Solution-processed Thin Films Research Articles

GO induced grain-boundary modification in transparent TiO2-GO nanocomposite thin films: Study by DC bias dependent impedance spectroscopy

Here we present the study on GO induced grain boundary modification of solution-processed transparent TiO2-GO nanocomposite thin films by dc bias dependent impedance spectroscopy. The experimental Nyquist plot obtained from impedance spectra is fitted by an appropriate model electrical circuit comprised of two parallel R-C circuits connected with a series resistance in order to evaluate different contributions arising from core grains and grain boundaries. Variation of relaxation times (τ = RC) with external bias and its modification after GO reinforcement confirms grain boundary modification and its influence on charge transport in nanocomposites.

Understanding on the formation mechanisms of quasi-2D Ruddlesden- Popper halide perovskites

Two dimensional (2D) or quasi-2D halide perovskites have attracted intense interest in recent years for the replacement of three dimensional(3D) perovskites, owing to their stability and structural/optoelectronic tunability. However, the control of their composition and orientation is challenging when solution processed for thin films, because of the similarity in formation energy for different layer numbers. In this review, we discuss the factors affecting formation of 2D and quasi 2D perovskites, especially Ruddlesden-Popper perovskites, in solution-processed thin films. All the factors are associated with nucleation and growth kinetics, which affect their phase distribution and orientation. Furthermore, the performances of the photovoltaics and light-emitting diodes based on the controlled quasi-2D perovskites are briefly discussed.

Enhanced Electrical Performance and Stability of Solution-Processed Thin-Film Transistors with In2O3/In2O3:Gd Heterojunction Channel Layer.

The use of the semiconductor heterojunction channel layer has been explored as a method for improving the performance of metal oxide thin-film transistors (TFTs). The excellent electrical performance and stability of heterojunction TFTs is easy for vacuum-based techniques, but difficult for the solution process. Here, we fabricated In2O3/In2O3:Gd (In2O3/InGdO) heterojunction TFTs using a solution process and compared the electrical properties with single-layer In2O3 TFTs and In2O3:Gd (InGdO) TFTs. The In2O3/InGdO TFT consisted of a highly conductive In2O3 film as the primary transmission layer and a subconductive InGdO film as the buffer layer, and exhibited excellent electrical performance. Furthermore, by altering the Gd dopant concentration, we obtained an optimal In2O3/InGdO TFT with a higher saturation mobility (µ) of 4.34 cm2V−1s−1, a near-zero threshold voltage (Vth), a small off-state current (Ioff) of A, a large on/off current ratio (Ion/Ioff) of , a small subthreshold swing (SS), and an appropriate positive bias stability (PBS). Finally, an aging test was performed after three months, indicating that In2O3/InGdO TFTs enable long-term air stability while retaining a high-mobility optimal switching property. This study suggests that the role of a high-performance In2O3/InGdO heterojunction channel layer fabricated by the solution process in the TFT is underlined, which further explores a broad pathway for the development of high-performance, low-cost, and large-area oxide electronics.

Improvement of Electrical Performance in Solution-Processed InZnO Thin-Film Transistor With a Radio Frequency O2 Triggered Multistacked Architecture

A method for radio frequency oxygen (O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) plasma-assisted treatment of indium zinc oxide (IZO) films with a multistacked architecture fabricated by a low-temperature (200 °C) solution process is investigated. We demonstrate that radio frequency O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma-assisted posttreatment technology can improve the mobility and stability of IZO thin-film transistors (TFTs). The activity layers were formed by spin coating and subsequently thermally annealed at a low temperature of 200 °C, followed by O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -plasma posttreatment process, to form the O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> triggered multistacked architecture. The TFT devices plasma-treated for 3 min exhibit improved electrical characteristics, with a mobility of 7.09 ± 0.24 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs, an ON OFF ratio of (1.21 ± 1.22) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{{6}}$ </tex-math></inline-formula> , a threshold voltage of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-0.22\,\,\pm \,\,0.35$ </tex-math></inline-formula> V, and a subthreshold swing of 0.34 ± 0.02 V/dec. The improved results offered by the O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma treatment led to a higher mobility and ON/ OFF ratio compared to sample device without this process. The successful fabrication of radio frequency O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> triggered multistacked architecture-based IZO TFTs showing the practical potential of high-performance low-temperature solution-processed preparation technology and radio frequency O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma-assisted treatment for electrical device application.

