Understanding the Degradation of Methylenediammonium and Its Role in Phase-Stabilizing Formamidinium Lead Triiodide.

Formamidinium lead triiodide (FAPbI3) is the leading candidate for single-junction metal-halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl2) has been used as an additive in FAPbI3. MDA2+ has been reported as incorporated into the perovskite lattice alongside Cl-. However, the precise function and role of MDA2+ remain uncertain. Here, we grow FAPbI3 single crystals from a solution containing MDACl2 (FAPbI3-M). We demonstrate that FAPbI3-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA2+ is not the direct cause of the enhanced material stability. Instead, MDA2+ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI3 crystals grown from a solution containing HMTA (FAPbI3-H) replicate the enhanced α-phase stability of FAPbI3-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA+ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H+). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H+ is selectively incorporated into the bulk of both FAPbI3-M and FAPbI3-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.

Recent Progress in Colloidal Quantum Dot Thermoelectrics.

Semiconducting colloidal quantum dots (CQDs) represent an emerging class of thermoelectric materials for use in a wide range of future applications. CQDs combine solution processability at low temperatures with the potential for upscalable manufacturing via printing techniques. Moreover, due to their low dimensionality, CQDs exhibit quantum confinement and a high density of grain boundaries, which can be independently exploited to tune the Seebeck coefficient and thermal conductivity, respectively. This unique combination of attractive attributes makes CQDs very promising for application in emerging thermoelectric generator (TEG) technologies operating near room temperature. Herein, recent progress in CQDs for application in emerging thin-film thermoelectrics is reviewed. First, the fundamental concepts of thermoelectricity in nanostructured materials are outlined, followed by an overview of the popular synthetic methods used to produce CQDs with controllable sizes and shapes. Recent strides in CQD-based thermoelectrics are then discussed with emphasis on their application in thin-film TEGs. Finally, the current challenges and future perspectives for further enhancing the performance of CQD-based thermoelectric materials for future applications are discussed.

Bismuth-based mixed-anion compounds for anode materials in rechargeable batteries.

A facile solvothermal synthesis approach for chemical composition control in ternary Bi-S-I systems is reported by simply controlling the sulfide concentration. We demonstrate the application of these bismuth-based ternary mixed-anion compounds as high capacity anode materials in rechargeable batteries. Cells utilising Bi13S18I2 achieved an initial capacity value of 807 mA h g-1, while those with BiSI/Bi13S18I2 a value of 1087 mA h g-1 in lithium-ion battery systems.

A Universal Cosolvent Evaporation Strategy Enables Direct Printing of Perovskite Single Crystals for Optoelectronic Device Applications.

Solution-processed metal halide perovskite (MHP) single crystals (SCs) are in high demand for a growing number of printed electronic applications due to their superior optoelectronic properties compared to polycrystalline thin films. There is an urgent need to make SC fabrication facile, scalable, and compatible with the printed electronic manufacturing infrastructure. Here, a universal cosolvent evaporation (CSE) strategy is presented by which perovskite SCs and arrays are produced directly on substrates via printing and coating methods within minutes at room temperature from drying droplets. The CSE strategy successfully guides the supersaturation via controlled drying of droplets to suppress all crystallization pathways but one, and is shown to produce SCs of a wide variety of 3D, 2D, and mixed-cation/halide perovskites with consistency. This approach works with commonly used precursors and solvents, making it universal. Importantly, the SC consumes the precursor in the droplet, which enables the large-scale fabrication of SC arrays with minimal residue. Direct on-chip fabrication of 3D and 2D perovskite photodetector devices with outstanding performance is demonstrated. The approach shows that any MHP SC can now be manufactured on substrates using precision printing and scalable, high-throughput coating methods.

