Imperial College London


Faculty of Natural SciencesDepartment of Chemistry

Research Associate







G22Molecular Sciences Research HubWhite City Campus





Publication Type

53 results found

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

An TK, Kang I, Yun HJ, Cha H, Hwang J, Park S, Kim J, Kim YJ, Chung DS, Kwon SK, Kim YH, Park CEet al., 2013, Solvent additive to achieve highly ordered nanostructural semicrystalline DPP copolymers: Toward a high charge carrier mobility, Advanced Materials, Vol: 25, Pages: 7003-7009, ISSN: 0935-9648

A facile spin-coating method in which a small percentage of the solvent additive, 1-chloronaphthalene (CN), is found to increase the drying time during film deposition, is reported. The field-effect mobility of a PDPPDBTE film cast from a chloroform-CN mixed solution is 0.46 cm2 V-1 s -1. The addition of CN to the chloroform solution facilitates the formation of highly crystalline polymer structures. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Journal article

Jeon CW, Kang SH, Yun HJ, An TK, Cha H, Park CE, Kim YHet al., 2013, Synthesis and characterization of poly(dialkylterthiophene-bithiophene) and poly(dialkylterthiophene-thienothiophene) for organic thin film transistors and organic photovoltaic cells, Synthetic Metals, Vol: 185-186, Pages: 159-166, ISSN: 0379-6779

We designed new polymers composed of dialkylated terthiophene and thieno[3,2-b]thiophene, 2,2′-bithiophene for the fabrication of organic thin film transistors and polymer solar cells. Poly[5-(thieno[3,2-b]thiophene-2- yl)3′,4′-dioctyl-2,2′:5′,2″-terthiophene] (PTTOT), and poly[5-((2,2′-bithiophene)-5-yl)3′,4′-dioctyl-2,2′: 5′,2″-terthiophene] (PBTOT) were synthesized via Stille coupling and Suzuki coupling reaction. The obtained polymers were confirmed by 1H NMR and FT-IR spectra. The weight average molecular, the thermal, optical and electronic properties of the polymers were investigated by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) spectroscopies and cyclic voltammetry (CV). The crystallinity of polymer films was investigated by X-ray diffraction (XRD). Field-effect hole mobilities of PTTOT and PBTOT based solution-processed OTFTs were 3.54 × 10-3 cm2 V-1 S-1 and 1.73 × 10-2 cm2 V-1 S-1, respectively. And, each of the polymers was investigated as an electron donor material with PC71BM as an electron acceptor in bulk hetero junction solar cells. The polymers of PTTOT and PBTOT showed the power conversion efficiency (PCE) of 0.096% (J sc = 0.1 mA/cm-2, FF = 83.49%) and 3.2% (JSC = 8.9 mA/cm-2, FF = 51.9%), respectively. © 2013 Elsevier B.V.

Journal article

An TK, Jang SH, Kim SO, Jang J, Hwang J, Cha H, Noh YR, Yoon SB, Yoon YJ, Kim LH, Chung DS, Kwon SK, Kim YH, Lee SG, Park CEet al., 2013, Synthesis and transistor properties of asymmetric oligothiophenes: Relationship between molecular structure and device performance, Chemistry - A European Journal, Vol: 19, Pages: 14052-14060, ISSN: 0947-6539

A series of three thiophene-naphthalene-based asymmetric oligomers-5-decyl-2, 2′:5′,2″:5″,2′″- quaterthio-phene (DtT), 5-decyl-5″-(naphthalen-2-yl)-2, 2′:5′,2″-terthiophene (D3TN), and 5-(4-decylphenyl)-5′- (naphthalen-2-yl)-2, 2′-bithiophene (DP2TN)-was synthesized by Suzuki cross-coupling reactions. The long alkyl side chains improved both the solubility of the oligomers in solvents and their tendency to self-assemble. UV/Vis absorption measurements suggested that DtT, D3TN, and DP2TN form H-type aggregates with a face-to-face packing structure. In addition, the three oligomers were found to adopt vertically aligned crystalline structures in films deposited on substrates, as revealed by grazing-incidence wide-angle X-ray scattering. These oligomers were used as the active layers of p-type organic field-effect transistors, and the resulting devices showed field-effect mobilities of 3.3×10-3 cm2V-1s -1 for DtT, 1.6 × 10-2cm2V -1s-1 for D3TN, and 3.7 × 10-2cm 2V-1s-1 for DP2TN. The differences in transistor performances were attributed to the degree of π overlap and the morphological differences determined by the molecular structures. © 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

