High Speed Ultraviolet Phototransistors Based on an Ambipolar Fullerene Derivative

Combining high charge carrier mobility with ambipolar transport in light-absorbing organic semiconductors is highly desirable as it leads to enhanced charge photogeneration, and hence improved performance, in various optoelectronic devices including solar cells and photodetectors. Here we report the development of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM)-based ultraviolet (UV) phototransistors with balanced electron and hole transport characteristics. The latter is achieved by fine-tuning the source–drain electrode work function using a self-assembled monolayer. Opto/electrical characterization of as-prepared ambipolar PC₆₁BM phototransistors reveals promising photoresponse, particularly in the UV-A region (315–400 nm), with a maximum photosensitivity and responsivity of 9 × 10³ and 3 × 10³ A/W, respectively. Finally, the temporal response of the PC₆₁BM phototransistors is found to be high despite the long channel length (10 s of μm) with typical switching times of <2 ms

Controlling Conformations of Diketopyrrolopyrrole-Based Conjugated Polymers: Role of Torsional Angle

Transport of charge carriers through conjugated polymers is strongly influenced by the presence and distribution of structural disorders. In the present work, structural defects caused by the presence of torsional angle were investigated in a diketopyrrolopyrrole (<b>DPP</b>)-based conjugated polymer. Two new copolymers of <b>DPP</b> were synthesized with varying torsional angles to trace the role of structural disorder. The optical properties of these copolymers in solution and thin film reveal the strong influence of torsional angle on their photophysical properties. A strong influence was observed on carrier transport properties of polymers in organic field-effect transistors (OFET) device geometry. The polymers based on phenyl DPP with higher torsional angle (<b>PPTDPP</b>-OD-TEG) resulted in high threshold voltage with less charge carrier mobility as compared to the polymer based on thiophene DPP (<b>2DPP</b>-OD-TEG) bearing a lower torsional angle. Carrier mobility and the molecular orientation of the conjugated polymers were correlated on the basis of grazing incidence X-ray scattering measurements showing the strong role of torsional angle introduced in the form of structural disorder. The results presented in this Article provide a deep insight into the sensitivity of structural disorder and its impact on the device performance of DPP-based conjugated polymers

Electrospray-Processed Soluble Acenes toward the Realization of High-Performance Field-Effect Transistors

Functionalized acenes have proven to be promising compounds in the field of molecular electronics because of their unique features in terms of the stability, performance, and ease of processing. The emerging concept of large-area-compatible techniques for flexible electronics has brought about a wide variety of well-established techniques for the deposition of soluble acenes, with spray-coating representing an especially fruitful approach. In the present study, electrostatic spray deposition is proposed as an alternative to the conventional spray-coating processes, toward the realization of high-performance organic field-effect transistors (OFETs), on both rigid and flexible substrates. Particularly, a thorough study on the effect of the solvent and spraying regime on the resulting crystalline film’s morphology is performed. By optimization of the process conditions in terms of control over the size as well as the crystallization scheme of the droplets, desirable morphological features along with high-quality crystal domains are obtained. The fabricated OFETs exhibit excellent electrical characteristics, with high field-effect mobility up to 0.78 cm²/(V s), <i>I</i>_on/<i>I</i>_off >10⁴, and near-zero threshold voltages. Additionally, the good performance of the OFETs realized on plastic substrates gives great potentiality to the proposed method for applications in the challenging field of large-area electronics

Fullerene/Cobalt Porphyrin Hybrid Nanosheets with Ambipolar Charge Transporting Characteristics

A novel supramolecular nanoarchitecture, comprising C₆₀/Co porphyrin nanosheets, was prepared by a simple liquid–liquid interfacial precipitation method and fully characterized by means of optical microscopy, AFM, STEM, TEM, and XRD. It is established that the highly crystalline C₆₀/Co porphyrin nanosheets have a simple (1:1) stoichiometry, and when incorporated in bottom-gate, bottom-contact field-effect transistors (FETs), they show ambipolar charge transport characteristics