14 GHz Schottky Diodes Using a p-Doped Organic Polymer.

The low carrier mobility of organic semiconductors and the high parasitic resistance and capacitance often encountered in conventional organic Schottky diodes hinder their deployment in emerging radio frequency (RF) electronics. Here, these limitations are overcome by combining self-aligned asymmetric nanogap electrodes (≈25 nm) produced by adhesion lithography, with a high mobility organic semiconductor, and RF Schottky diodes able to operate in the 5G frequency spectrum are demonstrated. C16 IDT-BT is used, as the high hole mobility polymer, and the impact of p-doping on the diode performance is studied. Pristine C16 IDT-BT-based diodes exhibit maximum intrinsic and extrinsic cutoff frequencies (fC ) of >100 and 6 GHz, respectively. This extraordinary performance is attributed to the planar nature of the nanogap channel and the diode's small junction capacitance (<2 pF). Doping of C16 IDT-BT with the molecular p-dopant C60 F48 improves the diode's performance further by reducing the series resistance resulting to intrinsic and extrinsic fC of >100 and ≈14 GHz respectively, while the DC output voltage of an RF rectifier circuit increases by a tenfold. Our work highlights the importance of the planar nanogap architecture and paves the way for the use of organic Schottky diodes in large-area RF electronics of the future.

Ruddlesden-Popper-Phase Hybrid Halide Perovskite/Small-Molecule Organic Blend Memory Transistors.

Controlling the morphology of metal halide perovskite layers during processing is critical for the manufacturing of optoelectronics. Here, a strategy to control the microstructure of solution-processed layered Ruddlesden-Popper-phase perovskite films based on phenethylammonium lead bromide ((PEA)2 PbBr4 ) is reported. The method relies on the addition of the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene (C8 -BTBT) into the perovskite formulation, where it facilitates the formation of large, near-single-crystalline-quality platelet-like (PEA)2 PbBr4 domains overlaid by a ≈5-nm-thin C8 -BTBT layer. Transistors with (PEA)2 PbBr4 /C8 -BTBT channels exhibit an unexpectedly large hysteresis window between forward and return bias sweeps. Material and device analysis combined with theoretical calculations suggest that the C8 -BTBT-rich phase acts as the hole-transporting channel, while the quantum wells in (PEA)2 PbBr4 act as the charge storage element where carriers from the channel are injected, stored, or extracted via tunneling. When tested as a non-volatile memory, the devices exhibit a record memory window (>180 V), a high erase/write channel current ratio (104 ), good data retention, and high endurance (>104 cycles). The results here highlight a new memory device concept for application in large-area electronics, while the growth technique can potentially be exploited for the development of other optoelectronic devices including solar cells, photodetectors, and light-emitting diodes.

Metal Halide Perovskites for High-Energy Radiation Detection.

Metal halide perovskites (MHPs) have emerged as a frontrunner semiconductor technology for application in third generation photovoltaics while simultaneously making significant strides in other areas of optoelectronics. Photodetectors are one of the latest additions in an expanding list of applications of this fascinating family of materials. The extensive range of possible inorganic and hybrid perovskites coupled with their processing versatility and ability to convert external stimuli into easily measurable optical/electrical signals makes them an auspicious sensing element even for the high-energy domain of the electromagnetic spectrum. Key to this is the ability of MHPs to accommodate heavy elements while being able to form large, high-quality crystals and polycrystalline layers, making them one of the most promising emerging X-ray and γ-ray detector technologies. Here, the fundamental principles of high-energy radiation detection are reviewed with emphasis on recent progress in the emerging and fascinating field of metal halide perovskite-based X-ray and γ-ray detectors. The review starts with a discussion of the basic principles of high-energy radiation detection with focus on key performance metrics followed by a comprehensive summary of the recent progress in the field of perovskite-based detectors. The article concludes with a discussion of the remaining challenges and future perspectives.

Rapid Photonic Processing of High-Electron-Mobility PbS Colloidal Quantum Dot Transistors.

Recent advances in solution-processable semiconducting colloidal quantum dots (CQDs) have enabled their use in a range of (opto)electronic devices. In most of these studies, device fabrication relied almost exclusively on thermal annealing to remove organic residues and enhance inter-CQD electronic coupling. Despite its widespread use, however, thermal annealing is a lengthy process, while its effectiveness to eliminate organic residues remains limited. Here, we exploit the use of xenon flash lamp sintering to post-treat solution-deposited layers of lead sulfide (PbS) CQDs and their application in n-channel thin-film transistors (TFTs). The process is simple, fast, and highly scalable and allows for efficient removal of organic residues while preserving both quantum confinement and high channel current modulation. Bottom-gate, top-contact PbS CQD TFTs incorporating SiO2 as the gate dielectric exhibit a maximum electron mobility of 0.2 cm2 V-1 s-1, a value higher than that of control transistors (≈10-2 cm2 V-1 s-1) processed via thermal annealing for 30 min at 120 °C. Replacing SiO2 with a polymeric dielectric improves the transistor's channel interface, leading to a significant increase in electron mobility to 3.7 cm2 V-1 s-1. The present work highlights the potential of flash lamp annealing as a promising method for the rapid manufacture of PbS CQD-based (opto)electronic devices and circuits.

