Solution Processing Techniques Research Articles

Solvent Engineering for Controlled Crystallization and Growth of All-Inorganic Pb-Free Rudorffite Absorbers of Perovskite Solar Cells.

Solution-processed Ag-Bi-I rudorffites with direct band gaps of <2 eV show promise for highly efficient and cost-effective Pb-free solar cells. However, relatively fast crystallization rates of Bi-based films and limited solubility of BiI3 in many solvents result in poor film morphologies, inhibiting their device performance. Here, we conduct a solvent-engineering method to adjust the dynamics of nucleation and growth during film formation. We fabricate Ag2BiI5, AgBiI4, and AgBi2I7 absorber layers using a mixed solvent of dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) and find that a volume percentage of 50% DMSO causes highly uniform and dense perovskite films via a BiI3-DMSO-AgI intermediate phase formation, leading to solar cells with an improved power conversion efficiency of 0.62% for the Ag2BiI5 absorber. These results provide valuable insights into the optimization of the solution processing technique to realize low-toxicity and efficient perovskite solar cells.

A Rapidly Stabilizing Water-Gated Field-Effect Transistor Based on Printed Single-Walled Carbon Nanotubes for Biosensing Applications.

Biosensors are expected to revolutionize disease management through provision of low-cost diagnostic platforms for molecular and pathogenic detection with high sensitivity and short response time. In this context, there has been an ever-increasing interest in using electrolyte-gated field-effect transistors (EG-FETs) for biosensing applications owing to their expanding potential of being employed for label-free detection of a broad range of biomarkers with high selectivity and sensitivity while operating at sub-volt working potentials. Although organic semiconductors have been widely utilized as the channel in EG-FETs, primarily due to their compatibility with cost-effective low-temperature solution-processing fabrication techniques, alternative carbon-based platforms have the potential to provide similar advantages with improved electronic performances. Here, we propose the use of inkjet-printed polymer-wrapped monochiral single-walled carbon nanotubes (s-SWCNTs) for the channel of EG-FETs in an aqueous environment. In particular, we show that our EG-CNTFETs require only an hour of stabilization before producing a highly stable response suitable for biosensing, with a drastic time reduction with respect to the most exploited organic semiconductor for biosensors. As a proof-of-principle, we successfully employed our water-gated device to detect the well-known biotin–streptavidin binding event.

Energy-Level Manipulation in Novel Indacenodithiophene-Based Donor–Acceptor Polymers for Near-Infrared Organic Photodetectors

Organic photodetectors (OPDs) are promising candidates for next-generation digital imaging and wearable sensors due to their low cost, tuneable optoelectrical properties combined with high-level performance, and solution-processed fabrication techniques. However, OPD detection is often limited to shorter wavelengths, whereas photodetection in the near-infrared (NIR) region is increasingly being required for wearable electronics and medical device applications. NIR sensing suffers from low responsivity and high dark currents. A common approach to enhance NIR photon detection is lowering the optical band gap via donor-acceptor (D-A) molecular engineering. Herein, we present the synthesis of two novel indacenodithiophene (IDT)-based D-A conjugated polymers, namely, PDPPy-IT and PSNT-IT via palladium-catalyzed Stille coupling reactions. These novel polymers exhibit optical band gaps of 1.81 and 1.27 eV for PDPPy-IT and PSNT-IT, respectively, with highly desirable visible and NIR light detection through energy-level manipulation. Moreover, excellent materials' solubility and thin-film processability allow easy incorporation of these polymers as an active layer into OPDs for light detection. In the case of PSNT-IT devices, a photodetection up to 1000 nm is demonstrated with a peak sensitivity centered at 875 nm, whereas PDPPy-IT devices are efficient in detecting the visible spectrum with the highest sensitivity at 660 nm. Overall, both OPDs exhibit spectral responsivities up to 0.11 A W-1 and dark currents in the nA cm-2 range. With linear dynamic ranges exceeding 140 dB and fast response times recorded below 100 μs, the use of novel IDT-based polymers in OPDs shows great potential for wearable optoelectronics.

Molecular Design of Luminescent Gold(III) Emitters as Thermally Evaporable and Solution-Processable Organic Light-Emitting Device (OLED) Materials.

