Solution-processed Materials Research Articles

3D printed high transconductance organic electrochemical transistors on flexible substrates

Solution-processable organic semiconducting materials have been utilized for low-cost disposable flexible electronics applications through additive printing techniques. Organic electrochemical transistors (OECTs) are powerful devices for biosensing due to their simplicity, stability in aqueous media and high transconductance. Here, we report fully 3D printed functional OECTs using fused deposition modelling (FDM) and direct writing 3D printing techniques using PEDOT:PSS as the channel material with silver as the source/drain material. These devices demonstrate high transconductance of 12.1 ± 0.4 mS, low operating voltage and high current ON/OFF ratio. They have a threshold voltage in the range of 0.66 ± 0.01 V. These printed OECTs demonstrate good stability and robust behavior after several measurement cycles. We demonstrate pH and K+ ion sensing using these printed OECTs with high sensitivity of −26.4 mV/pH and 240 mV/decade (higher than the Nernst limit of 59.2 mV/decade). These devices can be utilized for biosensing and logic circuitry for different wearable and standalone sensing applications.

Interfacial engineering of front-contact with finely tuned polymer interlayers for high-performance large-area flexible perovskite solar cells

Inverted perovskite solar cells (PSCs) have stood out from the promising candidates for portable power source applications. Their advantageous features such as high efficiency, low cost, light weight and flexibility can be retained when a robust and compact hole transport layer (HTL) is fabricated using a low-temperature technology. To maximize the mechanical feature, the integration of graphene-derived nanomaterials is a method of choice owing to their superior physical and mechanical properties. Herein, we report an innovative method of fabricating efficient PSCs by means of inserting conjugated polyelectrolyte PTFTS interlayers between the perovskite and graphene oxide (GO) HTL. Results show that the amphiphilic polymer interlayer facilitates the growth of perovskite grains, passivates interfacial trap-states, forms better energetic alignment, and increases intermolecular interaction between PTFTS and perovskite. More importantly, strong binding force between GO and PTFTS enables uniform interlayer films obtained from a simple dip-coating method. The facile processability of the PTFTS-modified GO has been successfully extended to scalable fabrication of flexible device (1.08 cm2) with excellent device efficiency over 17.0%, the highest reported value so far for large-area flexible PSCs. This front-contact interfacial engineering offers an efficient strategy towards the widespread application of solution-processable graphene materials in practical perovskite devices.

Interfacial Effects of UV-Ozone Treated Sol-Gel Processable ZnO for Hybrid Photodetectors and Thin Film Transistors

ABSTRACTHybrid organic-inorganic semiconducting interfaces have attracted attention in photodiodes and field-effect transistors (FETs) due to the realization of intrinsic p-n junctions and their mechanical flexibility. With the difficulty of developing high-mobility n-type organic semiconductors due to the necessity of low LUMO levels and ambient environment stability, solution processable inorganic materials are an excellent alternative. ZnO is an intrinsic n-type semiconductor which is non-toxic and sol-gel processable, creating avenues for film patterning and fully solution processed devices. We report the improvement of electron mobilities in ZnO FETs through simple UV-Ozone processing which reduces lattice defects within the film and at the SiO2/ZnO interface. Treated ZnO films yield electron mobilities close to 10-2 cm2/Vs and on/off current ratios of 104 while non-treated films have mobilities on the order of 10-5 cm2/Vs and an order of magnitude lower on/off current ratios. Treated films also yield improved photoresponsivity and detectivity in hybrid ZnO-organic photodetectors.

Highly reliable low-temperature (180 °C) solution-processed passivation for amorphous In–Zn–O thin-film transistors

This paper presents a low-temperature solution-processed passivation material based on polysilsesquioxane (PSQ) which greatly improves electrical performance and stability of solution-processed amorphous InZnO (a-IZO) thin-film transistors (TFTs). PSQ-passivated TFTs exhibited maximum mobilities of 6.02 cm2 V−1 s−1 with the smallest threshold voltage shift of 2.8 V after positive bias stress test. We showed that hydrogen-related ions from PSQ passivation help passivate oxygen vacancies, thus improving the percolation path in a-IZO channel. Moreover, PSQ-passivated TFTs showed minimal change after being subjected to humidity stress. These results demonstrate the ability of low-temperature PSQ passivation as an effective barrier against atmospheric effects.

