C8-BTBT Films Research Articles

Aggregation induced strong photoluminescence at room temperature in large-area C8BTBT thin films

Study of optical properties of 2,7-dioctyl[1]benzothieno[2,3-b]benzothiophene (C8BTBT) is promising for applications in low-cost and flexible optoelectronic devices. Here, we systematically investigated the thickness dependent structural and photophysical properties of large-area C8BTBT films. The large-area C8BTBT thin films of different thickness below 80 nm were prepared by off-centre spin coating method. Grazing incidence X-ray diffraction measurements confirm the highly crystalline nature of the thin films and reveal that crystal coherence length increases with increase of film thickness. The film morphology and its thickness were studied using atomic force microscopy. Further, optical absorption and emission behaviors of the thin films were analyzed. The results show strong ultraviolet (UV) emissions at room temperature. The photoluminescence behaviour of higher-thickness films differs from that of lower-thickness films. The thickness dependent UV emission spectra are explained in terms of defects and aggregation behavior of C8BTBT thin films. We determine the impact of thickness and aggregation (J- and H-type) on emission behaviors of the C8BTBT thin films. Our findings open a new avenue for the future optimization and prospective applications in photonics based on two-dimensional organic semiconductors.

Enhanced performance in doped micro-nano porous organic thin-film transistors

Molecular doping, as an effective technique for controlling the electrical property of organic semiconductors (OSCs) by introducing additional charges, has been proven to adjust important device parameters in organic thin-film transistors (OTFTs). Doping highly crystalline OSCs without disrupting structural order is a crucial challenge, as it significantly affects the charge carrier mobility. Here, we demonstrate a molecular doping method without disrupting the molecular ordering to improve the charge carrier mobility of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) based OTFTs via a simple thermal spin-coating method. The key is to introduce micro-nano pores into C8-BTBT thin-film for channel doping, which is achieved by mixing with the unsubstituted BTBT as it can be easily removed from the thin-film through an ordinary annealing process. Micro-nano pores allow the dopant molecules (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, F4-TCNQ) to access the conductive channel of OTFT, which is beneficial for charge injection. Indeed, we further discover that F4-TCNQ doped porous C8-BTBT thin-films exhibit better charge mobility than those of neat and F4-TCNQ doped C8-BTBT films in OTFTs. This work proposes an effective way to expose OSC conjugated core to the dopant, which not only improves the charge transfer reaction between organic/dopant semiconductor through cofacial stacking, but also reduces the trap density and contact resistance.

Ultraviolet-selective organic phototransistors for low-power skin-inspired nociceptor

Skin-inspired nociceptors are particularly important for future neurorobotic technology. In this work, an artificial skin nociceptor based on ultraviolet (UV)-selective C8-BTBT phototransistors was reported. By using polymethyl methacrylate as an interfacial modification layer, a large number of standing crystals appeared in the C8-BTBT film, and the photo-dark current ratio of the devices exceeded the value of 1010. The main biological synaptic behaviors of the devices under UV light stimuli were also thoroughly investigated. Interestingly, the minimum power consumption per synaptic event (2.45 fJ) is comparable to the energy required by the biological synapses. The biological behavior of the skin-inspired nociceptor was further simulated, such as allodynia, hyperalgesia, and the self-protection behavior under UV light stimulation. Finally, as a proof of concept, the skin-inspired nociceptor was proposed to be integrated with the UV-transmittance modulator to protect against excess UV stimulation, which fully demonstrated its potential applications in bionic electronics technology.

Flexible Organic Electrolyte-Gated Transistors Based on Thin Polymer Blend Films of Crystalline C8-BTBT and Amorphous PTAA

Thin films of polymer blends consisting of a crystalline small-molecular semiconductor, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), and an amorphous polymer semiconductor, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), are studied and applied to organic electrolyte-gated transistors (OEGTs). The compositions can be easily controlled by adjusting the mixing ratios of the solutions in chloroform. In the thin-film state, PTAA acts as a structural matrix and a charge-transport bridge for C8-BTBT grains. The C8-BTBT:PTAA blends have a compositional effect on the thin-film properties and performance of OEGTs. As a result, the C8-BTBT:PTAA blend films with weight ratios of 8:2 exhibit a hole mobility twice that of pristine C8-BTBT films. Moreover, OEGTs based on the C8-BTBT:PTAA blend exhibit better charge-transport properties on flexible plastic substrates under bending conditions, compared to flat devices.

