In-Nam Kang

New deep-blue emitting materials based on fully substituted ethylene derivatives

New blue fluorescent compounds containing tetra-substituted ethylene moieties have been designed and synthesized. These materials, 1,2-di(4′-tert-butylphenyl)-1,2-bis(4′-(anthracene-9-yl)phenyl)ethene [BPBAPE, 1A], 1,2-diphenyl-1,2-bis(4′-(anthracene-9-yl)phenyl)ethene [PBAPE, 1B], 9,10-bis(4-(1,2,2-tris(4-tert-butylphenyl)vinyl)phenyl)anthracene [BTBPPA, 2A], and 9,10-bis(4-(1,2,2-triphenylvinyl)phenyl)anthracene [BTPPA, 2B], were synthesized through Suzuki and McMurry reactions. By fabricating multilayered non-doped OLED devices using these new blue materials, we achieved luminance efficiencies of 4.00 lm W−1 (10.33 cd A−1 at 8.1 V) for BPBAPE [1A] and 1.82 lm W−1 (3.93 cd A−1 at 6.8 V) for BTPPA [2B] at 10 mA cm−2. The maxima in the electroluminescence spectra of ITO/2-TNATA (60 nm)/NPB (15 nm)/BPBAPE [1A] and BTPPA [2B] (30 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (200 nm) devices were found to be 475 and 452 nm respectively. The BPBAPE [1A] and BTPPA [2B] devices exhibited sky blue emission (0.195, 0.303) and deep blue emission (0.159, 0.135) at 10 mA cm−2 respectively.

Electroluminescence of polymer blend composed of carbazole group contained PMA and MEH-PPV

Abstract Alkylstyryl carbazole group containing poly(methacrylate) (Cz-PMA) was prepared and blended with poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV). The blends were characterized by UV-Vis, photoluminescence (PL), and electroluminescence (EL) studies. The polymer blends showed two isolated PL emission peaks at 440 and 560 nm corresponding to their component polymers, but the only one EL emission peak of the MEH-PPV/Cz-PMA blended light-emitting diode (LED) appeared near 580 nm which corresponds to that of MEH-PPV. This result indicates that the energy of excited Cz-PMA was transferred into MEH-PPV, resulting in the enhancement of EL intensity.

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.

Observation of new wavelength electroluminescence from multilayer structure device using poly(p- phenylenevinylene) derivative

We observe a new electroluminescent (EL) peak from a two-layer polymer device, which does not appear in EL spectra of each layer. The polymers of both layers are poly(p- phenylenevinylene) derivative with monoalkoxy substituents, poly(2-methoxy-1,4-phenylene- vinylene, abbreviated as PMPV), but dialyzed for different periods respectively. A new peak is located at 590 nm and has comparable intensity. The origin of this peak is discussed. Absorption and photoluminescence spectra are also measured and device properties show typical diode characteristics.

New TIPS-substituted benzo[1,2-b:4,5-b ']dithiophene-based copolymers for application in polymer solar cells

Novel triisopropylsilylethynyl (TIPS)-substituted benzodithiophene-based copolymers, poly[4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4,6-(2-ethylhexyl-thieno[3,4-b]thiophene-2-carboxylate)] (P1), poly[4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-[4,6-{(1-thieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one}] (P2), and poly[4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4,6-(2-ethylhexyl(3-fluorothieno[3,4-b]thiophene)-2-carboxylate)] (P3), were designed and synthesized for use in polymer solar cells (PSCs). We describe the effects of the different acceptor segment side groups on the optical, electrochemical, field-effect hole mobility, and photovoltaic characteristics of the resulting TIPS-based copolymers. The side groups in the copolymers were found to significantly influence the carrier mobilities and photovoltaic properties of the copolymers. The field-effect mobilities of the holes varied from 9 × 10−5 cm2 V−1 s−1 in P2 to 3 × 10−3 cm2 V−1 s−1 in P1. Under optimized conditions, the TIPS-based polymers showed power conversion efficiencies (PCEs) for the PSCs in the range of 3.16–5.76%. Among the TIPS-based copolymers studied here, P1 showed the best photovoltaic performance, with an open-circuit voltage (Voc) of 0.82 V, a short-circuit current density (Jsc) of 12.75 mA cm−2, a fill factor (FF) of 0.55, and a power-conversion efficiency of 5.76% using a P1:PC71BM blend film as the active layer under AM 1.5G irradiation (100 mW cm−2).

New amorphous semiconducting copolymers containing fluorene and thiophene moieties for organic thin-film transistors

New amorphous semiconducting materials consisting of fluorene-based thiophene copolymers, 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, have been successfully synthesized via a palladium-catalyzed Suzuki coupling reaction. The number-average molecular weights (Mn) of P1 and P2 were found to be 18 300, and 15 800, respectively. These polymers dissolve in common organic solvents such as chloroform, chlorobenzene, and toluene. The UV-vis absorption maxima of P1 and P2 appeared at 436 and 427 nm in solution and 441 and 431 nm in the film state, respectively. X-Ray diffraction (XRD) analysis showed no reflection peaks indicating amorphous collections of randomly oriented polymer chains. Atomic force microscopy (AFM) images of P1 and P2 showed amorphous film morphologies. Field-effect transistor mobilities of stable amorphous OTFTs of P1 and P2 under ambient conditions have been achieved up to 5.4 × 10−4 cm2V−1 s−1 and 1.6 × 10−4 cm2V−1 s−1, respectively. The high stability and mobility of fluorene-based thiophene copolymers in the amorphous state make them a new family of promising candidates for organic thin-film transistors.