The advancement of high-efficiency luminescent and thermally stable organometallic complexes has offered opportunities for the commercialization of metal phosphors for fabricating organic light-emitting devices (OLEDs). Since the first report on the potential use of iridium(III) and platinum(II) complexes for applications in OLEDs in the late 1990s, extensive efforts have been made by researchers on the development of various heavy metal-containing compounds with rich photophysical and luminescence properties and the engineering of device architectures to improve device efficiencies. Apart from the more well-studied iridium(III) and platinum(II) complexes, complexes of gold(III) recently have demonstrated their capabilities to serve as phosphorescent or thermally stimulated delayed phosphorescent or thermally activated delayed fluorescent emitters, and their promising performances in OLEDs have attracted growing interest in the past decade. Nowadays, complexes of gold(III) with emission energies ranging from sky-blue to near-infrared with high electroluminescence performances have been obtained. In addition, high-efficiency vacuum-deposited and solution-processed OLEDs with benchmark efficiencies comparable to those of the iridium(III) and platinum(II) complexes have been realized. This Focus Review summarizes the development of various series of luminescent gold(III) complexes to date and highlights important milestones in the development and advancement of gold(III)-based OLEDs. Focus will be made on the molecular design strategies for gold(III) emitters for application as dopants in OLEDs, including those fabricated by vacuum-deposition and solution-processing techniques.

Ultrasensitive self-powered UV photodetector based on a novel p-n heterojunction of solution-processable organic semiconductors

This work reports a highly sensitive self-powered ultraviolet photodetector, which is based on the active layer of NPD:Alq3 with different Alq3 contents. All solution-processed technique was utilized to fabricate the active layers of the devices to achieve an optimized system that exhibits highly sensitive and stable photodetector under self-powered operation mode. The values of photosensitivity, responsivity, and detectivity of the photodiodes at zero-bias were estimated at 1.30 × 105, 1.07 mA/W, 1.04 × 1011 Jones, respectively. The rise and decay times of the optimized device were estimated at 0.34 s and 0.28 s, respectively. For practical application of the device sensing efficiency, the optimal load line of self-powered UV photodetector to deliver the maximum power was estimated of an internal impedence of 0.579 MΩ. In conclusion, the high sensitivity, good stability, and fast response/recovery speed of the self-powered UV photodetector make the devices applicable to resolve some of the current issues in UV detection.

Photo-patterned oxide films produced using polymeric metal acrylate for low-voltage thin-film transistors

Solution-processed zinc oxide (ZnO) thin-film transistors (TFTs) have received considerable attention as key elements for future electronics due to the ease of fabricating these TFTs and due to the natural abundance and hence low cost of ZnO. However, issues regarding the patterning of solution-processed ZnO semiconductor films unfortunately continue to pose a barrier to their commercialization. In this study, we optimized nanometer-thick photo-curable zinc acrylate (ZnA) films as ZnO precursors and used these precursors to prepare photo-patterned ZnO semiconductors for the fabrication of n-type TFTs. Solution-processed ZnA films were effectively micro-patterned under UV light, because UV-irradiated ZnA film was not solved in etching solvent as a result of photocrosslinking of diacrylate moiety. Through a sol-gel reaction, wurtzite ZnO crystals were stably formed from the photo-patterned ZnA films. After optimizing the film thickness and by using a photo-patterned ZrOx dielectric layer, the prepared ZnO TFT exhibited stable performances with a low-voltage operation within ±2 V. This approach for developing a ZnO semiconductor film can be used to enable the realization of simple, inexpensive, and environmentally friendly solution-processing techniques in electronic device fields.

