Polymer additive controlled morphology for high performance organic thin film transistors

Performance Enhancement for Indium-Free Metal Oxide Thin-Film Transistors with Double-Active-Layers by Magnetron Sputtering at Room Temperature

We prepared an indium-free metal oxide thin-film transistor (TFT) using a double-active-layers structure at room temperature. We changed the growth sequence of Al-doped zinc oxide (AZO) and zinc oxide (ZnO) double-active-layers on Si/SiO₂ substrates by magnetron sputtering deposition to regulate the field-effect performance of TFTs. According to the analysis of field-effect properties before and after annealing in different atmospheres, the performance of TFT devices with ZnO/AZO/SiO₂/Si double-active-layers was obviously better than that with single AZO or ZnO active layer and inverted AZO/ZnO/SiO₂/Si double-active-layers in the device structure. The active layer with higher carrier concentration (AZO in this case) was closer to the dielectric layer, which was more favorable for carrier regulation in TFT devices. In addition, the annealed device had a lower on/off ratio (I_on/I_off), easier-to-reach on-state, and higher mobility. Furthermore, the performance of the devices annealed under vacuum condition was obviously better than that annealed under air atmosphere. The I_on/I_off could reach 6.8 × 10⁵ and the threshold voltage was only 2.9 V.

Importance of Structural Relaxation on the Electrical Characteristics and Bias Stability of Solution-Processed ZnSnO Thin-Film Transistors

Effect of structural relaxation (SR) on the electrical characteristics and bias stability of solution-processed zinc-tin oxide (ZTO) thin-film transistors (TFTs) were systematically investigated by controlling the annealing time of the ZTO semiconductor films. Note that SR was found to increase with increased annealing time. Due to the increased SR, the ratio of oxygen vacancies (V_O) increased from 21.5% to 38.2%. According to increased V_O, the mobility in the saturation region was exhibited by a sixfold increase from 0.38 to 2.41 cm² V^-1 s^-1. In addition, we found that the threshold voltage negatively shifted from 3.08 to -0.95 V. Regarding the issue of bias stability, according to increased SR, positive-bias stress of the ZTO TFTs was enhanced, compared with reverse features of negative-bias stress. Our understanding is expected to provide a basic way to improve the electrical characteristics and bias stability of rare-metal-free oxide semiconductor TFTs, which have not been sufficiently studied.

Thermomechanical Behavior of Poly(3-hexylthiophene) Thin Films on the Water Surface

The thermomechanical behavior of a conjugated polymer (CP) in a thin film state has rarely been studied despite the importance of understanding the polymer morphologies and optimizing the thermal processes of organic semiconductors. Moreover, the seamless integration of multilayers without mechanical failures in CP-based electronic devices is crucial for determining their operational stability. Large differences in the coefficients of thermal expansion (CTEs) between the multilayers can cause serious degradation of devices under thermal stress. In this study, we measure the intrinsic thermomechanical properties of poly(3-hexylthiophene) (P3HT) thin films in a pseudo-freestanding state on the water surface. The as-cast P3HT thin films exhibited a large thermal shrinkage (-1001 ppm K^-1) during heating on the water surface. Morphological analyses revealed that the thermal shrinkage of the polymer films was caused by the rearrangement of the polymer chain networks accompanied by crystallization, thus indicating that preheating the polymer films is essential for estimating their intrinsic CTE values. Moreover, the rigidity of the substrate significantly influences the thermomechanical behavior of the polymer films. The polymer films that were preheated on the glass substrate showed nonlinear thermal expansion due to the substrate constraint inhibiting sufficient relaxation of the polymer chains. In comparison, a linear expansion behavior is observed after preheating the films on the water surface, exhibiting a consistent CTE value (185 ppm K^-1) regardless of the number of thermal strain measurements. Thus, this work provides a direct method for measuring in-plane CTE values and an in-depth understanding of the thermomechanical behaviors of CP thin films to design thermomechanically reliable organic semiconductors.

Self-Aligned Bilayers for Flexible Free-Standing Organic Field-Effect Transistors

Free-standing and flexible field-effect transistors based on 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)/polystyrene bilayers are obtained by well-controlled phase separation of both components. The phase separation is induced by solvent vapor annealing of initially amorphous blend films, leading to crystallization of TIPS-pentacene as the top layer. The crystallinity and blend morphology strongly depend on the molecular weight of polystyrene, and under optimized conditions, distinct phase separation with a well-defined and trap-free interface between both fractions is achieved. Due to the distinct bilayer morphology, the resulting flexible field-effect transistors reveal similar charge carrier mobilities as rigid devices and additionally pronounced environmental and bias stress stabilities. The performance of the flexible transistors remains stable up to a strain of 1.8%, while above this deformation, a close relation between current and strain is observed that is required for applications in strain sensors.