Highly stable printed polymer field-effect transistors and inverters via polyselenophene conjugated polymers

We report the use of two polyselenophene-based conjugated polymers, poly(3,3′′-didodecyl-2,2′:5,2′′-terselenophene) (P3Se) and poly(3,3′′,3′′′,3′′′′-tetradodecyl-2,5′:2′,2′′:5′′,2′′′-pentaselenophene) (P5Se), as an active layer of printed p-channel organic field-effect transistors (OFETs). Top-gate/bottom-contact (TG/BC) P5Se OFETs showed a high-saturation hole mobility of up to ∼0.1 cm2 V−1 s−1 and a high on/off ratio of ∼105 with no hysteresis. In addition, polyselenophene-based OFETs exhibited a much better bias and ambient stability when compared with poly(3-hexylthiophene)-based OFETs. The excellent air stability of those polyselenophenes enables the realization of complementary metal-oxide semiconductor (CMOS) inverters via extended periods of ink-jetting under ambient conditions. CMOS inverters were demonstrated using p-[P5Se] and n-channel [poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-s,6-diyl]-alt-5,5′-(2,2′-dithiophene)}] ([P(NDI2OD-T2)], Polyera ActivInk N2200/OFETs) by inkjet printing of conjugated polymers. Printed CMOS inverters exhibited a stable voltage transfer characteristic with negligible hysteresis, a DC voltage gain of ∼10, and a power consumption of ∼0.025 mW at VDD = −60 V.

Impact of the Crystalline Packing Structures on Charge Transport and Recombination via Alkyl Chain Tunability of DPP-Based Small Molecules in Bulk Heterojunction Solar Cells.

A series of small compound materials based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) with three different alkyl side chains were synthesized and used for organic photovoltaics. These small compounds had different alkyl branches (i.e., 2-ethylhexyl (EH), 2-butyloctyl (BO), and 2-hexyldecyl (HD)) attached to DPP units. Thin films made of these compounds were characterized and their solar cell parameters were measured in order to systematically analyze influences of the different side chains of compounds on the film microstructure, molecular packing, and hence, charge-transport and recombination properties. The relatively shorter side chains in the small molecules enabled more ordered packing structures with higher crystallinities, which resulted in higher carrier mobilities and less recombination factors; the small molecule with the EH branches exhibited the best semiconducting properties with a power conversion efficiency of up to 5.54% in solar cell devices. Our study suggested that tuning the alkyl chain length of semiconducting molecules is a powerful strategy for achieving high performance of organic photovoltaics.

Influential effects of π-spacers, alkyl side chains, and various processing conditions on the photovoltaic properties of alkylselenyl substituted benzodithiophene based polymers

π-spacers and alkyl side chains play a key role in the optical, electrochemical, and photovoltaic properties of π-conjugated polymers. To investigate the collective effects of π-spacers, alkyl side chains, and various processing conditions on the photovoltaic properties, an array of four new low bandgap (LBG) π-conjugated polymers (P1–P4) was designed and synthesized for their application as donor materials in bulk heterojunction polymer solar cells (BHJ PSCs). These π-conjugated polymers contain a benzodithiophene (BDT) donor unit substituted with 2-ethylhexylselenyl or 2-hexyldecylselenyl as π-conjugated side chains and a dialkoxybenzothiadiazole (dialkoxyBT) electron deficient unit connected with thiophene or selenophene as π-spacers. Among the four polymers, the absorption spectra of P3 with the thiophene π-spacer showed a well enhanced vibronic shoulder peak between 620 and 650 nm, indicating that P3 possesses a strong interchain aggregation tendency and attains a planar backbone structure due to the non-covalent interactions arising between the sulfur atom in thiophene and the oxygen atom in dialkoxyBT. Under suitable device processing conditions optimized pristine PSCs of P3 showed a maximum power conversion efficiency (PCE) of 4.09%. After employing 1,8-diiodooctane as an additive, one of the PSC devices based on P2 displayed a PCE of 5.34%. The active layers of P1–P4 showed a positive response towards methanol treatment, especially the P3-based devices delivered an improved PCE of 5.63%, which was further assessed by electrical impedance spectroscopy. These findings in the current article provide a good specimen for efficiently fine tuning the optical and photovoltaic properties of π-conjugated polymers via varying the size of alkyl chains, π-spacer groups and device processing conditions for the imminent growth of LBG π-conjugated polymers.

Electrical and third-order nonlinear optical properties of poly(2-fluoro-1,4-phenylenevinylene) and its copolymers

SummaryPoly(2-fluoro-1,4-phenylenevinylene), PFPV, and its copolymers have been synthesized via water soluble precursor route and their electrical and optical propreties were measured. It seems that electron-withdrawing fluorine substituent on phenylene ring increases the band-gap of PFPV and it affects electrical and optical properties. The conductivity values of FeCl3-doped drawn polymer films ranged from 10-1 to 101 Scm-1 depending on their composition, and were 10 times larger than those of undrawn ones. The χ(3) value for undrawn PFPV, using THG technique at 1907nm fundamental wavelength, was 4.76x10-12 esu.