High Performance Organic Thin Film Research Articles

Performance improvement of α-6T thin film sensors based on F16CuPc buffer layer

A F16CuPc buffer layer is proposed to be added to improve the performance of the sensors. The change of relative responsivity of sensors with and without buffer layer was investigated. The relative responsivity of sensors with buffer layer reached 448.32 % from the off-state current of transistors under 20 ppm NO2 gas. The result was 51.45 % higher than that of the sensors without buffer layer. The sensitivity for NO2 gas reaches 2087.92 %/ppm, which is 25.80 % higher than the sensors without buffer layer. The addition of buffer layer is beneficial for the transmission of charge carriers. Therefore, it is possible to obtain highly responsive sensors, which provide a feasible solution for the study of high-performance organic thin film sensors.

Photolithographic Rugged Electrode for High-Density Low-Contact-Resistance Coplanar Organic Transistors.

The realization of high-performance photolithographic coplanar organic thin film transistors (OTFTs) is fundamental to boost cosmicallycommercial applications of organic electronics. However, photolithographic coplanar OTFTs generally suffer from poor charge injection and therefore poor filed-effect performance. Here, a simple and effective strategy is developed to fabricate photolithographic rugged electrodes, and successfully achieve high-density low-contact-resistance photolithographic coplanar OTFTs. Based on this versatile electrode, the wafer-scale photolithographic rugged electrode can be easily achieved, and the device density of the coplanar OTFTs is as high as 28000cm-2 . The device shows excellent electrical properties with mobility up to 2.01cm2 V-1 s-1 and Rc as low as 7.8kΩcm, which is superior to all the reported Ag-electrode coplanar OTFTs. This work shows a reliable strategy to reduce the contact resistance of photolithographic coplanar OTFTs and elucidates the effect of injection resistance (Rinj ) and access resistance (Racc ) on coplanar OTFTs.

Diketopyrrolopyrrole-based Conjugated Polymers as Representative Semiconductors for High-Performance Organic Thin-Film Transistors and Circuits

Diketopyrrolopyrrole-based Conjugated Polymers as Representative Semiconductors for High-Performance Organic Thin-Film Transistors and Circuits

Poly(ionic liquid) Gating Materials for High-Performance Organic Thin-Film Transistors: The Role of Block Copolymer Self-Assembly at the Semiconductor Interface.

The widespread realization of wearable electronics requires printable active materials capable of operating at low voltages. Polymerized ionic liquid (PIL) block copolymers exhibit a thickness-independent double-layer capacitance that makes them a promising gating medium for the development of organic thin-film transistors (OTFTs) with low operating voltages and high switching speed. PIL block copolymer structure and self-assembly can influence ion conductivity and the resulting OTFT performance. In an OTFT, self-assembly of the PIL gate on the semiconducting polymer may differ from bulk self-assembly, which would directly influence electrical double-layer formation. To this end, we used 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)) as a model semiconductor for our OTFTs, on which our PILs exhibited self-assembly. In this study, we explore this critical interface by grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) of P(NDI2OD-T2) and a series of poly(styrene)-b-poly(1-(4-vinylbenzyl)-3-butylimidazolium-random-poly(ethylene glycol) methyl ether methacrylate) (poly(S)-b-poly(VBBI+[X]-r-PEGMA)) block copolymers with varying PEGMA/VBBI+ ratios and three different mobile anions (where X = TFSI-, PF6-, or BF4-). We investigate the thin-film self-assembly of block copolymers as a function of device performance. Overall, a mixed orientation at the interface leads to improved device performance, while predominantly hexagonal packing leads to nonfunctional devices, regardless of the anion present. These PIL gated OTFTs were characterized with a threshold voltage below 1 V, making understanding of their structure-property relationships crucial to enabling the further development of high-performance gating materials.

Flexible high-performance organic thin film transistors and PMOS inverters: Trap controlled grain boundaries and contact resistance effect in different channel length devices

Printed high-performance organic thin film transistors (OTFTs) are fabricated on flexible polyethylene naphthalate (PEN) substrate for low-voltage operation. Gate, source and drain electrodes are inkjet-printed using conductive silver ink, while chemical vapor deposition grown parylene is used as dielectric layer. We used a blend of 2,7-dihexyl-dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene (DTBDT-C6) and polystyrene (PS), for active p-type organic semiconducting material, and deposited by a dispenser system to achieve 0.66 cm2/Vs mobility at VGS = −6 V. For a set of 30 OTFT devices, the uniformity in device characteristics is illustrated by plotting the histograms for on current, mobility, and threshold voltage (VTh). Mobility decrease for small channel length due to greater contribution of contact resistance, while negative VTh shift occurs because of the generation of hole traps at dielectric-semiconductor interface attributed to moisture present. Electron traps on grain boundaries of the active semiconducting layer causes positive shift in VTh. By using these high mobility OTFT devices, two different low-voltage operation PMOS (P-channel metal-oxide-semiconductor) inverters are fabricated for which high signal gain value of 24.86 is achieved at VDD = −5 V.