Flexible and Transparent Agnws-Based Microelectrode Arrays and Interconnects for Multifunctional Electrical and Optical Biointerfacing

Transparent microelectrode provides a promising approach combining electrophysiology and optogenetics for multi-functional biointerfacing. Flexible platforms with high performance which can achieve seamlessly integrating with soft tissue systems are highly needed. Herein, we demonstrate silver nanowires (Ag NWs)-based transparent microelectrode arrays (MEAs) and interconnects to meet this demand by using an innovative photolithography-based solution-processing technique. The resulting microelectrodes exhibit a high transparency of 76.1-90.0% over the visible spectrum, and low normalized electrochemical impedance of 3.4-15 Ω cm2 at 1 kHz which is comparable to that of opaque solid Ag films (12.4 Ω cm2). 93% transparency at 550 nm can be achieved through patterning the nanowire networks. Additionally, the microelectrodes possess excellent sheet resistance of 11-25 Ω sq−1, superior mechanical stability for 100,000 bending cycles at 5 mm radius, and chemical stability after 30-day soaking at 37 oC. In vivo histology study reveals the biocompatibility of the device. Studies on rat and transgenic mouse hearts show the ability of Ag NWs MEAs to optical mapping with negligible light-induced artifacts and real-time monitoring of heart rhythm under simultaneously optogenetic pacing with high-fidelity, respectively. This work illustrates that the Ag NWs-based platform can serve as a promising candidate for the next-generation of large-scale multi-functional biointerfaces for biological and biomedical research. Figure 1

Facile Synthesis and Characterizations of Silver Nanoparticles-Reduced Graphene Oxide Hybrid

Here, we report the synthesis of silver nanoparticles-reduced graphene oxide (AgNPs-rGO) hybrid for simple and eco-friendly method. Silver nanoparticles (AgNPs) were successfully deposited on reduced graphene oxide (rGO) sheets to form (AgNPs-rGO) hybrid using lemon extract as a reducing and stabilizing agent. The products form a stable aqueous solution without any surfactant stabilizers and this makes it possible to produce (AgNPs-rGO) hybrid on a large scale using low-cost solution processing technique. The synthesis of nanohybrid was examed at different ratio of reducing agent (1:1, 1:2, 1:4) and characterized using UV-Visible (UV-Vis) absorption spectrum, X-ray diffraction (XRD), Raman spectroscopy analyses and transmission electron microscopy (TEM) analyses. From UV-Vis absorption spectrum, the (AgNPs-rGO) (1:1) hybrid result shows the sharp peak at 433 nm indicating the accomplishment formation of AgNPs on the surface of rGO sheets. Crystalline and spherical AgNPs with an average particle size of 21 nm were found in the (AgNPs-rGO) hybrid with the assistance of 1:1 reducing agent. Furthermore, (AgNPs-rGO) (1:1) hybrid exhibit fast electron-transfer kinetics for electrochemical reaction of Fe (CN)63-/4- redox couple compared to other controlled modified electrodes, suggesting the potential applications for electrocatalysis and electrochemical sensor.

Achieving over 15% Efficiency in Solution-Processed Cu(In,Ga)(S,Se)2 Thin-Film Solar Cells via a Heterogeneous-Formation-Induced Benign p-n Junction Interface.

Cu(In,Ga)(S,Se)2 (CIGS) thin-film solar cells have attracted considerable interest in the field of photovoltaic devices due to their high efficiency and great potential for diverse applications. While CdS has been the most favorable n-type semiconductor because of its excellent lattice-match and electronic band alignment with p-type CIGS, its narrow optical band gap (∼2.4 eV) has limited light absorption in underlying CIGS absorber films. Reducing the thickness of CdS films to increase the short-circuit current-density has been less effective due to the following decrease in the open-circuit voltage. To overcome this trade-off between the main parameters, we controlled the formation mechanism of CdS films in chemical bath deposition and established its direct correlation with the properties of p-n junctions. Interestingly, a heterogeneous CdS film formation was found to have a synergetic effect with its ammonia bath solution, effectively reducing charge carrier loss from the shunt paths and interface recombination of CIGS/CdS junctions. With these electrical benefits, the trade-off was successfully alleviated and our best device achieved a power conversion efficiency of 15.6%, which is one of the state-of-the-art CIGS thin-film solar cells prepared using solution-processing techniques.