Benzotrithiophene Copolymers: Influence of Molecular Packing and Energy Levels on Charge Carrier Mobility

The planar benzotrithiophene unit (<b>BTT</b>) was incorporated into four different donor polymers, and by systematically changing the nature and positioning of the solubilizing alkyl side chains along the conjugated backbone, the polymers’ frontier energy levels and optoelectronic properties were controlled. Reducing the steric hindrance along the polymer backbone lead to strong interchain aggregation and highly ordered thin films, achieving hole mobilities of 0.04 cm²/(V s) in organic thin film transistors. In an attempt to increase the polymer’s processability and reduce chain aggregation, steric hindrance between alkyl side chains was exploited. As a result of the increased solubility, the film forming properties of the polymer could be improved, but at the cost of reduced hole mobilities in OFET devices, due to the lack of long-range order in the polymer films

Edmund Spenser

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Influence of Side-Chain Regiochemistry on the Transistor Performance of High-Mobility, All-Donor Polymers

Three novel polythiophene isomers are reported whereby the only difference in structure relates to the regiochemistry of the solubilizing side chains on the backbone. This is demonstrated to have a significant impact on the optoelectronic properties of the polymers and their propensity to aggregate in solution. These differences are rationalized on the basis of differences in backbone torsion. The polymer with the largest effective conjugation length is demonstrated to exhibit the highest field-effect mobility, with peak values up to 4.6 cm² V^–1 s^–1

Conjugated Copolymers of Vinylene Flanked Naphthalene Diimide

We report the synthesis of a novel naphthalene diimide (NDI) monomer containing two (tributyl­stannyl)­vinyl groups. The utility of this building block is demonstrated by its copolymerization with five different electron-rich comonomers under Stille conditions. The resulting high molecular weight polymers show red-shifted optical absorptions in comparison to the analogous polymers without the vinylene spacer and a significant increase in the intensity of the low-energy intramolecular charge transfer band. The polymers all exhibit ambipolar behavior in bottom-gate, top-contact organic thin-film transistors. The insertion of a solution-processed barium hydroxide layer between the polymer and the gold electrode led to unipolar behavior with improved electron mobilities

Comparative Optoelectronic Study between Copolymers of Peripherally Alkylated Dithienosilole and Dithienogermole

Here we report a simple methodology for the synthesis of dithienosilole and dithienogermole monomers in which the necessary solubilizing long chain alkyl groups are incorporated into the peripheral 3,5-positions of the fused ring. We report four novel monomers in which methyl or butyl groups are attached to the bridging Si and Ge atom. Copolymers with bithiophene were synthesized by a Stille polymerization in high molecular weight. We report the optical, electrical, electrochemical and morphological properties of the resulting polymers. We find that the nature of the bridging heteroatom (Si or Ge) has only a minor influence on these properties, whereas the nature of the alkyl chain attached to the bridging atom is found to have a much larger effect

Design and piezoelectric energy harvesting properties of a ferroelectric cyclophosphazene salt

Cyclophosphazenes offer a robust and easily modifiable platform for a diverse range of functional systems that have found applications in a wide variety of areas. Herein, for the first time, it reports an organophosphazene-based supramolecular ferroelectric [(PhCH2NH)6P3N3Me]I, [PMe]I. The compound crystallizes in the polar space group Pc and its thin-film sample exhibits remnant polarization of 5 µC cm−2. Vector piezoresponse force microscopy (PFM) measurements indicated the presence of multiaxial polarization. Subsequently, flexible composites of [PMe]I are fabricated for piezoelectric energy harvesting applications using thermoplastic polyurethane (TPU) as the matrix. The highest open-circuit voltages of 13.7 V and the maximum power density of 34.60 µW cm−2 are recorded for the poled 20 wt.% [PMe]I/TPU device. To understand the molecular origins of the high performance of [PMe]I-based mechanical energy harvesting devices, piezoelectric charge tensor values are obtained from DFT calculations of the single crystal structure. These indicate that the mechanical stress-induced distortions in the [PMe]I crystals are facilitated by the high flexibility of the layered supramolecular assembly.</p