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This paper assesses the interface stability of the perovskite CsPbBr3 and transport layer CuI using density functional theory and band offset calculations. As a low-cost, more stable alternative to current hole transport materials, CuI may be used to template the epitaxial growth of perovskites such as CsPbBr3 owing to a 1% lattice constant mismatch and larger bulk modulus. We compare all eight atomic terminations of the interfaces between the (100) low-energy facet for both CsPbBr3 and CuI, increasing material thickness to consider charge density redistribution and bonding characteristics between surface and bulk-like regions. A low energy atomic termination is found to exist between these materials where alternating charge accumulation and depletion regions stabilize bonds at the interface. Band offset calculations reveal a type I straddling gap offset in the bulk shifting to a type II staggered gap offset as the thickness of the materials is increased, where the built-in potential changes as layer thickness increases, indicating the tunability of charge separation at the interface. CuI may, thus, be used as an alternative hole transport layer material in CsPbBr3 optoelectronic devices.

Metal halide perovskites (MHPs) have rapidly emerged as leading contenders in photovoltaic technology and other optoelectronic applications owing to their outstanding optoelectronic properties. After a decade of intense research, an in-depth understanding of the charge carrier transport in MHPs is still an active topic of debate. In this Perspective, we discuss the current state of the field by summarizing the most extensively studied carrier transport mechanisms, such as electron-phonon scattering limited dynamics, ferroelectric effects, Rashba-type band splitting, and polaronic transport. We further extensively discuss the emerging experimental and computational evidence for dominant polaronic carrier dynamics in MHPs. Focusing on both small and large polarons, we explore the fundamental aspects of their motion through the lattice, protecting the photogenerated charge carriers from the recombination process. Finally, we outline different physical and chemical approaches considered recently to study and exploit the polaron transport in MHPs.

Controlled and tunable growth of chemically active edge sites over inert in-plane MoS2 flakes is the key requirement to realize their vast number of applications in catalytic activities. Thermodynamically, growth of inert in-plane MoS2 is preferred due to fewer active sites on its surface over the edge sites. Here, we demonstrate controlled and tunable growth from in-plane MoS2 flakes to dense and electrically connected edge-enriched three-dimensional (3D) network of MoS2 flakes by varying the gas flow rate using tube-in-tube chemical vapor deposition technique. Field emission scanning electron microscope results demonstrated that the density of edge-enriched MoS2 flakes increase with increase in the gas flow rate. Raman and transmission electron microscopy analyses clearly revealed that the as-synthesized in-plane and edge-enriched MoS2 flakes are few layers in nature. Atomic force microscopy measurement revealed that the growth of the edge-enriched MoS2 takes place from the in-plane MoS2 flakes. On the ...

Toxic gases are produced during the burning of fossil fuels. Room temperature (RT) fast detection of toxic gases is still challenging. Recently, MoS2 transition metal dichalcogenides have sparked great attention in the research community due to their performance in gas sensing applications. However, MoS2 based gas sensors still suffer from long response and recovery times, especially at RT. Considering this challenge, here, we report photoactivated highly reversible and fast detection of NO2 sensors at room temperature (RT) by using mixed in-plane and edge-enriched p-MoS2 flakes (mixed MoS2). The sensor showed fast response with good sensitivity of ∼10.36% for 10 ppm of NO2 at RT without complete recovery. However, complete recovery was obtained with better sensor performance under UV light illumination at RT. The UV assisted NO2 sensing showed improved performance in terms of fast response and recovery kinetics with enhanced sensitivity to 10 ppm NO2 concentration. The sensor performance is also investig...