Regulation of peripheral tert-butyl position: Approaching efficient blue OLEDs based on solution-processable hole-transporting materials

Abstract Solution-processable hole transporting materials (HTMs) play a vital role in organic light emitting diodes (OLEDs) with the purpose of reducing costs and improving productivity. Herein, two twisted HTMs N2,N2,N7,N7-tetrakis(4-(3,6-di-tert-butyl-9H- carbazol-9-yl)phenyl)-9,9-dimethyl-9H-fluoren-2,7-diamine (p-BCz-F) and N2,N2,N7, N7-tetrakis(4-(2,7-di-tert-butyl-9H-carbazol-9-yl)phenyl)-9,9-dimethyl-9H-fluoren-2,7-diamine (m-BCz-F), have been successfully synthesized. The two compounds are endowed with superior solubility in common organic solvents, facilitating solution-processed OLEDs. By simply changing position of peripheral tert-butyl moiety, the material performance could be regulated. The HTMs p-BCz-F exhibits superior glass transition temperature (Tg) of 168 °C. Particularly, in terms of mobility, 1.42-fold higher could be also found for p-BCz-F (4.35 × 10−4 cm2V–1S−1) than m-BCz-F (3.05 × 10−4 cm2V–1S−1). Similarly, the p-BCz-F-based blue devices with BCzVBi as the emitter show better performance with maximum current efficiency (CE) of 7.05 cd/A and luminance (L) of 23365 cd/m2, corresponding CE of 6.12 cd/A and L of 21055 cd/m2 for m-BCz-F-based devices. In short, the different position of peripheral tert-butyl on carbazolyl moiety will lead to different performance and it provided a simple strategy to design solution-processable HTMs for efficient OLEDs.

Enhanced gas barrier property of stacking-controlled reduced graphene oxide films for encapsulation of polymer solar cells

Compact stacking of building blocks is essential in order to utilize the impermeable nature of reduced graphene oxide (rGO) for encapsulating barriers in organic electronics. To obtain the desired accumulation characteristic, rGO films were fabricated via spraying process using phase-controlled graphene oxide (GO) dispersion. The rGO laminate prepared from the isotropic-phase dispersion exhibits a more favorable face-on orientation of rGO building blocks than that from the nematic-phase GO, which implies effective stacking of building blocks. As a result, the resulting gas barrier performance of the low-temperature-annealed rGO film is superior to previously reported low-temperature solution-processed materials. Fabricated gas barriers are adopted to seal the polymer solar cell with 57.2 times longer lifetime than that of the unsealed device. A device covered by an optimized rGO/polyethylene naphthalate gas barrier maintains photovoltaic performance as high as 88% of the initial efficiency after 28 days.