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.

Understanding molecular surface doping of large bandgap organic semiconductors and overcoming the contact/access resistance in organic field-effect transistors.

The contact resistance (Rc) and the effective carrier mobility (μeff) are considered as the important indicators of the performance of organic field-effect transistors (OFETs). Conventionally, the contact resistance is regarded as the interface effect between the metal electrodes and the organic semiconductors, while the carrier mobility is correlated to the crystallinity and π-π stacking of the organic molecules. In the staggered OFETs, Rc is actually closely correlated to μeff through the channel sheet resistance. Besides, the accuracy of the carrier mobility directly extracted from the non-ideal transfer curves with significant contact effect is always questionable. Herein, a diffusion-lead surface doping approach is employed to improve the contact resistance and mobility issues simultaneously. By suppressing the trap states in the sublimated 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) organic semiconductor with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), we observed a 3-fold increase in the carrier mobility from 0.5 to 1.6 cm2 V-1 s-1, and the Rc also drops remarkably from 25.7 kΩ cm to 5.2 kΩ cm. Moreover, the threshold voltage (VTH), subthreshold swing (SS) and the bias stability of the OFETs are also significantly improved. Based on the detailed characterization of the C8-BTBT film upon surface doping, including X-ray diffraction (XRD) for the film crystallinity, Kelvin probe force microscopy (KPFM) for the surface potential, trap state investigation by density of states (DOS) measurement and electrical circuit modeling for partial doping analysis, we confirmed that the spontaneous charge transfer process due to the diffusion of the F4-TCNQ dopants in the C8-BTBT matrix can lead to an effective trap filling. This technique and findings can be potentially developed into a general approach for the improvement of different performance parameters of OFETs.

Solution-processed organic field-effect transistors using directed assembled carbon nanotubes and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT)

Achieving low-cost fabrication of organic field-effect transistors (OFETs) has long been pursued in the semiconductor industry. Solution-based process allows the fabrication of OFETs cost-effective because of its merit of vacuum-free and room temperature operation. Here, we show a facile and scalable fabrication of solution-processed OFETs using carbon nanotube (CNT) as source/drain electrodes and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) as semiconducting layer on silicon as well as on flexible and transparent polyethylene terephthalate (PET) substrates. The CNT electrodes and the C8-BTBT film are fabricated using a dip coating-based directed assembly process, and two dip coating parameters, the pulling speed and the solution concentration, are carefully chosen so that the thickness of the C8-BTBT film is close to that of the CNT electrodes. The fabricated OFET devices show typical p-channel behavior. Low-cost, ease of processing, wafer level scalability and good compatibility with various substrates make the fabrication process presented in this paper well suited for next-generation electronics and sensors.

Controllable microstructure of polymer-small molecule blend thin films for high-performance organic field-effect transistors

The carrier mobility is an important figure of merit and is sensitively influenced by the microstructure of semiconducting films such as crystallization and morphology. Here, a method of blending semiconducting small molecule 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and insulating polymer binder is reported to well control the microstructures of blend semiconducting films. It is found that the insulating polymer in the blends prevents the dewetting of C8-BTBT from the SiO2 substrate and provides a platform for C8-BTBT molecules to segregate and crystallize at the air-film interface. By controlling the spinning speed, polymer type and ratio of the blend solution, it is possible to regulate the domain size and the surface roughness of C8-BTBT films, which in turn determines the performances of OTFTs. The bottom gate transistors based on C8-BTBT/polystyrene(PS) blend demonstrate excellent stability and a mobility up to 6.80 cm2/Vs due to the large domain size, smooth grain boundary and the phase-separated interface, which shows the potential applications in high-performance flexible and printed electronics.