Methylammonium lead iodide is a benchmark hybrid organic perovskite material used for low-cost printed solar cells with a power conversion efficiency of over 20%. Nevertheless, the nature of light–matter interaction in hybrid perovskites and the exact physical mechanism underlying device operation are currently debated. Here, we report room temperature, ultrafast photocurrent generation, and free-space terahertz emission from unbiased hybrid perovskites induced by femtosecond light pulses. The polarization dependence of the observed photoresponse is consistent with the bulk photovoltaic effect caused by a combination of injection and shift currents. Observation of this type of photocurrents sheds light on the low recombination and long carrier diffusion lengths arising from the indirect bandgap in CH3NH3PbI3. Naturally ballistic shift and injection photocurrents may enable third-generation perovskite solar cells with efficiency exceeding the Shockley–Queisser limit. The demonstrated control over photocurrents with light polarization also opens new venues toward perovskite spintronics and tunable THz devices. The study provides new insight into the light-matter interactions of organic based perovskites. The authors do so by investigating the hybrid perovskite methylammonium lead iodide, which is considered a benchmark material for solar cells with high power conversion efficiency.

Neutral-π radical based open shell molecules foster new potential in light emitting diodes because of their theoretical near-equity quantum efficiencies. In this study, we report organic light emitting diodes (OLEDs) based on a novel open shell molecule (2,4,6-trichlorophenyl)methyl (TTM) as the electroluminescent layer. The singly occupied molecular orbital (SOMO) level and optical bandgap of TTM was calculated using cyclic voltammetry and UV–visible absorption respectively. Thermogravimetric analysis showed a stable molecule capable of sublimation. Two decidedly different approaches, thermal evaporation and solution processing, were employed to deposit TTM:host blend thin films for OLED device fabrication. OLED devices fabricated via thermal evaporation and solution processing yielded external quantum efficiencies of 2.78% and 0.28% and luminances of 330 cdm -2 and 78 cdm -2 respectively. Further, the effect of doping ratios of the host materials on OLED device performance were investigated and optimal ratios were established. We report for the first time solution processed open shell organic molecules for light emitting diode applications. Our results elucidate the potential for low-cost and high efficiency optoelectronic devices.

The organo-halide lead perovskite solar cells (PSC) provide a cost effective method of acquiring sunlight energy and converting directly into electrical power with high conversion efficiency. However, the chemical stability of the PSC is a significant issue that is limiting practical application and prevents further device development. The device is extremely sensitive to moisture and may degrade during a few days. Recent investigation demonstrated that the materials, architecture and structure of the interface might play a crucial role in degradation rates and durability of the device. A systematic understanding of interfacial layers in stability of perovskite solar cells is essential to achieve highly stable devices for extensive industrial applications. This report is reviewing the role of interface in stability of perovskite solar cells and discussed their future trends and aspects.

The nucleation and growth of organic–inorganic hybrid perovskite films induced by the molar ratio of precursor components and their role in optoelectronic performance are investigated. Lead iodide (PbI2), methylammonium acetate (MAAc) and methylammonium iodide (MAI) mixed in different ratios were used as starting precursors for active layer deposition. While all the as cast films showed the presence of a precursor intermediate (PbIx·MAAc·MAI), the films derived from 1 : 1 : 1 and 1 : 0.5 : 1 ratios also exhibited crystallization of the perovskite (CH3NH3PbI3) phase during the spin coating process. The nucleation and crystallization of the perovskite phase in films derived from the equimolar precursor ratio were found to be the key to growing larger, smoother and single crystalline grains along the thickness leading to higher carrier lifetimes. Perovskite solar cells fabricated utilizing such a morphology showed a much higher power conversion efficiency irrespective of the active layer thickness. Furthermore, methyl ammonium bromide based perovskite light emitting diodes were also fabricated using the aforementioned equimolar precursor ratio which exhibited superior device performance compared to diodes with active layers derived from other precursor ratios. This work highlights a facile deposition technique and growth mechanism to attain reproducible, low cost and high-performance perovskite optoelectronic devices.

