Organic thin-film transistors with bilayer of rubbed and evaporated hydrocarbon-based acene as active layer

We have investigated organic thin-film transistors (OTFTs) with a bilayer of rubbed and evaporated hydrocarbon-based acene 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) as an active layer. Using a rubbing process after spin-coating the C8-BTBT, crystallinity of the C8-BTBT thin film was improved and resultant superior OTFT characteristics were realized. We obtained a field-effect mobility of 1.6 cm²/Vs, a threshold voltage of −8.2 V, an on-off ratio of 10⁶, and a subthreshold swing of 55 mV/decade.     

C8 BTBT薄膜结晶形貌及OTFT器件性能研究

以p型共轭有机小分子2,7二辛基[1]苯并噻吩并[3,2‐b]苯并噻吩(C8‐BTBT)作为底栅顶接触有机薄膜晶体管(OTFT)的有源层,采用浸渍提拉法、喷墨打印法和真空蒸镀法三种制备工艺,探究半导体薄膜载流子迁移率与结晶形貌的关系,发现不同工艺下有机小分子呈现出不同的生长行为和结晶情况,在很大程度上决定了OTFT器件性能的优劣;此外,通过XRD分析研究了退火处理对C8‐BTBT结晶的影响。结果表明,真空蒸镀制备的薄膜具有更高的结晶度、衬底覆盖率高,并且呈现出SK(Stranski‐Krastanov)模式的结晶生长特征,相应器件中陷阱密度最低,迁移率高达5.44 cm^2·V^-1·s^-1,开关比超过106;且退火处理会严重破坏C8‐BTBT薄膜的结晶。因此,控制半导体层的生长行为,提升半导体层的覆盖率和结晶度是制备高性能共轭小分子OTFT器件的有效途径。     

Controllable thin-film morphology and structure for 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8BTBT) based organic field-effect transistors

Organic field-effect transistors (OFETs) based on organic semiconductor material 2,7-dioctyl[1]benzothieno[3,2-b] benzothiophene (C8BTBT) as the active layer were fabricated by using organic molecular beam deposition (OMBD) and solution-processed methods, in which the C8BTBT thin-film morphology could be well controlled. In OMBD method, C8BTBT thin-film morphology could be controlled by the thickness of organic semiconductor layer and the deposition rate, of which the high-quality C8BTBT thin film was obtained at a thickness of about 20 nm and at a deposition rate of 1.2 nm/min, resulting in an obvious mobility improvement from 2.8 × 10⁻³ cm² V⁻¹ s⁻¹ to 1.20 cm² V⁻¹ s⁻¹. While in the solution-processing, C8BTBT thin-film morphology and thickness are related to the spin-coating speed and the substrate position in spin coater, i.e., in-centre and off-centre position. The off-centre spin-coating with an optimized speed produced large-size domain C8BTBT thin film and accordingly resulted in a mobility of 1.47 cm² V⁻¹ s⁻¹. Furthermore, an additive polystyrene (PS) was added into C8BTBT solution could further improve the thin-film morphology with more metal-stable phase as well as improve the interface contact with the substrate SiO₂, resulting in the highest mobility up to 3.56 cm² V⁻¹ s⁻¹. The research suggested that C8BTBT-based OFETs with the mobility over 1.20 cm² V⁻¹ s⁻¹ could be fabricated by using both OMBD and solution-processed methods through the thin-film morphology and structure optimization, which shows the potential applications in high-performance flexible and printed electronics.     

High-performance solution-processed organic transistors with electroless-plated electrodes

Electroless-plated gold and platinum films are used as source and drain electrodes in high-performance solution-processed organic field-effect transistors (OFETs), representing a promising large-area, near-room-temperature and vacuum-free technique to form low-resistance metal-to-semiconductor interfaces in ambient atmosphere. Developing non-displacement conditions using a Pt-colloidal catalyst for soft electroless plating, the electrodes are deposited on crystallized thin films of 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C₁₀-DNTT) without significant damage to the semiconductor material. The top-contact OFETs show remarkable performance, with a mobility of 6.0 cm² V^?1 s^?1. The method represents a practical fabrication technique to mass-produce circuitry arrays of nearly best-performing OFETs for the printed electronics industry.     

