Air-stable and high-mobility n-channel organic transistors based on small-molecule/polymer semiconducting blends

Novel Dissymmetric Formal Quinoidal Molecules End-Capped by Dicyanomethylene and Triphenylphosphonium

A number of quinoidal molecules with symmetric end-capping groups, particularly dicyanomethylene units, have been synthesized for organic optoelectronic materials. In comparison, dissymmetric quinoidal molecules, characterized by end-capping with different groups, are less explored. In this paper, we present the unexpected formation of new formal quinoidal molecules, which are end-capped with both dicyanomethylene and triphenylphosphonium moieties. The structures of these dissymmetric quinoidal molecules were firmly verified by single crystal structural analyses. On the basis of the control experiments and DFT calculations, we proposed the reaction mechanism for the formation of these dissymmetric quinoidal molecules. The respective zwitterionic forms should make contributions to the ground state structures of these quinoidal molecules based on the analysis of their bond lengths and aromaticity and Mayer Bond Orbital (MBO) calculation. This agrees well with the fact that negative solvatochromism was observed for these quinoidal molecules. Although these new quinoidal molecules are non-emissive both in solutions and crystalline states, they become emissive with quantum yields up to 51.4 % after elevating the solvent viscosity or dispersing them in a PMMA matrix. Interestingly, their emissions can also be switched on upon binding with certain proteins, in particular with human serum albumin.

Ultra-Narrowband Near-Infrared Responsive J-Aggregates of Fused Quinoidal Tetracyanoindacenodithiophene

Narrowband photoresponsive molecules are highly coveted in high-resolution imaging, sensing, and monochromatic photodetection, especially those extending into the near-infrared (NIR) spectral range. Here, a new class of J-aggregating materials based on quinoidal indacenodithiophenes (IDTs) that exhibit an ultra-narrowband (full width half maxima of 22 nm) NIR absorption peak centered at 770 nm is reported. The spectral width is readily tuned by the length of the solubilizing alkyl group, with longer chains resulting in significant spectral narrowing. The J-aggregate behavior is confirmed by a combination of excited state lifetime measurements and single-crystal X-ray diffraction measurements. Their utility as electron-transporting materials is demonstrated in both transistor and phototransistor devices, with the latter demonstrating good response at NIR wavelengths (780 nm) over a range of intensities.

Computational study of electron transport in halogen incorporated diindenotetracene compounds: crystal structure, charge transport and optoelectronic properties

The crystal structure, charge transport and optoelectronic properties of newly designed air-stable halogenated diindenotetracene (DIT) based OSCs are reported in this article. The structural, electronic and charge transport properties of the compounds are investigated using density functional theory (DFT) formalism. The air-stability and n-type characteristics are validated from their low lying LUMO energies (<-3.9 eV) and large electron affinity (EA) values (>3.0 eV). Compared with the parent DIT, the designed DIT-X compounds (except for DIT-I) exhibit larger electronic coupling (V^e is found to be ∼1.5 times larger than that of the bare DIT) and higher electron mobilities because of the effect of electron-withdrawing groups substituted at the peripheral positions of the DIT derivatives. The designed DIT-X compounds (except DIT-I) show high electron mobilities (∼2.4-5.4 cm² V^-1 s^-1), implying that the compounds can serve as promising electron transport materials. In addition, the UV-visible optical spectra of DIT derivatives (except DIT-F) display bathochromic shifts as compared to the bare DIT compound.

Mixed Ionic and Electronic Conduction in Small-Molecule Semiconductors

Small-molecule organic semiconductors have displayed remarkable electronic properties with a multitude of π-conjugated structures developed and fine-tuned over recent years to afford highly efficient hole- and electron-transporting materials. Already making a significant impact on organic electronic applications including organic field-effect transistors and solar cells, this class of materials is also now naturally being considered for the emerging field of organic bioelectronics. In efforts aimed at identifying and developing (semi)conducting materials for bioelectronic applications, particular attention has been placed on materials displaying mixed ionic and electronic conduction to interface efficiently with the inherently ionic biological world. Such mixed conductors are conveniently evaluated using an organic electrochemical transistor, which further presents itself as an ideal bioelectronic device for transducing biological signals into electrical signals. Here, we review recent literature relevant for the design of small-molecule mixed ionic and electronic conductors. We assess important classes of p- and n-type small-molecule semiconductors, consider structural modifications relevant for mixed conduction and for specific interactions with ionic species, and discuss the outlook of small-molecule semiconductors in the context of organic bioelectronics.