Exploration of Alkyl Group Effects on the Molecular Packing of 5,15-Disubstituted Tetrabenzoporphyrins toward Efficient Charge-Carrier Transport.

The high design flexibility of organic semiconductors should lead to diverse and complex electronic functions. However, currently available high-performance organic semiconductors are limited in variety; most of p-type materials are based on thienoacenes or related one-dimensionally (1D) extended π-conjugated systems. In an effort to expand the diversity of organic semiconductors, we are working on the development of tetrabenzoporphyrin (BP) derivatives as active-layer components of organic electronic devices. BP is characterized by its large, rigid two-dimensionally (2D) extended π-framework with high light absorptivity and therefore is promising as a core building unit of organic semiconductors for optoelectronic applications. Herein, we demonstrate that BP derivatives can afford field-effect hole mobilities of >4 cm2 V-1 s-1 upon careful tuning of substituents. Comparative analysis of a series of 5,15-bis(n-alkyldimethylsilylethynyl)tetrabenzoporphyrins reveals that linear alkyl substituents disrupt the π-π stacking of BP cores, unlike the widely observed "fastener effect" for 1D extended π-systems. The n-octyl and n-dodecyl groups have the best balance between high solution processability and minimal π-π stacking disruption, leading to superior hole mobilities in solution-processed thin films. The resulting thin films show high thermal stability wherein the field-effect hole mobility stays above 1 cm2 V-1 s-1 even after heating at 160 °C in air, reflecting the tight packing of large BP units. These findings will serve as a good basis for extracting the full potential of 2D extended π-frameworks and thus for increasing the structural or functional diversities of high-performance organic semiconductors.

Humidity effect on electrical performance and bias stability of solution-processed In2O3 thin film transistor

ABSTRACT Relative humidity (RH) in storage environment has a great impact on the electrical performance of thin film transistors (TFTs), and the RH is critical for semiconductor manufacturing and device packaging. In this work, solution-processed indium oxide (In2O3) TFT is fabricated, and the electrical performance of the device is investigated after being exposed to various RH conditions. It is found that the threshold voltage (Vth) and the mobility of In2O3 TFT exhibit clear responses to the RH ranging from 23% to 85%. The inherent mechanism about the humidity effect on the electrical performance is elaborated, and this is due to the “donor effect” from the absorbed water molecules. In addition to the effect on Vth and mobility, RH also exhibits a significant effect on the electrical stability of the TFT. The experimental result indicates that the In2O3 TFT exposed to RH <50% exhibits excellent stability with an applied positive bias stressing for 2000s.

Band-Gap Tuning Induced by Germanium Introduction in Solution-Processed Kesterite Thin Films.

In the last few decades, the attention of scientific community has been driven toward the research on renewable energies. In particular, the photovoltaic (PV) thin-film technology has been widely explored to provide suitable candidates as top cells for tandem architectures, with the purpose of enhancing current PV efficiencies. One of the most studied absorbers, made of earth-abundant elements, is kesterite Cu2ZnSnS4 (CZTS), showing a high absorption coefficient and a band gap around 1.4–1.5 eV. In particular, thanks to the ease of band-gap tuning by partial/total substitution of one or more of its elements, the high-band-gap kesterite derivatives have drawn a lot of attention aiming to find the perfect partner as a top absorber to couple with silicon in tandem solar cells (especially in a four-terminal architecture). In this work, we report the effects of the substitution of tin with different amounts of germanium in CZTS-based solar cells produced with an extremely simple sol–gel process, demonstrating how it is possible to fine-tune the band gap of the absorber and change its chemical–physical properties in this way. The precursor solution was directly drop-cast onto the substrate and spread with the aid of a film applicator, followed by a few minutes of gelation and annealing in an inert atmosphere. The desired crystalline phase was obtained without the aid of external sulfur sources as the precursor solution contained thiourea as well as metal acetates responsible for the in situ coordination and thus the correct networking of the metal centers. The addition of KCl in dopant amounts to the precursor solution allowed the formation of well-grown compact grains and enhanced the material quality. The materials obtained with the optimized procedure were characterized in depth through different techniques, and they showed very good properties in terms of purity, compactness, and grain size. Moreover, solar-cell prototypes were produced and measured, exhibiting poor charge extraction due to heavy back-contact sulfurization as studied in depth and experimentally demonstrated through Kelvin probe force microscopy.

