Jangwhan Cho

Morphology-Driven High-Performance Polymer Transistor-based Ammonia Gas Sensor

Developing high-performance gas sensors based on polymer field-effect transistors (PFETs) requires enhancing gas-capture abilities of polymer semiconductors without compromising their high charge carrier mobility. In this work, cohesive energies of polymer semiconductors were tuned by strategically inserting buffer layers, which resulted in dramatically different semiconductor surface morphologies. Elucidating morphological and structural properties of polymer semiconductor films in conjunction with FET studies revealed that surface morphologies containing large two-dimensional crystalline domains were optimal for achieving high surface areas and creating percolation pathways for charge carriers. Ammonia molecules with electron lone pairs adsorbed on the surface of conjugated semiconductors can serve as efficient trapping centers, which negatively shift transfer curves for p-type PFETs. Therefore, morphology optimization of polymer semiconductors enhances their gas sensing abilities toward ammonia, leading to a facile method of manufacturing high-performance gas sensors.

Analysis of charge transport in high-mobility diketopyrrolopyrole polymers by space charge limited current and time of flight methods

The hole mobility of the widely studied diketopyrrolopyrole-based polymers (PDPPDTSE) was examined using space charge limited current (SCLC) and time of flight (TOF) methods. The mobility of the hole-only device based on PDPPDTSE was found to be dependent upon the e-field over the range of 10−3 to 10−2 cm2 V−1 s−1 with nearly identical Poole–Frenkel coefficients. In addition, we found that the mobility strongly depended on the thickness of the PDPPDTSE. By analyzing the temperature dependence of transport characteristics, we argued that the charge transport in this polymer was greatly influenced by trap distribution at the electrode/semiconductor interface.

Thin Film Transistor Gas Sensors Incorporating High-Mobility Diketopyrrolopyrole-Based Polymeric Semiconductor Doped with Graphene Oxide

In this work, we fabricated a diketopyrrolopyrole-based donor-acceptor copolymer composite film. This is a high-mobility semiconductor component with a functionalized-graphene-oxide (GO) gas-adsorbing dopant, used as an active layer in gas-sensing organic-field-effect transistor (OFET) devices. The GO content of the composite film was carefully controlled so that the crystalline orientation of the semiconducting polymer could be conserved, without compromising its gas-adsorbing ability. The resulting optimized device exhibited high mobility (>1 cm(2) V(-1) s(-1)) and revealed sensitive response during programmed exposure to various polar organic molecules (i.e., ethanol, acetone, and acetonitrile). This can be attributed to the high mobility of polymeric semiconductors, and also to their high surface-to-volume ratio of GO. The operating mechanism of the gas sensing GO-OFET is fully discussed in conjunction with charge-carrier trap theory. It was found that each transistor parameter (e.g., mobility, threshold voltage), responds independently to each gas molecule, which enables high selectivity of GO-OFETs for various gases. Furthermore, we also demonstrated practical GO-OFET devices that operated at low voltage (<1.5 V), and which successfully responded to gas exposure.

High Charge-Carrier Mobility of 2.5 cm(2) V(-1) s(-1) from a Water-Borne Colloid of a Polymeric Semiconductor via Smart Surfactant Engineering.

Semiconducting polymer nanoparticles dispersed in water are synthesized by a novel method utilizing non-ionic surfactants. By developing a smart surfactant engineering technique involving a selective post-removal process of surfactants, an unprecedentedly high mobility of 2.51 cm(2) V(-1) s(-1) from a water-borne colloid is demonstrated for the first time.

Low dark current inverted organic photodiodes using anionic polyelectrolyte as a cathode interlayer

We demonstrate the effect of anionic polyelectrolyte as a cathode interlayer to enhance charge selectivity of the electrode/semiconductor junction of organic photodiodes. Poly(styrenesulfonate) (PSS) was used as a cathode interlayer to tune the energy level of an ITO/ZnO electrode, so that hole injection can be minimized while electron extraction can be maximized. Optimized photodiodes with a PSS interlayer showed lower and flatter dark current density curves compared to the reference devices, which implies that tunneling currents at the electrode/active layer interface were dramatically suppressed. Moreover, PSS as an interlayer enabled lower charge recombination yield, as confirmed by the ideality factor and linear dynamic range analysis. As a result, we could realize the near-ideal organic photodiodes with a high performance of specific detectivity up to 3.3 × 1012 Jones at −5 V.