Enhancing the performance of solution-processed organic thin-film transistors by blending binary compatible small molecule semiconductors

Physical blending is a facile and effective way to improve the performance of solution processed organic thin-film transistors (OTFTs). Blending small molecule semiconductors with soluble polymers has been extensively studied in recent years. However, blending between binary small molecule semiconductors is rare due to the difficulty to obtain ideal thin films. Herein, we systematically investigate the blending effects on the morphologies of thin films and their field-effect performance by using two small molecule semiconductors, 2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT) and 2-(4-dodecylphenyl) [1]benzothieno[3,2-b]benzothiophene, (C12-Ph-BTBT), which have the same aromatic skeleton. Molecular ordering and better crystallinity are observed in most of spin-coated blend thin films, thanks to the enhanced molecular interaction after blending. As a result, OTFTs based on blend thin films exhibit improved performance in most cases, with the highest average hole mobility about 1.5 cm² V⁻¹ s⁻¹ demonstrated. Further device performance improvements are demonstrated by blending polystyrene with Ph-BTBT and C12-Ph-BTBT blends. The results here indicate that blending between small molecule semiconductors with compatible fused ring structures may be a promising strategy to enhance the performance of organic transistors.     

Bar-coated high-performance organic thin-film transistors based on ultrathin PDFDT polymer with molecular weight independence

We report high-performance organic thin-film transistors (OTFTs) with an ultrathin active layer of difluorobenzothiadiazole-dithienosilole copolymer (PDFDT) form by using the wire bar-coating process. The top-gate/bottom contact (TG/BC) OTFTs based on bar-coated PDFDT polymer as channel material and poly(methyl methacrylate) (PMMA) as gate dielectric show a hole mobility of up to 2.2 cm² V⁻¹s⁻¹ with a current ON/OFF ratio (I_on/I_off) of 10⁴∼10⁵, with the mobility being two times larger than that of the spin-coated PDFDT based OTFTs. The higher mobility of the bar-coated PDFDT polymer films can be attributed to the well-organized fibril structures of the polymer chains. Importantly, two different molecular weight polymers (M_n = 23 and 34 kDa) were employed to conduct these experiments and both batches showed about the same performance, which mitigates the typical batch-to-batch variation in OTFT performance. Furthermore, we explored the operational stability of the bar-coated OTFTs in ambient air and nitrogen environments. The bias-stress and cycling tests between the ON/OFF states of the bar-coated devices showed high stability in both nitrogen and air. Conclusively, here we demonstrate that (i) a simple bar-coating process is a better method to control and obtain good polymer morphology in comparison to spin-coating, and (ii) the PDFDT polymer has great potential to provide good reproducibility and stability in large-area OTFT devices.     

Triacetate cellulose gate dielectric organic thin-film transistors

Here, we report on the performance and the characterization of all solution-processable top-contact organic thin-film transistors (OTFTs) consisting of a natural-resourced triacetate cellulose gate dielectric and a representative hole-transport poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT) semiconductor layer on rigid or flexible substrates. The bio-based triacetate cellulose layer has an important role in the OTFT fabrication because it provides the pBTTT semiconducting polymer with highly suitable gate dielectric properties including a low surface roughness, hydrophobic surface, appropriate dielectric constant, and low leakage current. The triacetate cellulose gate dielectric-based pBTTT OTFTs exhibit an average filed-effect mobility of 0.031 cm²/Vs similar to that obtained from a SiO₂ gate dielectric-based OTFT device in ambient conditions. Even after a bending stimulation of 100 times and in an outward bending state, the flexible triacetate cellulose gate pBTTT OTFT device still showed excellent electrical device performance without any hysteresis.     

Orientation-Engineered Small-Molecule Semiconductors as Dopant-Free Hole Transporting Materials for Efficient and Stable Perovskite Solar Cells

Crystallized p-type small-molecule semiconductors have great potential as an efficient and stable hole transporting materials (HTMs) for perovskite solar cells (PSCs) due to their relatively high hole mobility, good stability, and tunable highest occupied molecular orbitals. Here, a thienoacene-based organic semiconductor, 2,9-diphenyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DPh-DNTT), is thermally evaporated and employed as the dopant-free HTM that can be scaled up for large-area fabrication. By controlling the deposition temperature, the molecular orientation is modulated into a dominant face-on orientation with π–π stacking direction perpendicular to the substrate surface, maximizing the out-of-plane carrier mobility. With an engineered face-on orientation, the DPh-DNTT film shows an improved out-of-plane mobility of 3.3 × 10⁻² cm² V⁻¹ s⁻¹, outperforming the HTMs reported so far. Such orientation-reinforced mobility contributes to a remarkable efficiency of 20.2% for CH₃NH₃PbI₃ inverted PSCs with enhanced stability. The results reported here provide insights into engineering the orientation of molecules for the dopant-free organic HTMs for PSCs.     