Journal article

Kim YJ, Lee YJ, Jang JW, Cha H, Kim YH, Kwon SK, Park CEet al., 2013, Synthesis and characterization of naphtho[2,1-b:3,4-b′]dithiophene- based polymers with extended p-conjugation systems for use in bulk heterojunction polymer solar cells, Journal of Polymer Science, Part A: Polymer Chemistry, Vol: 51, Pages: 4742-4751, ISSN: 0887-624X

Two novel polymeric semiconductor materials based on naphtho[2,1-b:3,4- b′]dithiophene (NDT), PNDT-TTT and PNDT-TET, were designed and synthesized. These synthesized polymers were tested in bulk heterojunction solar cells as blends with the acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). PNDT-TTT contained tri-thiophene units, and PNDT-TET contained bi-thiophene units coupled by ethylenic linkages. Comparison to the properties of PNDT-T, which contained single thiophene units, these polymers exhibit red-shifted absorption spectra as a result of the enhanced conjugation lengths. These effects resulted in high short circuit currents (JSC) in the organic solar cells. The PNDT-TET- and PNDT-TTT-based devices exhibited considerably better photovoltaic performances, with power conversion efficiencies of 3.5 and 3.3%, respectively, compared to the PNDT-T-based device (1.3%). © 2013 Wiley Periodicals, Inc.

Journal article

An TK, Park SM, Nam S, Hwang J, Yoo SJ, Lee MJ, Yun WM, Jang J, Cha H, Hwang J, Park S, Kim J, Chung DS, Kim YH, Kwon SK, Park CEet al., 2013, Thin film morphology control via a mixed solvent system for high-performance organic thin film transistors, Science of Advanced Materials, Vol: 5, Pages: 1323-1327, ISSN: 1947-2935

The electrical performance of a thin film in an organic thin film transistor depends on the film morphology. A mixed solvent system was demonstrated to provide a useful method for controlling the morphologies of organic semiconductor layers in organicJhin film transistor. To overcome the limitations on solvent properties, control over the morphology of a 2-(6-((2-ethylhexyl)oxy)naphthalen-2-yl)anthracene film using a tetrahydrofu-ran/chloroform mixed system (with ratio/of 1:5 w/w).cHere, innan effortItohachieve improved the device electrical performance properties for use in organic thin film transistors, we have examined a mixed solvent system that is suitable for the fabrication of organic semiconductors. © 2013 American Scientific Publishers.

Journal article

Cha H, Lee GY, Fu Y, Kim YJ, Park CE, Park Tet al., 2013, Simultaneously grasping and self-organizing photoactive polymers for highly reproducible organic solar cells with improved efficiency, Advanced Energy Materials, Vol: 3, Pages: 1018-1024, ISSN: 1614-6832

The incorporation of 3,6-carbazole units into photoactive copolymers results in conjugation breaks along the polymer backbones and thus in strong intermolecular interactions. Such copolymers provide thermally reproducible morphologies and enhanced device performances over a wide range of annealing temperatures, 100 to 160 °C, with high power conversion efficiencies of over 6%. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Journal article

Cha H, Chung DS, Bae SY, Lee MJ, An TK, Hwang J, Kim KH, Kim YH, Choi DH, Park CEet al., 2013, Complementary absorbing star-shaped small molecules for the preparation of ternary cascade energy structures in organic photovoltaic cells, Advanced Functional Materials, Vol: 23, Pages: 1556-1565, ISSN: 1616-301X