Unraveling the nonlinear optical behaviors of indium selenide thin films prepared by spin coating method

We report the nonlinear optical properties of solution-processed indium selenide thin films. The films were synthesized by spin coating method at various annealing temperatures using thiol-amine cosolvents. During annealing, a facile mechanical stress on the thin films resulted in a transformation from β -In3Se2 to γ -In2Se3 phase. The X-ray diffraction (XRD) study confirmed the polycrystalline nature of both phases. The Scanning Electron Microscope (SEM) study revealed the uniform and smooth surface morphologies of the films. The linear optical properties including extinction coefficient, refractive index, dielectric function, and dispersion parameters for both β -In3Se2 and γ -In2Se3 phases were investigated. The dispersion parameters were calculated using the Wemple-DiDomenico model. Third-order nonlinear susceptibility and nonlinear refractive index of both phases were calculated to find their potential in nonlinear optical applications.

Solution-Processed Silicon Doped Tin Oxide Thin Films and Thin-Film Transistors Based on Tetraethyl Orthosilicate

Recently, tin oxide (SnO2) has been the preferred thin film material for semiconductor devices such as thin-film transistors (TFTs) due to its low cost, non-toxicity, and superior electrical performance. However, the high oxygen vacancy (VO) concentration leads to poor performance of SnO2 thin films and devices. In this paper, with tetraethyl orthosilicate (TEOS) as the Si source, which can decompose to release heat and supply energy when annealing, Si doped SnO2 (STO) films and inverted staggered STO TFTs were successfully fabricated by a solution method. An XPS analysis showed that Si doping can effectively inhibit the formation of VO, thus reducing the carrier concentration and improving the quality of SnO2 films. In addition, the heat released from TEOS can modestly lower the preparation temperature of STO films. By optimizing the annealing temperature and Si doping content, 350 °C annealed STO TFTs with 5 at.% Si exhibited the best device performance: Ioff was as low as 10−10 A, Ion/Ioff reached a magnitude of 104, and Von was 1.51 V. Utilizing TEOS as an Si source has a certain reference significance for solution-processed metal oxide thin films in the future.

Semi-interpenetrating network of polymer semiconductor and tetrapod nanocrystal assemblies with balanced ambipolar transport

Most organic semiconductors exhibit p-type electrical characteristics, whereas inorganic semiconductor nanocrystals typically possess n-type characteristics. Therefore, in principle, blending these two different materials can yield ambipolar transport for both electron and hole conduction if a percolation pathway for each charge carrier can be developed within the blended material. As the dimensions of these materials are on the nanometer scale, forming such a percolation pathway is challenging. Herein, we report a solution-processed method to construct an ambipolar channel by combining tetrapod-shaped CdSe nanocrystals and polythiophene. A fully percolated network of tetrapod CdSe nanocrystals were first produced by benefitting from the extended geometry of the nanostructure with four arms. The voids of the network were then filled with polythiophene, leading to a semi-interpenetrating network containing percolation pathways for both electrons and holes. A balanced ambipolar transport was achieved with electron and hole conductivities of 10.6 S/cm and 16.2 S/cm, respectively. The hybridization strategy for organic/inorganic semiconductors presented herein provides an innovative route to prepare solution-processable thin films for both electron and hole transport electronic devices.

