, Cha H, Jiang Y, Park CE, Durrant JR, An TK, Kwon SK, Kim YHet al., 2019, Acene-Modified Small-Molecule Donors for Organic Photovoltaics, Chemistry - A European Journal, Vol: 25, ISSN: 0947-6539

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Invited for the cover of this issue is the group of Tae Kyu An at the Korea National University of Transportation, Soon-Ki Kwon and Yun-Hi Kim at the Gyeongsang National University. The image depicts organic photovoltaics, in which fused acene cores have been used to modulate the conjugation lengths and the bulk heterojunction morphologies. Read the full text of the article at 10.1002/chem.201902177.

Journal article

Sung MJ, Hong J, Cha H, Jiang Y, Park CE, Durrant JR, An TK, Kwon S-K, Kim Y-Het al., 2019, Acene-Modified Small-Molecule Donors for Organic Photovoltaics, CHEMISTRY-A EUROPEAN JOURNAL, Vol: 25, Pages: 12316-12324, ISSN: 0947-6539

Journal article

Cha H, Fish G, Luke J, Alraddadi A, Lee HH, Zhang W, Dong Y, Limbu S, Wadsworth A, Maria IP, Francas L, Sou HL, Du T, Kim J-S, McLachlan MA, McCulloch I, Durrant JRet al., 2019, Suppression of Recombination Losses in Polymer:Nonfullerene Acceptor Organic Solar Cells due to Aggregation Dependence of Acceptor Electron Affinity, ADVANCED ENERGY MATERIALS, Vol: 9, ISSN: 1614-6832

Journal article

Green JP, Cha H, Shahid M, Creamer A, Durrant JR, Heeney Met al., 2019, Dithieno[3,2-b:2,3-d]arsole-containing conjugated polymers in organic photovoltaic devices, Dalton Transactions, Vol: 48, Pages: 6676-6679, ISSN: 1477-9234

Arsole-derived conjugated polymers are a relatively new class of materials in the field of organic electronics. Herein, we report the synthesis of two new donor polymers containing fused dithieno[3,2-b:2′,3′-d]arsole units and report their application in bulk heterojunction solar cells for the first time. Devices based upon blends with PC71BM display high open circuit voltages around 0.9 V and demonstrate power conversion efficiencies around 4%.

Journal article

Speller EM, Clarke AJ, Aristidou N, Wyatt MF, Francàs L, Fish G, Cha H, Lee HKH, Luke J, Wadsworth A, Evans AD, McCulloch I, Kim JS, Haque SA, Durrant JR, Dimitrov SD, Tsoi WC, Li Zet al., 2019, Toward improved environmental stability of polymer:fullerene and polymer:non-fullerene organic solar cells: a common energetic origin of light and oxygen induced degradation, ACS Energy Letters, Vol: 4, Pages: 846-852, ISSN: 2380-8195

With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation.

Journal article

Seok H, Jeong S-G, Kim K-S, Junyent-Ferre AA, Cha H, Kim Met al., 2019, Instantaneous Reactive Power Reduction of Ripple-Free Resonant Buck Converter Using Bidirectional Switch, 34th Annual IEEE Applied Power Electronics Conference and Exposition (APEC), Publisher: IEEE, Pages: 3174-3179, ISSN: 1048-2334

Conference paper

Dong Y, Cha H, Zhang J, Pastor E, Tuladhar PS, McCulloch I, Durrant JR, Bakulin AAet al., 2019, The binding energy and dynamics of charge-transfer states in organic photovoltaics with low driving force for charge separation, Journal of Chemical Physics, Vol: 150, ISSN: 0021-9606