Configuring device architecture with new solution-processable host for high performance low color-temperature OLEDs with ultra-low driving voltage

After the revelation of nonvisual lighting impact on human health, the lighting engineers are more concerned about the human benign light with low cost and high efficiency. The solution-processed fabrication technique with smart device engineering for high efficacy OLED devices is being anticipated to drastically reduce the fabrication cost leading to affordable desired end product. The co-host matrix could be a potential solution to improve device performance multi-folds with suitable band-gap engineering and most effective energy transfer from mixed host to guest. Here, rationally configured device architecture with two novel host materials possessing wide energy gap, high triplet energy, and excellent thermal and morphological stability resulted in highly-efficient solution-processed green, red and low color temperature (CT) OLEDs with sub-bandgap level driving voltage of 2.1 V i.e. record lowest within its own category. Perfect triplet energy match of our newly developed host materials with commercial p-type host m-MTDATA and the common phosphors enabled efficient energy transfer with very low energy loss led to high efficiency of resulting OLED. The resultant solution-processed triplet emitter based green and red OLED devices displayed a maximum efficacy of 75.0 and 30.2 lm W−1 without using light extraction out-coupling techniques, respectively. The designed blue-hazard free 2048 K low CT OLED exhibited a ηPE of 44.5 lmW−1, a ηCE of 47.6 cdA-1 and an EQE of 20.7% as the maximum value, the highest known so far within its own category. The melatonin suppression sensitivity (MSS) of the engineered low CT OLED is only 1.0% to that of the 480 nm blue light, which is much safer compared to other light sources. The impact of our design engineering was established by fabricating 1 cm × 1 cm device area prototype.

Present Status of Solution‐Processing Routes for Cu(In,Ga)(S,Se)2 Solar Cell Absorbers

AbstractPhotovoltaic technologies offer a sustainable solution to the challenge of meeting increasing energy demands. Chalcopyrite Cu(In,Ga)(S,Se)2, short CIGS—thin‐film solar cells—having intrinsically p‐type absorbers with a tunable direct bandgap—exhibits one of the highest stabilized power conversion efficiencies of 23.35%, utilizing absorbers typically fabricated via vacuum deposition methods. Research is increasingly devoted to absorbers deposited by solution processing techniques, which may inherently improve material usage, increase throughput, and lower financial barriers to commercialization. However, the performance of current devices with solution‐processed absorbers is still falling short of their vacuum‐processed counterparts with record power conversion efficiencies up to 18.7% reported to date. While hydrazine solvent‐based routes offer reduced residual impurities, their toxicity poses hindrances to widespread adoption. Alternatively, less toxic and environmentally friendly routes based on protic and aprotic solvents are being researched and are showing promising device efficiencies well above 14%. This review describes the current status of CIGS solar cell absorber layers fabricated by pure solution‐based deposition methods, provides a comparison of champion solution‐processed devices (with and without hydrazine), and offers an outlook for future improvements.

Solution-processed electronics for artificial synapses.

Artificial synaptic devices and systems have become hot topics due to parallel computing, high plasticity, integration of storage, and processing to meet the challenges of the traditional Von Neumann computers. Currently, two-terminal memristors and three-terminal transistors have been mainly developed for high-density storage with high switching speed and high reliability because of the adjustable resistivity, controllable ion migration, and abundant choices of functional materials and fabrication processes. To achieve the low-cost, large-scale, and easy-process fabrication, solution-processed techniques have been extensively employed to develop synaptic electronics towards flexible and highly integrated three-dimensional (3D) neural networks. Herein, we have summarized and discussed solution-processed techniques in the fabrication of two-terminal memristors and three-terminal transistors for the application of artificial synaptic electronics mainly reported in the recent five years from the view of fabrication processes, functional materials, electronic operating mechanisms, and system applications. Furthermore, the challenges and prospects were discussed in depth to promote solution-processed techniques in the future development of artificial synapse with high performance and high integration.

Inkjet Printing of an Electron Injection Layer: New Role of Cesium Carbonate Interlayer in Polymer OLEDs.