Tailored Polymer Gate Dielectric Engineering to Optimize Flexible Organic Field-Effect Transistors and Complementary Integrated Circuits

The increasing demand for solution-processed and flexible organic electronics has promoted the fabrication of integrated logic circuits using organic field-effect transistors (OFETs) instead of fundamental unit devices. This has been made possible through the rapid development of materials and processes in the past few decades. It is important for the p- and n-type OFETs using different organic semiconductors (OSCs) to have complementarily matched electrical characteristics, which significantly improve the performance of organic logic circuits. In this study, an efficient strategy to optimize the performance of flexible organic electronics, such as OFETs and complementary inverters, is proposed using a combination of polymer insulators tailored to each OSC type. Photopatternable soluble copolyimides (ScoPIs), which exhibit excellent insulating properties and chemical resistance, are synthesized and applied as gate dielectric layers in the OFETs. The material and electrical properties are systematically investigated by varying the molecular ratio of ScoPIs to determine the optimal conditions for each OFET type. As a result, complementary inverters report 1.67 times higher integration density compared to the conventional ones while maintaining gain, switching threshold, and static noise margin of 23.7 V/V, 22.1 V, and 12.1 V, respectively, at a supply voltage of 40 V. The flexible complementary inverters are successfully demonstrated by fully exploiting the advantages of ScoPIs.

New Synthetic Approaches to N-Aryl and π-Expanded Diketopyrrolopyrroles as New Building Blocks for Organic Optoelectronic Materials

Diketopyrrolopyrrole (DPP) as a building block has been intensively investigated for organic semiconductors and light emitting materials. The synthesis of N-aryl DPPs remains challenging. Herein, we firstly report a new easily handled synthetic method toward N-aryl DPPs through H-DPP with diaryliodonium salt in the presence of CuI, which shows broad reaction scope. Sixteen N-aryl DPPs, including phenyl, furan and thiophene as flanking aromatic groups, were synthesized with yields up to 78 %. These N-aryl DPPs are fluorescent in both solutions and solid states, with quantum yields up to 96 % and 40 %, respectively. Moreover, we show that the reaction between H-DPP and diaryliodonium salt can lead to π-expanded DPPs by using Pd(OAc)₂ as catalyst. Nine π-expanded DPPs were obtained in 27-61 % yields. These π-expanded DPPs exhibit good semiconducting properties with hole mobility of 0.71 cm²  V^-1  s^-1 , demonstrating that they are useful building blocks for high performance organic semiconductors.

A Solution Processable Dithioalkyl Dithienothiophene (DSDTT) Based Small Molecule and Its Blends for High Performance Organic Field Effect Transistors

The 3,5-dithiooctyl dithienothiophene based small molecular semiconductor DDTT-DSDTT (1), end functionalized with fused dithienothiophene (DTT) units, was synthesized and characterized for organic field effect transistors (OFET). The thermal, optical, electrochemical, and computed electronic structural properties of 1 were investigated and contrasted. The single crystal structure of 1 reveals the presence of intramolecular locks between S(alkyl)···S(thiophene), with a very short S-S distance of 3.10 Å, and a planar core. When measured in an OFET device compound 1 exhibits a hole mobility of 3.19 cm² V^-1 s^-1, when the semiconductor layer is processed by a solution-shearing deposition method and using environmentally acceptable anisole as the solvent. This is the highest value reported to date for an all-thiophene based molecular semiconductor. In addition, solution-processed small molecule/insulating polymer (1/PαMS) blend films and devices were investigated. Morphological analysis reveals a nanoscopic vertical phase separation with the PαMS layer preferentially contacting the dielectric and 1 located on top of the stack. The OFET based on the blend comprising 50% weight of 1 exhibits a hole mobility of 2.44 cm² V^-1 s^-1 and a very smaller threshold voltage shift under gate bias stress.

Keep glowing and going: recent progress in diketopyrrolopyrrole synthesis towards organic optoelectronic materials

Recent progress in the syntheses of DPP derivatives is summarized as well as the structure–property relationships of the derivatives, including the syntheses of DPP cores, N-functionalization reactions, and π-extensions on and along the DPP cores.