The strong continuous induction effect based on isotype heterojunction transistors with different polymer modifications

Abstract Organic thin film transistors (OTFTs) with a p-type pentacene/dioctylbenzothienobenzothiophene (C8BTBT) heterojunction have been fabricated. Insulating materials, including inorganic silicon oxide (SiO2), three polymers of Poly (4-vinylphenol) (PVP), Polystyrene (PS) and Polymethylmethacrylate (PMMA) were used as either dielectrics or modification layers. Our experiment shows that the morphology of C8BTBT films is highly relevant with the selection of the modification layer, which is due to the different physical/chemical properties of these polymers. The morphology difference of C8BTBT film then leads the pentacene film morphologies deposited upon C8BTBT to change again. Such fact clearly shows a to first-semiconductor to second-semiconductor continuous induction effect in our heterojunction OTFTs. Finally, it has been found the SiO2/PS double-layer insulator promotes the growth of the C8BTBT film and upper pentacene film. And a high mobility of 3.6 cm2/Vs for OTFTs with this dielectric and low cost copper (Cu) source/drain electrodes can be achieved.

Effective performance improvement based on dioctylbenzothienobenzothiophene/pentacene isotype organic heterojunction transistors

Organic thin film transistors (OTFTs) with isotype heterojunction semiconducting layers composed of p-type pentacene and dioctylbenzothienobenzothiophene (C8BTBT) have been fabricated. In our heterojunction OTFTs, continuous pentacene film is deposited on top of C8BTBT as a secondary semiconducting layer, which not only can play the role of interfacial layer for hole carriers injection from copper (Cu) source/drain (S/D) electrodes to C8BTBT but also provide another hole transport channel itself. The device performance is then effectively improved. According to the careful study of the C8BTBT/pentacene heterojunction from the film morphologies, it is found that the C8BTBT film facilitates the growth of the upper pentacene film which can further contribute to performance enhancement of the OTFTs. Moreover, diode characteristics with the C8BTBT/pentacene heterojunction show a negligible heterojunction effect that makes the off-state current of OTFTs easy to control. The resulting OTFT exhibits high performance with a mobility of 1.5 cm2/Vs and an on/off ratio of 105, even with low work function Cu S/D electrodes.

Effect of non-chlorinated solvents on the enhancement of field-effect mobility in dioctylbenzothienobenzothiophene-based top-gate organic transistors processed by spin coating

Abstract The use of non-chlorinated solvents for film deposition in organic field-effect transistors (OFETs) is highly desirable to fabricate flexible and printable electronic circuits by environmentally friendly processes. Here, we investigate the influence of the use of non-chlorinated solvents, such as toluene, p-xylene, cyclohexanone, and mesitylene, for the deposition of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) films by spin coating on the morphologies of these semiconductor films and the electrical characteristics of OFETs with a top-gate configuration. The results show that the size of polycrystalline domains in spin-coated C8-BTBT films tends to increase as the boiling point (b.p.) of the solvent increases, whereas an improvement in the field-effect mobility is restricted by the decrease in the extent to which the C8-BTBT film covers the substrate due to the long evaporation time of high b.p. solvents. The addition of the high b.p. solvent p-xylene to toluene allows a significant increase in polycrystalline domain sizes in spin-coated C8-BTBT films. Further, spin coating C8-BTBT using the high b.p. solvent mesitylene at a high spin rate is effective for the production of thinner C8-BTBT films with diminished surface roughness. Top-gate C8-BTBT FETs processed by spin coating using a mixed binary solvent and by high-speed spin coating achieved high average field-effect mobilities of 4.9 and 5.4 cm2 V−1 s−1, respectively.

Scalable Directed Assembly of Highly Crystalline 2,7-Dioctyl[1]benzothieno[3,2- b][1]benzothiophene (C8-BTBT) Films.