In this work, we report the process of building an accurate ellipsometry model from scratch for benchmark hybrid perovskite MAPbI 3 . This model can be effectively utilized for a wide range of film thickness measurement. Optical constants from UV to infrared and bandgap of the film were extracted to implement the model. As developed models were evaluated by comparing the measured thicknesses using other direct measurement techniques such as profilometer, cross sectional SEM and AFM. Considering the level of complexity, time required for the measurement, area under measurement, sample preparation and accuracy, a comparison has been presented among different thickness measurement techniques. The process of ellipsometry model building can be used for other noble film with unknown pattern of optical constant with wavelength.

Despite the recent increase in PPDT2FBT polymer thin film applications for optoelectronic devices, a comprehensive study of this material's optical dispersion properties is unavailable. The optical properties of the PPDT2FBT thin film is investigated using variable-angle spectroscopic ellipsometry (VASE) at ambient conditions. Knowledge of optical dispersion properties is essential for designing and fabricating optoelectronic devices such as solar cells, photodetectors, and photodiodes. In this research, we determined the dielectric function of PPDT2FBT thin film using the B-spline model and then reproduced the dielectric function using Psemi-Tri oscillators. We estimated the refractive index (n) of the thin film to be between 2.00 and 2.15 and the extinction coefficient (k) to be in the range of 1.14–1.39 at a wavelength of 632.8 nm. We further verified the estimated optical properties from the model using directly measured quantities such as transmission and absorption data obtained using the ultraviolet–visible (UV–Vis) spectrometer and thicknesses obtained using a surface profilometer. In addition, we determined the optical band gap of PPDT2FBT using the absorption coefficient.

CH3NH3PbI3 deposited on a ZnO electron transport layer is chemically unstable and decomposes at >80 °C, leaving PbI2 on the substrate along with traces of iodine. We found that this decomposition was reversible and could be prevented if a restricted volume solvent annealing procedure was applied. We also found that decomposition requires the presence of a certain amount of the processing solvent within the film. Finally, we developed a reliable annealing protocol for depositing perovskite film on ZnO, which resulted in the generation of repeatable solar cell devices.

We report on the use of SrTiO3 films on GaAs(001) substrates grown by molecular beam epitaxy (MBE) as intermediate buffer layers for the heteroepitaxial growth of ferromagnetic La0.7Sr0.3MnO3 (LSMO) and room temperature multiferroic (ferroelectric/antiferromagnetic) BiFeO3 (BFO) thin films using the pulsed laser deposition technique. The exchange bias coupling effect in the BFO/LSMO heterostructure has been investigated. The magnetization measurements with field cooling exhibit a surprising increment in the magnetic moment with enhanced magnetic hysteresis squareness. This we believe is the consequence of exchange interactions between the antiferromagnetic BFO and the ferromagnetic LSMO at the interface. The integration of BFO materials with LSMO on GaAs substrates facilitated the demonstration of resistive switching based non-volatile memory (NVM) devices which can be faster with lower energy consumption compared to present commercial technologies. Ferroelectric switching observations using piezoresponse force microscopy show polarization switching demonstrating its potential for read–write operation in NVM devices. The ferroelectric and electrical characterization exhibits strong resistive switching with low set/reset voltages. Furthermore, we demonstrate a prototypical epitaxial field effect transistor based on multiferroic BFO as the gate dielectric and ferromagnetic LSMO as the conducting channel. The device exhibits a modulation in channel conductance with a high ON/OFF ratio. This work also demonstrates the first step towards the development of magneto-electronic devices integrated with a compound semiconductor.