Two anthracene-based star-shaped conjugated small molecules, 5',5a″-(9,10-bis((4-hexylphenyl)ethynyl)anthracene-2,6-diyl)bis(5-hexyl-2, 2'-bithiophene), HBantHBT, and 5',5a″-(9,10-bis(phenylethynyl)anthracene- 2,6-diyl)bis(5-hexyl-2,2'-bithiophene), BantHBT, are used as electron-cascade donor materials by incorporating them into organic photovoltaic cells prepared using a poly((5,5-E-alpha-((2-thienyl)methylene)-2-thiopheneacetonitrile)-alt-2, 6-[(1,5-didecyloxy)naphthalene])) (PBTADN):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) blend. The small molecules penetrate the PBTADN:PC71BM blend layer to yield complementary absorption spectra through appropriate energy level alignment and optimal domain sizes for charge carrier transfer. A high short-circuit current (JSC) and fill factor (FF) are obtained using solar cells prepared with the ternary blend. The highest photovoltaic performance of the PBTADN:BantHBT:PC71BM blend solar cells is characterized by a JSC of 11.0 mA cm-2, an open circuit voltage (VOC) of 0.91 V, a FF of 56.4%, and a power conversion efficiency (PCE) of 5.6% under AM1.5G illumination (with a high intensity of 100 mW-2). The effects of the small molecules on the ternary blend are investigated by comparison with the traditional poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) system. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Journal article

An TK, Hahn SH, Nam S, Cha H, Rho Y, Chung DS, Ree M, Kang MS, Kwon SK, Kim YH, Park CEet al., 2013, Molecular aggregation-performance relationship in the design of novel cyclohexylethynyl end-capped quaterthiophenes for solution-processed organic transistors, Dyes and Pigments, Vol: 96, Pages: 756-762, ISSN: 0143-7208

The synthesis and characterization of cyclohexylethenyl end-capped quaterthiophenes is reported. Additionally, an investigation of the performance of organic field-effect transistors based on these quaterthiophenes in view of the relationship between the solid-state (or aggregate) order and the electronic performance is described. UV-vis absorption measurements revealed that the quaterthiophene with an asymmetrically substituted cyclohexylethynyl end-group induced the formation of H-type aggregates, whereas the quaterthiophene with a symmetrically substituted cyclohexylethynyl end-groups favored the formation of J-type aggregates. Two-dimensional grazing-incidence wide-angle X-ray scattering studies were performed to support the molecular structure-dependent packing of films of the new quaterthiophenes. Solution-processed quaterthiophenes were tested as the active layers of p-type organic field-effect transistors with a bottom gate/top contact geometry. The field-effect mobility of devices that incorporated asymmetric quaterthiophene molecules was quite high, exceeding 0.02 cm2/V s, due to H-aggregation and good in-plane ordering. In contrast, the field-effect mobility of devices that incorporated symmetrical quaterthiophenes, was low, above 5 × 10-4 cm2/(V s), due to the formation of J-aggregates and poor in-plane ordering. A comparison of the symmetrical and asymmetrical quaterthiophene derivatives revealed that the molecular aggregation-dependent packing, determined by the cyclohexylethynyl end groups, was responsible for influencing the organic field-effect transistor performance. © 2012 Elsevier Ltd. All rights reserved.