Wettability Control of Interfaces for High-Performance Organic Thin-Film Transistors by Soluble Insulating Polymer Films

Organic small-molecule semiconductors have higher carrier mobility compared to polymer semiconductors, while the actual performances of these materials are susceptible to morphological defects and misalignment of crystalline grains. Here, a new strategy is explored to control the crystallization and morphologies of a solution-processed organic small-molecule semiconductor 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) using soluble polymer films to control the wettability of substrates. Different from the traditional surface modification method, the polymer layer as a modification layer is soluble in the semiconductor solution during the fabrication of organic thin-film transistors (OTFTs). The dissolved polymer alters the state of the semiconductor solution, which in turn, changes the crystallographic morphologies of the semiconductor films. By controlling the solubility and thickness of the polymer modification layers, it is possible to regulate the grain boundary and domain size of C8-BTBT films, which determine the performances of OTFTs. The bottom-gate transistors modified by a thick PS layer exhibit a mobility of >7 cm²/V·s and an on/off ratio of >10⁷. It is expected that this new modification method will be applicable to high-performance OTFTs based on other small molecular semiconductors and dielectrics.

High performance p-channel and ambipolar OFETs based on imidazo[4,5-f]-1,10-phenanthroline-triarylamines

A series of phenanthroline functionalized triarylamines (TAA) has been designed and synthesised to evaluate their OFET characteristics. Solution processed OFET devices have exhibited p-channel/ambipolar behaviour with respect to the substituents, in particular methoxyphenyl substitution resulted with highest mobility (μ h) up to 1.1 cm2 V-1 s-1 with good I on/off (106) ratio. These compounds can be potentially utilized for the fabrication of electronic devices.

Synthesis, Molecular Packing, and Electrical Properties of New Regioisomeric n-type Semiconducting Molecules with Modification of Alkyl Substituents Position

We design and synthesize a series of regioisomeric n-type small molecules, which have an identical diketopyrrolopyrrole (DPP) core and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile (INCN) terminal groups with octyl substituents at different positions. The isomeric structures are confirmed by two-dimensional NMR spectroscopy based on the heteronuclear multiple-bond coupling method. Incorporation of the electron-deficient DPP and strongly electron-withdrawing INCN groups yields deep frontier molecular orbitals with n-type charge-transport properties in solution-processed organic field-effect transistors (OFETs). Interestingly, a minor change in the substitution position of the octyl side chains significantly influences the optoelectronic and morphological properties of the thin film. The polycrystalline morphology of the as-cast films is reorganized differently with thermal annealing depending on the octyl topology, significantly affecting the OFET performance. With thermal treatment at 200 °C, the kinked DPP(EH)-INCNO1 (EH = 2-ethylhexyl) structures transform into single crystalline-like structures, exhibiting a remarkably improved electron mobility up to ∼0.6 cm²V^-1 s^-1 compared with DPP(EH)-INCNO2 isomers. The more linear DPP(EH or HD)-INCNO2 (HD = 2-hexyldecyl) molecules become more crystalline with thermal treatments, but their polycrystalline packing structures with large grain boundaries are the main reason for their lower electron mobility. When the solubilizing alkyl substituents are selected, careful molecular design is needed, with consideration of both the solubility and intermolecular packing, for optimizing the optoelectronic properties.

An A-D-A'-D-A Conjugated Molecule Entailing Diazapentalene Unit for an n-Type Organic Semiconductor

Conjugated molecules with low lying LUMO levels are demanding for the development of air stable n-type organic semiconductors. In this paper, we report a new A-D-A'-D-A conjugated molecule (DAPDCV) entailing diazapentalene (DAP) and dicyanovinylene groups as electron accepting units. Both theoretical and electrochemical studies manifest that the incorporation of DAP unit in the conjugated molecule can effectively lower the LUMO energy level. Accordingly, thin film of DAPDCV shows n-type semiconducting behavior with electron mobility up to 0.16 cm² ⋅V^-1 ⋅s^-1 after thermal annealing under N₂ atmosphere. Moreover, thin film of DAPDCV also shows stable n-type transporting property in air with mobility reaching 0.078 cm² ⋅V^-1 ⋅s^-1 .