Recent progress in organic photovoltaics (OPVs) has been enabled by optimization of the energetic driving force for charge separation, and thus maximization of open-circuit voltage, using non-fullerene acceptor (NFA) materials. In spite of this, the carrier dynamics and relative energies of the key states controlling the photophysics of these systems are still under debate. Herein, we report an in-depth ultrafast spectroscopic study of a representative OPV system based on a polymer donor PffBT4T-2OD and a small-molecule NFA EH-IDTBR. Global analysis of the transient absorption data reveals efficient energy transfer between donor and acceptor molecules. The extracted kinetics suggest that slow (∼15 ps) generation of charge carriers is followed by significant geminate recombination. This contrasts with the "reference" PffBT4T-2OD:PC71BM system where bimolecular recombination dominates. Using temperature-dependent pump-push-photocurrent spectroscopy, we estimate the activation energy for the dissociation of bound charge-transfer states in PffBT4T-2OD:EH-IDTBR to be 100 ± 6 meV. We also observe an additional activation energy of 14 ± 7 meV, which we assign to the de-trapping of mobile carriers. This work provides a comprehensive picture of photophysics in a system representing new generation of OPV blends with a small driving force for charge separation.

Journal article

Hong J, Wang C, Cha H, Kim HN, Kim Y, Park CE, An TK, Kwon S-K, Kim Y-Het al., 2019, Morphology Driven by Molecular Structure of Thiazole-Based Polymers for Use in Field-Effect Transistors and Solar Cells., Chemistry, Vol: 25, Pages: 649-656

The effects of the molecular structure of thiazole-based polymers on the active layer morphologies and performances of electronic and photovoltaic devices were studied. Thus, thiazole-based conjugated polymers with a novel thiazole-vinylene-thiazole (TzVTz) structure were designed and synthesized. The TzVTz structure was introduced to extend the π conjugation and coplanarity of the polymer chains. By combining alkylthienyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDT) or dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) electron-donating units and a TzVTz electron-accepting unit, enhanced intermolecular interactions and charge transport were obtained in the novel polymers BDT-TzVTz and DTBDT-TzVTz. With a view to using the polymers in transistor and photovoltaic applications, the molecular self-assembly in and their nanoscale morphologies of the active layers were controlled by thermal annealing to enhance the molecular packing and by introducing a diphenyl ether solvent additive to improve the miscibility between polymer donors and [6,6]phenyl-C71-butyric acid methyl ester (PC71 BM) acceptors, respectively. The morphological characterization of the photoactive layers showed that a higher degree of π-electron delocalization and more favorable molecular packing in DTBDT-TzVTz compared with in BDT-TzVTz leads to distinctly higher performances in transistor and photovoltaic devices. The superior performance of a photovoltaic device incorporating DTBDT-TzVTz was achieved through the superior miscibility of DTBDT-TzVTz with PC71 BM and the improved crystallinity of DTBDT-TzVTz in the nanofibrillar structure.

Journal article

Cha H, Tan C-H, Wu J, Dong Y, Zhang W, Chen H, Rajaram S, Narayan KS, McCulloch I, Durrant JRet al., 2018, An analysis of the factors determining the efficiency of photocurrent generation in polymer:nonfullerene acceptor solar cells, Advanced Energy Materials, Vol: 8, ISSN: 1614-6832

Herein, a meta‐analysis of the device performance and transient spectroscopic results are undertaken for various donor:acceptor blends, employing three different donor polymers and seven different acceptors including nonfullerene acceptors (NFAs). From this analysis, it is found that the primary determinant of device external quantum efficiency (EQE) is the energy offset driving interfacial charge separation, ΔECS. For devices employing the donor polymer PffBT4T blended with NFA and fullerene acceptors, an energy offset ΔECS = 0.30 eV is required to achieve near unity charge separation, which increases for blends with PBDTTT‐EFT and P3HT to 0.36 and ≈1.2 eV, respectively. For blends with PffBT4T and PBDTTT‐EFT, a 100 meV decrease in the LUMO of the acceptor is observed to result in an approximately twofold increase in EQE. Steady state and transient optical data determine that this energy offset requirement is not associated with the need to overcome the polymer exciton binding energy and thereby drive exciton separation, with all blends studied showing efficient exciton separation. Rather, the increase in EQE with larger energy offset is shown to result from suppression of geminate recombination losses. These results are discussed in terms of their implications for the design of donor/NFA interfaces in organic solar cells, and strategies to achieve further advances in device performance.