Journal article

Fu Y, Cha H, Song S, Lee GY, Eon Park C, Park Tet al., 2013, Low-bandgap quinoxaline-based D - A-type copolymers: Synthesis, characterization, and photovoltaic properties, Journal of Polymer Science, Part A: Polymer Chemistry, Vol: 51, Pages: 372-382, ISSN: 0887-624X

Three classes of quinoxaline (Qx)-based donor-acceptor (D-A)-type copolymers, poly[thiophene-2,5-diyl-alt-2,3-bis(4-(octyloxy)phenyl-quinoxaline- 5,8-diyl] P(T-Qx), poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′] dithiophene-2,6-diyl-alt-2,3-bis(4-(octyloxy)phenyl-quinoxaline-5,8-diy} P(BDT-Qx), and poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′] dithiophene-2,6-diyl-alt-(5′,8′-di-2-thienyl-2,3-bis(4-octyloxyl) phenyl)-quinoxaline-5,5-diyl} P(BDT-DTQx), were synthesized via a Stille coupling reaction. The Qx unit was functionalized at the 2- and 3-positions with 4-(octyloxy)phenyl to provide good solubility and to reduce the steric hindrance. The absorption spectra of the Qx-containing copolymers could be tuned by incorporating three different electron-donating moieties. Among these, P(T-Qx) acted as an electron donor and yielded a high-performance solar cell by assuming a rigid planar structure, confirmed by differential scanning calorimetry, UV-vis spectrophotometer, and density functional theory study. In contrast, the P(BDT-Qx)-based solar cell displayed a lower power conversion efficiency (PCE) with a large torsional angle (34.7 ) between the BDT and Qx units. The BDT unit in the P(BDT-DTQx) backbone acted as a linker and interfered with the formation of charge complexes or quinoidal electronic conformations in a polymer chain. The PCEs of the polymer solar cells based on these copolymers, in combination with [6,6]-phenyl C70 butyric acid methyl ester (PC 71BM), were 3.3% [P(T-Qx)], 1.9% [P(BDT-Qx)], and 2.3% [P(BDT-DTQx)], respectively, under AM 1.5G illumination (100 mW cm-2). © 2012 Wiley Periodicals, Inc.

Journal article

Cha H, Park JW, Chung DS, An TK, Kim YH, Kwon SK, Park CEet al., 2012, A side chain-modified quaterthiophene derivative for enhancing the performance of organic solar cell devices, Journal of Materials Chemistry, Vol: 22, Pages: 15141-15145, ISSN: 0959-9428

A conjugated polymer donor material, poly(3,4′′′- di(decylthiophenyl)quaterthiophene) (PDTQT), featuring decylthiophenyl side chains on the polymer backbone, was introduced to reduce the crystallinity of poly(quaterthiophene) (PQT-C12) layers in organic photovoltaic cells. The resulting PDTQT:PC<inf>71</inf>BM blend active layer formed a well-interpenetrated nanoscale morphology that increased the power conversion efficiency (PCE) relative to devices based on highly crystalline PQT-C12. Bulk heterojunction solar cells fabricated using the PDTQT:PC<inf>71</inf>BM blend thin films yielded the best photovoltaic performances with a high short-circuit current density of 9.8 mA cm<sup>-2</sup>, a high open-circuit voltage of 0.91 V, a fill factor of 0.36, and a high PCE of 3.2% under AM 1.5G illumination with an intensity of 100 mW cm<sup>-2</sup>. Decylthiophenyl side chain substitution appeared to be an effective strategy for obtaining high organic photovoltaic cell device performances. © The Royal Society of Chemistry 2012.

Journal article

Fu Y, Cha H, Lee GY, Moon BJ, Park CE, Park Tet al., 2012, 3,6-carbazole incorporated into poly[9,9-dioctylfluorene-alt-(bisthienyl) benzothiadiazole]s improving the power conversion efficiency, Macromolecules, Vol: 45, Pages: 3004-3009, ISSN: 0024-9297

A novel concept of D-A-type copolymers based on fluorene polymer incorporated with 3,6-carbazole unit enhances the device performance for organic photovoltaic cells. P(F 45C 5-DTBT), incorporating 5 mol % 3,6-carbazole into P(2,7F-DTBT), shows an almost 2-fold improvement (5.1%) in power conversion efficiency relative to P(2,7F-DTBT) (2.6%). This results is ascribed to the good balance between electron and hole mobilities in the devices (μ e/μ h ∼ 1.8 for P(F 45C 5-DTBT) vs 152 for P(2,7F-DTBT)), and the formation of a nanoscale morphology in the blend of the copolymer and [6,6]-phenyl C71-butyric acid methyl ester (PC 71BM). © 2012 American Chemical Society.