(Invited) Garment Integrated Sensors Created Using Reactive Vapor Deposition

Wearable sensors are uniquely suited for personalized health monitoring and activity tracking. In particular, fiber- and fabric-based sensors that can be seamlessly integrated into everyday clothing have the power to create smart garments that can monitor health statistics and alert a wearer to impending medical problems or chemical/biological threats. However, natural fibers and fabrics--which would be the preferred platform for such garment-integrated sensors--are demanding substrates, and printing electronic materials and electrochemically-active sensors onto natural fibers and fabrics is nontrivial. Solution-processed active materials either soak into natural fibers or nonhomogeneously wet their surfaces, thus creating buried/inaccessible active sites or broken conduction pathways, respectively, that prevent isolation of functioning sensors. Further, the rough and disordered structures of fibers and fabrics also attenuate the baseline bulk conductivities of any electronic material, creating troublesome series resistances that are otherwise absent in conventional, glass- or plastic-backed devices. Our lab uses reactive vapor coating to form conjugated polymer films directly on the surface of any premade garment, prewoven fabric or fiber/yarn, without the need for specialized processing conditions, surface pretreatments, detergents or fixing agents. This feature allows for electronic coatings to be applied at the end of existing, high-throughput textile and garment manufacturing routines, irrespective of dye content or surface finish of the final textile. Further, reactive vapor coating produces conductive materials without any insulating moieties and yields uniform and conformal films on fiber/fabric surfaces that are notably wash- and wear-stable and withstand mechanically-demanding textile manufacturing routines. These unique features mean that rugged and practical textile electronic devices can be created using sewing, weaving or knitting procedures without compromising or otherwise affecting the surface electronic coating. In this talk, we will highlight selected electronic garments and garment-integrated sensors created by melding reactive vapor deposition with traditional textile manipulation processes, including: (1) smart joint braces for movement/activity sensing; (2) fabric electrodes for bioimpedance spectroscopy; (3) fiber-based radiation monitors; and (4) garment-integrated chemiresistive sensors for detecting selected biomarkers in sweat.

Controlled Steric Hindrance Enables Efficient Ligand Exchange for Stable, Infrared-Bandgap Quantum Dot Inks

Colloidal quantum dots (CQDs), which benefit from a size-tuned bandgap, are a solution-processed material for infrared energy harvesting. This characteristic enables the fabrication of solar cells ...

PEDOT nanoparticles fully covered on natural tubular clay for hierarchically porous electrochromic film

In this work, we develop a facile and feasible strategy to prepare natural tubular clay halloysites nanotubes (HNT)@poly(3,4-ethylenedioxythiophene) (PEDOT) nanocomposites as an electrochromic material. The HNT@PEDOT nanocomposites were formed by coating PEDOT on the outer surface of HNT through in-situ chemical oxidative polymerization. Due to the hollow tubular and hierarchical porous structure of the hybrid nanocomposites, the surface area of active materials is enlarged and the diffusion distance of ions is shortened. Consequently, the HNT@PEDOT film delivers higher optical contrast (59.3%), coloration efficiency value (174.3 cm2/C) and long-cycle stability compared to pure PEDOT film. Besides, the as-prepared solution processable electrochromic material enables the utilization of printing or spraying methods and applies for large area and patterned films.

Arylacetylene end capped naphthodithiophene diimide (NDTI)-based semiconductors for air-stable, solution-processed n-channel organic field-effect transistors: Effect of terminal aryl groups on charge transport

Solution-processable n-channel organic field-effect transistors (OFETs) are crucial for realizing low-cost, large-area, and all organic logic/complementary circuits. Herein, we present the synthesis, characterization and OFET characteristics of a series of arylacetylene end capped NDTI-based n-channel small molecule semiconductors with four different arylacetylene groups, i.e., thiophenylethynyl (TE, NDTI-BTE), phenylethynyl (PE, NDTI-BPE), 2-pyridinylethynyl (2PdE, NDTI-B2PdE), 4-pyridinylethynyl (4PdE, NDTI-B4PdE). All four NDTIs exhibit excellent solubility behavior in common organic solvents. Electrochemical and optical studies reveal that the introduction of the arylacetylene groups at the thiophene α-positions of NDTI endowed these molecules have sufficiently low-lying LUMO energy levels from −4.12 to −4.24 eV, suitable for better air-stable electron transport. Under ambient atmosphere, we found that solution-processed bottom-gate top-contact (BGTC) transistors based on 120 °C-annealed thin-films of these NDTIs exhibit the best performance with the maximum electron mobilities of 0.049, 0.023, 0.011 and 0.033 cm2 V−1 s−1 for NDTI-BTE, NDTI-BPE, NDTI-B2PdE and NDTI-B4PdE, respectively, due to better thin film microstructures. AFM measurements reveal that the spin-coated films of these materials exhibited quite smooth and uniform morphology although these are small molecules forming crystalline structures. These results clearly demonstrate that arylacetylene groups are potential functional moieties for developing ambient-stable solution-processable small molecular electron-transporting materials for printable organic electronics.

