High-Mobility Ambipolar Organic Thin-Film Transistor Processed From a Nonchlorinated Solvent

Synthesis and Optoelectronic Characterizations of Conjugated Polymers Based on Diketopyrrolopyrrole and 2,2'-(thieno[3,2-b]thiophene-2,5-diyl)diacetonitrile Via Knoevenagel Condensation

Conjugated polymers have attracted extensive attention as semiconducting materials in wearable and flexible electronics. In this study, we utilize atom-economical Knoevenagel reaction to construct two conjugated polymers, PTDPP-CNTT and PFDPP-CNTT, based on dialdehyde-thiophene/furan-flanked diketopyrrolopyrrole (DPP) and 2,2'-(thieno[3,2-b]thiophene-2,5-diyl)diacetonitrile (CNTT). The resulting polymers exhibited suitable highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) energy levels, small bandgaps, and broad UV-vis-NIR absorptions (≈400-1000 nm), endowing them with photothermal and balanced ambipolar semiconducting properties with hole and electron mobilities over 10-3 cm2V-1s-1. Additionally, PTDPP-CNTT-based organic field-effect transistors (OFETs) devices show photo-responsive characteristics at 808 and 980 nm in the hole transport channel with the photo-responsivenesses of 9.0 × 10-3 A/W and 0.4 A/W, respectively, suggesting potential application in organic NIR-phototransistors.

Organic Semiconductors Processed from Synthesis-to-Device in Water

Organic semiconductors (OSCs) promise to deliver next-generation electronic and energy devices that are flexible, scalable and printable. Unfortunately, realizing this opportunity is hampered by increasing concerns about the use of volatile organic compounds (VOCs), particularly toxic halogenated solvents that are detrimental to the environment and human health. Here, a cradle-to-grave process is reported to achieve high performance p- and n-type OSC devices based on indacenodithiophene and diketopyrrolopyrrole semiconducting polymers that utilizes aqueous-processes, fewer steps, lower reaction temperatures, a significant reduction in VOCs (>99%) and avoids all halogenated solvents. The process involves an aqueous mini-emulsion polymerization that generates a surfactant-stabilized aqueous dispersion of OSC nanoparticles at sufficient concentration to permit direct aqueous processing into thin films for use in organic field-effect transistors. Promisingly, the performance of these devices is comparable to those prepared using conventional synthesis and processing procedures optimized for large amounts of VOCs and halogenated solvents. Ultimately, the holistic approach reported addresses the environmental issues and enables a viable guideline for the delivery of future OSC devices using only aqueous media for synthesis, purification and thin-film processing.

Azaisoindigo-Based Polymers with a Linear Hybrid Siloxane-Based Side Chain for High-Performance Semiconductors Processable with Nonchlorinated Solvents

Developing nonchlorinated solvent-processed polymeric semiconductors to avoid environmental concerns and health hazards caused by chlorinated solvents is especially urgent. Here, a molecular design strategy, composed of backbone fluorination and side chain optimization, is used for preparing high-solubility and high-performance azaisoindigo-based polymers. The effects of different backbones and side chains on the solubility, film crystallinity, molecular stacking, and charge transport properties are mainly investigated. A long linear hybrid siloxane-based chain (C₆-Si₇) is chosen to improve the solubility, while the incorporation of fluorine (F) is used to enhance the film crystallinity and charge mobility. By optimizing the backbone and side chain, both solubility and charge mobility of the azaisoindigo-based polymer are significantly improved. As a result, PAIIDBFT-Si films processed with toluene, tetrahydrofuran, ether, and alkanes, achieved charge mobilities of 4.14, 3.78, 2.14, and 2.34 cm² V^-1 s^-1, respectively. The current study provides an effective strategy for the design and synthesis of high-performance polymeric semiconductors processed with nonchlorinated solvents.

Biodegradable Materials and Green Processing for Green Electronics

There is little question that the "electronic revolution" of the 20th century has impacted almost every aspect of human life. However, the emergence of solid-state electronics as a ubiquitous feature of an advanced modern society is posing new challenges such as the management of electronic waste (e-waste) that will remain through the 21st century. In addition to developing strategies to manage such e-waste, further challenges can be identified concerning the conservation and recycling of scarce elements, reducing the use of toxic materials and solvents in electronics processing, and lowering energy usage during fabrication methods. In response to these issues, the construction of electronic devices from renewable or biodegradable materials that decompose to harmless by-products is becoming a topic of great interest. Such "green" electronic devices need to be fabricated on industrial scale through low-energy and low-cost methods that involve low/non-toxic functional materials or solvents. This review highlights recent advances in the development of biodegradable materials and processing strategies for electronics with an emphasis on areas where green electronic devices show the greatest promise, including solar cells, organic field-effect transistors, light-emitting diodes, and other electronic devices.

Developments of Diketopyrrolopyrrole-Dye-Based Organic Semiconductors for a Wide Range of Applications in Electronics

In recent times, fused aromatic diketopyrrolopyrrole (DPP)-based functional semiconductors have attracted considerable attention in the developing field of organic electronics. Over the past few years, DPP-based semiconductors have demonstrated remarkable improvements in the performance of both organic field-effect transistor (OFET) and organic photovoltaic (OPV) devices due to the favorable features of the DPP unit, such as excellent planarity and better electron-withdrawing ability. Driven by this success, DPP-based materials are now being exploited in various other electronic devices including complementary circuits, memory devices, chemical sensors, photodetectors, perovskite solar cells, organic light-emitting diodes, and more. Recent developments in the use of DPP-based materials for a wide range of electronic devices are summarized, focusing on OFET, OPV, and newly developed devices with a discussion of device performance in terms of molecular engineering. Useful guidance for the design of future DPP-based materials and the exploration of more advanced applications is provided.