Journal article

Hong J, Sung MJ, Cha H, Park CE, Durrant JR, An TK, Kim Y-H, Kwon S-Ket al., 2018, Understanding Structure-Property Relationships in All-Small-Molecule Solar Cells Incorporating a Fullerene or Nonfullerene Acceptor, ACS APPLIED MATERIALS & INTERFACES, Vol: 10, Pages: 36037-36046, ISSN: 1944-8244

Journal article

Francas Forcada L, Burns E, Steier L, Cha H, Solà-Hernández L, Li X, Shakya Tuladhar P, Bofill R, García-Antón J, Sala X, Durrant Jet al., 2018, Rational design of a neutral pH functional and stable organic photocathode., Chemical Communications, Vol: 2018, ISSN: 1359-7345

In this work we lay out design guidelines for catalytically more efficient organic photocathodes achieving stable hydrogen production in neutral pH. We propose an organic photocathode architecture employing a NiO hole selective layer, a PCDTBT:PCBM bulk heterojunction, a compact TiO2 electron selective contact and a RuO2 nanoparticle catalyst. The role of each layer is discussed in terms of durability and function. With this strategically designed organic photocathode we obtain stable photocurrent densities for over 5 h and discuss routes for further performance improvement.

Journal article

Kim K, Jeon J, Ha YH, Cha H, Park CE, Kang M, Jhon H, Kwon SK, Kim YH, An TKet al., 2018, Ambipolar charge transport of diketopyrrolepyrrole-silole-based copolymers and effect of side chain engineering: Compact model parameter extraction strategy for high-voltage logic applications, Organic Electronics, Vol: 54, Pages: 1-8, ISSN: 1566-1199

© 2017 The copolymers P24DPP-silole and P29DPP-silole, each composed of diketopyrrolopyrrole (DPP) and silole derivatives, were synthesized using a Stille coupling reaction, and their electrical performances in organic field-effect transistors (OFETs) and circuits were investigated. While both the as-spun OFETs exhibited quite low field-effect hole mobility values, the OFETs subjected to thermal annealing at 150 °C exhibited typical ambipolar transport characteristics with average hole and electron mobility values of 1 × 10−1 cm2/(V s) and 2 × 10−3 cm2/(V s). Because the compact model was necessary to perform circuit design with the synthesized OFETs, a strategy for extracting compact model parameters was proposed for high-voltage logic circuit applications by using the industry standard compact Berkeley short-channel IGFET model (BSIM).

Journal article

Cha H, Wheeler S, Holliday S, Dimitrov SD, Wadsworth A, Lee HH, Baran D, McCulloch I, Durrant JRet al., 2018, Influence of blend morphology and energetics on charge separation and recombination dynamics in organic solar cells incorporating a nonfullerene acceptor, Advanced Functional Materials, Vol: 28, ISSN: 1616-301X

Nonfullerene acceptors (NFAs) in blends with highly crystalline donor polymers have been shown to yield particularly high device voltage outputs, but typically more modest quantum yields for photocurrent generation as well as often lower fill factors (FF). In this study, we employ transient optical and optoelectronic analysis to elucidate the factors determining device photocurrent and FF in blends of the highly crystalline donor polymer PffBT4T-2OD with the promising NFA FBR or the more widely studied fullerene acceptor PC71BM. Geminate recombination losses, as measured by ultrafast transient absorption spectroscopy, are observed to be significantly higher for PffBT4T-2OD:FBR blends. This is assigned to the smaller LUMO-LUMO offset of the PffBT4T-2OD:FBR blends relative to PffBT4T-2OD:PC71BM, resulting in the lower photocurrent generation efficiency obtained with FBR. Employing time delayed charge extraction measurements, these geminate recombination losses are observed to be field dependent, resulting in the lower FF observed with PffBT4T-2OD:FBR devices. These data therefore provide a detailed understanding of the impact of acceptor design, and particularly acceptor energetics, on organic solar cell performance. Our study concludes with a discussion of the implications of these results for the design of NFAs in organic solar cells.