A Versatile Method to Enhance the Operational Current of Air-Stable Organic Gas Sensor for Monitoring of Breath Ammonia in Hemodialysis Patients

Point-of-care (POC) application for monitoring of breath ammonia (BA) in hemodialysis (HD) patients has emerged as a promising noninvasive health monitoring approach. In this context, many organic gas sensors have been reported for BA detection. However, one of the major challenges for its integration with affordable household POC application is to achieve stable performance for accuracy and high operational current at low voltage for low-cost read-out circuitry. Herein, we exploited the stability of the Donor-Acceptor polymer on the cylindrical nanopore structure to realize the sensors with a high sensitivity and stability. Then, we proposed a double active layer (DL) strategy that exploits an ultrathin layer of Poly(3-hexylthiophene-2,5-diyl) (P3HT) to serve as a work function buffer to enhance the operational current. The DL sensor exhibits a sustainable enhanced operational current of microampere level and a stable sensing response even with the presence of P3HT layer. This effect is carefully examined with different aspects, including vertical composition profile of DL configuration, lifetime testing on different sensing layer, morphological analysis, and the versatility of the DL strategy. Finally, we utilize the DL sensor to conduct a tracing of BA concentration in two HD patients before and after HD, and correlate it with the blood urea nitrogen (BUN) levels. A good correlation coefficient of 0.96 is achieved. Moreover, the feasibility of DL sensor integrated into a low-cost circuitry was also verified. The results demonstrate the potential of this DL strategy to be used to integrate organic sensor for affordable household POC devices.

Recent Progress in High-Mobility Organic Transistors: A Reality Check

Over the past three decades, significant research efforts have focused on improving the charge carrier mobility of organic thin-film transistors (OTFTs). In recent years, a commonly observed nonlinearity in OTFT current-voltage characteristics, known as the "kink" or "double slope," has led to widespread mobility overestimations, contaminating the relevant literature. Here, published data from the past 30 years is reviewed to uncover the extent of the field-effect mobility hype and identify the progress that has actually been achieved in the field of OTFTs. Present carrier-mobility-related challenges are identified, finding that reliable hole and electron mobility values of 20 and 10 cm2 V-1 s-1 , respectively, have yet to be achieved. Based on the analysis, the literature is then reviewed to summarize the concepts behind the success of high-performance p-type polymers, along with the latest understanding of the design criteria that will enable further mobility enhancement in n-type polymers and small molecules, and the reasons why high carrier mobility values have been consistently produced from small molecule/polymer blend semiconductors. Overall, this review brings together important information that aids reliable OTFT data analysis, while providing guidelines for the development of next-generation organic semiconductors.

Thiadizoloquinoxaline-Based N-Heteroacenes as Active Elements for High-Density Data-Storage Device

A novel thiadiazoloquinoxaline (TQ)-based donor-acceptor (D-A)-type N-heteroacene (Py-1-TQ) has been demonstrated for promising applications in organic multilevel resistive memory devices. Compared with its counterparts (Py-0-TQ and Py-2-TQ), which show flash-type binary memory behaviors, Py-1-TQ exhibits excellent nonvolatile write-once-read-many-times-type ternary memory effects with high ON2/ON1/OFF current ratios (10^5.8:10^3.4:1), which can be attributed to the different electron-withdrawing abilities between the pyrazine unit and TQ species that can induce stepwise D-A charge-transfer processes. These results suggest that TQ-based N-heteroacenes can be potentially useful in ultrahigh-density data-storage devices through the rational D-A tuning.

Reduction of Charge Traps and Stability Enhancement in Solution-Processed Organic Field-Effect Transistors Based on a Blended n-Type Semiconductor

Solution-processed n-type organic field-effect transistors (OFETs) are essential elements for developing large-area, low-cost, and all organic logic/complementary circuits. Nonetheless, the development of air-stable n-type organic semiconductors (OSCs) lags behind their p-type counterparts. The trapping of electrons at the semiconductor-dielectric interface leads to a lower performance and operational stability. Herein, we report printed small-molecule n-type OFETs based on a blend with a binder polymer, which enhances the device stability due to the improvement of the semiconductor-dielectric interface quality and a self-encapsulation. Both combined effects prevent the fast deterioration of the OSC. Additionally, a complementary metal-oxide semiconductor-like inverter is fabricated depositing p-type and n-type OSCs simultaneously.