Among solution-processable techniques, inkjet printing is a potential method for manufacturing low-cost and high-resolution polymer organic light-emitting diodes (PLEDs) for displays/solid-state lighting applications. Herein, we demonstrate use of the inkjet printed cesium carbonate (Cs2CO3) film as an electron injection interlayer. We have elaborated the Cs2CO3 ink using an alcohol-based solvent for the industrial-grade printhead. The printed Cs2CO3 layer morphology was investigated by means of an optical microscope and an atomic force microscope. The PLEDs based on emissive polymer (Super Yellow) with printed Cs2CO3 interlayer show a remarkable current efficiency and luminance compared to the PLEDs made without the Cs2CO3 layer. Such results suggest that the Cs2CO3 is a promising material for the formulation of the electron injecting inkjet inks. The possibility of inkjet printing of an efficient electron injecting layer enables in situ patterning of PLEDs’ emission area. Such a simple and flexible technique can be applied for a wide range of applications such as signage, pictograms, advertising, smart packaging, etc.

Vacuum-free fabrication of organic solar cell on assembled glass substrates

Vacuum-free fabrication is essential to realize large-scale production of organic solar cells. Blade-coating and printing are important solution-processing techniques to realize this idea. Preparation devices on large substrates is important for mass production; however, it is not easy to handle large substrates during fabrication procedures. In this work, we demonstrate a fabrication method for all-solution vacuum-free organic solar cells on an assembled glass substrates. Small pieces of glasses were assembled into a mosaic of larger total size for device fabrication. A gap-prefilling method was also developed to realize uniform thickness distribution. For the device with blade-coated ZnO electron transport layer, blade-coated active layer, spray-coated hole transport layer and spray-coated top electrode, power conversion efficiency of 4.19% is obtained. This showed that, by combining spray and blade coating, ultimate scale of the vacuum-free fabrication of OPV on glass substrate will not be limited by the substrate size itself, and can be scaled up to multi-meter by assembling the glasses into a mosaic. A method for all-solution vacuum-free organic solar cells on an assembled glass substrates were developed. Devices fabricated by blade-coating and spray-coating with power conversion efficiency of 4.19% was obtained. • ‧A technique for all-solution vacuum-free organic solar cells on an assembled glass substrates is presented. • ‧A gap-prefilling method was also developed to realize uniform thickness distribution. • ‧Power conversion efficiency of 4.19% is obtained for the device with blade-coated PEDOT:PSS, blade-coated active layer, spray-coated hole transport layer and spray-coated top electrode.

Enhanced Nanostructured ZnO-Based Photocatalyst Immobilized by Ink-Jet Printing for Methylene Blue Degradation

Exploring facile fabrication methods for stable, ordered-structure photocatalysts with high performance is critical to realize their practical applications. In this work, a ZnO-based photocatalyst was deposited using a solution-processing ink-jet printing technique. Novel water-based inks were developed with pure ZnO and Mg-doped ZnO nanoparticles, which were synthesized by the polyacrylamide gel method and then modified by polyethylene glycol. The influence of organic solvents, surfactants, and polymers on the surface tension and viscosity of the inks was investigated using an orthogonal experimental design. The printed films annealed at low temperatures exhibited low resistance and high photocatalytic performance for the degradation of methylene blue. These results indicated that ink-jet printed Mg-doped ZnO photocatalyst thin film is promising as a feasible candidate for treating organic pollutants in waste water.

Prioritization of Cartagena Coastal Military Batteries to Transform Them into Scientific, Tourist and Cultural Places of Interest: A GIS-MCDM Approach

This study presents a combination of multi-criteria decision-making (MCDM) methodologies with geographic information systems (GIS) to carry out a prioritization of obsolete military coastal batteries with the aim of transforming them into touristic, scientific, and cultural places of interest. The study area is located in the Municipality of Cartagena, in Southeast Spain. Such a prioritization requires taking into account transport criteria (distance to roads or train stations), infrastructure criteria (distance to electrical grids or distance to water tanks), touristic or scientific criteria (distance to towns, beaches, archaeological sites, assets of cultural interest, etc.), and orography criteria (area, altitude, and slope of each battery). Therefore, this decision problem involves a set of alternatives (coastal military batteries) to be prioritized based on a group of criteria that should be considered. To tackle this, GIS software is used to provide the attribute table of alternatives and criteria (decision matrix), and the proposed decision problem is solved through a combination of MCDM methodologies based on the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) and Analytic Hierarchy Process (AHP) techniques. The AHP approach is applied to determine the weights of the criteria whilst the TOPSIS method provides a ranking of alternatives in order to obtain a prioritization.