Ultranarrow Bandgap Naphthalenediimide-Dialkylbifuran-Based Copolymers with High-Performance Organic Thin-Film Transistors and All-Polymer Solar Cells.

A new polymer acceptor poly{(N,N'-bis(2-ethylhexyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl)-alt-5,5-(3,3'-didodecyl-2,2'-bifuran)} (NDI-BFR) made from naphthalenediimide (NDI) and furan-derived head-to-head-linked 3,3'-dialkyl-2,2'-bifuran (BFR) units is reported in this study. Compared to the benchmark polymer poly(naphthalenediimide-alt-bithiophene) (N2200), NDI-BFR exhibits a larger bathochromic shift of absorption maxima (842 nm) with a much higher absorption coefficient (7.2 × 104 m-1 cm-1 ), leading to an ultranarrow optical bandgap of 1.26 eV. Such properties ensure good harvesting of solar light from visible to the near-infrared region in solar cells. Density functional theory calculation reveals that the polymer acceptor NDI-BFR possesses a higher degree of backbone planarity versus the polymer N2200. The polymer NDI-BFR exhibits a decent electron mobility of 0.45 cm2 V-1 s-1 in organic thin-film transistors (OTFTs), and NDI-BFR-based all-polymer solar cells (all-PSCs) achieve a power conversion efficiency (PCE) of 4.39% with a very small energy loss of 0.45 eV by using the environmentally friendly solvent 1,2,4-trimethylbenzene. These results demonstrate that incorporating head-to-head-linked BFR units in the polymer backbone can lead to increased planarity of the polymer backbone, reduced optical bandgap, and improved light absorbing. The study offers useful guidelines for constructing n-type polymers with narrow optical bandgaps.

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.

Tuning the alignment of pentacene on copper substrate by annealing-assistant surface functionalization**Project supported by the Natural Science Foundation of Zhejiang Province, China (Grant No. LY19F040005) and the National Natural Science Foundation ofChina (Grant Nos. 61474077 and 51802355).

Controlling the alignment and packing structure of organic molecules on solid substrate surfaces at molecule level is essential to develop high-performance organic thin film (OTF) devices. Pentacene, which is a typical p-type semiconductor material usually adopts lying-down geometry on metal substrates owning to π–d coupling between pentacene and metal substrates. However, in this study, we found that pentacene molecules can be adsorbed on an anneal-treated Cu (111) surface with their long axis perpendicular to substrate surface. Highly ordered single-layer pentacene film with stand-up molecular geometry was achieved on this substrate. It was found that the functionalization of Cu surface with C = O groups due to annealing treatment should be accounted for standing-up geometry of pentacene on Cu substrate. This observation shed light on the tuning of the alignment and packing structure of organic molecules.

Control of dielectric surface energy by dry surface treatment for high performance organic thin film transistor based on dibenzothiopheno[6,5-b:6′,5′-f]thieno[3,2-b]thiophene semiconductor

We studied organic thin film transistors using vacuum-deposited para-sexiphenyl (p-6P) as a sublayer to reduce the surface energy of the dielectric material. The correlation between the growth mode of a thin film of the organic semiconductor dibenzothiopheno[6,5-b:6′,5′-f]thieno[3,2-b]thiophene and the number of p-6P sublayers could be explained by a surface energy difference resulting from molecular orientation changes in the p-6P layer. A local surface energy difference was confirmed by measuring the adhesion force using a nanomechanical force measurement method.