Journal article

Nam S, Jang J, Cha H, Hwang J, An TK, Park S, Park CEet al., 2012, Effects of direct solvent exposure on the nanoscale morphologies and electrical characteristics of PCBM-based transistors and photovoltaics, Journal of Materials Chemistry, Vol: 22, Pages: 5543-5549, ISSN: 0959-9428

We investigated the effects of direct solvent exposure on the properties of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films and poly(3-hexylthiophene) (P3HT)/PCBM blend films employed as active layers in, respectively, organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). The crystallinity, morphology, and OFET characteristics of the PCBM thin films were significantly influenced by direct exposure to solvent, especially to select alcohols. Control over the nanoscale morphology of the PCBM film, achieved via direct solvent exposure, yielded highly efficient poly(3-hexylthiophene) (P3HT)/PCBM OPVs with a short-circuit current density of 10.2 mA cm -2, an open-circuit voltage of 0.64 V, and a power conversion efficiency of 3.25% under AM 1.5 illumination with a light intensity of 100 mW cm -2. These results indicated that optimal phase separation in the P3HT/PCBM films could be obtained simply by exposing the active layer films for a few seconds to solvent. © The Royal Society of Chemistry 2012.

Journal article

Kim SO, Chung DS, Cha H, Hwang MC, Park JW, Kim YH, Park CE, Kwon SKet al., 2011, Efficient polymer solar cells based on dialkoxynaphthalene and benzo[c][1,2,5]thiadiazole: A new approach for simple donoracceptor pair, Solar Energy Materials and Solar Cells, Vol: 95, Pages: 1678-1685, ISSN: 0927-0248

We reported the synthesis of novel polymeric semiconductor materials based on [poly(4-(5-(1,5-bis(alkoxy)naphthalen-2-yl)thiophen-2-yl)-7-(thiophen-2-yl) benzo[c][1,2,5]-thiadiazole)] (PANTBT) and the fabrication of solar cells with a power conversion efficiency of 4.2% using the synthesized polymers blended with [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) in bulk heterojunction geometry. By varying the side chains, three polymers were synthesized [poly(4-(5-(1,5-bis(2-ethylhexyloxy)naphthalen-2-yl)thiophen-2-yl)- 7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PENTBT), [poly(4-(5-(1,5- bis(decyloxy)naphthalen-2-yl)thiophen-2-yl)-7-(thiophen-2-yl)benzo[c][1,2,5] thiadiazole)] (PDNTBT), and [poly(4-(5-(1,5-bis(tetradecyloxy)naphthalen-2-yl) thiophen-2-yl)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PTDNTBT), maintaining a low highest occupied molecular orbital (HOMO) energy level and relatively low band gap, which lead to a high open circuit voltage and short circuit current of the resulting devices. Due to the superior miscibility of PANTBT derivatives with PC70BM, favorable phase separation with a domain size of 1020 nm was achieved regardless of the crystalline nature of the pristine polymers. PDNTBT with alkyl side chain C10 and PTDNTBT with alkyl side chain C14 showed higher photovoltaic performances. In addition, the effects of the crystalline nature of polymers on the thermal stability of the resulting solar cell devices were discussed in terms of the influence of side chains. © 2011 Elsevier B.V. All rights reserved.