Molecular Dynamics Study of Ionomer Aggregate Structure during Solvent Evaporation

Cathode catalyst layer (CL) is one of the principal components for polymer electrolyte fuel cells. CL is typically prepared by solution processing techniques where a catalyst ink consisting of Pt-C (platinum on a carbon powder support), ionomer, and solvents (water/alcohol mixtures) is exposed to evaporation. The structure and stability of ionomer aggregates in solutions are the key to the physical properties of ionomer thin films. Thus, understanding the ionomer aggregation behaviors in solutions is of great importance to gain a deep insight into fundamental morphological characteristics of ionomers. In our previous studies, we have observed ionomer formation into cylindrical bundle-like aggregates in solutions. The size of ionomer bundles decreases with increasing alcohol content, indicating that the ionomers tend to be more dispersed at higher alcohol content, which is in good agreement with the trend inferred from the scattering experiments. In this study, evolution of ionomer aggregate structure in water solutions have been investigated using solvent evaporation simulations based on the course-grained molecular dynamics method, as shown in FIgure 1. The balance between evaporation, sedimentation, and diffusion was controlled by the dimensionless Peclet number, providing a link between experiments and simulations. The effects of substrate wettability on the ionomer thin film structure will be discussed in the presentation. Figure 1

Crystallization and Stability of Flexible P-Type Semiconductors and Their Thin Film Transistor Application

In recent years, electronic devices that require superior mechanical flexibility have been extremely sought for potential applications in next-generation displays, sensors, bio-implantable and disposable medical devices. Polymeric materials, of which the flexibility far exceeds that of conventional brittle inorganic counterparts, are widely considered for various flexible devices due to their excellent stretchable, foldable and bendable characteristics.Oxidative chemical vapor deposition (oCVD) is an emerging technique that provides not only a capability to process mechanically flexible polymeric films but also strategies to address current challenges claimed in typical solution-based polymer processing1-2. The oCVD technique allows for greater uniformity, excellent conformality, enhanced performance (e.g., conductivity) and an ability of large area polymer processing that are limited in conventional solution-processed polymers3-4.Crystallization and air stability of organic materials have been issues that have to be mitigated to warrant high performance and sustainable device applications. The oCVD technique, in addition to the advantages described above, could also address the issues of crystallization and air stability to enhance the carrier transport and overall conductivity, and to secure sustainability. We recently found that an in-situ oCVD capability of substrate heating during depositions resulted in an substantial increase in ordered structures in conjugated polymers, leading to enhanced crystallization of the resulting oCVD polymers5. Consequently, the enhancement of the carrier transport and conductivity was obtained due to the increase in favorable conduction paths in the crystalline phase. Further, the vapor phase processing of oCVD may be of crucial relevance to achieve air stability since oCVD does not require any monomer solubility in specific solvents and, therefore, is able to polymerize insoluble materials6 that are not available for conventional solution processing techniques such as spin-coating or dip-casting. The reactivity of insoluble monomers is quite limited due to their rigid backbone structures; however, this limited reactivity is the key to achieve air stability since the reactions between insoluble materials and water (H2O)/oxygen, which are the main origins to cause instabilities of organic materials in air, are significantly prohibited.In this presentation, we report our recent findings of enhanced crystallization and air-stability of oCVD polymers. Particularly, the structure, chemical and electrical properties of two oCVD polymers, poly(3,4-ethylenedioxythiophene) (i.e., PEDOT) and unsubstituted polythiophene, will be compared with focuses on the crystalline state, dope-ability and air-stability. Further, thin film transistors (TFTs) that employ the two oCVD polymers as TFT channel are presented and the device performance of output and transfer characteristics will be compared with focuses on the carrier mobility and on/off switching properties. Acknowledgement: This work was supported by Purdue University and partly by an NSF grant, Award No. ECCS-193108. References Lock, J. P.; Im, S. G.; Gleason, K. K., Oxidative chemical vapor deposition of electrically conducting poly(3,4-ethylenedioxythiophene) films. Macromolecules 2006, 39 (16), 5326-5329.Im, S. G.; Gleason, K. K., Systematic control of the electrical conductivity of poly(3,4-ethylenedioxythiophene) via oxidative chemical vapor deposition. Macromolecules 2007, 40 (18), 6552-6556.Coclite, A. M.; Howden, R. M.; Borrelli, D. C.; Petruczok, C. D.; Yang, R.; Yaguee, J. L.; Ugur, A.; Chen, N.; Lee, S.; Jo, W. J.; Liu, A.; Wang, X.; Gleason, K. K., 25th Anniversary Article: CVD Polymers: A New Paradigm for Surface Modification and Device Fabrication. Adv. Mater. 2013, 25 (38), 5392-5422.Kovacik, P.; Hierro, G. d.; Livernois, W.; Gleason, K. K., Scale-up of oCVD: large-area conductive polymer thin films for next-generation electronics. Materials Horizons 2015, 2 (2), 221-227.Wang, X.; Zhang, X.; Sun, L.; Lee, D.; Lee, S.; Wang, M.; Zhao, J.; Shao-Horn, Y.; Dincă, M.; Palacios, T.; Gleason, K. K., High electrical conductivity and carrier mobility in oCVD PEDOT thin films by engineered crystallization and acid treatment. Science Advances 2018, 4 (9), eaat5780.Borrelli, D. C.; Lee, S.; Gleason, K. K., Optoelectronic properties of polythiophene thin films and organic TFTs fabricated by oxidative chemical vapor deposition. Journal of Materials Chemistry C 2014, 2 (35), 7223-7231. Figure 1