Self-assembly crystal microribbons with nucleation additive for high-performance organic thin film transistors

We demonstrate fabrication of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) crystal microribbon based organic thin film transistors (OTFTs) with a solvent exchange method and the addition of a nucleation agent. TIPS pentacene crystals were formed via self-assembly as a result of solubility difference between the double solvents. At the same time, 4-hexylbenzoic acid (HBA) was added as a nucleation agent and formed an interfacial layer on the silicon dioxide gate dielectrics, which dispersed the aggregated crystals and enhanced its morphology uniformity. TIPS pentacene crystal microribbon based OTFTs with HBA nucleation additive showed a hole mobility of up to 0.36 cm2 V−1 s−1, and particularly, a 9-fold enhancement in average mobilities, as compared to pristine crystal based OTFTs. The remarkable enhancement in device performance can be attributed to the improved crystallinity, reduced defects and charge trap centers located at the grain boundaries, and controlled crystallization and dispersion effect due to the addition of the HBA nucleation agent.

Head-to-Head Linked Dialkylbifuran-Based Polymer Semiconductors for High-Performance Organic Thin-Film Transistors with Tunable Charge Carrier Polarity

A planar backbone conformation is essential for enabling polymer semiconductors with high charge carrier mobility in organic thin-film transistors. Benefiting from the smaller van der Waals radius ...

Highly conductive, transparent and metal-free electrodes with a PEDOT:PSS/SWNT bilayer for high-performance organic thin film transistors

Abstract Conductive organic materials including polymers, small molecules, and carbon nanotubes (CNTs) are a promising alternative to inorganic materials in electronic devices. Conventionally, organic electrodes employing CNTs are designed using functionalization of their surfaces or formation of nanocomposites with a conductive polymer. However, phase separation limits the concentration of CNTs in a polymer matrix, hindering the formation of highly dense CNT networks and leading to poor electrical conductivity. In this paper, we introduce bilayer electrodes comprising poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and single-walled CNTs (SWNTs) chemically modified by HNO3 treatment. Impressive conductivities of 2432 and 2438 S cm−1 are found for the SWNT/PEDOT:PSS (S/P) and PEDOT:PSS/SWNT (P/S) electrodes, respectively. Further, an increase in the work function of the electrodes after HNO3 treatment lowers the hole injection barrier, which facilitates hole injection from pentacene. The smooth surface of PEDOT:PSS also contributes to growth of large pentacene grains; consequently, the field-effect mobility of pentacene-based thin film transistors is 1.88 cm2 V−1 s−1 when the P/S electrode is employed. The metal-free electrodes also exhibit a high optical transparency of 88.7%, which suggests that they have great potential for applications in optoelectronics.

Polymer additive controlled morphology for high performance organic thin film transistors.

Solution-crystallizable small-molecule organic semiconductors, such as 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), 5,11-bis(triethylgermylethynyl)anthradithiophene (diF-TEG-ADT), 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), and N,N'-1H,1H-perfluorobutyl dicyanoperylenecarboxydiimide (PDIF-CN2), demonstrate various practical advantages including high mobility, air stability and solution processibility. In this article, we review various polymer additive based approaches to control the crystal morphology and the resultant charge transport of some bench-mark, high performance, solution crystallizable, small-molecule organic semiconductors. The polymer additives are discussed under the categories of non-conjugated polymers and conjugated polymers. The approaches and structure-performance correlations that we discussed here may be applied far beyond the examples shown in this review and have important implications for high performance organic semiconductors in general.

Polyimide-based gate dielectrics for high-performance organic thin film transistors

In this work, polyimide-based novel polymer dielectric materials, containing a cross-linkable olefin group and a long alkyl chain with biphenyl, were designed and synthesized by a mild chemical synthesis method to avoid thermal imidization so that they can be widely utilized.

Photocrosslinkable zinc diacrylate-based gate insulators for reliable operation of organic thin film transistors

Abstract We report new organic-inorganic hybrid gate insulator materials containing solution-processable zinc diacrylate (ZnDA) and dipentaerythritol penta-/hexa-acrylate (DPHA) to fabricate high-performance and reliable organic thin film transistors (OTFTs). ZnDA-DPHA organic-inorganic hybrid materials can be crosslinked by ultraviolet illumination (λmax = 365 nm) within 7 min in the presence of a photoinitiator. Hence, the crosslinked insulator film provides not only good insulating properties (such as a dielectric constant of ca. 4.95 and low leakage current of

Solution-processed high performance organic thin film transistors enabled by roll-to-roll slot die coating technique

Abstract In this study, we achieved high performance organic small molecule transistors by using roll-to-roll slot die coating technique and polymer dielectric layers. By carefully controlling the coating conditions, highly ordered single crystalline thin films could be achieved. Hence, the organic thin film transistor (OTFT) device exhibited an average mobility of 4.2 cm2V−1s−1 and maximum mobility of 6.5 cm2V−1s−1 based on polystyrene (PS) dielectric layer in the ambient air condition, which is among the best values for TIPS-pentacene based devices. The thin film morphology, crystallinity, and optical properties were studied in details. A complementary inverter based on p-type TIPS-pentacene and n-type 6,13-Dicyano pentacene-2,3:9,10-bis(dicarboximide) (PDI-CN) with a high voltage gain of 54 was achieved. Finally, the flexible devices based on roll-to-roll slot die coated PS dielectric and TIPS-pentacene were achieved with a charge carrier mobility of 1.2 cm2V−1s−1.