Water-Surface-Mediated Precise Patterning of Organic Single-Crystalline Films via Double-Blade Coating for High-Performance Organic Transistors

Application-oriented growth of patterned organic semiconductor (OSC) thin films with a single domain is a nonnegotiable requirement for the manufacturing of high-performance organic electronic devices. However, the prevalent selective-wetting patterning method remains a challenge in controlling the density of nucleation events in microscale spaces, resulting in thin films with high grain boundary density and no preferential orientation spherulites. Herein, a simple double-blade-coating printing technique using a combination of wetting-patterned substrates to produce an array of highly crystalline OSC thin films is developed. Specifically, the approach confines the OSC crystallization on a molecular-flat water surface in specific areas, enabling a significant reduction in the number of nuclei. Consequently, patterned 2,7-dioctyl[1]benzothieno[3,2-b] benzothiophene (C₈-BTBT) thin films comprising single-crystal domains are achieved with an exceptionally high yield of 62.5%. The organic field-effect transistor array developed from such patterns of C₈-BTBT single-crystalline films exhibits an excellent average mobility of 11.5 cm² V⁻¹ s⁻¹ which is 12.5-fold higher compared to that of the reference sample fabricated via conventional single-blade coating. It is believed that this approach can be widely applied to other soluble organic materials, thereby opening up opportunities for fabricating multicomponent integrated electronics.     

High performance tetrathienoacene-DDP based polymer thin-film transistors using a photo-patternable epoxy gate insulating layer

High performance organic thin-film transistors (OTFTs) are fabricated on an epoxy based photo-patternable organic gate insulating layer (p-OGI) using a top contact thin-film transistor configuration. This negative tone p-OGI material is composed of an epoxy type polymer resin, a polymeric epoxy cross-linker, and a sulfonium photoacid generator (PAG). Features from p-OGI can be precisely patterned down to ∼3 μm via i-line photolithography. In order to evaluate the potential of this epoxy type resin as a gate insulator, we evaluated the dielectric properties of the p-OGI and its gate insulating performance upon fabricating solution processed OTFTs using an organic semiconductor (OSC), namely tetrathienoacene-DPP copolymer (PTDPPTFT4). Results show that the PTDPPTFT4 based OTFTs with this p-OGI exhibit field-effect mobilities up to 1 cm² V⁻¹ s⁻¹, indicating the potential of high performance solution processed OTFT based on an epoxy based p-OGI/OSC system.     

An integrated organic passive pixel sensor

We demonstrate an imaging passive pixel sensor circuit consisting of a bottom-gate, top-contact pentacene organic thin-film transistor (OTFT) integrated with a top-illuminated, inverted subphthalocyanine/C₆₀ organic photodetector (OPD). The vacuum-deposited OTFT utilizes parylene as the gate insulator, achieving a drain current ON/OFF ratio of 10⁵. The transistor hole mobility is 0.09 ± 0.02 cm²/V s. The inverted OPD has a dark current of 20 pA at a reverse bias of 1.5 V. By integrating the two components, a 12-bit dynamic range passive pixel sensor is achieved, with an OFF current of 31 ± 5 pA and a pixel readout time of 0.4 ± 0.05 ms, limited by the discharge time of the OTFT channel. The integrated pixel has potential for use in large-scale focal plane array imagers.     

Interfacial modifying layer-driven high-performance organic thin-film transistors and their nitrogen dioxide gas sensors

Organic thin-film transistors (OTFTs) based on bottom-gate bottom-contact configuration were fabricated by inserting two kinds of modifying layers at the interface of source/drain electrode and organic semiconductor, while nitrogen dioxide (NO₂) sensing capability was also evaluated based on the obtained OTFTs. Compared to OTFT without interfacial layer, the field-effect mobility (μ) was enhanced from 0.018 cm²/Vs to 0.15 cm²/Vs by incorporating with MoO_x interfacial layer. Moreover, when exposed to 30 ppm NO₂, the saturation current and μ of OTFT with MoO_x interfacial layer increase 22.7% and 26.7%, respectively, while in original OTFT, the values are only 3.0% and 3.7%, respectively. The mechanism of performance improvement of OTFT sensor was systematically studied by focusing on the interface of source/drain electrode and organic semiconductor. The reduced contact resistance leads to higher μ, meanwhile, pentacene morphology modulation on MoO_x contributes to better diffusion of NO₂ molecules. As a result, higher μ and more diffused gas molecules enhance the gas sensing property of the transistor.     

High-performance thin-film transistor with 6,13-bis(triisopropylsilylethynyl) pentacene by inkjet printing

We have studied the performance improvement of organic thin-film transistor (OTFT) with a solution based TIPS pentacene (6,13-bis(triisopropylsilylethynyl)pentacene) by inkjet printing. The TIPS pentacene with 1.0 wt.% solution in 1,2-dichlorobenzene was used for printing of an active layer of OTFT. The OTFT printed at room temperature shows a shoulder-like behavior but it disappears for the OTFT printed at the substrate temperature of 60 °C. The OTFT on plastic exhibited an on/off current ratio of ∼10⁷, a threshold voltage of −2.0 V, a gate voltage swing of 0.6 V/decade and a field-effect mobility of 0.24 cm²/Vs in the saturation region.     