The synthesis and properties of linear A-π-D-π-A type organic small molecule containing diketopyrrolopyrrole terminal units

A novel linear A-π-D-π-A-type organic small molecule Ph2(PDPP)₂ consisting diketopyrrolopyrrole (DPP) as acceptor unit, biphenylene as donor unit and acetylene unit as π-linkage has been successfully designed and synthesized. Its corresponding thermal, photophysical and electrochemical properties as well as the photoinduced charge-separation process were investigated. Ph2(PDPP)₂ exhibits high thermal stability and it can be soluble in common organic solvents such as chloroform and tetrahydrofuran. The photophysical properties show that DPP₂Ph₂ harvests sunlight over the entire visible spectrum range in the thin-film state (300-800nm). DPP₂Ph₂ has lower band gaps and appropriate energy levels to satisfy the requirement of solution-processable organic solar cells. The efficient photoinduced charge separation process was clearly observed between DPP₂Ph₂ with PC₆₁BM and the K_sv value was found to be as high as 2.13×10⁴M^-1. Therefore, these excellent properties demonstrate that the designed A-π-D-π-A-type small molecule Ph2(PDPP)₂ is the prospective candidate as donor material for organic photovoltaic material.

Solvent-Free Processable and Photo-Patternable Hybrid Gate Dielectric for Flexible Top-Gate Organic Field-Effect Transistors

We report a novel solvent-free and direct photopatternable poly[(mercaptopropyl)methyl-siloxane] (PMMS) hybrid dielectric for flexible top-gate organic field-effect transistors (OFETs) utilizing a photoactivated thiol-ene reaction under UV irradiation of 254 nm to induce cross-linking, even in air and at low temperatures. In particular, a solvent-free PMMS-f dielectric film, for which an optimal cross-linking density is shown by a well-organized molar ratio between thiol and vinyl in the thiol-ene reaction, exhibited a high dielectric constant (5.4 @ 100 Hz) and a low leakage current (<1 nA mm^-2 @ 2 MV cm^-1). The excellent dielectric characteristics of the solvent-free PMMS-hybrid dielectrics, along with their other unique characteristics of a direct photopatternability for which UV-nanoimprint lithography is used and a high surface energy of 45.6 mJ m^-2, allowed the successful application of the dielectrics to flexible solvent-free top-gate OFETs with a high reliability against the radius of curvature (9.5, 7.0, and 5.5 mm) and repetitive bending cycles at the radius of curvature of 5.5 mm. This will eventually enable the proposed dielectric design to be used in a variety of applications such as flexible displays and soft organic sensors including chemical and tactile capability.

Ambipolar Small-Molecule:Polymer Blend Semiconductors for Solution-Processable Organic Field-Effect Transistors

We report on the fabrication of an organic thin-film semiconductor formed using a blend solution of soluble ambipolar small molecules and an insulating polymer binder that exhibits vertical phase separation and uniform film formation. The semiconductor thin films are produced in a single step from a mixture containing a small molecular semiconductor, namely, quinoidal biselenophene (QBS), and a binder polymer, namely, poly(2-vinylnaphthalene) (PVN). Organic field-effect transistors (OFETs) based on QBS/PVN blend semiconductor are then assembled using top-gate/bottom-contact device configuration, which achieve almost four times higher mobility than the neat QBS semiconductor. Depth profile via secondary ion mass spectrometry and atomic force microscopy images indicate that the QBS domains in the films made from the blend are evenly distributed with a smooth morphology at the bottom of the PVN layer. Bias stress test and variable-temperature measurements on QBS-based OFETs reveal that the QBS/PVN blend semiconductor remarkably reduces the number of trap sites at the gate dielectric/semiconductor interface and the activation energy in the transistor channel. This work provides a one-step solution processing technique, which makes use of soluble ambipolar small molecules to form a thin-film semiconductor for application in high-performance OFETs.

Facile synthesis of low band-gap DPP–EDOT containing small molecules for solar cell applications

Four donor–donor–acceptor–donor–donor type small molecules were synthesized (by direct alkylation) for photovoltaic applications.

Air-Stable n-channel Diketopyrrolopyrrole-Diketopyrrolopyrrole Oligomers for High Performance Ambipolar Organic Transistors