Thin Film Photodetectors Based on Zinc Oxide Nanoinks

We report on the fabrication and characterization of ZnO thin film photodetectors using planetary ball-milled nanoparticle suspensions (nanoinks). Milling parameters, speed and time, were varied between 200 rpm and 1000 rpm, and 10 and 60 minutes, respectively to produce suspensions with particles below 100 nm in diameter. The resulting PBM nanoinks were used to create ZnO thin films whose photoconductance was measured under broadband UV/visible illumination. Thin film ZnO photodetector devices showed strong response upon light exposure with up to an order of magnitude increase in photocurrent compared to baseline (dark) for the conditions studied. This work demonstrates the use of low-cost PBM nanoinks for the active materials in solution-processed thin film photodetectors based on ZnO and allows film properties to be tailored and optimized for different applications in a straightforward manner by adjusting grinding conditions.

Atomic Structure Evaluation of Solution-Processed a-IZO Films and Electrical Behavior of a-IZO TFTs.

Understanding the chemical reaction pathway of the metal–salt precursor is essential for modifying the properties of solution-processed metal-oxide thin films and further improving their electrical performance. In this study, we focused on the structural growth of solution-processed amorphous indium-zinc-oxide (a-IZO) films and the electrical behavior of a-IZO thin-film transistors (TFT). To this end, solution-processed a-IZO films were prepared with respect to the Zn molar ratio, and their structural characteristics were analyzed. For the structural characteristic analysis of the a-IZO film, the cross-section, morphology, crystallinity, and atomic composition characteristics were used as the measurement results. Furthermore, the chemical reaction pathway of the nitrate precursor-based IZO solution was evaluated for the growth process of the a-IZO film structure. These interpretations of the growth process and chemical reaction pathway of the a-IZO film were assumed to be due to the thermal decomposition of the IZO solution and the structural rearrangement after annealing. Finally, based on the structural/chemical results, the electrical performance of the fabricated a-IZO TFT depending on the Zn concentration was evaluated, and the electrical behavior was discussed in relation to the structural characteristics.

Local Disordering in the Amorphous Network of a Solution-Processed Indium Tin Oxide Thin Film.

The polyhedra unit structure (MOx) in an amorphous metal oxide network has more freedom and flexibility than the same unit structure in a crystalline phase. Consequently, a mild external stimulus (e.g., instant photonic and acoustic energy) could affect and change this network parameter, thereby enhancing and modulating the electrical properties. However, it is difficult to tune these atomic parameters solely while maintaining the metal oxide's initial global amorphous phase and thereby preventing mechanical instability at the film-substrate interface (i.e., cracking or distortion). Here, we report local disordering in an amorphous network of a solution-processable indium tin oxide (ITO) film, where the disordering is triggered by mild-light irradiation (<0.1 mJ/cm2). Through a combination of systematic characterizations of the global structural and chemical compositional changes in conjunction with extended X-ray absorption fine structure analyses, we revealed the distortion of the atomic structure in the amorphous network of the ITO film led to the formation of additional structural oxygen vacancies. Our findings enabled us to fabricate mechanical-instability-free, perfect amorphous-phase ITO thin films on plastic substrates, where the sheet resistance substantially decreased to ∼ 2 × 103 Ω/□. Furthermore, this sheet resistance did not vary when the film and substrate were bent to a radius of 2 mm and could operate at low temperatures. This work can pave the novel way to fabricate high-quality flexible transparent electrodes suitable for rapid, cost-effective, and patternable processing on plastic substrates, and the domain can be extended to flexible electronics.

