Jihun Hwang

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.

Solvent-free solution processed passivation layer for improved long-term stability of organic field-effect transistors

In an effort to realize organic field-effect transistors (OFETs) that are stable over long periods of time, we have designed an organic–inorganic hybrid passivation material (TGD622t) prepared via a non-hydrolytic sol–gel process that does not require the use of solvents. Fourier-transform infrared spectroscopy, atomic force microscopy, and UV-visible spectroscopy demonstrated the high density and low porosity of the organic–inorganic hybrid transparent TGD622t film after low-temperature curing (below 100 °C). The dense TGD622t passivation layer, which exhibited a water vapor transmission rate (WVTR) of 0.434 g m−2 per day, effectively protected the poly[9,9-dioctylfluorenyl-2,7-diyl]-co-(bithiophene)]-based OFETs from humidity and oxygen in ambient air, resulting in a much more robust OFET performance with long-term stability relative to the operation of unpassivated devices.

Photocurable polymer gate dielectrics for cylindrical organic field-effect transistors with high bending stability

Here we describe the use of photocurable poly(vinyl cinnamate) (PVCN) as a gate dielectric in high-performance cylindrical organic field-effect transistors (OFETs) with high bending stability. A smooth-surface metallic fiber (Al wire) was employed as a cylindrical substrate, and polymer dielectrics (PVCN and poly(4-vinyl phenol) (PVP)) were formed viadip-coating. The PVCN and PVP dielectrics deposited on the Al wire and respectively cross-linked via UV irradiation and thermal heating were found to be very smooth and uniform over the entire coated area. Pentacene-based cylindrical OFETs with the polymer dielectrics exhibited high-performance hysteresis-free operation. Devices made with the PVCN dielectric showed superior bending stability than devices made with PVP dielectrics or previously reported cylindrical OFETs due to the good flexibility of the PVCN dielectric. The devices maintained their excellent performance under bending at a bending radius comparable to the lowest value reported for planar OFETs.

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.

Photopatternable ultrathin gate dielectrics for low-voltage-operating organic circuits

We report here a photopatternable ultrathin gate dielectric for the fabrication of low-voltage-operating organic field-effect transistors (OFETs) and inverters. The gate dielectric material is composed of a photocrosslinkable polymer, poly(vinyl cinnamate), and a thermally crosslinkable silane crosslinking reagent, 1,6-bis(trichlorosilyl)hexane. The spin-coated dielectric is photocured with ultraviolet light, which enables fine film patterning via regular photolithography. After thermal curing (at 110 °C), the dielectric showed excellent insulating properties (a leakage current density of ≈10−7 A/cm2 at 2.0 MV/cm) for an ultrathin film thickness of 70 nm, thus reducing the operating voltage of the OFETs and inverters to −5 V.

A composite of a graphene oxide derivative as a novel sensing layer in an organic field-effect transistor

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.

Facile method for the environmentally friendly fabrication of reduced graphene oxide films assisted by a metal substrate and saline solution

In this study, a graphene oxide film supported on a steel sheet was prepared by electrophoretic deposition and then treated with a NaCl solution to form a reduced graphene oxide (rGO) film. The rGO film displayed an electrical conductivity of 40.7 S m−1, and was prepared using benign and environmentally friendly reduction conditions.