n-channel organic semiconductors are prone to oxidation upon exposed to ambient conditions. Herein, we report design and synthesis of diketopyrrolopyrrole (DPP)-based oligomers for ambipolar organic thin-film transistors (OFETs) with excellent air and bias stability at ambient conditions. The cyclic voltammetry measurements reveal exceptional electrochemical stability during the redox cycle of oligomers. Structural properties including aggregation, crystallinity, and morphology in thin film were investigated by UV-visible spectroscopy, atomic force microscopy (AFM), thin-film X-ray diffraction (XRD), and grazing incidence small-angle X-ray scattering (GISAXS) measurements. AFM reveals morphological changes induced by different processing conditions whereas GISAXS measurements show an increase in the population of face-on oriented crystallites in films subjected to a combination of solvent and thermal treatments. These measurements also highlight the significance of chalcogen atom from sulfur to selenium on the photophysical, optical, electronic, and solid-state properties of DPP-DPP oligomers. Charge carrier mobilities of the oligomers were investigated by fabricating top-gate bottom-contact (TG-BC) thin-film transistors by annealing the thin films under various conditions. Combined solvent and thermal annealing of DPP-DPP oligomer thin films results in consistent electron mobilities as high as ∼0.2 cm(2) V(-1) s(-1) with an on/off ratio exceeding 10(4). Field-effect behavior was retained for up to ∼4 weeks, which illustrates remarkable air and bias stability. This work paves the way toward the development of n-channel DPP-DPP-based oligomers exhibiting retention of field-effect behavior with superior stability at ambient conditions.

Nanoscale Concentration Quantification of Pharmaceutical Actives in Amorphous Polymer Matrices by Electron Energy-Loss Spectroscopy

We demonstrated the use of electron energy-loss spectroscopy (EELS) to evaluate the composition of phenytoin:hydroxypropyl methylcellulose acetate succinate (HPMCAS) spin-coated solid dispersions (SDs). To overcome the inability of bright-field and high-angle annular dark-field TEM imaging to distinguish between glassy drug and polymer, we used the π-π* transition peak in the EELS spectrum to detect phenytoin within the HPMCAS matrix of the SD. The concentration of phenytoin within SDs of 10, 25, and 50 wt % drug loading was quantified by a multiple least-squares analysis. Evaluating the concentration of 50 different regions in each SD, we determined that phenytoin and HPMCAS are intimately mixed at a length scale of 200 nm, even for drug loadings up to 50 wt %. At length scales below 100 nm, the variance of the measured phenytoin concentration increases; we speculate that this increase is due to statistical fluctuations in local concentration and chemical changes induced by electron irradiation. We also performed EELS analysis of an annealed 25 wt % phenytoin SD and showed that the technique can resolve concentration differences between regions that are less than 50 nm apart. Our findings indicate that EELS is a useful tool for quantifying, with high accuracy and sub-100 nm spatial resolution, the composition of many pharmaceutical and soft matter systems.

Crystal structure and modeled charge carrier mobility of benzobis(thiadiazole) derivatives

Benzobis(thiadiazole) derivatives showed a large diffusion coefficient between the molecules located in the co-planar and the standard π–π stacking directions.

Influence of alkyl chain branching point on the electron transport properties of di(perylene diimides) thin film transistors

4CldiPDIs with different side-chain lengths and positions of the branching point was designed, synthesized, and characterized to study how the electron transporting properties vary with the branching point away from the perylene backbone.

Direct C–H arylation for various Ar-cored diketopyrrolopyrrole containing small molecules in solution-processed field-effect transistors

D–A–π–A–D small molecules Ar(DPPT₂)₂ are designed and synthesized by direct (hetero) C–H arylation, with hole mobility as high as 0.12 cm² V⁻¹ s⁻¹.

Improved electron transport properties of n-type naphthalenediimide polymers through refined molecular ordering and orientation induced by processing solvents

Two new NDI-based polymers, P(NDI2SiC₅-T2) and P(NDI2SiC₅-TTh) were synthesized to determine the role played by the choice of processing solvents.

High-Performance Organic Vertical Thin Film Transistor Using Graphene as a Tunable Contact

Here we present a general strategy for the fabrication of high-performance organic vertical thin film transistors (OVTFTs) based on the heterostructure of graphene and different organic semiconductor thin films. Utilizing the unique tunable work function of graphene, we show that the vertical carrier transport across the graphene-organic semiconductor junction can be effectively modulated to achieve an ON/OFF ratio greater than 10(3). Importantly, with the OVTFT design, the channel length is determined by the organic thin film thickness rather than by lithographic resolution. It can thus readily enable transistors with ultrashort channel lengths (<200 nm) to afford a delivering current greatly exceeding that of conventional planar TFTs, thus enabling a respectable operation frequency (up to 0.4 MHz) while using low-mobility organic semiconductors and low-resolution lithography. With this vertical device architecture, the entire organic channel is sandwiched and naturally protected between the source and drain electrodes, which function as the self-passivation layer to ensure stable operation of both p- and n-type OVTFTs in ambient conditions and enable complementary circuits with voltage gain. The creation of high-performance and highly robust OVTFTs can open up exciting opportunities in large-area organic macroelectronics.