Effect of hydrochloric acid addition on dielectric properties of solution-processed aluminum titanium oxide thin film annealed at low temperature

This study investigated the effect of hydrochloric acid (HCl) addition on the dielectric properties of solution-processed aluminum titanium oxide (ATO) applied to thin-film transistors (TFTs) as a gate insulator layer. The formation of the solution-processed ATO film can be accelerated by adding HCl to the precursor solution because of the hydrolysis catalyst effect of HCl. The solution-processed ATO film exhibits higher crystallinity as the amount of HCl added increases. Furthermore, the addition of HCl suppresses the formation of oxygen vacancies, which can act as defect states in the ATO film. Consequently, the optimized ATO film annealed at 450 ℃ exhibited minimum leakage current density level of 5.14×10−9 A/cm2 at 0.33 MV/cm and dielectric constant of 13.9 when HCl of 2 ml was added to the precursor solution. These results are comparable with the dielectric properties of the ATO film annealed at 750 ℃ without HCl addition, because HCl addition catalyses hydrolysis and enhances the formation of metal-oxide bonding. Solution-processed amorphous indium-gallium-zinc oxide (IGZO) TFTs using the optimized ATO/AlOx film as gate insulator displayed excellent electrical properties such as threshold voltage of 0.17 V, field-effect mobility of 2.59 cm2/V∙s, subthreshold swing of 0.22 V/dec, and on/off ratio of 4.54×105. In addition, the IGZO TFT with the ATO gate insulator exhibited superior current driving capability compared to that of silicon dioxide (SiO2). These results indicate that solution-processed ATO films with HCl addition could be promising candidates for gate insulator layers in achieving low power consumption, high performance, and low cost TFTs.

Low Temperature Sol-Gel Processed Zirconium Oxide Based Transparent Resistive Random Access Memory Devices

In this work, solution-processed ZrO2-based resistive random-access memory (RRAM) device with ITO (Indium Tin Oxide) as a bottom electrode (BE) was fabricated at a low thermal budget by sol-gel spin coating technique. The grown devices (ZrO2/ITO) were found to be amorphous by XRD measurements and exhibit 80% transmittance in the visible region. The resistive switching characteristics of solution-processed ZrO2 thin films were investigated. The W/ZrO2/ITO device exhibits good cyclic endurance with an on-off ratio (~10) and reliable switching and good retention at room temperature. The conduction mechanism for the low resistance state is ohmic, whereas the conduction mechanism of the high resistance state is ohmic at low applied voltages, while a space charge limited conduction (SCLC) at the high applied voltages. The combined ohmic and space charge limited conduction mechanisms together indicate that the RS behavior in the ZrO2 layer is due to the formation and rupture of conducting filaments. Results suggest that solution-processed ZrO2 has great potential to develop transparent and flexible resistive random access memory devices.

Enhancement of solution-processed Cu2ZnSnS4 film properties via a facile approach of sodium incorporation

Properties of solution-processed thin-films of Cu2ZnSnS4 (CZTS), a promising photo-absorber material for the photovoltaic application, were investigated as a function of sodium (Na) incorporation. This was achieved via a facile approach in which Na source was introduced at the bottom of the CZTS precursor as well as at the top. A significant increase in the grain size, from 319 to 1125 nm, and a large drop in the resistivity from 7.9 × 103 to 5.0 × 101 Ω cm have been observed. Large grains are desirable for reducing the recombination loss of the carriers at the defect sites at grain boundaries. Additionally, the approach incorporated in this work was also found beneficial for the suppression of tin loss. The effect of Na incorporation was also measured on the optical band gap, and the results indicate that Nai is the dominant defect present in these CZTS films.

Solution-processed tin oxide thin film for normally-off hydrogen terminated diamond field effect transistor

The solution processed method has been wildly used in the thin film fabrication because of the advantages of low cost, high efficiency, large scale for production, and long-term stability. In this paper, a normally-off hydrogen-terminated diamond field-effect transistor (FET) has been realized by using a solution processed SnO2 (sp-SnO2) film as an insulator layer. X-ray photoelectron spectroscopy results demonstrated the stoichiometry of the sp-SnO2 film, which shows good insulator properties with leakage current density less than 2.1 × 10−5 A·cm−2 at gate voltages from −6.0 to 6.0 V. The drain–source current maximum, threshold voltage, extrinsic transconductance maximum, and effective mobility of the FET with a gate length of 10 μm are −17.6 mA·mm−1, −0.5 V, 5.7 mS·mm−1, and 41.3 cm2/V s, respectively. According to the capacitance voltage characteristic, the enhancement mode could be ascribed to the high positive fixed charge density in the sp-SnO2 film, which will repel the hole in the channel. This paper provides a simple method and a low temperature process to fabricate an insulator layer.

