Moo-Jin Park

New selenophene-based semiconducting copolymers for high performance organic thin-film transistors

A series of new selenophene-based organic semiconducting copolymers, poly(5,5′-bis(3-dodecylthiophen-2-yl)-2,2′-biselenophene) (PDT2Se2) and poly(5,5′-bis(4,4′-didodecyl-2,2′-bithiophen-5-yl)-2,2′-biselenophene) (PDT4Se2), were successfully synthesized by Stille and oxidative coupling reactions. Our aim was to investigate the effects of the selenophene units and inserted dodecylthiophenes on the optical and electrochemical properties of these copolymers, their intermolecular ordering in the film state, and hence their thin-film transistor (TFT) performance. X-ray-diffraction (GIXRD and XRD) and theoretical calculations for models of these polymers were used to show that PDT2Se2 films have well organized interlayer packing and π–π stacking, whereas the films of PDT4Se2, which contain regioregularly inserted additional dodecylthiophenes next to the repeat units of PDT2Se2, have a long-range amorphous structure. The TFT characteristics of these polymers are strongly dependent on the intermolecular ordering of the polymer chains. PDT2Se2 exhibited a high hole transporting mobility of 0.02 cm2V−1 s−1 due to its excellent intermolecular ordering, whereas PDT4Se2 exhibited a very poor mobility of 1.4 × 10−5 cm2V−1 s−1 due to its amorphous characteristics, which result from the repulsion between the additional dodecylthiophenes. These results confirm that it is important to consider intermolecular ordering in the design of semiconducting materials for high performance OTFTs.

Charge-transfer character of excitons in poly[2,7-(9,9-di-n-octylfluorene)(1−x)-co-4,7-(2,1,3-benzothiadiazole)(x)]

Quantum-chemical calculations performed on poly[2,7-(9,9-di-n-octylfluorene)(1-x)-co-4,7-(2,1,3-benzothiadiazole)(x)] copolymers (0 < or = x < or = 0.5) show that the lowest unoccupied molecular orbital is always highly localized on the benzothiadiazole (BT) units while the highest occupied molecular orbital is delocalized over the whole chain. Chains with a low BT content are characterized by a reduced oscillator strength of the lowest optical transition and by an increased charge-transfer character of the exciton. These results are supported experimentally by a blueshift of the lowest energy absorption band upon reduction in the BT ratio, lower photoluminescence efficiency, longer excited state lifetimes, and greater solvent dependence of the emission properties.

Characterization of white electroluminescent devices fabricated using conjugated polymer blends

We report the characterization of white light emitting devices fabricated using conjugated polymer blends. Blue emissive poly[9,9-bis(4-n-octyloxyphenyl)fluorene-2, 7-diyl-co-10-(2-ethylhexyl)phenothiazine-3,7-diyl] [poly(BOPF-co-PTZ)] and red emissive poly(2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene) (MEH-PPV) were used in the blends. The inefficient energy transfer between these blue and red light emitting polymers (previously deduced from the photoluminscence (PL) spectra of the blend films) enables the production of white light emission through control of the blend ratio. The PL and electroluminescence (EL) emission spectra of the blend systems were found to vary with the blend ratio. The EL devices were fabricated in the indium tin oxide [poly(3,4-ethylenedioxy-thiophene)-poly(styrenesulfonate)] (ITO/PEDOT-PSS)blend/LiF/Al configuration, and white light emission was obtained for one of the tested blend ratios.

Thermal annealing induced bicontinuous networks in bulk heterojunction solar cells and bipolar field-effect transistors

Polymer bulk heterojunction solar cells fabricated from poly(2,5-bis(3′-dodecyl-2,2′-bithiophen-5-yl)-3,6-dimethylthieno [3,2-b] thiophene):[6,6]-phenyl-C61-butric acid methyl ester (1:1, w/w) blend showed significantly improved power conversion efficiency (PCE), from 0.96% to 2.32%, with post-thermal annealing at 140 °C. Charge transport properties obtained from bipolar field-effect transistors indicated that post-thermal annealing induced the assembly of significantly improved bicontinuous networks and excellently balanced hole (7.2×10−3 cm2 V−1 s−1) and electron (5.8×10−3 cm2 V−1 s−1) mobilities (due, particularly, to improved electron mobility), thereby improving PCE.

Synthesis and light‐emitting properties of poly (fluorene) copolymers containing EDOT comonomer

Abstract A series of statistical random copolymers of dioctylfluorene (DOF) and 3,4‐ethylenedioxythiophene (EDOT) were synthesized by Ni (0) mediated polymerization and their light‐emitting properties were compared with poly (9,9‐di‐n‐octylfluorene) (PDOF). The synthesized polymers were characterized using UV‐vis spectroscopy, TGA, photoluminescence (PL) & electroluminescence (EL) spectroscopy and by conducting molecular weight studies. The resulting polymers were found to be thermally stable and readily soluble in organic solvents. The UV‐visible absorption and PL emission spectra of the copolymers were gradually red‐shifted as the fraction of EDOT in copolymers increased. Light‐emitting devices were fabricated in an ITO (indium‐tin oxide)/PEDOT/polymer/Ca/Al configuration. Interestingly, the EL spectra of these devices were similar to the PL spectra of the corresponding polymer film. However, the EL devices constructed from the copolymer showed more than 10 times higher efficiency level than the devices constructed from the PDOF homopolymer. This higher efficiency is possibly the result of better charge carrier balance in the copolymer systems due to the lower HOMO levels of the copolymers in comparison to that of PDOF homopolymer.

Light-Emitting Properties of Fluorene Based Copolymers

A series of random copolymers of 2,7-dibromo-9,9-bis(4′-n-octyloxyphenyl) fluorene (BOPF) and 2,7-dibromo-N-(2′-ethylhexyl)carbazole (EHC) were synthesized through Ni(0)-mediated polymerization. Carbazole comonomer was introduced to improve the hole-transporting properties of PBOPF. The synthesized poly(BOPF-co-EHC)s showed similar UV-visible absorption and PL emission to PBOPF. EL devices were fabricated in an ITO/PEDOT/polymer/Ca/Al configuration. EL devices which used copolymers showed improved device performance over devices which used PBOPF homopolymers due to a more balanced charge transport.