High-performance Polymer Field-effect Transistors Research Articles

Enhanced Performance of Cyclopentadithiophene-Based Donor-Acceptor-Type Semiconducting Copolymer Transistors Obtained by a Wire Bar-Coating Method.

Herein, we report the fabrications of high-performance polymer field-effect transistors (PFETs) with wire bar-coated semiconducting polymer film as an active layer. For an active semiconducting material of the PFETs, we employed cyclopentadithiophene-alt-benzothiadiazole (CDT-BTZ) that is a D-A-type-conjugated copolymer consisting of a repeated electron-donating unit and an electron-accepting unit, and the other two CDT-based D-A-type copolymer analogues are cyclopentadithiophene-alt-fluorinated-benzothiadiazole (CDT-FBTZ) and cyclopentadithiophene-alt-thiadiazolopyridine (CDT-PTZ). The linear field-effect mobility values obtained from the transfer curve of the PFETs fabricated with the spin-coating were 0.04 cm2/Vs, 0.16 cm2/Vs, and 0.31 cm2/Vs, for CDT-BTZ, CDT-FBTZ, and CDT-PTZ, respectively, while the mobility values measured from the PFETs with the wire bar-coated CDT-BTZ film, CDT-FBTZ film, and CDT-PTZ film were 0.16 cm2/Vs, 0.28 cm2/Vs, and 0.95 cm2/Vs, respectively, which are about 2 to 4 times higher values than those of the PFETs with spin-coated films. These results revealed that the aligned molecular chain is beneficial for the D-A-type semiconducting copolymer even though the charge transport in the D-A-type semiconducting copolymer is known to be less critical to the degree of disorder in film.

High-performance polymer field-effect transistors: from the perspective of multi-level microstructures.

The multi-level microstructure of conjugated polymers is the most critical parameter determining the charge transport property in field-effect transistors (FETs). However, controlling the hierarchical microstructures and the structural evolution remains a significant challenge. In this perspective, we discuss the key aspects of multi-level microstructures of conjugated polymers towards high-performance FETs. We highlight the recent progress in the molecular structures, solution-state aggregation, and polymer crystal structures, representing the multi-level microstructures of conjugated polymers. By tuning polymer hierarchical microstructures, we attempt to provide several guidelines for developing high-performance polymer FETs and polymer electronics.

Thiophene-Thiazole-Based Semiconducting Copolymers for High-Performance Polymer Field-Effect Transistors.

We report a newly synthesized donor (D)-acceptor (A)type semiconducting copolymer, consisting of thiophene as an electron-donating unit and thiazole as an electron-accepting unit (PQTBTz-TT-C8) for the active layer of the organic field-effect transistors (OFETs). Specifically, this study investigates the structure and electrical property relationships of PQTBTz-TT-C8 with comprehensive analyses on the charge-transporting properties corresponding to the spin rate of the spin coater during the formation of the PQTBTz-TT-C8 film. The crystallinity of PQTBTz-TT-C8 films is examined with grazing incidence X-ray diffraction. Temperature-dependent transfer measurements of the OFETs are conducted to extract the density of states (DOS) and characterize the charge-transport properties. Comparative analyses on charge transports within the framework of the physical model, based on polaron hopping and Gaussian DOS, reveal that the prefactors of both physical charge-transport models are independent of the spin-coating condition for the films. For staggered structural transistors, however, the thickness of the PQTBTz-TT-C8 films, which strongly affect the series resistance along the charge-transfer path in a vertical direction, is changed in accordance with the spin-coating rate. In other words, the spin-coating rate of the PQTBTz-TT-C8 films influences the thickness of the polymer films, yet any significant changes in the crystallinity of the film or electronic coupling between the neighboring molecules upon the spin-coating condition were barely noticeable. Because the PQTBTz-TT-C8 backbone chains inside the thin film are stacked up with the edge-on, the series resistances are changed according to the thickness of the film and thus the performance of the device varies depending on the thickness.

Manganese Oxide Nanoparticle as a New p-Type Dopant for High-Performance Polymer Field-Effect Transistors.