Journal article

Hong J, Ha YH, Cha H, Kim R, Kim YJ, Park CE, Durrant JR, Kwon S-K, An TK, Kim Y-Het al., 2017, All-Small-Molecule Solar Cells Incorporating NDI-Based Acceptors: Synthesis and Full Characterization, ACS APPLIED MATERIALS & INTERFACES, Vol: 9, Pages: 44667-44677, ISSN: 1944-8244

Journal article

Cha H, Wu J, Wadsworth A, Nagitta J, Limbu S, Pont S, Li Z, Searle J, Wyatt MF, Baran D, Kim J-S, McCulloch I, Durrant JRet al., 2017, An efficient, "burn in" free organic solar cell employing a nonfullerene electron acceptor, Advanced Materials, Vol: 29, ISSN: 0935-9648

A comparison of the efficiency, stability, and photophysics of organic solar cells employing poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3'″-di(2-octyldodecyl)-2,2';5',2″;5″,2'″-quaterthiophen-5,5'″-diyl)] (PffBT4T-2OD) as a donor polymer blended with either the nonfullerene acceptor EH-IDTBR or the fullerene derivative, [6,6]-phenyl C71 butyric acid methyl ester (PC71 BM) as electron acceptors is reported. Inverted PffBT4T-2OD:EH-IDTBR blend solar cell fabricated without any processing additive achieves power conversion efficiencies (PCEs) of 9.5 ± 0.2%. The devices exhibit a high open circuit voltage of 1.08 ± 0.01 V, attributed to the high lowest unoccupied molecular orbital (LUMO) level of EH-IDTBR. Photoluminescence quenching and transient absorption data are employed to elucidate the ultrafast kinetics and efficiencies of charge separation in both blends, with PffBT4T-2OD exciton diffusion kinetics within polymer domains, and geminate recombination losses following exciton separation being identified as key factors determining the efficiency of photocurrent generation. Remarkably, while encapsulated PffBT4T-2OD:PC71 BM solar cells show significant efficiency loss under simulated solar irradiation ("burn in" degradation) due to the trap-assisted recombination through increased photoinduced trap states, PffBT4T-2OD:EH-IDTBR solar cell shows negligible burn in efficiency loss. Furthermore, PffBT4T-2OD:EH-IDTBR solar cells are found to be substantially more stable under 85 °C thermal stress than PffBT4T-2OD:PC71 BM devices.

Journal article

Cha H, Park CE, Kwon S-K, An TKet al., 2017, Ternary blends to achieve well-developed nanoscale morphology in organic bulk heterojunction solar cells, ORGANIC ELECTRONICS, Vol: 45, Pages: 263-272, ISSN: 1566-1199

Journal article

Sung MJ, Kim Y, Lee SB, Lee GB, An TK, Cha H, Kim SH, Park CE, Kim YHet al., 2016, New dithienophosphole-based donor-acceptor alternating copolymers: Synthesis and structure property relationships in OFET, Dyes and Pigments, Vol: 125, Pages: 316-322, ISSN: 0143-7208

© 2015 Elsevier Ltd. Two donor-acceptor conjugated copolymers comprised of a novel dithienophosphole (DTP) as the acceptor unit and bithiophene (BT) or decylthiophenebenzodithiophene (DTBDT) as the donor unit were synthesized and characterized to elucidate the relationship between donor-acceptor (D-A) architecture and organic field-effect transistors (OFETs) performance. These characteristics of D-A copolymers were affected by the structural suitability of donor and acceptor units. Poly(DTP-BT) had an ordered structure that facilitated charge carrier transfer. The crystallinity of poly(DTP-BT) increased as annealing temperature (Tann) increased. In contrast, poly(DTP-DTBDT) was amorphous regardless of Tann due to the long alkyl chains of the DTBDT units. OFET devices made with poly(DTP-BT) and annealed at 200°C exhibit a highly crystalline morphology and a relatively high field-effect mobility (4.9 × 10-3 cm2/(V·s)).