Ligand-mediated synthesis of colloidal Cs2SnI6 three-dimensional nanocrystals and two-dimensional nanoplatelets

Cs2SnI6 is a variant on tin-iodide solution-processable materials and may lead to a lead-free material for use in next-generation photovoltaic cells and other optoelectronics. So far, only a few studies have been conducted where shape and geometry control of Cs2SnI6 nanocrystals is demonstrated. Here we report a general approach to directly synthesize Cs2SnI6 of two-dimensional (2D) layered nanoplatelets as well as three-dimensional (3D) nanocrystals. The shape of Cs2SnI6 nanocrystals could be engineered into 3D nanoparticles and different 2D nanoplatelets with well-defined morphology by choosing different organic acid and amine ligands via a hot injection process. Moreover, the thickness of layered 2D nanoplatelets could be adjusted by changing the amount of Cs-oleate present during the synthesis. The photoluminescence emission peaks changed from 643 to 742 nm based on nanomaterial shape. Our method provides a facile and versatile route to rationally control the shape of the Cs2SnI6 nanocrystals, which will create opportunities for applications in lead-free optoelectronics.

Body Heat Powers Future Electronic Skins

Ruoming Tian recently joined the Mark Wainwright Analytical Centre as an XRD research assistant at the University of New South Wales. She was formerly a Research Scientist at the Toyota Physical and Chemical Research Institute in Japan (2015–2018) and received her PhD from the University of New South Wales in 2014. Her research interests focus on the nano- and micro-materials, flexible hybrid materials, and smart devices for harvesting both high- and low-grade thermal energy. She has published 2 book chapters and 18 peer-reviewed articles and delivered more than 15 talks at international conferences and leading research institutes. Yuqing Liu is currently an Associate Research Fellow and received her PhD (2018) in the Intelligent Polymer Research Institute (IPRI), University of Wollongong, Australia. Her research interest is on the fabrication of wearable energy storage and conversion devices (e.g. thermos-cells, batteries, supercapacitors, etc.), mainly focusing on nanostructured electrode materials, gel electrolyte/electrode interface, and device fabrication via printing techniques. Kunihito Koumoto, Professor Emeritus of Nagoya University, received his BS, MS, and PhD from the University of Tokyo (UT). He served as an Assistant Professor, Lecturer, and Associate Professor at UT (1979–1992) and a full professor at Nagoya University (1992–2015). He has published 495 papers and reviews and 58 book chapters in inorganic materials chemistry. He was awarded Medal with Purple Ribbon by the Emperor of Japan in 2013 and Outstanding Achievement Award by the International Thermoelectric Society in 2018. His current research focuses on inorganic/organic nanohybrid thermoelectrics for energy harvesting. Koumoto can be reached by email at koumoto@apchem.nagoya-u.ac.jp . Jun Chen is currently a Full Professor at IPRI|ACES, Australian Institute for Innovative Materials, University of Wollongong (UOW). He received BE from Zhejiang University of Technology (1995) and PhD from UOW (2003). Professor Chen has published more than 180 papers and has been named a Highly Cited Researcher for 2018 (Clarivate Analytics). His research interests include sustainable energy devices, electro-/bio-interfaces, nano-/micro-materials, 2D/3D printing, and smart wearable electronic devices. Apart from academic achievements, Chen has also been involved in two spin-off companies: AQUAHYDREX as one of the key inventors, and IMAGINE as one of the scientific founders.

Achieving 20% External Quantum Efficiency for Fully Solution-Processed Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Dendrimers with Flexible Chains.