On the Role of Contact Resistance and Electrode Modification in Organic Electrochemical Transistors.

Contact resistance is renowned for its unfavorable impact on transistor performance. Despite its notoriety, the nature of contact resistance in organic electrochemical transistors (OECTs) remains unclear. Here, by investigating the role of contact resistance in n-type OECTs, the first demonstration of source/drain-electrode surface modification for achieving state-of-the-art n-type OECTs is reported. Specifically, thiol-based self-assembled monolayers (SAMs), 4-methylbenzenethiol (MBT) and pentafluorobenzenethiol (PFBT), are used to investigate contact resistance in n-type accumulation-mode OECTs made from the hydrophilic copolymer P-90, where the deliberate functionalization of the gold source/drain electrodes decreases and increases the energetic mismatch at the electrode/semiconductor interface, respectively. Although MBT treatment is found to increase the transconductance three-fold, contact resistance is not found to be the dominant factor governing OECT performance. Additional morphology and surface energy investigations show that increased performance comes from SAM-enhanced source/drain electrode surface energy, which improves wetting, semiconductor/metal interface quality, and semiconductor morphology at the electrode and channel. Overall, contact resistance in n-type OECTs is investigated, whilst identifying source/drain electrode treatment as a useful device engineering strategy for achieving state of the art n-type OECTs.

One-Dimensional Porphyrin-Fullerene (C60 ) Assemblies: Role of Central Metal Ion in Enhancing Ambipolar Mobility.

One-dimensional (1D) nanostructures of π-conjugated molecules exhibiting excellent charge carrier mobilities have attracted much interest for use in organic electronic devices. Although it is tedious to form such structures, the availability of highly delocalized electron and hole carriers in these donor (D)-acceptor (A) coassemblies realize ambipolar charge transport. Here we demonstrate the use of a simple solution casting method to create an ambipolar donor-acceptor single-crystalline assembly. 1D assemblies of 5,10,15,20-tetraphenylporphyrins (H2 TPP, ZnTPP) and fullerene (C60 ) exhibit high ambipolar mobility in the range of 0.8-3.4 cm2 Vs-1 for electrons and holes with high ON/OFF ratio and low threshold voltage. A direct experimental proof for the pivotal role of the central Zn2+ in tetraphenyl porphyrin, which enables a strong D-A charge transfer interaction in the cocrystal and thereby induces electron (1.35 cm2 Vs-1 ), hole (3.42 cm2 Vs-1 ) mobilities, the highest reported for two component D-A assemblies using solution casting, is demonstrated.

A Supramolecular Nanofiber-Based Passive Memory Device for Remembering Past Humidity.

Memorizing the magnitude of a physical parameter such as relative humidity in a consignment may be useful for maintaining recommended conditions over a period of time. In relation to cost and energy considerations, it is important that the memorizing device works in the unpowered passive state. In this article, we report the fabrication of a humidity-responsive device that can memorize the humidity condition it had experienced while being unpowered. The device makes use of supramolecular nanofibers obtained from the self-assembly of donor-acceptor (D-A) molecules, coronene tetracarboxylate salt (CS) and dodecyl methyl viologen (DMV), respectively, from aqueous medium. The fibers, while being highly sensitive to humidity, tend to develop electrically induced disorder under constant voltage, leading to increased resistance with time. The conducting state can be regained via self-assembly by exposing the device to humidity in the absence of applied voltage, the extent of recovery depending on the magnitude of the humidity applied under no bias. This nature of the fibers has been exploited in reading the humidity memory state, which interestingly is independent of the lapsed time since the humidity exposure as well as the duration of exposure. Importantly, the device is capable of differentiating the profiles of varying humidity conditions from its memory. The device finds use in applications requiring stringent condition monitoring.

Effects of Dielectric Material, HMDS Layer, and Channel Length on the Performance of the Perylenediimide-Based Organic Field-Effect Transistors.