Journal article

Lee J, Cha H, Kong H, Seo M, Heo J, Jung IH, Kim J, Shim HK, Park CE, Kim SYet al., 2015, Synthesis of triarylamine-based alternating copolymers for polymeric solar cell, Polymer, Vol: 55, Pages: 4837-4845, ISSN: 0032-3861

© 2014 Elsevier Ltd. Two donor-acceptor alternating copolymers based on electron-rich triarylamine, di(1-(6-(2-ethylhexyl))naphthyl)phenylamine (DNPA), and electron-deficient benzothiadiazole and benzoselenadiazole derivatives were designed and synthesized via Suzuki coupling reaction. The resulting triarylamine-based alternating copolymers PDNPADTBT and PDNPADTBS showed good solubility in common organic solvents and good thermal stability. The optical band gaps determined from the onset absorption were 1.93 and 1.81 eV, respectively. By introducing the naphthalene ring into the triarylamine, copolymers had relatively deep HOMO energy levels of -5.48 and -5.45 eV, which led to a high open circuit voltage (Voc) and good air stability for photovoltaic application. Bulk heterojunction solar cells were fabricated with a structure of ITO/PEDOT-PSS/copolymers-PC70BM/LiF/Al by blending the copolymer with PC70BM. Both blend systems showed remarkably high Voc near 0.9 V, and the highest performance of 2.2% was obtained from PDNPADTBT, with Voc = 0.88 V, Jsc = 7.4 mA/cm2, and a fill factor of 34.4% under AM 1.5 G.

Journal article

An TK, Park SJ, Ahn ES, Jang SH, Kim Y, Kim K, Cha H, Kim YJ, Kim SH, Park CE, Kim YHet al., 2015, Solvent boiling point affects the crystalline properties and performances of anthradithiophene-based devices, Dyes and Pigments, Vol: 114, Pages: 60-68, ISSN: 0143-7208

© 2014 Elsevier Ltd. All rights reserved. We investigated the effects of the solvent boiling point on organic field-effect transistor (OFET) device performances in devices prepared using two newly synthesized anthradithiophene derivatives: 9,10-di(4′-pentylphenylethynyl)-anthra[2,3-b:6,7-b′]dithiophene (DPPEADT) and 9,10-bi([9′,9′-dimethyl-fluoren-2-ylethynyl])-anthra[2,3-b:6,7-b′]dithiophene (DFEADT). DPPEADT exhibited a distinct crystalline morphology whereas DFEADT was amorphous. We characterized the relationship between the molecular structures, film morphologies, and OFET device performances in devices prepared using solvents having one of three different boiling points (chlorobenzene, 1,2-dichlorobenzene, and 1,2,4-trichlorobenzene). 1,2,4-Trichlorobenzene, which provided the highest boiling point among the solvents tested and acted as a good solvent for DPPEADT, significantly improved the field-effect mobilities of DPPEADT devices up to 0.16 cm2/V by enhancing the crystallinity of the film. OFETs based on amorphous DFEADT films prepared using the three solvents did not provide enhanced electrical performances. The differences between the transistor performances were attributed to the degree of π-overlap, the molecular structures, and the morphological properties of the films.

Journal article

Kim MJ, An TK, Kim SO, Cha H, Kim HN, Xiofeng T, Park CE, Kim YHet al., 2015, Molecular design and ordering effects of alkoxy aromatic donor in a DPP copolymer on OTFTs and OPVs, Materials Chemistry and Physics, Vol: 153, Pages: 63-71, ISSN: 0254-0584

© 2014 Elsevier B.V. All rights reserved. Two p-type polymers, PONDPP and PTADPP were synthesized by Suzuki coupling reaction to investigate the effect of alkoxy aromatic donor units in diketopyrrolopyrrole (DPP)-based copolymers on OTFTs and OPVs. PONDPP containing dialkoxynaphthalene exhibits excellent field-effect performances, with a hole mobility of 0.324 cm2/V while PTADPP containing dialkoxyanthracene exhibits mobility of 4.5 × 10-3 cm2/V at 200 °C annealing. Bulk heterojunction type polymer solar cells based on these polymers as the electron donor materials, with PC71BM as the acceptor, showed maximum power conversion efficiency (PCE) of 0.9% for PONDPP and 1.1% for PTADPP under AM 1.5 illumination. From photophysical and structural studies, we found that naphthalene unit was introduced to the DPP unit to enhance more the molecular ordering compared to anthracene unit.