Journal article

Kwon JH, An JY, Jang H, Choi S, Chung DS, Lee MJ, Cha HJ, Park JH, Park CE, Kim YHet al., 2011, Development of a new conjugated polymer containing dialkoxynaphthalene for efficient polymer solar cells and organic thin film transistors, Journal of Polymer Science, Part A: Polymer Chemistry, Vol: 49, Pages: 1119-1128, ISSN: 0887-624X

A new semiconducting polymer, poly((5,5-E-α-((2-thienyl)methylene)-2- thiopheneacetonitrile)-alt-2,6-[(1,5-didecyloxy)naphthalene])) (PBTADN), an alternating copolymer of 2,3-bis-(thiophene-2-yl)-acrylronitrile and didecyloxy naphthalene, is synthesized and used as an active material for organic thin film transistors (OTFTs) and organic solar cells. The incorporation of 2,3-bis-(thiophene-2-yl)-acrylronitrile as an electron deficient group and didecyloxy naphthalene as an electron rich group resulted in a relatively low bandgap, high charge carrier mobility, and finally good photovoltaic performances of PBTADN solar cells. Because of the excellent miscibility of PBTADN and PC71BM, as confirmed by Grazing Incident X-ray Scattering (GIXS) measurements and Transmission Electron Microscopy (TEM), homogeneous film morphology was achieved. The maximum power conversion efficiency of the PBTADN:PC71BM solar cell reached 2.9% with a Voc of 0.88 V, a short circuit current density (Jsc) of 5.6 mA/cm2, and a fill factor of 59.1%. The solution processed thin film transistor with PBTADN revealed a highest saturation mobility of 0.025 cm2/Vs with an on/off ratio of 104. The molecular weight dependence of the morphology, charge carrier mobility, and finally the photovoltaic performances were also studied and it was found that high molecular weight PBTADN has better self assembly characteristics, showing enhanced performance. © 2010 Wiley Periodicals, Inc.

Journal article

Kim SO, Sung Chung D, Cha H, Wan Jang J, Kim YH, Kang JW, Jeong YS, Park CE, Kwon SKet al., 2011, Thermally stable organic bulk heterojunction photovoltaic cells incorporating an amorphous fullerene derivative as an electron acceptor, Solar Energy Materials and Solar Cells, Vol: 95, Pages: 432-439, ISSN: 0927-0248

A highly soluble amorphous fullerene derivative substituted with dihexylfluorene (DHFCBM) was synthesized and used as an electron acceptor material for P3HT-based bulk heterojunction solar cells. By fitting the experimental JV curves with space charge limited current equation, the electron mobility of DHFCBM was determined to be 4×10-4 cm 2/Vs, possibly leading to balanced charge transport with P3HT. From structural and morphological analysis using X-ray diffraction, UVvis absorption, and atomic force microscopy, we found that the amorphous nature of DHFCBM stabilized the nanomorphology of P3HT:DHFCBM blend films under high temperature annealing. By optimizing blend ratios and annealing conditions, P3HT:DHFCBM-based solar cells yielded power conversion efficiencies in excess of 3%. In addition, the fabricated cells maintained their initial performances even after high temperature annealing for long times, as predicted from the stable nanomorphology. We believe that the use of thermally stable amorphous fullerene as an electron acceptor can be a promising strategy for commercialization of organic solar cells. © 2010 Elsevier B.V. All rights reserved.

Journal article

Kwon JH, Yeo HD, Cha HJ, Lee MJ, Park HT, Park JH, Park CE, Kim YHet al., 2011, Synthesis and characterization of dialkoxynaphthalene-based new π-conjugated copolymer for photovoltaic solar cell, Macromolecular Research, Vol: 19, Pages: 197-202, ISSN: 1598-5032

A π-conjugated dialkoxynaphthalene-based donor-acceptor copolymer was synthesized by a Suzuki coupling reaction, and its photophysical, thermal and electrochemical properties and photovoltaic characteristics were characterized. The copolymer showed good solubility and a weight average molecular weight of 11,500. The new copolymer was characterized by two absorption bands at 315 and 403 nm in the dilute solution, which were assigned to the π→π* transition and an intramolecular charge transfer band, respectively. The wide band gap of the donor-acceptor copolymer was explained by the 3-dimensional structure and tortional angle between the dialkoxynaphthalene and benzothiadiazole of the model compound. The PCE of the photovoltaic solar cell device fabricated using this copolymer reached 0.06% with a V oc, J sc, and FF of 0.59, 0.3, and 33%, respectively. © 2011 The Polymer Society of Korea and Springer Netherlands.