Assembly of organic semiconductors with ordered crystal structure has been actively pursued for electronics applications such as organic field-effect transistors (OFETs). Among various film deposition methods, solution-based film growth from small molecule semiconductors is preferable because of its low material and energy consumption, low cost, and scalability. Here, we show scalable and controllable directed assembly of highly crystalline 2,7-dioctyl[1]benzothieno[3,2- b][1]benzothiophene (C8-BTBT) films via a dip-coating process. Self-aligned stripe patterns with tunable thickness and morphology over a centimeter scale are obtained by adjusting two governing parameters: the pulling speed of a substrate and the solution concentration. OFETs are fabricated using the C8-BTBT films assembled at various conditions. A field-effect hole mobility up to 3.99 cm2 V-1 s-1 is obtained. Owing to the highly scalable crystalline film formation, the dip-coating directed assembly process could be a great candidate for manufacturing next-generation electronics. Meanwhile, the film formation mechanism discussed in this paper could provide a general guideline to prepare other organic semiconducting films from small molecule solutions.

Fully transparent conformal organic thin-film transistor array and its application as LED front driving.

A fully transparent conformal organic thin-film field-effect transistor array is demonstrated based on a photolithography-compatible ultrathin metallic grid gate electrode and a solution-processed C8-BTBT film. The resulting organic field-effect transistor array exhibits a high optical transparency of >80% over the visible spectrum, mobility up to 2 cm2 V-1 s-1, on/off ratio of 105-106, switching current of >0.1 mA, and excellent light stability. The transparent conformal transistor array is demonstrated to adhere well to flat and curved LEDs as front driving. These results present promising applications of the solution-processed wide-bandgap organic semiconductor thin films in future large-scale transparent conformal active-matrix displays.

Solvent-dependent performance of solution-processed small-molecule organic field-effect transistors

We investigated the morphology and crystallinity of 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) thin films formed by the solution-shearing method using dichlorobenzene (DCB) or toluene as the solvent for the C8-BTBT solution. In addition, we investigated the effects of the solvents on the performance of C8-BTBT-based organic field-effect transistors (OFETs). Based on the electrical properties of the C8-BTBT-based OFETs, it was observed that the performance of the OFETs with the C8-BTBT films formed using the DCB solvents, which have a high boiling point (180.5 °C), depended strongly on the processing method and the surface condition of the substrate, affording high-performance C8-BTBT-based OFETs with a reasonably high mobility of 0.52 cm2V−1s−1; however, the OFETs fabricated with toluene-based C8-BTBT solutions, which have a relatively low boiling point (111 °C), resulted in similar mobility values from 0.01 to 0.05 cm2V−1s−1, regardless of the coating method and the surface state of the substrate. The experimental results reveal that the process conditions that allow the solvent to be confined for a sufficient amount of time in order to form C8-BTBT thin films play an important role in the fabrication of solution-processed high-performance OFETs.

Fullerene (C60) interlayer modification on the electronic structure and the film growth of 2,7-diocty[1]benzothieno-[3,2-b]benzothiophene on SiO2

Modification by ultra-thin fullerene (C60) insertion layer of about 1 monolayer on the electronic structure and the film growth of 2,7-diocty[1]benzothieno-[3,2-b]benzothiophene (C8-BTBT) on silicon oxide (SiO2) is investigated using ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and X-ray diffraction (XRD). It is found that the ultra-thin C60 insertion layer doesn’t induce an ambipolar charge transport characteristic but it indeed lifts up the energy levels of C8-BTBT, resulting in p-doping and hole accumulation at the semiconductor-dielectric interface. The charge transfer from trap states residing in the band gap of C8-BTBT to the lowest unoccupied molecular orbital (LUMO) of C60 most likely leads to a decrease of threshold voltage for corresponding organic field effect transistors (OFETs) based on such a configuration. Band bending and decrease of the ionization potential (IP) are observed in the C8-BTBT film attributed to the increasing ordering of molecular orientation with the deposition of C8-BTBT. The HOMO downward shift is clearly bigger with the C60 insertion layer than that without. A typical Volmer-Weber (VW) growth mode is detected due to the weaker van der Waals interaction between C8-BTBT and the underlying layer of C60/C8-BTBT than that between the C8-BTBT molecules. Compared to without C60 interlayer, the molecular packing is a little more disordered at low coverages, resulting in more C8-BTBT molecules to lean away from the interface normal. The C8-BTBT film in such a structure phase has the HOMO closer to the EF than that at an almost perfectly vertically standing up configuration. Our results show that the introduction of C60 insertion layer lifts up the HOMO of C8-BTBT and improves the energy level alignment at the interface.