It is well-known that the improvement in the performance of organic field-effect transistors (OFETs) relies primarily on growth properties of organic molecules on gate dielectrics, their interface behavior, and on understanding the physical processes occurring during device operation. In this work, the relation of varying the dielectric materials in an n-type OFET device based on 1,7-dibromo-N,N'-dioctadecyl-3,4,9,10-perylenetetracarboxylic diimide (Br2PTCDI-C18) molecule on a low-cost glass substrate at different channel lengths is reported, which is conceptually very important and fundamental in the context of device performance. Anodized alumina (Al2O3) along with dielectric films of polyvinyl alcohol (PVA) or polymethylmethacrylate (PMMA) was used to fabricate the devices and study their influence on various transistor properties. In addition, the effects of a thin hexamethyldisilazane (HMDS) layer on the performance of OFETs including their contact resistances were studied with the channel length variations. The devices with PVA dielectric material exhibited the maximum mobility values of 0.012-0.025 cm2 V-1 s-1 irrespective of varying channel lengths from 25 to 190 µm. The bias-stress measurements were recorded to realize the effects of the channel length and HMDS layer on the stability of the devices. The on/off ratios and electrical stabilities of these devices were enhanced significantly by modifying the surface of the PVA dielectric layer using a thin layer of HMDS. Similarly, in the case of PMMA dielectric layer, a drastic enhancement in the on/off ratio and bias-stress stability was observed. Characterization of all devices at different channel lengths using different dielectric materials permitted us to identify the effects of contact resistance on OFET devices. The stability of the devices in relation to the bias-stress measurements of devices by varying channel lengths and surface modification was systematically investigated. A careful analysis of oxide gate dielectrics modified with polymer-based dielectric materials, contact resistance, influence of thin HMDS layer on the electrical properties, and other parameters on top-contact bottom-gated configured n-type OFET devices is presented herein.

Enhanced environmental stability induced by effective polarization of a polar dielectric layer in a trilayer dielectric system of organic field-effect transistors: a quantitative study.

We report a concept fabrication method that helps to improve the performance and stability of copper phthalocyanine (CuPc) based organic field-effect transistors (OFETs) in ambient. The devices were fabricated using a trilayer dielectric system that contains a bilayer polymer dielectrics consisting of a hydrophobic thin layer of poly(methyl methacrylate) (PMMA) on poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) or polystyrene (PS) with Al2O3 as a third layer. We have explored the peculiarities in the device performance (i.e., superior performance under ambient humidity), which are caused due to the polarization of dipoles residing in the polar dielectric material. The anomalous behavior of the bias-stress measured under vacuum has been explained successfully by a stretched exponential function modified by adding a time dependent dipole polarization term. The OFET with a dielectric layer of PVA or PVP containing hydroxyl groups has shown enhanced characteristics and remains highly stable without any degradation even after 300 days in ambient with three times enhancement in carrier mobility (0.015 cm(2)·V(-1)·s(-1)) compared to vacuum. This has been attributed to the enhanced polarization of hydroxyl groups in the presence of absorbed water molecules at the CuPc/PMMA interface. In addition, a model has been proposed based on the polarization of hydroxyl groups to explain the enhanced stability in these devices. We believe that this general method using a trilayer dielectric system can be extended to fabricate other OFETs with materials that are known to show high performances under vacuum but degrade under ambient conditions.

Local diffusion induced roughening in cobalt phthalocyanine thin film growth.

We have studied the kinetic roughening in the growth of cobalt phthalocyanine (CoPc) thin films grown on SiO2/Si(001) surfaces as a function of the deposition time and the growth temperature using atomic force microscopy (AFM). We have observed that the growth exhibits the formation of irregular islands, which grow laterally as well as vertically with coverage of CoPc molecules, resulting rough film formation. Our analysis further disclosed that such formation is due to an instability in the growth induced by local diffusion of the molecules following an anomalous scaling behavior. The instability relates the (ln(t))(1/2), with t as deposition time, dependence of the local surface slope as described in nonequilibrium film growth. The roughening has been characterized by calculating different scaling exponents α, ß, and 1/z determined from the height fluctuations obtained from AFM images. We obtained an average roughness exponent α = 0.78 ± 0.04. The interface width (W) increases following a power law as W ∼ t(ß), with growth exponent ß = 0.37 ± 0.05 and lateral correlation length (ξ) grows as ξ ∼ t(1/z) with dynamic exponent 1/z = 0.23 ± 0.06. The exponents revealed that the growth belongs to a different class of universality.