Journal article

Back JY, An TK, Cheon YR, Cha H, Jang J, Kim Y, Baek Y, Chung DS, Kwon SK, Park CE, Kim YHet al., 2015, Alkyl chain length dependence of the field-effect mobility in novel anthracene derivatives, ACS Applied Materials and Interfaces, Vol: 7, Pages: 351-358, ISSN: 1944-8244

© 2014 American Chemical Society. We report six asymmetric alkylated anthracene-based molecules with different alkyl side chain lengths for use in organic field-effect transistors (OFETs). Alkyl side chains can potentially improve the solubility and processability of anthracene derivatives. The crystallinity and charge mobility of the anthracene derivatives may be improved by optimizing the side chain length. The highest field-effect mobility of the devices prepared here was 0.55 cm2/(V s), for 2-(p-pentylphenylethynyl)anthracene (PPEA). The moderate side chain length appeared to be optimal for promoting self-organization among asymmetric anthracene derivatives in OFETs, and was certainly better than the short or long alkyl side chain lengths, as confirmed by X-ray diffraction measurements.

Journal article

Kim Y, An TK, Kim J, Hwang J, Park S, Nam S, Cha H, Park WJ, Baik JM, Park CEet al., 2014, A composite of a graphene oxide derivative as a novel sensing layer in an organic field-effect transistor, Journal of Materials Chemistry C, Vol: 2, Pages: 4539-4544, ISSN: 2050-7534

We report the fabrication of a gas sensor with an oleylamine-modified graphene oxide (OA-GO)/poly(9-9′-dioctyl-fluorene-co-bithiophene) (F8T2) composite as an active layer and demonstrate that it has better sensing ability than a comparable device with an F8T2-only active layer. OA-GO was chosen as the receptor material because of its enhanced interaction with gas analytes and its easy mixing with F8T2. OA-GO was synthesized by a simple condensation reaction between GO and oleylamine (9-octadecylamine), and characterized by Fourier transform infrared spectroscopy. The sensitivities of the gas sensors with respect to acetone and ethanol analytes were investigated by measuring the electrical parameters of the corresponding organic field effect transistor at room temperature. The sensitivity of the OA-GO/F8T2 composite device was up to 34 times that of the F8T2 device for the mobility change of acetone. © 2014 the Partner Organisations.

Journal article

Ma JY, Yun HJ, Kim SO, Lee GB, Cha H, Park CE, Kwon SK, Kim YHet al., 2014, Novel alkoxyanthracene donor and benzothiadiazole acceptor for organic thin film transistor and bulk heterojunction organic photovoltaic cells, Journal of Polymer Science, Part A: Polymer Chemistry, Vol: 52, Pages: 1306-1314, ISSN: 0887-624X

Novel alkoxy anthracene (ODA)-based polymeric semiconductors were designed for polymer solar cell applications. Alkoxyanthracene, which contains many π electrons and electron donating group, was easily synthesized. The copolymers, poly(alkoxy anthracene-alt-thiophene benzothiadiazole thiophene) poly(ODA-TBT) and poly(alkoxy anthracene-alt-benzothiadiazole) poly(ODA-BT), have been obtained by Suzuki coupling polymerization. Both polymers have ODA unit as a donor and benzothiadiazole as an acceptor. ODA-TBT has thiophene linkages between ODA and benzothiadiazole. The optical, thermal, and electrochemical properties have been investigated by UV-visible absorption, thermal gravimetric analysis, differential scanning calorimetry, and CV. Organic thin-film transistor using polymers showed that the hole mobility of poly(ODA-alt-TBT) was around 3.6 × 10-3 cm2/Vs with on/off ratio of 9.91 × 105 while that of poly(ODA-alt-BT) was around 1.21 × 10-2 cm2/Vs with on/off ratio of 2.64 × 10 6. Organic photovoltaic performance based on polymers were evaluated with a configuration of ITO/PEDOT:PSS/active layer/LiF/Al. Poly(ODA-TBT) exhibits a short circuit current (Jsc) of 3.9 mA/cm2 and power conversion efficiency (PCE) of 1.4%, and poly(ODA-BT) exhibits the J sc of 6.4 mA/cm2 and PCE of 2.2%. The better device performance of poly(ODA-BT) is attributed to its charge transfer ability and enhanced mobility and crystallinity although poly(ODA-BT) does not have extended π-conjugation due to twisted structure compared with poly(ODA-TBT). © 2014 Wiley Periodicals, Inc.