Advanced Electron Microscopy of Nanophased Synthetic Polymers and Soft Complexes for Energy and Medicine Applications

After decades of developments, electron microscopy has become a powerful and irreplaceable tool in understanding the ionic, electrical, mechanical, chemical, and other functional performances of next-generation polymers and soft complexes. The recent progress in electron microscopy of nanostructured polymers and soft assemblies is important for applications in many different fields, including, but not limited to, mesoporous and nanoporous materials, absorbents, membranes, solid electrolytes, battery electrodes, ion- and electron-transporting materials, organic semiconductors, soft robotics, optoelectronic devices, biomass, soft magnetic materials, and pharmaceutical drug design. For synthetic polymers and soft complexes, there are four main characteristics that differentiate them from their inorganic or biomacromolecular counterparts in electron microscopy studies: (1) lower contrast, (2) abundance of light elements, (3) polydispersity or nanomorphological variations, and (4) large changes induced by electron beams. Since 2011, the Center for Nanophase Materials Sciences (CNMS) at Oak Ridge National Laboratory has been working with numerous facility users on nanostructured polymer composites, block copolymers, polymer brushes, conjugated molecules, organic-inorganic hybrid nanomaterials, organic-inorganic interfaces, organic crystals, and other soft complexes. This review crystalizes some of the essential challenges, successes, failures, and techniques during the process in the past ten years. It also presents some outlooks and future expectations on the basis of these works at the intersection of electron microscopy, soft matter, and artificial intelligence. Machine learning is expected to automate and facilitate image processing and information extraction of polymer and soft hybrid nanostructures in aspects such as dose-controlled imaging and structure analysis.

Crystallization Control of N,N'-Dioctyl Perylene Diimide by Amphiphilic Block Copolymers Containing poly(3-Hexylthiophene) and Polyethylene Glycol

The preparation of micron- to nanometer-sized functional materials with well-defined shapes and packing is a key process to their applications. There are many ways to control the crystal growth of organic semiconductors. Adding polymer additives has been proven a robust strategy to optimize semiconductor crystal structure and the corresponding optoelectronic properties. We have found that poly(3-hexylthiophene) (P3HT) can effectively regulate the crystallization behavior of N,N'-dioctyl perylene diimide (C8PDI). In this study, we combined P3HT and polyethylene glycol (PEG) to amphiphilic block copolymers and studied the crystallization modification effect of these block copolymers. It is found that the crystallization modification effect of the block copolymers is retained and gradually enhanced with P3HT content. The length of C8PDI crystals were well controlled from 2 to 0.4 μm, and the width from 210 to 35 nm. On the other hand, due to the water solubility of PEG block, crystalline PEG-b-P3HT/C8PDI micelles in water were successfully prepared, and this water phase colloid could be stable for more than 2 weeks, which provides a new way to prepare pollution-free aqueous organic semiconductor inks for printing electronic devices.

Doped Highly Crystalline Organic Films: Toward High-Performance Organic Electronics

Today's organic electronic devices, such as the highly successful OLED displays, are based on disordered films, with carrier mobilities orders of magnitude below those of inorganic semiconductors like silicon or GaAs. For organic devices such as diodes and transistors, higher charge carrier mobilities are paramount to achieve high performance. Organic single crystals have been shown to offer these required high mobilities. However, manufacturing and processing of these crystals are complex, rendering their use outside of laboratory-scale applications negligible. Furthermore, doping cannot be easily integrated into these systems, which is particularly problematic for devices mandating high mobility materials. Here, it is demonstrated for the model system rubrene that highly ordered, doped thin films can be prepared, allowing high-performance organic devices on almost any substrate. Specifically, triclinic rubrene crystals are created by abrupt heating of amorphous layers and can be electrically doped during the epitaxial growth process to achieve hole or electron conduction. Analysis of the space charge limited current in these films reveals record vertical mobilities of 10.3(49) cm2 V-1 s-1. To demonstrate the performance of this materials system, monolithic pin-diodes aimed for rectification are built. The f 3 d b of these diodes is over 1 GHz and thus higher than any other organic semiconductor-based device shown so far. It is believed that this work will pave the way for future high-performance organic devices based on highly crystalline thin films.

Polyferrocenylsilane Semicrystalline Polymer Additive for Solution-Processed p-Channel Organic Thin Film Transistors

In this study, we demonstrated for the first time that a metal-containing semicrystalline polymer was used as an additive to mediate the thin film morphology of solution-grown, small-molecule organic semiconductors. By mixing polyferrocenylsilane (PFS) with an extensively-studied organic semiconductor 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene), PFS as a semicrystalline polymer independently forms nucleation and crystallization while simultaneously ameliorating diffusivity of the blend system and tuning the surface energies as a result of its partially amorphous property. We discovered that the resultant blend film exhibited a 6-fold reduction in crystal misorientation angle and a 3-fold enlargement in average grain width. Enhanced crystal orientation considerably reduces mobility variation, while minimized defects and trap centers located at grain boundaries lessen the adverse impact on the charge transport. Consequently, bottom-gate, top-contact organic thin film transistors (OTFTs) based on the TIPS pentacene/PFS mixture yielded a 40% increase in performance consistency (represented by the ratio of average mobility to the standard deviation of mobility). The PFS semicrystalline polymer-controlled crystallization can be used to regulate the thin film morphology of other high-performance organic semiconductors and shed light on applications in organic electronic devices.

