Gyoungsik Kim

A Thienoisoindigo-Naphthalene Polymer with Ultrahigh Mobility of 14.4 cm2/V·s That Substantially Exceeds Benchmark Values for Amorphous Silicon Semiconductors

By considering the qualitative benefits associated with solution rheology and mechanical properties of polymer semiconductors, it is expected that polymer-based electronic devices will soon enter our daily lives as indispensable elements in a myriad of flexible and ultra low-cost flat panel displays. Despite more than a decade of research focused on designing and synthesizing state-of-the-art polymer semiconductors for improving charge transport characteristics, the current mobility values are still not sufficient for many practical applications. The confident mobility in excess of ∼10 cm(2)/V·s is the most important requirement for enabling the realization of the aforementioned near-future products. We report on an easily attainable donor-acceptor (D-A) polymer semiconductor: poly(thienoisoindigo-alt-naphthalene) (PTIIG-Np). An unprecedented mobility of 14.4 cm(2)/V·s, by using PTIIG-Np with a high-k gate dielectric poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)), is achieved from a simple coating processing, which is of a magnitude that is very difficult to obtain with conventional TFTs by means of molecular engineering. This work, therefore, represents a major step toward truly viable plastic electronics.

Ultra-narrow-bandgap thienoisoindigo polymers: structure–property correlations in field-effect transistors

From a structural point of view, the newly conceived thienoisoindigo (TIIG) moiety can serve as an ideal building block for the synthesis of high-performance polymers. To expand the range of available TIIG-based conjugated polymers, herein we report the synthesis and characterization of two new TIIG-based donor–acceptor polymers (PTIIG-TT and PTIIG-TVT), containing either the thieno[3,2-b]thiophene (TT) or the (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) moiety. In addition, we conducted a systematic investigation on the relationship between the microstructure of the polymer film and charge transport in organic field-effect transistors (OFETs) fabricated using these polymers. It was observed that the incorporation of a TVT moiety into the TIIG backbone imparts higher crystallinity and increases the molecular packing density, leading to an increased hole mobility (∼0.45 cm2 V−1 s−1) in PTIIG-TVT, compared with PTIIG-TT. When an Al electrode is used instead of a Au electrode in the OFET devices, both polymers exhibit outstanding ambipolar characteristics. This study advances the understanding of the structural features of TIIG-based polymers, which will potentially accelerate the improvement in the mobility of TIIG-based polymers.

Influence of Simultaneous Tuning of Molecular Weights and Alkyl Substituents of Poly(thienoisoindigo-alt-naphthalene)s on Morphology and Change Transport Properties

To simultaneously assess the impact of molecular weight (Mn) and alkyl substituent variations of polymers on the structural and optoelectronic properties, herein, we conduct a systematic study of a series of poly(thienoisoindigo-alt-naphthalene) (PTIIG-Np)-based polymers containing different alkyl substituents (2-hexyldecyl (HD), 2-octyldodecyl (OD), and 2-decyltetradecyl (DT) chains) and Mns (low (L) and high (H)). All of the polymers produce almost identical energy levels, whereas their optical spectra show a clear dependence on Mns and the alkyl substituents. Interestingly, increasing the alkyl substituent sizes of the polymers steadily increases the lamellar d-spacings (d100), ultimately leading to a densely packed lamellar structure for PTIIGHD-Np. In addition, both H-PTIIGOD-Np and H-PTIIGDT-Np exhibit larger π-stacking crystallites than the corresponding low-Mn polymers, while for PTIIGHD-Np, their size increases in the low-Mn batch. Ultimately, L-PTIIGHD-Np shows the best hole mobility of 1.87 cm2 V-1 s-1 in top-gate and bottom-contact organic field-effect transistors (OFETs) with a poly(methyl methacrylate), which is nearly 1 order of magnitude higher than other polymers tested in this study. Our results demonstrate that the simultaneous Mn and alkyl substituent engineering of the polymers can optimize their film morphology to produce high-performance OFETs.

Thienoisoindigo-Based Semiconductor Nanowires Assembled with 2-Bromobenzaldehyde via Both Halogen and Chalcogen Bonding

We fabricated nanowires of a conjugated oligomer and applied them to organic field-effect transistors (OFETs). The supramolecular assemblies of a thienoisoindigo-based small molecular organic semiconductor (TIIG-Bz) were prepared by co-precipitation with 2-bromobenzaldehyde (2-BBA) via a combination of halogen bonding (XB) between the bromide in 2-BBA and electron-donor groups in TIIG-Bz, and chalcogen bonding (CB) between the aldehyde in 2-BBA and sulfur in TIIG-Bz. It was found that 2-BBA could be incorporated into the conjugated planes of TIIG-Bz via XB and CB pairs, thereby increasing the π − π stacking area between the conjugated planes. As a result, the driving force for one-dimensional growth of the supramolecular assemblies via π − π stacking was significantly enhanced. TIIG-Bz/2-BBA nanowires were used to fabricate OFETs, showing significantly enhanced charge transfer mobility compared to OFETs based on pure TIIG-Bz thin films and nanowires, which demonstrates the benefit of nanowire fabrication using 2-BBA.

Ambipolar organic phototransistors based on 6,6′-dibromoindigo

Ambipolar organic phototransistors were fabricated using a natural pigment 6,6′-dibromoindigo (6-BrIG) as the active channel. These phototransistors yielded significantly enhanced currents upon light illumination with photoresponsivities and external quantum efficiencies as high as 10.3 A W−1 and 2437% for the n-channel, and 55.4 mA W−1 and 13.1% for the p-channel, respectively. In addition, simple inverter complementary circuits were fabricated by integrating two ambipolar phototransistors. Channel current was dependent on light intensity and voltage bias. This study provides a basis for an in-depth understanding of the optoelectronic characteristics of 6-BrIG, and introduces this material as an ecofriendly candidate for optoelectronic applications.