Journal article

Hong K, Kim SH, Yang C, An TK, Cha H, Park C, Park CEet al., 2011, Photopatternable, highly conductive and low work function polymer electrodes for high-performance n-type bottom contact organic transistors, Organic Electronics, Vol: 12, Pages: 516-519, ISSN: 1566-1199

We demonstrate the use of n-type N,N′-ditridecyl-3,4,9,10- perylenetetracarboxylic diimide (PTCDI-C13) bottom contact organic field-effect transistors that employ photopatternable highly conductive poly(3,4- ethylenedioxythiophene):tosylate (PEDOT:Tos) source/drain electrodes characterized by a very low work function (4.3 eV). Due to the low work function of this material, the electron injection barrier between PTCDI-C13 and PEDOT:Tos was 0.25 eV lower than that between PTCDI-C13 and gold. The low injection barrier reduced the contact resistance, yielding a high field effect mobility in transistors based on PEDOT:Tos (0.145 cm2/Vs); the field effect mobility was 16 times higher than that in transistors based on gold (0.009 cm2/Vs). © 2011 Elsevier B.V. All rights reserved.

Journal article

Kim SO, Chung DS, Kim PS, Cha H, Koh HJ, Kim YH, Park CE, Kwon SKet al., 2010, Synthesis and characterization of narrow-band-gap copolymers containing alkoxyanthracene as electron donor for organic photovoltaic applications, Pages: 703-704, ISSN: 1738-7558

A new electron donor polymer (PTFTEA) was synthesized by the Suzuki coupling reaction. PTFREA has both high molecular weight (Mn 7.8 × 10 3 g/mol PDI 1.47) and narrow band gap. We found optical wavelength absorption using UV spectrum at 618 and 650 nm in solution and at 668 and 723 nm in film, obtained narrow band gap (1.39 eV). Moreover, we obtain HOMO energy of PTFTEA at - 5.36 eV by cyclic voltammetry (CV) studies, calculated LUMO using band gap. As a results, PTFTEA has narrow band gap, so that to be suitable for performance (1.1%).

Conference paper

Hong K, Kim SH, Yang C, Jang J, Cha H, Park CEet al., 2010, Improved n-type bottom-contact organic transistors by introducing a poly(3,4-ethylenedioxythiophene):Poly(4-styrene sulfonate) coating on the source/drain electrodes, Applied Physics Letters, Vol: 97, ISSN: 0003-6951

We improved the device performance of N,N′-ditridecyl-3,4,9,10- perylenetetracarboxylic diimide (PTCDI-C13) n-type field-effect transistors, increasing electron-mobility from 0.003 to 0.101 cm2/Vs, by applying a coating of poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) to gold source/drain (S/D) electrodes, thereby reducing contact resistance in the devices. Crystallinity and electronic structure studies suggested that the improved device performance resulted from higher crystallinity of PTCDI-C13 on the PEDOT:PSS-coated S/D electrodes at the interface between the electrode and the channel. © 2010 American Institute of Physics.

Journal article

Chung DS, Park JW, Yun WM, Cha H, Kim Y-H, Kwon S-K, Park CEet al., 2010, Solution-processed organic photovoltaic cells with anthracene derivatives., ChemSusChem, Vol: 3, Pages: 742-748

Solution-processed small-molecule bulk heterojunction photovoltaic cells are fabricated by using [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) as electron acceptor and triisopropylsilylethynyl anthracene (TIPSAnt) derivatives substituted with naphthalene (TIPSAntNa) and bithiophene (TIPSAntBT) as electron donors. In contrast to TIPS-pentacene, the TIPSAnt derivatives are not susceptible to Diels-Alder reactions with PCBM when processed in solution, as confirmed by UV/Vis measurements. Photoluminescence quenching measurements show exciton diffusion lengths of 5 and 3 nm for TIPSAntBT and TIPSAntNa, respectively. Blending TIPSAntBT and TIPSAntNa with PCBM (1:1, 1:2, 1:3, and 1:4 weight ratios) produces films that possess adequate hole and electron mobilities. The morphological changes that result from varying the blending ratio range from obvious phase-segregated crystalline domains at a 1:1 ratio to homogeneous, nearly amorphous phases at a 1:4 ratio. Bulk heterojunction solar cells prepared by using a TIPSAntBT:PCBM blend reach power conversion efficiencies as high as 1.4 %.