The Importance of Nonequilibrium to Equilibrium Transition Pathways for the Efficiency and Stability of Organic Solar Cells.

Controlled morphology of solution-processed thin films have realized impressive achievements for non-fullerene acceptor (NFA)-based organic solar cells (OSCs). Given the large set of donor-acceptor pairs, employing various processing conditions to realize optimal morphology for high efficiency and stable OSCs is a strenuous task. Therefore, comprehensive correlations between processing conditions and morphology evolution pathways have to be developed for efficient performance and stability of devices. Within the framework of the blend system, crystallization transitions of NFA molecules are tracked utilizing the first heating scan of differential scanning calorimeter (DSC) measurement correlating with respective morphology evolution of blend films. Real-time dynamics measurements and morphology characterizations are combined to provide optimal morphology transition pathways as NFA molecules are shown to be released from the mixed-phase to form balanced ordered packing with variant processing conditions. Polymer:NFA films are fabricated using blade coating incorporating solvent additive or thermal annealing as processing conditions as a correlation is formulated between performance and stability of solar cells with morphology transition pathways. This work demonstrates the significance of processing condition-controlled transition pathways for the realization of optimal morphology leading to superior OSC devices.

Stereoisomer-dependent conversion of dinaphthothienothiophene precursor films

Soluble precursor materials of organic semiconductors are employed for fabricating solution-processable thin film devices. While the so-called precursor approach has already been tried for various organic electronic devices such as transistors and solar cells, understanding of the conversion process in the film lags far behind. Here, we report that molecular aggregation of the precursor compound significantly influences the thermal conversion reaction in the film. For this study, two stereoisomers of a dinaphthothienothiophene (DNTT) precursor that are the endo- and exo-DNTT-phenylmaleimide monoadducts are focused on. The structural change during the thermal conversion process has been investigated by a combination of infrared spectroscopy and X-ray diffraction techniques. The results show that the endo-isomer is readily converted to DNTT in the film by heating, whereas the exo-isomer exhibits no reaction at all. This reaction suppression is found to be due to the self-aggregation property of the exo-isomer accompanying the intermolecular C–HcdotsO interactions. This finding shows a new direction of controlling the on-surface reaction, as well as the importance of analyzing the film structure at the initial stage of the reaction.

Stable and Solution-Processable Cumulenic sp-Carbon Wires: A New Paradigm for Organic Electronics.

Solution-processed, large-area, and flexible electronics largely relies on the excellent electronic properties of sp2 -hybridized carbon molecules, either in the form of π-conjugated small molecules and polymers or graphene and carbon nanotubes. Carbon with sp-hybridization, the foundation of the elusive allotrope carbyne, offers vast opportunities for functionalized molecules in the form of linear carbon atomic wires (CAWs), with intriguing and even superior predicted electronic properties. While CAWs represent a vibrant field of research, to date, they have only been applied sparingly to molecular devices. The recent observation of the field-effect in microcrystalline cumulenes suggests their potential applications in solution-processed thin-film transistors but concerns surrounding the stability and electronic performance have precluded developments in this direction. In the present study, ideal field-effect characteristics are demonstrated for solution-processed thin films of tetraphenyl[3]cumulene, the shortest semiconducting CAW. Films are deposited through a scalable, large-area, meniscus-coating technique, providing transistors with hole mobilities in excess of 0.1cm2 V-1 s-1 , as well as promising operational stability under dark conditions. These results offer a solid foundation for the exploitation of a vast class of molecular semiconductors for organic electronics based on sp-hybridized carbon systems and create a previously unexplored paradigm.