On the Colloidal Stability of PbS Quantum Dots Capped with Methylammonium Lead Iodide Ligands.

Phase-transfer exchange of pristine organic ligands for inorganic ones is essential for the integration of colloidal quantum dots (CQDs) in optoelectronic devices. This method results in a colloidal dispersion (ink) which can be directly deposited by various solution-processable techniques to fabricate conductive films. For PbS CQDs capped with methylammonium lead iodide ligands (MAPbI3), the most commonly employed solvent is butylamine, which enables only a short-term (hours) colloidal stability and thus brings concerns on the possibility of manufacturing CQD devices on a large scale in a reproducible manner. In this work, we studied the stability of alternative inks in two highly polar solvents which impart long-term colloidal stability of CQDs: propylene carbonate (PC) and 2,6-difluoropyridine (DFP). The aging and the loss of the ink’s stability were monitored with optical, structural, and transport measurements. With these solvents, PbS CQDs capped with MAPbI3 ligands retain colloidal stability for more than 20 months, both in dilute and concentrated dispersions. After 17 months of ink storage, transistors with a maximum linear mobility for electrons of 8.5 × 10–3 cm2/V s are fabricated; this value is 17% of the one obtained with fresh solutions. Our results show that both PC- and DFP-based PbS CQD inks offer the needed shelf life to allow for the development of a CQD device technology.

Controllable deposition of organic metal halide perovskite films with wafer-scale uniformity by single source flash evaporation

Conventional solution-processing techniques such as the spin-coating method have been used successfully to reveal excellent properties of organic–inorganic halide perovskites (OHPs) for optoelectronic devices such as solar cell and light-emitting diode, but it is essential to explore other deposition techniques compatible with large-scale production. Single-source flash evaporation technique, in which a single source of materials of interest is rapidly heated to be deposited in a few seconds, is one of the candidate techniques for large-scale thin film deposition of OHPs. In this work, we investigated the reliability and controllability of the single-source flash evaporation technique for methylammonium lead iodide (MAPbI3) perovskite. In-depth statistical analysis was employed to demonstrate that the MAPbI3 films prepared via the flash evaporation have an ultrasmooth surface and uniform thickness throughout the 4-inch wafer scale. We also show that the thickness and grain size of the MAPbI3 film can be controlled by adjusting the amount of the source and number of deposition steps. Finally, the excellent large-area uniformity of the physical properties of the deposited thin films can be transferred to the uniformity in the device performance of MAPbI3 photodetectors prepared by flash evaporation which exhibited the responsivity of 51 mA/W and detectivity of 9.55 × 1010 Jones.