Electronic structure and microscopic charge-transport properties of a new-type diketopyrrolopyrrole-based material

Recently, diketopyrrolopyrrole (DPP)-based materials have attracted much interest due to their promising performance as a subunit in organic field effect transistors. Using density functional theory and charge-transport models, we investigated the electronic structure and microscopic charge transport properties of the cyanated bithiophene-functionalized DPP molecule (compound 1). First, we analyzed in detail the partition of the total relaxation (polaron) energy into the contributions from each vibrational mode and the influence of bond-parameter variations on the local electron-vibration coupling of compound 1, which well explains the effects of different functional groups on internal reorganization energy (λ). Then, we investigated the structural and electronic properties of compound 1 in its isolated molecular state and in the solid state form, and further simulated the angular resolution anisotropic mobility for both electron- and hole-transport using two different simulation methods: (i) the mobility orientation function proposed in our previous studies (method 1); and (ii) the master equation approach (method 2). The calculated electron-transfer mobility (0.00003-0.784 cm(2) V(-1) s(-1) from method 1 and 0.02-2.26 cm(2) V(-1) s(-1) from method 2) matched reasonably with the experimentally reported value (0.07-0.55 cm(2) V(-1) s(-1) ). To the best of our knowledge, this is the first time that the transport parameters of compound 1 were calculated in the context of band model and hopping models, and both calculation results suggest that the intrinsic hole mobility is higher than the corresponding intrinsic electron mobility. Our calculation results here will be instructive to further explore the potential of other higher DPP-containing quinoidal small molecules.

New organic semiconductors with imide/amide-containing molecular systems

Due to their high electron affinities, chemical and thermal stabilities, π-conjugated molecules with imide/amide frameworks have received considerable attentions as promising candidates for high-performance optoelectronic materials, particularly for organic semiconductors with high carrier mobilities. The purpose of this Research News is to give an overview of recent advances in development of high performance imide/amide based organic semiconductors for field-effect transistors. It covers naphthalene diimide-, perylene diimide- and amide-based conjugated molecules and polymers for organic semiconductors.

High mobility polymer based on a π-extended benzodithiophene and its application for fast switching transistor and high gain photoconductor

Here we present synthesis and electronic properties of a new alternating copolymer composed of dithieno[2,3-d;2',3'-d']benzo[1,2-b;4,5-b']dithiophene (DTBDT) and diketopyrrolopyrrole units, poly dithienobenzodithiophene-co-diketopyrrolopyrrolebithiophene (PDPDBD). The resulting polymer showed hysteresis free, fast switching and highly reliable organic thin-film transistor properties comparable to a-Si. Hole mobility of the polymer is about 2.7 cm(2)V(-1)s(-1), which is remarkably improved compared with its benzodithiophene (BDT)-analougue that contains a smaller aromatic ring of BDT in the place of DTBDT. This is mainly due to much increased intramolecular charge transport originated from PDPDBD's rigid molecular backbone. Furthermore, photoconductor devices fabricated by using PDPDBD as an active layer showed a high performance with the highest photoconductive gain of ~10(5). Taken together, the successful PDPDBD's transistor and photoconductor performances with high device stability demonstrated practical applicability of PDPDBD in low-cost and flexible optoelectronic devices.

Pyrene and diketopyrrolopyrrole-based oligomers synthesized via direct arylation for OSC applications

In this report, an atom efficient and facile synthetic strategy for accessing multi-diketopyrrolopyrrole (DPP)-based oligomers used in solution-processed organic field effect transistors (OFETs) and organic solar cells (OSCs) has been developed. The DPP units were successfully installed onto benzene and pyrene cores via palladium-catalyzed dehydrohalogenative coupling of mono-capped DPPs with multi-bromo-benzene or -pyrene (direct arylation), affording four oligomer small molecules (SMs 1-4) containing bis-, tri-, tri-, and tetra-DPP, respectively, in high yields of 78-96%. All the designed linear or branched DPP-based oligomers exhibit broad light absorptions, narrow band-gaps (1.60-1.73 eV), deep highest occupied molecular orbital (HOMO) levels (-5.26∼-5.18 eV), and good thermal stability (Td=390-401 °C). OFETs based on SMs 1-4 showed hole mobilities of 0.0033, 0.0056, 0.0005, and 0.0026 cm2 V(-1) s(-1), respectively. OSCs based on SMs 1-4 under one sun achieved power conversion efficiencies of 3.00%, 3.71%, 2.47%, and 1.86% accordingly, along with high open-circuit voltages of 0.86-0.94 V. For OSC devices of SM 1, SM 3, and SM 4, the solvent CHCl3 was solely employed to the formation of active layers; neither high boiling point additives nor annealing post-treatment was needed. Such a simple process benefits the large-scale production of OSCs via roll to roll technology.