Journal article

Lee GB, Kim R, Cha HJ, Park CE, Kim JH, Kim YHet al., 2014, New donor-acceptor copolymer containing dialkoxy naphthalene and carbonylated thieno[3,4-b]thiophene for OTFT and OPV, Macromolecular Research, Vol: 22, Pages: 569-573, ISSN: 1598-5032

A new copolymer (PONTET), composed of dialkoxy napthalene as donor and a carbonylated thieno [3,4-b]thiophene group as an acceptor, was obtained by Suzuki coupling polymerization. The number average molecular weight (M n ) of PONTET is 6.4 kg mol-1 with a polydispersity index of 1.20, determined by gel permeation chromatography using chloroform. PONTET showed broad absorption with a maximum at 534 nm in solution and 618 nm in a thin film, respectively. The hole mobility of PONTET was 1.5×10 -3 cm2/Vs and the I on /I off ratio was 7.7×104. The maximum power conversion efficiency of a PONTET:PC70BM (1:4)-based solar cell reached 1.3% with an open circuit voltage (V oc ) of 0.56 V, a short circuit current density (J sc ) of 5.6 mA/cm2, and a fill factor (FF) of 41.3%.[Figure not available: see fulltext.] © 2014 The Polymer Society of Korea and Springer Sciene+Business Media Dordrecht.

Journal article

Cha H, Kim HN, An TK, Kang MS, Kwon SK, Kim YH, Park CEet al., 2014, Effects of cyano-substituents on the molecular packing structures of conjugated polymers for bulk-heterojunction solar cells, ACS Applied Materials and Interfaces, Vol: 6, Pages: 15774-15782, ISSN: 1944-8244

© 2014 American Chemical Society. The molecular packing structures of two conjugated polymers based on alkoxy naphthalene, one with cyano-substituents and one without, have been investigated to determine the effects of electron-withdrawing cyano-groups on the performance of bulk-heterojunction solar cells. The substituted cyano-groups facilitate the self-assembly of the polymer chains, and the cyano-substituted polymer:PC71BM blend exhibits enhanced exciton dissociation to PC71BM. Moreover, the electronwithdrawing cyano-groups lower the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the conjugated polymer, which leads to a higher open circuit voltage (VOC) and a lower energy loss during electron transfer from the donor to the acceptor. A bulk-heterojunction device fabricated with the cyano-substituted polymer:PC71BM blend has a higher VOC (0.89 V), a higher fill factor (FF) (51.4%), and a lower short circuit current (JSC) (7.4 mA/cm2) than that of the noncyano-substituted polymer:PC71BM blend under AM 1.5G illumination with an intensity of 100 mW cm-2. Thus, the cyano-substitution of conjugated polymers may be an effective strategy for optimizing the domain size and crystallinity of the polymer:PC71BM blend, and for increasing VOC by tuning the HOMO and LUMO energy levels of the conjugated polymer.

Journal article

An TK, Yun HJ, Narote R, Kim R, Lee SU, Kim Y, Nam S, Cha H, Jeong YJ, Kim K, Cho S, Kwon SK, Kim YH, Park CEet al., 2014, Synthesis and characterization of an ester-terminated organic semiconductor for ethanol vapor detection, Organic Electronics, Vol: 15, Pages: 2277-2284, ISSN: 1566-1199

An organic-field effect transistor (OFET) based on a newly synthesized organic molecule with an ester end group (ANOAT) was tested in a sensor for the detection of ethanol vapor. The linearly fused acene in ANOAT forms channel layers in the OFET and the ester end group of ANOAT interacts with ethanol vapor. The ethanol sensing ability of the ANOAT-based OFET derives not from changes in the crystalline structure of the organic semiconductor but from the interaction between ethanol vapor and the ester end group, which we demonstrated by using FT-IR analysis, computational modeling, X-ray scattering, and AFM analysis. © 2014 Elsevier B.V. All rights reserved.