Actualizing high-efficiency thermally activated delayed fluorescent (TADF) organic light-emitting diodes (OLEDs) with fully wet processes is of great significance to the development of purely organic electroluminescence and the application of large-area OLED displays. Herein, new strategies are proposed to develop the TADF dendrimers with tunable colors by adjusting the way of linking branches to the core and the numbers of peripheral branches. Due to an energy gradient and efficient exciton utilization in the core-dendron system, the solution-processed OLEDs with the four dendrimers 5CzBN-O-Cz, 5CzBN-O-2Cz, 5CzBN-Cz, and 5CzBN-2Cz all give rise to low turn-on voltages and great device efficiency. Notably, 5CzBN-2Cz affords record-high fully solution-processed TADF OLEDs with external quantum efficiency of above 20%, which is significantly comparable to the efficiency of TADF OLEDs based on vacuum deposition. The work offers a guideline for designing solution-processable materials, paving the way toward practical applications of large-area fully solution-processed OLEDs.

2D-σ-2A type cruciform host material with silane core for highly efficient solution-processable green thermally activated delayed fluorescence organic light emitting diodes

A new solution processable cruciform tetraphenylsilane-based bipolar host material, bis(4-(9H-carbazol-9-yl)phenyl)bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)silane (SiCT), was designed and synthesized. Two carbazoles as donors and two diphenyl triazine moieties as acceptors were tethered to the tetraphenylsilane. SiCT showed great solubility in common organic solvents, high thermal stability, high triplet energy (T1 ∼3.0 eV), and a superior bipolar electronic nature, which are sufficient for it to function as a host material in thermally activated delayed fluorescence organic light emitting diodes (TADF-OLEDs). Consequently, the solution-processed green TADF-OLEDs employed SiCT as a bipolar host exhibited a low turn-on voltage and high external quantum efficiency (EQE) of 19.17% with a high power efficiency (PE) of 41.13 lm W−1, while control device based-on SiC as unipolar host showed a low EQE of 12.74% with a low PE of 16.13 lm W−1. This work highlights the potential of a bipolar tetraphenylsilane derivative as a host material in solution-processable green TADF-OLEDs.

Improved Hole Injection in Bulk Heterojunction (BHJ) Hybrid Solar Cells by Applying a Thermally Reduced Graphene Oxide Buffer Layer

In this work, the utilization of graphene oxide (GO), reduced graphene oxide (rGO), and carbon nanotube (CNT) thin films as hole transport and electron-blocking layers in polymer/nanocrystal hybrid solar cells is demonstrated. A simple method has been used to modify the anode of hybrid solar cells by depositing these two solution-processable nanocarbon materials between poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) and transparent indium tin oxide (ITO) layers. Upon the use of an rGO interlayer, we found a substantial improvement in power conversion efficiency (PCE) from 2.5% to 3.2% due to a decrease in series resistance (Rs). This decrease has been obtained by a careful tuning of the reduction degree of rGO, inducing optimization of the energy band alignment at the solar cell anode. In addition, charge extraction by linearly increasing voltage (CELIV) measurements show an increase in light-induced charge extraction of ca. 50%. Finally, the utilization of rGO as replacement for PEDOT:PSS is also presented. The findings reported in this work demonstrate the excellent potential of rGO as an efficient hole transport material in hybrid solar cells.

Field Effect and Photoconduction in Au25 Nanoclusters Films.

Quantum-confined Au nanoclusters exhibit molecule-like properties, including atomic precision and discrete energy levels. The electrical conductivity of Au nanocluster films can vary by several orders of magnitude and is determined by the strength of the electronic coupling between the individual nanoclusters in the film. Similar to quantum-confined, semiconducting quantum dots, the electrical coupling in films is dependent on the size and structure of the Au core and the length and conjugation of the organic ligands surrounding it. Unlike quantum dots, however, semiconducting transport has not been reported in Au nanocluster films. Here, it is demonstrated that through a simple yet careful choice of cluster size and organic ligands, stable Au nanocluster films can electronically couple and become semiconducting, exhibiting electric field effect and photoconductivity. The molecule-like nature of the Au nanoclusters is evidenced by a hopping transport mechanism reminiscent of doped, disordered organic semiconductor films. These results demonstrate the potential of metal nanoclusters as a solution-processed material for semiconducting devices.