Analysis of Fractal Structures in Dehydrated Films of Protein Solutions

The article deals with dendritic structures resulting from self-organization processes in aqueous solutions of albumin proteins. The methods for obtaining the structures and experimental results are presented. It is shown that dendrites are fractal structures that are symmetric under certain conditions of their formation and can have different characteristics depending on the isothermal dehydration of liquid samples. The fractal dimension of the structures in films of the albumin protein solution has been calculated. Dependences of the fractal dimension on the concentrations of salts and protein in the initial solutions and also on the dehydration temperature have been revealed. It has been shown that as the protein concentration in the solution grows, the salt concentration for the initiation of the dendritic structure formation increases. It has been found that the temperature dependences of the fractal dimension of the structures become smoother with increasing protein concentration in solutions. The relationship between geometric characteristics of dendrites and self-organization parameters during drying is discussed.

Conjugated Polymer Controlled Morphology and Charge Transport of Small-Molecule Organic Semiconductors

In this study, we report an effective approach to tune the crystallization, microstructure and charge transport of solution-processed organic semiconductors by blending with a conjugated polymer additive poly(3-hexylthiophene) (P3HT). When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was used as a model semiconductor material to mix with different amount of P3HT, their intermolecular interactions led to distinctive TIPS pentacene film morphologies, including randomly-oriented crystal ribbons, elongated needles with enhanced long-range order, and grass-like curved microwires with interlinkages. Each type of morphology was found to further correlate to considerably different charge transport and device performance. As compared to pristine TIPS pentacene devices, bottom-gate, top-contact OTFTs with 2% in weight P3HT additive showed a 2-fold and 5-fold improvement of average field-effect mobility and performance consistency (defined as the ratio of average mobility to the standard deviation), respectively. The improvement in transistor electrical performance can be attributed to the combined effect of enhanced crystal orientation and uniformity, as well as increased areal coverage. This work can be applied beyond the particular example demonstrated in this study and to tune the charge transport of other small-molecule organic semiconductors in general.

Retracted Article: Wavelength modulation of ZnO nanowire based organic light-emitting diodes with ultraviolet electroluminescence

Although organic light emitting diodes (OLEDs) can find important applications in display-related fields, it still remains a challenge to fabricate high-efficiency ultraviolet (UV) OLEDs with tunable wavelength. In this work, we demonstrate a facile method to adjust the electroluminescence (EL) peak from an inverted UV-OLED device that has zinc oxide nanowires (ZnO NWs) as an electron injection layer. The organic–inorganic interface between ZnO NWs and the 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ) emission layer employed in this work allows a reduction of the diffusion length of excitons, which further results in a hampered relaxation process of higher energy states as well as a blue shift of the EL spectrum. As a result, the emission peaks of the UV-OLED can be easily adjusted from 383 nm to 374 nm by tuning both the length of the ZnO NWs and the thickness of the TAZ emission layer. Our work reveals an important correlation between emission peaks and exciton diffusion, and presents a novel approach to fabricate high-performance UV-OLEDs with the capability of facilely modifying the emission wavelength.

As organic light emitting diodes (OLEDs) find important applications in display-related fields, we demonstrate the fabrication of an inverted UV-OLED device with tunable wavelength that composes zinc oxide nanowires as an electron injection layer.

Rapid template-free synthesis of nanostructured conducting polymer films by tuning their morphology using hyperbranched polymer additives

Nanostructures in conducting polymer films can enhance charge carrier and ion transfer, provide porosity with high specific area and confer unique optoelectronic properties for potential applications. A general and facile synthesis has been developed to prepare nanostructured conducting polymer films without the need for using templates. This simple approach employs hyperbranched polymers as additives to tune the morphology of conducting polymer films into a continuous nanofibril network. Nanostructured conducting polymer films with improved crystallinity exhibit good charge carrier transport and stable nanofibril network, without sacrificing either property upon removing residual additives. Polymer field-effect transistor sensors have been used to demonstrate the benefits of the large surface area provided by the nanofibril network. The sensors with porous nanostructures exhibit lower detection limits (two times lower) and faster response times (33% faster) compared to the sensors without nanostructures. This general approach can advance the knowledge and development of nanostructured conducting polymer films for energy harvesting and storage, electronics, catalysts, sensors and biomedical applications.