Tae Kyu An

Effect of Selenophene in a DPP Copolymer Incorporating a Vinyl Group for High‐Performance Organic Field‐Effect Transistors

A new polymeric semiconductor, PDPPDTSE, is reported which is composed of a diketopyrrolopyrrole moiety and selenophenylene vinylene selenophene, with a high field-effect mobility achieved through intermolecular donor-acceptor interactions. The field-effect mobility of OFET devices based on PDPPDTSE by spin-casting is 4.97 cm(2) V(-1) s(-1) , which is higher than predecessor polymeric semiconductors.

Effects of direct solvent exposure on the nanoscale morphologies and electrical characteristics of PCBM-based transistors and photovoltaics

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.

Solvent Additive to Achieve Highly Ordered Nanostructural Semicrystalline DPP Copolymers: Toward a High Charge Carrier Mobility

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 cm(2) V(-1) s(-1). The addition of CN to the chloroform solution facilitates the formation of highly crystalline polymer structures.

Alkyl Chain Length Dependence of the Field-Effect Mobility in Novel Anthracene Derivatives

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 cm(2)/(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.

Solution-processed flexible ZnO transparent thin-film transistors with a polymer gate dielectric fabricated by microwave heating

We report the development of solution-processed zinc oxide (ZnO) transparent thin-film transistors (TFTs) with a poly(2-hydroxyethyl methacrylate) (PHEMA) gate dielectric on a plastic substrate. The ZnO nanorod film active layer, prepared by microwave heating, showed a highly uniform and densely packed array of large crystal size (58 nm) in the [002] direction of ZnO nanorods on the plasma-treated PHEMA. The flexible ZnO TFTs with the plasma-treated PHEMA gate dielectric exhibited an electron mobility of 1.1 cm(2) V(-1) s(-1), which was higher by a factor of approximately 8.5 than that of ZnO TFTs based on the bare PHEMA gate dielectric.

Polymer–nanocrystal hybrid photodetectors with planar heterojunctions designed strategically to yield a high photoconductive gain

We demonstrate an approach to enhancing the photoresponsivity of a polymer photodetector (PPD). Both conventional bulk heterojunction (BHJ) and planar heterojunction (PHJ) PPDs were fabricated considering that the interface between a CdSe nanocrystal and a polymer can create photoconductive gain. A systematic study of the illumination wavelength and light power dependence of the photocurrent gain, combined with the charge carrier transport analysis, suggested that the PHJ-PPD could yield a higher hole mobility than could be achieved in a BHJ-PPD without compromising on the selective electron trapping effects. The optimized PHJ-PPD led to a photoconductive detectivity of 1.3 × 1010 cm Hz1/2/W.

Highly stable fluorine-rich polymer treated dielectric surface for the preparation of solution-processed organic field-effect transistors

Highly stable fluorine-rich polymer dielectrics were fabricated using cross-linked poly(3-(hexafluoro-2-hydroxyl) propyl) styrene (PFS), which shows excellent electrical stability, good adhesive surface properties, and good wettability on deposited solution-processed materials. Solution-processed triethylsilylethynyl anthradithiophene (TES-ADT) could be deposited onto the cross-linked PFS dielectrics to yield highly ordered crystalline structured films that did not delaminate. The field-effect mobilities were as high as 0.56 cm2 V−1 s−1, and negligible hysteresis was observed in the organic field-effect transistors (OFETs). The threshold voltage, the ON/OFF ratio, and the subthreshold slope were −0.043 V, ∼107, and −0.3 V per decade, respectively. The OFETs demonstrated excellent device reliability under gate-bias stress conditions due to the presence of highly stable fluorine groups in the cross-linked PFS dielectrics.

Thieno[3,4‐c]pyrrole‐4,6‐dione‐Based Small Molecules for Highly Efficient Solution‐Processed Organic Solar Cells

Two small molecules named BT-TPD and TBDT-TTPD with a thieno[3,4-c]pyrrole-4,6-dione (TPD) unit were designed and synthesized for solution-processed bulk-heterojunction solar cells. Their thermal, electrochemical, optical, charge-transport, and photovoltaic characteristics were investigated. These compounds exhibit strong absorption at 460-560 nm and low highest occupied molecular orbital levels (-5.36 eV). Field-effect hole mobilities of these compounds are 1.7-7.7×10(-3) cm(2) V(-1) s(-1). Small-molecule organic solar cells based on blends of these donor molecules and a acceptor display power conversion efficiencies as high as 4.62% under the illumination of AM 1.5G, 100 mW cm(-2).

High Tgcyclic olefin copolymer/Al2O3 bilayer gate dielectrics for flexible organic complementary circuits with low-voltage and air-stable operation

Here we present solution-processed ultrathin cyclic olefin copolymer (COC)/Al2O3 bilayer gate dielectrics for low-voltage and flexible N,N′-ditridecyl perylene diimide (PTCDI-C13)-based n-type organic field-effect transistors (OFETs) and their complementary circuits. The PTCDI-C13 thin films grown on the COC/Al2O3 bilayer gate dielectrics formed large and flat grains with thermal treatment, resulting in high OFET performance, and stability in an ambient air atmosphere. Despite the high glass transition temperature of the COC, the COC thin films showed good mechanical flexibility with the application of bending strain, and OFETs with bilayer gate dielectrics showed stable operation up to a strain of 1.0%. Complementary inverters based on the PTCDI-C13 and pentacene OFETs with bilayer dielectrics functioned at a low voltage of 5 V, and exhibited a high gain of 63.

Effects of Cyano-Substituents on the Molecular Packing Structures of Conjugated Polymers for Bulk-Heterojunction Solar Cells

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 electron-withdrawing 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 (V(OC)) 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 V(OC) (0.89 V), a higher fill factor (FF) (51.4%), and a lower short circuit current (J(SC)) (7.4 mA/cm(2)) 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 V(OC) by tuning the HOMO and LUMO energy levels of the conjugated polymer.