Fused Bithiophene Imide Oligomer and Diketopyrrolopyrrole Copolymers for n-Type Thin-Film Transistors

Diketopyrrolopyrrole (DPP)-based copolymers have received considerable attention as promising semiconducting materials for high-performance organic thin-film transistors (OTFTs). However, these polymers typically exhibit p-type or ambipolar charge-transporting characteristics in OTFTs due to their high-lying highest occupied molecular orbital (HOMO) energy levels. In this work, a new series of DPP-based n-type polymers have been developed by incorporating fused bithiophene imide oligomers (BTIn) into DPP polymers. The resulting copolymers BTIn-DPP show narrow band gaps as low as 1.27 eV and gradually down-shifted frontier molecular orbital energy levels upon the increment of imide group number. Benefiting from the coplanar backbone conformation, well-delocalized π-system, and favorable polymer chain packing, the optimal polymer in the series shows promising n-type charge transport with an electron mobility up to 0.48 cm² V^-1 s^-1 in OTFTs, which is among the highest values for the DPP-based n-type polymers reported to date. The results demonstrate that incorporating fused bithiophene imide oligomers into polymers can serve as a promising strategy for constructing high-performance n-type polymeric semiconductors.

A comparative analysis of symmetric diketopyrrolopyrrole-cored small conjugated molecules with aromatic flanks: From geometry to charge transport

Diketopyrrolopyrrole (DPP) derivatives are promising compounds for application in organic electronics. Here, we investigate several symmetrical N-unsubstituted and N-methyl substituted DPPs which differ in the heteroatom in the aromatic flanks. The conformational, electronic, and optical properties are characterized for single molecules in vacuum or a solvent. The intermolecular interactions are evaluated for interacting dimers. Here, a number of stacking geometries is tested, and dimers with mutual orientation of the molecules corresponding to the minimal binding energies are determined. The predicted charge carrier mobilities for stacks having minimal binding energies corroborate experimentally measured values. We conclude that DFT prediction of such stacks is a promising and computationally inexpensive approach to a rough estimation of transport properties. Additionally, the super-cell of the experimentally resolved crystal structure is used to study the dynamics and to compute the charge transport along the hopping pathways. We discuss obtained high mobilities and relate them to the symmetry of DPP core. © 2018 Wiley Periodicals, Inc.

2,1,3-Benzothiadiazole-5,6-dicarboxylicimide-Based Polymer Semiconductors for Organic Thin-Film Transistors and Polymer Solar Cells

A series of polymer semiconductors incorporating 2,1,3-benzothiadiazole-5,6-dicarboxylicimide (BTZI) as strong electron-withdrawing unit and an alkoxy-functionalized head-to-head linkage containing bithiophene or bithiazole as highly electron-rich co-unit are designed and synthesized. Because of the strong intramolecular charge transfer characteristics, all three polymers BTZI-TRTOR (P1), BTZI-BTOR (P2), and BTZI-BTzOR (P3) exhibit narrow bandgaps of 1.13, 1.05, and 0.92 eV, respectively, resulting in a very broad absorption ranging from 350 to 1400 nm. The highly electron-deficient 2,1,3-benzothiadiazole-5,6-dicarboxylicimide and alkoxy-functionalized bithiophene (or thiazole) lead to polymers with low-lying lowest unoccupied molecular orbitals (-3.96 to -4.28 eV) and high-lying highest occupied molecular orbitals (-5.01 to -5.20 eV). Hence, P1 and P3 show substantial and balanced ambipolar transport with electron mobilities/hole mobilities of up to 0.86/0.51 and 0.95/0.50 cm² V^-1 s^-1, respectively, and polymer P2 containing the strongest donor unit exhibited unipolar p-type performance with an average hole mobility of 0.40 cm² V^-1 s^-1 in top-gate/bottom-contact thin-film transistors with gold as the source and drain electrodes. When incorporated into bulk heterojunction polymer solar cells, the narrow bandgap (1.13 eV) polymer P1 shows an encouraging power conversion efficiency of 4.15% with a relatively large open-circuit voltage of 0.69 V, which corresponds to a remarkably small energy loss of 0.44 eV. The power conversion efficiency of P1 is among the highest reported to date with such a small energy loss in polymer:fullerene solar cells.

High-Performance Furan-Containing Conjugated Polymer for Environmentally Benign Solution Processing

Developing semiconducting polymers that exhibit both strong charge transport capability via highly ordered structures and good processability in environmentally benign solvents remains a challenge. Given that furan-based materials have better solubility in various solvents than analogous thiophene-based materials, we have synthesized and characterized furanyl-diketopyrrolopyrrole polymer (PFDPPTT-Si) together with its thienyl-diketopyrrolopyrrole-based analogue (PTDPPTT-Si) to understand subtle changes induced by the use of furan instead of thiophene units. PTDPPTT-Si films processed in common chlorinated solvent exhibit a higher hole mobility (3.57 cm² V^-1 s^-1) than PFDPPTT-Si films (2.40 cm² V^-1 s^-1) under the same conditions; this greater hole mobility is a result of tightly aggregated π-stacking structures in PTDPPTT-Si. By contrast, because of its enhanced solubility, PFDPPTT-Si using chlorine-free solution processing results in a device with higher mobility (as high as 1.87 cm² V^-1 s^-1) compared to that of the corresponding device fabricated using PTDPPTT-Si. This mobility of 1.87 cm² V^-1 s^-1 represents the highest performances among furan-containing polymers reported to the best of our knowledge for nonchlorinated solvents. Our study demonstrates an important step toward environmentally compatible electronics, and we expect the results of our study to reinvigorate the furan-containing semiconductors field.