Towards Infrared Electronic Eyes: Flexible Colloidal Quantum Dot Photovoltaic Detectors Enhanced by Resonant Cavity

Electronic eye cameras are receiving increasing interest due to their unique advantages such as wide field of view, low aberrations, and simple imaging optics compared to conventional planar focal plane arrays. However, the spectral sensing ranges of most electronic eyes are confined to the visible, which is limited by the energy gaps of the sensing materials and by fabrication obstacles. Here, a potential route leading to infrared electronic eyes is demonstrated by exploring flexible colloidal quantum dot (CQD) photovoltaic detectors. Benefitting from their tunable optical response and the ease of fabrication as solution processable materials, mercury telluride (HgTe) CQD detectors with mechanical flexibility, wide spectral sensing range, fast response, and high detectivity are demonstrated. A strategy is provided to further enhance the light absorption in flexible detectors by integrating a Fabry-Perot resonant cavity. Integrated short-wave IR detectors on flexible substrates have peak D* of 7.5 × 1010 Jones at 2.2 µm at room temperature and promise the development of infrared electronic eyes with high-resolution imaging capability. Finally, infrared images are captured with the flexible CQD detectors at varying bending conditions, showing a practical approach to sensitive infrared electronic eyes beyond the visible range.

Encapsulation of methylammonium lead bromide perovskite in nanoporous GaN

Halide perovskites hold exceptional promise as cheap, low temperature solution-processed optoelectronic materials. Yet they are hindered by poor structural and chemical stability, rapidly degrading when exposed to moisture or air. We demonstrate a solution-phase method for infiltrating methylammonium lead bromide perovskite (CH3NH3PbBr3, or MAPbBr3) into nanoporous GaN which preserved the green photoluminescence of the perovskite after up to 1 year of storage under ambient conditions. Besides a protective effect, confinement within the porous GaN matrix also resulted in a blueshift of the perovskite emission with decreasing pore size, suggesting an additional templating effect of the pores on the size of the perovskite crystals within. We anticipate that our method may be generalised to related perovskite materials, offering a route to producing composites of interest for use in optoelectronic devices for various applications.

Fully Inkjet-Printed, Mechanically Flexible MoS2 Nanosheet Photodetectors.

Solution-processed two-dimensional materials offer a scalable route toward next-generation printed devices. In this report, we demonstrate fully inkjet-printed photodetectors using molybdenum disulfide (MoS2) nanosheets as the active material and graphene as the electrodes. Percolating films of semiconducting MoS2 with high electrical conductivity are achieved with an ethyl cellulose-based ink formulation. Two classes of photodetectors are fabricated, including thermally annealed devices on glass with fast photoresponse of 150 μs and photonically annealed devices on flexible polyimide with high photoresponsivity exceeding 50 mA/W. The photonically annealed photodetector also reduces the curing time to milliseconds and maintains functionality over 500 bending cycles.

Naphthalene-diimide selenophene copolymers as efficient solution-processable electron-transporting material for perovskite solar cells

Abstract Three naphthalene-diimide semiconducting polymers (NDI-Se, NDI-BiSe, NDI-TriSe) were synthesized via copolymerization of naphthalene-diimide and selenophene building blocks (selenophene, biselenophene, and triselenophene). The physical, optical, electrochemical and electronic properties of these polymers are characterized by thermogravimetric analysis (TGA), UV–Vis, X-ray powder diffraction (XRD), cyclic voltammetry (CV) and space charge limited current (SCLC). These polymers show excellent thermal stability, high crystallinity, and well matched energy level to that of perovskites. Utilizing these polymers as single and solution processed electron transporting layer (ETL), p-i-n type perovskite solar cells with a configuration of ITO/NiOx/MAPbCl0.2I2.8/ETL/Ag were successfully fabricated with the highest efficiency (14.0%) achieved from NDI-TriSe solar cell, which is comparable to the standard cell (14.6%) using thermally deposited C60/BCP bilayer ETL. Our results demonstrate NDI-selenophene based copolymers are very promising electron transporting materials for highly efficient and solution processable perovskite solar cells.