Wafer-scale and environmentally-friendly deposition methodology for extremely uniform, high-performance transistor arrays with an ultra-low amount of polymer semiconductors

We report on a new class of microliter-scale solution processes for fabricating highly uniform and large-area transistor arrays with extremely low consumption of semiconducting polymers. These processes are accomplished by applying a vertical phase separation of polymers with an environmentally benign solvent, a random copolymerization strategy between two highly conductive repeating units, and a meniscus-dragging deposition technique. The successful realization of these three processes, as confirmed by the structural and morphological in-depth characterizations, has enabled the fabrication of high-performance polymeric field-effect transistors that were uniformly distributed, without a single failure, on a 4 inch wafer using only 40 μg of semiconducting polymers. The resulting transistor arrays showed an average mobility of 0.28 cm2 V−1 s−1, with a low standard deviation of 0.04, as well as ultra-uniform near-zero threshold voltages. Our simple strategy shows great promise for fabricating large-scale organic electronic devices in the future using a truly low-cost process.

Synergetic Evolution of Diketopyrrolopyrrole-Based Polymeric Semiconductor for High Reproducibility and Performance: Random Copolymerization of Similarly Shaped Building Blocks

A new random copolymer consisting of similarly shaped donor-acceptor building blocks of diketopyrrolopyrrole-selenophene-vinylene-selenophene (DPP-SVS) and DPP-thiophene-vinylene-thiophene (DPP-TVT) is designed and synthesized. The resulting P-DPP-SVS(5)-TVT(5) with an equal molecular ratio of the two building blocks produced significantly enhanced solubility when compared to that of the two homopolymers, PDPP-SVS and PDPP-TVT. More importantly, despite the maximum segmental randomness of the PDPP-SVS(5)-TVT(5) copolymer, its crystalline perfectness and preferential orientation are outstanding, even similar to those of the homopolymers thanks to the similarity of the two building blocks. This unique property produces a high charge carrier mobility of 1.23 cm2 V-1 s-1 of PDPP-SVS(5)-TVT(5), as determined from polymer field-effect transistor (PFET) measurements. The high solubility of PDPP-SVS(5)-TVT(5) promotes formulation of high-viscosity solutions which could be successfully processed to fabricate large-areal PFETs onto hydrophobically treated 4 in. wafers. A total of 269 individual PFETs are fabricated. These devices exhibit extremely narrow device-to-device deviations without a single failure and demonstrate an average charge carrier mobility of 0.66 cm2 V-1 s-1 with a standard deviation of 0.064. This is the first study to report on successfully realizing large-areal reproducibility of high-mobility polymeric semiconductors.

Effects of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane doping on diketopyrrolopyrrole-based, low crystalline, high mobility polymeric semiconductor

The effects of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) doping on diketopyrrolo-pyrrole-based polymeric semiconductors in terms of charge transport behavior and structural ordering are systematically investigated. Although the energy level offset between the polymeric semiconductor and the F4TCNQ acceptor was not particularly large, ultraviolet photoelectron spectroscopy analyses revealed that a low doping ratio of 1 wt. % is sufficient to tune the energy distance between the Fermi level and the HOMO level, reaching saturation at roughly 5 wt. %, which is further confirmed by the depletion mode measurements of field effect transistors (FETs). Structural analyses using grazing-incidence X-ray diffraction (GIXD) show that the overall degree of edge-on orientation is disturbed by the addition of dopants, with significant influence appearing at high doping ratios (>3 wt. %). The calculated charge carrier mobility from accumulation mode measurements of FETs showed a maximum value of 2 cm2/...