Improved performance in diketopyrrolopyrrole-based transistors with bilayer gate dielectrics

There has been significant progress in the past 2 decades in the field of organic and polymer thin-film transistors. In this paper, we report a combination of stable materials, device architecture, and process conditions that resulted in a patterned gate, small channel length (<5 μm) device that possesses a scaled field-induced conductivity in air that is higher than any organic/polymer transistor reported thus far. The operating voltage is below 10 V; the on-off ratio is high; and the active materials are solution-processable. The semiconducting polymer is a new donor-acceptor polymer with furan-substituted diketopyrrolopyrrole and thienyl-vinylene-thienyl building blocks in the conjugated backbone. One of the major striking features of our work is that the patterned-gate device architecture is suitable for practical applications. We also propose a figure of merit to meaningfully compare polymer/organic transistor performance that takes into account mobility and operating voltage. With this figure of merit, we compare leading organic and polymer transistors that have been hitherto reported. The material and device architecture have shown very high mobility and low operating voltage for such short channel length (below 5 μm) organic/polymer transistors.

A perfluorohexyl containing diketopyrrolopyrrole (DPP) small molecule for high performance ambipolar transistors with balanced hole and electron mobilites

Diketopyrrolopyrrole (DPP) small molecules DPPTT-F and DPPTT-H were synthesized. DPPTT-F displayed ambipolar characteristics with balanced hole and electron mobilities in air.

Pyromellitic Diimide-Ethynylene-Based Homopolymer Film as an N-Channel Organic Field-Effect Transistor Semiconductor

We report the synthesis and characterization of two solution-processable pyromellitic diimide (PyDI)-acetylene-based conjugated homopolymers. Adjacent PyDI cores were connected with triple bond linkages by reacting 3,6-dibromo-N,N'-dialkyl pyromellitic diimides with bis(tributylstannyl)acetylene under Stille coupling conditions. Cyclic voltammetry revealed that these polymers have sufficient electron affinity to accept electrons. Absorption spectra revealed that one polymer, with a simple octyl chain, has greater intermolecular interaction or conjugation after forming a thin film, and that film exhibited electron transport in top-gate bottom-contact mode organic field-effect transistor (OFET) devices. X-ray diffraction (XRD) and atomic force microscopy (AFM) results show that the octyl polymer is amorphous on the bulk scale. The polymer exhibited electron mobility of about 2 × 10^-4 cm² V^-1 s^-1 with on/off ratio of 10³ and is the simplest n-channel polymer yet reported. A 4-trifluoromethylphenethyl side chain did not result in measurable electron mobility. The octyl polymer exhibited negative Seebeck coefficient on the order of -40 μV/K in thermoelectric devices, further substantiating its n-channel activity. The demonstration of electron transport from such a simple polymer has opened a new path for obtaining n-channel semiconducting activity from polymer films.

Enhanced hole carrier transport due to increased intermolecular contacts in small molecule based field effect transistors

Small molecules and oligomers can be synthesized with very high purity and precise molecular weights, but they often do not form uniform thin films while processed from solution. Decreased intermolecular contacts between the small molecules are another disadvantage. To increase the intermolecular contacts in small molecules, we have chosen i-indigo, as one of the conjugated molecular units. The electron poor i-indigo has been connected with electron rich triphenylamine to synthesize a donor-acceptor-donor type small molecule. The propeller shaped triphenylamine helps to increase the solubility of the small molecule as well as isotropic charge transport. The intermolecular spacing between the molecules has been found to be low and did not vary as a function of thermal annealing. This implies that the intermolecular contacts between the small molecules are enhanced, and they do not vary as a function of thermal annealing. Organic field effect transistors (OFET) fabricated using a small molecule exhibited a hole carrier mobility (μ) of 0.3 cm(2)/(V s) before thermal annealing. A marginal increase in μ was observed upon thermal annealing at 150 °C, which has been attributed to changes in thin film morphology. The morphology of the thin films plays an important role in charge transport in addition to the intermolecular spacing that can be modulated with a judicious choice of the conjugated molecular unit.