Photochemical Molecular Tailoring for Efficient Diffusion and Reorganization of Organic Nanocrystals for Ultra-Flexible Organic Semiconductor Arrays.

Solution-processed organic single crystals with high carrier mobility have been actively investigated for diverse applications such as displays, sensors, and next generation electronics on a flexible platform. However, the lack of precise alignment and growth control of organic single crystals impedes the widespread adoption of organic materials in an industrial perspective. Here, a photochemical modification approach is reported tailoring the solubility and molecular diffusivity of polymeric sacrificial layer and sequential batch-type vapor annealing to implement high-performance (average saturation mobility: 8.01 cm2 V-1 s-1 ) organic single-crystal thin film transistors with large channel width including multiple aligned single crystals. Additionally, the mechanical properties of the organic single crystals are systematically investigated with extreme strain conditions such as bending radius of 150 μm.

Blending crystalline/liquid crystalline small molecule semiconductors: A strategy towards high performance organic thin film transistors

Solution processed small molecule polycrystalline thin films often suffer from the problems of inhomogeneity and discontinuity. Here, we describe a strategy to solve these problems through deposition of the active layer from a blended solution of crystalline (2-phenyl[1]benzothieno[3,2-b][1]benzothiophene, Ph-BTBT) and liquid crystalline (2-(4-dodecylphenyl) [1]benzothieno[3,2-b]benzothiophene, C12-Ph-BTBT) small molecule semiconductors with the hot spin-coating method. Organic thin film transistors with average hole mobility approaching 1 cm2/V s, much higher than that of single component devices, have been demonstrated, mainly due to the improved uniformity, continuity, crystallinity, and stronger intermolecular π-π stacking in blend thin films. Our results indicate that the crystalline/liquid crystalline semiconductor blend method is an effective way to enhance the performance of organic transistors.

(Invited) A New Materials Concept for High Performance Organic Thin Film Transistors

Various organic semiconductor materials for field effect transistors have been developed in these decades, since comparable FET mobility to that of amorphous silicon FETs was demonstrated in Pentacene FETs fabricated with its crystalline thin films. These includes small molecules and polymer materials for solution processes, some of which exhibit high mobility of over 10cm2/Vs. [1], [2] However, the small molecules have remained the problems to be solved such as quality and thermal durability of films, while the polymers need easy control of molecular orientation in a practical way, in addition to reliablities on material preparation from lot to lot and material purification. We have established a new approach to solve the problems e in the small molecules, that is, utilization of highly-ordered smectic liquid crystals exhibiting self-organization and solid-like nature. 2-Phenyl-7-decyl-benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) is a typical example in this approach and exhibits SmE phase at the temperature range from 142oC to 200oC, which allows high thermal durability at that range for thermal processes in device fabrication and high mobility of over 10cm2/Vs in solution processed polycrystalline FETs in bottom-gate and bottom-contact configuration [3]. In addition, we have succeeded in operating the FETs at low voltage lower than 5V with low-dielectric gate materials [4]. In this talk, we will discuss how the liquid crystallinity can solve the problems in small molecules for FET applications, and report the newest results on the FETs fabricated with this new material.

(Invited) A New Materials Concept for High Performance Organic Thin Film Transistors

We have established a new materials concept for organic field effect transistors (OFETs) solution-processed, i.e., utilization of highly ordered smectic liquid crystals, which exhibit self-organization and solid-like nature, in order to solve the problems such as film uniformity and thermal durability in small molecule OFET materials. 2-Phenyl-7-decyl-benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) exhibiting SmE phase was designed and synthesized to apply OFETs. Bottom-gate and bottom-contact OFETs fabricated with its polycrystalline thin films exhibited high mobility of over 10cm2/Vs with its small variation from device to device and high thermal durability up to 200oC . In addition, we succeeded in operating the OFETs at a few volt with a low-dielectric gate insulator material of SiO2.