Novel organic-perovskite hybrid structure forward photo field effect transistor

To enhance photosensitive performances for an organic thin film transistor, we fabricated a hybrid structured transistor with C8BTBT film as organic active layer, onto which, a CH3NH3PbI3 layer was formed through the vacuum deposition. The phototransistor showed the best photo responsivity as high as 33 A/W, much higher than most other organic based thin film transistors, in addition to keeping fast response time and well gate tunable ability. The working mechanism were further investigated with the temperature dependence measurement. The organic-perovskite hybrid transistor may open up a path way for the optimization of organic photo sensitive transistors.

Thickness-dependent electronic structure of the interface of 2,7-dioctyl[1]benzothieno[3,2-b][1] benzothiophene/Ni(100)

Combining ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy and atomic force microscopy (AFM), we perform a systematic investigation on the correlation of energy level alignment, film growth and molecular orientation of 2, 7-dioctyl[1]benzothieno-[3, 2-b][1]benzothiophene (C8-BTBT) on Ni(100). The molecules lie down at the first layer and are partly devulcanized by the substrate. Chemical adsorption of reaction products of sulfur atoms on the Ni substrate and the evaporation of the hydrocarbon products into vacuum make the C/S ratio as low as 11.5 : 1 in the XPS of the initially deposited C8-BTBT film of 1-4 thickness, far less than the stoichiometric of 15 : 1. With the thickness increasing from 4 to 8 , there are sharp downward shifts of Evac, HOMO and core levels of C 1s, S 2p, and a sharp increase of C/S ratio, which can be ascribed to the change of molecular orientations from lying down at 4 to standing up at 8 . From 8 onward, the C/S ratio increases steadily till it reaches 15 : 1. The energy levels show relatively less changes when the thickness increases from 8 to 32 . When the thickness increases over 32 , the energy band starts bending downward apparently because of the charging effect during the photoelectron emission processes. The poor conductivity along the standing alkyl chain of C8-is the main cause for the charging. The standing up configurations of the C8-BTBT molecules are confirmed by the AFM investigation in which the heights of the upper layers of C8-BTBT are around 30 , close to the length of the long c-axis. AFM image also indicates that the molecules tend to grow into islands for larger thickness, which is consistent with the slower decrease of the (I/I0) of Ni 2p3/2 with the C8-BTBT film thickness. Our results suggest that a buffer layer be inserted between Ni and C8-BTBT and the thickness of the C8-BTBT film be controlled as thin as possible in related devices.

Effect of a MoO3buffer layer between C8-BTBT and Co(100) single-crystal film

The effect of a MoO3buffer layer inserted between 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) and Co single-crystal film has been investigated using X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS).

The importance of spinning speed in fabrication of spin-coated organic thin film transistors: Film morphology and field effect mobility

We have investigated the film morphology and the field effect mobility of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) thin films which were formed by spin coating on the SiO2 substrate with solution-processed graphene electrodes. The domain size and the density of aggregates in the C8-BTBT film showed the same dependence on the spinning speed. These competitive two factors (domain size and density of aggregates) give an optimum spinning speed, at which the field effect mobility of C8-BTBT transistor showed a maximum (2.6 cm2/V s). This result indicates the importance of spinning speed in the fabrication of solution processed organic thin film transistors by spin coating.

Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method

Organic semiconductors with higher carrier mobility and better transparency have been actively pursued for numerous applications, such as flat-panel display backplane and sensor arrays. The carrier mobility is an important figure of merit and is sensitively influenced by the crystallinity and the molecular arrangement in a crystal lattice. Here we describe the growth of a highly aligned meta-stable structure of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) from a blended solution of C8-BTBT and polystyrene by using a novel off-centre spin-coating method. Combined with a vertical phase separation of the blend, the highly aligned, meta-stable C8-BTBT films provide a significantly increased thin film transistor hole mobility up to 43 cm(2) Vs(-1) (25 cm(2) Vs(-1) on average), which is the highest value reported to date for all organic molecules. The resulting transistors show high transparency of >90% over the visible spectrum, indicating their potential for transparent, high-performance organic electronics.