Journal article

Cha H, Baek JY, An TK, Kim SO, Kwon SK, Kim YH, Park CEet al., 2014, Development of bulk heterojunction morphology by the difference of intermolecular interaction behaviors, Organic Electronics, Vol: 15, Pages: 3558-3567, ISSN: 1566-1199

© 2014 Elsevier B.V. All rights reserved. The morphology of a bulk heterojunction can be controlled by adding a processing additive in order to improve its power conversion efficiency (PCE) in photovoltaic devices. The phase-separated morphologies of blends of PONTBT or P3HT with fullerene derivatives are systematically examined in the presence of processing additives that possess various alkane alkyl chain lengths or end-group electronegativities. We determined the morphologies of the bulk heterojunction layers by using atomic force microscopy (AFM) and grazing incidence wide angle X-ray scattering (GIWAXS). The photocurrent-voltage characteristics of the bulk heterojunction solar cells were found to be strongly dependent on the intermolecular interactions between the conjugated polymers, the fullerene derivatives, and the processing additives in the photoactive layer. The optimal PONTBT:fullerene derivative blend morphology was obtained with a processing additive, 1,3-diiodopropane (1,3-DIP), that possesses a short alkyl chain and an end group with weak electronegativity, and was found to exhibit a high fill factor (FF) and a high current density (JSC). In contrast, in blends of P3HT with the fullerene derivative, PCEs with higher FF and JSC values were achieved by incorporating the processing additive, 1,8-dibromooctane (1,8-DBrO), which has a long alkyl chain and a strong electronegative end group. Thus the selection of the processing additive with the aim of enhancing photovoltaic performance needs to take into account the intermolecular interaction of the conjugated polymer.

Journal article

Lee W, Cha H, Kim YJ, Jeong JE, Hwang S, Park CE, Woo HYet al., 2014, Amorphous thieno[3,2-b]thiophene and benzothiadiazole based copolymers for organic photovoltaics, ACS Applied Materials and Interfaces, Vol: 6, Pages: 20510-20518, ISSN: 1944-8244

© 2014 American Chemical Society. Three types of amorphous thienothiophene (TT)-benzothiadiazole (BT) based copolymers ( PFTTBT) were synthesized by incorporating alkyl-substituted fluorene moieties as a third component in the polymer backbone. Their optical, electrochemical, morphological, and photovoltaic properties were examined by a comparison with those of a crystalline TT-BT derivative (PTTBT14). PTTBT14 was reported to have a high hole mobility (0.26 cm2/(V s)) due to the pronounced interchain ordering but poor photovoltaic power conversion efficiency (PCE) of 2.4-2.6% was reported due to excessively strong self-interactions with poor miscibility with fullerene structures. By incorporating fluorene units, the UV-vis spectra showed an increased bandgap (∼1.9 eV) with the disappearance of the packing-originated shoulder peak, and the valence band decreased compared to crystalline PTTBT14. The amorphous PFTTBT polymers showed substantially improved photovoltaic properties compared to PTTBT14, even though they showed poor hole mobility (∼10-6 cm2/(V s)) and fill factor. The optimal devices were achieved by blending with excess PC71BM (polymer:PC71BM = 1:4 by weight), showing little improvement in the thermal and additive treatments. Under simulated solar illumination of AM 1.5 G, the best PCE of 6.6% was achieved for a PFehTTBT:PC71BM device with an open-circuit voltage of 0.92 V, a short-circuit current of 15.1 mA/cm2, and a fill factor of 0.48. These results suggest that it is useful to disrupt partially the interchain organizations of excessively crystalline polymers, enabling fine-control of intermolecular ordering and the morphological properties (i.e., miscibility with fullerene derivatives, etc.) to utilize the advantages of both crystalline and amorphous materials for further improving PCE of polymer solar cells. (Chemical Equation Presented).

Journal article

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