Journal article

Chung DS, Kong H, Yun WM, Cha H, Shim HK, Kim YH, Park CEet al., 2010, Effects of selenophene substitution on the mobility and photovoltaic efficiency of polyquaterthiophene-based organic solar cells, Organic Electronics, Vol: 11, Pages: 899-904, ISSN: 1566-1199

We investigated thin films comprised of a blend of poly(5,5′-bis(3-dodecylthiophene-2-yl)2-2′-biselenophene)/[6,6] and phenyl C61 butyric acid methyl ester (PDT2Se2/PCBM) for use in bulk heterojunction photovoltaic cells. The charge transport characteristics of PDT2Se2 and its analog, poly(3,3′′′-didodecyl quaterthiophene) (PQT-12), were elucidated through analysis of the space charge limited current behavior at various temperatures. PDT2Se2 showed higher mobilities, lower field activation parameters, and a lower temperature dependence of these parameters than did PQT-12, indicating better charge transport in PDT2Se2. Optimization of the PDT2Se2:PCBM composition ratio produced a bicontinuous donor-acceptor network with domain sizes on the order of 10 nm, which afforded power conversion efficiencies of 1.4%, a short circuit current density of 4.3 mA cm-2, an open circuit voltage of 0.69 V, and a fill factor of 47%. This performance was much better than the performance achieved previously using PQT-12:PCBM blend systems. Selenophene substitution appears to be an effective strategy for enhancing the photovoltaic effect of thiophene-based polymeric semiconductors for high performance organic solar cells (OSCs). © 2010 Elsevier Ltd. All rights reserved.

Journal article

Cha H, Kong H, Chung DS, Yun WM, An TK, Hwang J, Kim YH, Shim HK, Park CEet al., 2010, Thermally stable amorphous polymeric semiconductors containing fluorene and thiophene for use in organic photovoltaic cells, Organic Electronics, Vol: 11, Pages: 1534-1542, ISSN: 1566-1199

We report amorphous polymeric semiconductors containing fluorene and thiophene, specifically poly(2-(5-(9,9-dibutyl-9 H-fluoren-2-yl)-3- hexylthiophen-2-yl)-5-(3-hexylthiophen-2-yl)thieno[3,2-b]thiophene) (P1) and poly(2-(5-(9,9-dibutyl-9 H-fluoren-2-yl)-4-hexylthiophen-2-yl)-5-(4- hexylthiophen-2-yl)thieno[3,2-b]thiophene) (P2), for use as electron donor materials in organic photovoltaic cells. The optical properties of the polymers blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were characterized by UV-Visible absorption and photoluminescence (PL) measurements. A maximum power conversion efficiency (PCE) of 1.8% and a high open circuit voltage (VOC) of 0.97 eV were obtained from the P2:PCBM blend. Morphological and structural analysis of the polymer:PCBM blend films, by atomic force microscopy (AFM) and transmission electron microscopy (TEM), demonstrated that the range of possible side chain arrangements produced distinctly different nano-morphologies and, therefore, different thermal stabilities of the resulting devices. P2 was characterized by long distance between alkyl chains along the polymer backbone that yielded a uniformly amorphous morphology without PCBM clusters to hinder charge transport to electrode and steady photovoltaic performance exhibiting remarkable thermal stability, even at elevated temperatures. © 2010 Elsevier B.V. All rights reserved.

Journal article

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