We report a new p-type dopant, manganese oxide (Mn3O4) nanoparticle, to enhance the performance of organic field-effect transistors (OFETs) with conjugated polymers, including poly(3-hexylthiophene-2,5-diyl), poly[[N,N 9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)], and poly[[2,5-bis(2-octyldodecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[[2,2'-(2,5-thiophene)bis-thieno(3,2b) thiophene]-5,5'-diyl]] (DPPT-TT). Incorporating a small amount of Mn3O4 nanoparticles in the semiconductor film significantly improved the hole mobility and decreased the threshold voltage for all OFETs, indicating efficient Mn3O4 nanoparticle p-type doping. The Mn3O4 nanoparticle showed a better doping efficiency than the widely used FeCl3 dopant due to better mixability with the host conjugated polymers. In particular, doped DPPT-TT OFETs showed significantly improved mobility up to 2.35 (±0.4) cm2/(V·s) with enhanced air and operational stability at 0.1 wt % doping concentration from 1.2 cm2/(V·s) for pristine devices.

Tailoring molecular weight of polymeric dielectric to enhance electron and hole mobilities in polymer field-effect transistors

Abstract Polymer field-effect transistors (PFETs) are promising candidates for future electronics due to easy and inexpensive fabrication. Poly{[ N , N ′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) [P(NDI2OD-T2)] based PFET devices with poly (methyl 2-methylpropenoate) (PMMA) as dielectric were fabricated successfully and it was found that the charge transporting behaviors were greatly impacted by the molecular weight of PMMA. Both hole and electron mobilities were improved simultaneously with increasing molecular weight of PMMA. In addition, similar result was also obtained in the PFETs based on a semiconductor, poly{[ N , N ′-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl-alt-5,5′-di(thiophen-2-yl)-2,2′-(E)-2-(2-(thiophen-2-yl)vinyl)thiophene]}. Furthermore, the corresponding complementary-like inverters with PMMA displayed a gain value of 100. We studied the interface between the semiconducting and dielectric layers and found that the low trap density with higher-molecular-weight PMMA attributes to high mobilities of electrons and holes. This work provides a new way to fabricate high-performance PFETs and to improve performance of the complementary circuits through studying the effects of polymeric gate dielectrics, compared to the conventional view of semiconducting materials.

Water-based high-performance polymer field effect transistors enabled by heat-assisted surfactant elimination

Environmentally benign processing technology of polymeric semiconductor is demonstrated to facilitate industrial application of organic electronics. By employing sodium dodecyl sulfate (SDS) as a surfactant and Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) as a polymeric semiconductor, a water-borne colloid was synthesized and used for forming the active layer of a field effect transistor. Traditionally, this technology was not successful in realizing high charge carrier mobility, owing to difficulties in identifying an optimal temperature for eliminating the surfactant while maintaining the long-range-ordered π–π stacked structure of the semiconductors. In this work, by utilizing the liquid crystalline nature of the PBTTT, we identified the optimal thermal treatment condition (∼270°C), which is sufficient both for eliminating the SDS and stimulating a second phase transition of the PBTTT. As a result, the PBTTT nanoparticle film cast from the water-borne colloid was successfully sintered to form continuously π–π stacked polymeric semiconductor films, whose high charge carrier mobility of 0.19cm2/Vs, is comparable to those of organic solvents.

Synthetic Tailoring of Solid-State Order in Diketopyrrolopyrrole-Based Copolymers via Intramolecular Noncovalent Interactions

Understanding the microstructures of semiconducting polymers is critical for improving the charge transport properties of polymer field-effect transistors (PFETs). A series of diketopyrrolopyrrole-based copolymers designed by implementing the concept of intramolecular noncovalent conformational locks through the functionalization of polymer backbones with fluorine atoms or methoxy groups were synthesized and compared with their unfunctionalized analogue. In contrast to the bimodal texture of the unfunctionalized polymer, the thin films of the polymer with fluorine atoms exhibit predominantly edge-on texture with much improved crystalline ordering. The thin films of the polymer modified with methoxy groups have a principally face-on texture. These dramatic differences in thin-film texture can be correlated with the polymers’ solubilities. Furthermore, the improved crystalline ordering of these semiconductor polymers enables the fabrication of high-performance PFETs: the hole mobility of the methoxy-modified polymer is reduced by half with respect to that of the unmodified polymer, whereas the hole mobility values of the fluorine-modified polymer are up to ∼6 times higher, approximately 1.32 cm2 V–1 s–1, and exhibit pronounced thermal stability. These results provide new guidelines for the molecular design of semiconducting polymers with noncovalent conformational locks.