Transparent organic thin-film transistors based on high quality polycrystalline rubrene film as active layers

Transparent organic thin-film transistors (OTFTs) with high performance are demonstrated by using high quality polycrystalline 5,6,11,12-Tetraphenylnaphthacene (rubrene) as an active layer, which is prepared by weak epitaxy growth (WEG) method. Benefiting from epitaxial relationship is formed between the inducing layer and the rubrene films, highly oriented and continuous organic polycrystalline thin films with large grains were obtained, which enhances the carrier transport in the film plane. The mobility of devices reaches 1.3 cm²/Vs, the threshold voltage is lower than ?0.9 V and the on–off current ratio (I_on/I_off) is higher than 10⁶ after the photolithography process. Moreover, the array consisting of the transparent thin-film transistors displays a high optical transparency more than 65% in visible light regions. The high-performance transparent OTFTs promote the practical applications for large-area and flexible active-matrix organic light-emitting diodes (AMOLEDs) display.     

Pressure Control Organic Vapor Deposition Methods for Fabricating Organic Thin‐Film Transistors

In this letter, we report on the development progress of a pressure control organic vapor deposition (PCOVD) technology used to design and build a large area deposition system. We also investigate the growth characteristics of a pentacene thin film by PCOVD. Using the PCOVD method, the mobility and on/off current ratio of an organic thin‐film transistor (OTFT) on a plastic substrate are 0.1 cm²/Vs and 10⁶, respectively. The developed OTFT can be applied to a flexible display on a plastic substrate.     

Organic thin film transistors with double insulator layers

We have investigated a double-layer structured gate dielectric for the organic thin films transistor (OTFT) with the purpose of improving the performance of the SiO₂ gate insulator. A 50 nm PMMA layer was coated on top of the SiO₂ gate insulator as organic insulator layer. The results demonstrated that using inorganic/organic compound insulator as the gate dielectric layers is an effective method to fabricate OTFTs with improved electric characteristics and decreased leakage current. Electrical parameters such as carrier mobility and on/off ratio by field effect measurement have been calculated. OTFT based on highly doped Si substrate with a field-effect mobility of 0.004 cm²/V s and on/off ratio of 10⁴ have been obtained.     

High performance submicrometer pentacene-based organic thin-film transistor using planar bottom-contact structure

This paper presents a pentacene-based organic thin-film transistor (OTFT) with a submicrometer channel length of 0.5 μm that uses a planar bottom-contact (pBC) structure to achieve high electrical performance. The performance of the submicrometer OTFT is dominantly influenced by the growth continuity of pentacene near the edge of the source/drain (S/D). The pBC structure with a bilayer dielectric can provide a continuous plane for improving the growth continuity and quality of pentacene near the edge of the S/D. This results in high electrical performance for the submicrometer OTFT with pBC structure, such as a mobility of 0.14 cm²/V s and an on/off current ratio of 1.9 × 10⁵.     

Synergistic effect of polymer and oligomer blends for solution-processable organic thin-film transistors

The thin-film morphologies and thin-film transistor (TFT) characteristics of a series of binary blends of poly(9,9′-dioctylfluorene-alt-bithiophene) (F8T2) and α,ω-dihexylquarterthiophene (DH4T) are reported. The blends of F8T2 and DH4T exhibit good solubility and produce TFT devices with better performances than F8T2 and DH4T devices. The 50% DH4T blend device was found to have a hole mobility of 0.011 cm ² V⁻¹ s⁻¹, which is four times higher than the mobility of the F8T2 device, with a high-on/off ratio of about 10⁵ and a low-off current of 17 pA. The polymer and oligomer domains are phase-separated with large domain size and arranged in characteristic molecular alignments. It was found that carrier transport in the blend systems is mainly controlled by the polymer component, and that the nature of the blended oligomer affects the OTFT performance of the blends.     

Effect of active layer thickness on environmental stability of printed thin-film transistor

We have studied the effect of active layer thickness on the performance and environmental stability of the 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) thin-film transistor. The organic thin-film transistors (OTFTs) were fabricated by inkjet printing using a solution based TIPS pentacene. To get thick organic semiconductor, the surface of gate insulator was treated with n-octyltrichlorosilane (OTS-C₈) before jetting. The on-currents of the OTFT with ～1 μm active layer decreases a little in air, but the OTFT with 0.05 μm TIPS pentacene shows a significant degradation in drain currents.