High-performance polymer field-effect transistors fabricated with low-bandgap DPP-based semiconducting materials

New π-conjugated D–A copolymers PDMOTT-n combining a diketopyrrolopyrrole unit and a 3,6-dimethoxythieno[3,2-b]thiophene moiety were synthesized, and their field-effect performances were successfully characterized.

Design, Synthesis, and Structure–Property Relationships of Isoindigo-Based Conjugated Polymers

Conjugated polymers have developed rapidly due to their promising applications in low-cost, lightweight, and flexible electronics. The development of the third-generation donor-acceptor (D-A) polymers greatly improved the device performance in organic solar cells (OSCs) and field-effect transistors (FETs). However, for further improvement of device performance, scientists need to develop new building blocks, in particular electron-deficient aromatics, and gain an in-depth understanding of the structure-property relationships. Recently, isoindigo has been used as a new acceptor of D-A conjugated polymers. An isomer of indigo, isoindigo is a less well-known dye and can be isolated as a by-product from certain biological processes. It has two lactam rings and exhibits strong electron-withdrawing character. This electron deficiency gives isoindigo-based polymers intriguing properties, such as broad absorption and high open circuit voltage in OSCs, as well as high mobility and good ambient stability in FETs. In this Account, we review our recent progress on the design, synthesis, and structure-property relationship study of isoindigo-based polymers for FETs. Starting with some discussion on carrier transport in polymer films, we provide some basic strategies towards high-performance polymer FETs. We discuss the stability issue of devices, the impediment of the alkyl side chains, and the choice of the donor part of conjugated polymers. We demonstrate that introducing the isoindigo core effectively lowers the HOMO levels of polymers and provides FETs with long-time stability. In addition, we have found that when we use inappropriate alkyl side chains or non-centrosymmetric donors, the device performance of isoindigo polymers suffers. To further improve device performance and ambient stability, we propose several design strategies, such as using farther branched alkyl chains, modulating polymer energy levels, and extending π-conjugated backbones. We have found that using farther branched alkyl chains can effectively decrease interchain π-π stacking distance and improve carrier mobility. When we introduce electron-deficient functional groups on the isoindigo core, the LUMO levels of the polymers markedly decrease, which significantly improves the electron mobility and device stability. In addition, we present a new polymer system called BDOPV, which is based on the concept of π-extended isoindigo. By application of some strategies successfully used in isoindigo-based polymers, BDOPV-based polymers exhibit high mobility and good stability both in n-type and in ambipolar FETs. We believe that a synergy of molecular engineering strategies towards the isoindigo core, donor units, and side chains may further improve the performance and broaden the application of isoindigo-based polymers.

Electron-Deficient Poly(p-phenylene vinylene) Provides Electron Mobility over 1 cm2 V–1 s–1 under Ambient Conditions

Poly(p-phenylene vinylene) derivatives (PPVs) are one of the most widely investigated p-type polymers in organic electronics. PPVs generally exhibit electron mobilities lower than 10(-4) cm(2) V(-1) s(-1), thus hindering their applications in high-performance polymer field-effect transistors and organic photovoltaics. Herein, we design and synthesize a novel electron-deficient PPV derivative, benzodifurandione-based PPV (BDPPV). This new PPV derivative displays high electron mobilities up to 1.1 cm(2) V(-1) s(-1) under ambient conditions (4 orders of magnitude higher than those of other PPVs), because it overcomes common defects in PPVs, such as conformational disorder, weak interchain interaction, and a high LUMO level. BDPPV represents the first polymer that can transport electrons over 1 cm(2) V(-1) s(-1) under ambient conditions.

Influence of intensive light exposure on polymer field-effect transistors

The influence of intensive light exposure on high performance polymer field-effect transistors containing highly regioregular poly(3-alkylthiophene) as a semiconductor is reported. While the transistors show high stability under ambient air and light, a distinct degradation upon exposure to intensive light could be detected by its impact on the on currents. UV-Vis spectra were used to interpret the current decrease as being caused by a decrease of the conjugation length of the semiconductor, which is supported by IR spectrometry. The role of ambient air, in particular, oxygen, in the degradation process is shown.