We report the growth of zirconium oxide (ZrO2) as a high-k gate dielectric for an inkjet-printed transistor using a low-temperature atomic layer deposition (ALD) from tetrakis(dimethylamido)zirconium (TDMAZr) and water precursors. All the samples are deposited at low-temperature ranges of 150–250 °C. The films are very uniform with RMS roughness less than 4% with respect to their thickness. The atomic force microscopy (AFM) shows a significant change in surface morphology from tapered posts to undulating mountain-like structures with several hundreds of ALD cycles. The results from X-ray diffraction (XRD) analysis exhibit an amorphous to the crystalline structure with temperature variation, which is independent of the thickness of the films. All our samples are hydrophilic as contact angles are less than 90°. The capacitance–voltage (C–V) and conductance–voltage (Gp/ω–V) characteristics of ZrO2 dielectrics for silicon metal–oxide–semiconductor (MOS) capacitors are studied for different temperatures. For the n-type substrate MOS capacitors, the dielectric constants are estimated to be 7.5–11. Due to the low deposition temperature, a hydrophilic surface, and high k value, the ALD-ZrO2 dielectric can be compatible for printed transistors. The processes of fabrication and characterization of inkjet-printed graphene transistors is demonstrated using the ZrO2 dielectric. The possible solvents, surfactant, and the dielectric induced modifications in graphene flakes are demonstrated by Raman spectra. The graphene flakes spread uniformly on the ZrO2 surface. The functional inkjet-printed graphene transistor characteristics are demonstrated to illustrate the field effect behavior with the ALD-ZrO2 dielectric.

The rapid surge in perovskite solar cell efficiency has necessitated the development of viable metrology techniques during device integration, paving the way for commercialization. Ellipsometry is considered the most appropriate technique for fast and accurate thickness measurement for large scale production. However, a precise and well-calibrated model is a prerequisite for this technique. While ellipsometry of individual device layers has been reported in recent perovskite literature, a comprehensive multilayer modeling approach is thus far unavailable. Perovskite optoelectronic devices generally consist of a six-layer film stack with three transparent layers required for optical absorption in the perovskite layer. Spin casted thin films, now common in this line of research, impart their own difficulties into ellipsometric modeling. Roughnesses at each heterointerface, similarities in optical spectra of transparent layers, and anomalous dispersion of perovskite are just a few of such challenges. In this work, we report the process of building an ellipsometry model from scratch for thickness measurement of methylammonium lead iodide (MAPI) perovskite and indium tin oxide (ITO)/hole transport layer (HTL) bilayer thin film stacks on a glass substrate. Three promising representatives of HTLs (CuI, Cu2O, and PEDOT:PSS) were studied. The models were extended to measure the individual layer thicknesses of the MAPI/HTL/ITO film stack on a glass substrate using the models developed for individual layers. Optical constants of all the representative thin films were thus extracted for a wide wavelength range (300 nm–900 nm).

The second-harmonic generation in CH3NH3PbI3 (MAPI) perovskite multidomain thin films is experimentally studied. The MAPI nonlinear second-order susceptibility is estimated. Polarization of MAPI SHG signal is experimentally observed and hyper-Rayleigh scattering angle value is estimated.

Organohalide lead (hybrid) perovskites have emerged as competitive semiconducting materials for photovoltaic devices due to their high performance and low cost. To further the understanding and optimization of these materials, solution-based methods for interrogating and modifying perovskite thin films are needed. In this work, we report a hydrofluoroether (HFE) solvent-based electrolyte for electrochemical processing and characterization of organic-inorganic trihalide lead perovskite thin films. Organic perovskite films are soluble in most of the polar organic solvents, and thus until now, they were not considered suitable for electrochemical processing. We have enabled electrochemical characterization and demonstrated a processing toolset for these materials utilizing highly fluorinated electrolytes based on a HFE solvent. Our results show that chemically orthogonal electrolytes based on HFE solvents do not dissolve organic perovskite films and thus allow electrochemical characterization of the electronic structure, investigation of charge transport properties, and potential electrochemical doping of the films with in situ diagnostic capabilities.

The advent of organic electronics has made available a host of materials and devices with unique properties for interfacing with biology.1–2 One example is the use of conducting polymer coatings on metal electrodes that are implanted in the central nervous system and interface electrically with neurons, providing stimulation and recording the neuron's electrical activity.3–5 Coating a metal electrode with a conducting polymer has been shown to lower the electrical impedance and decrease the mechanical properties mismatch at the interface with tissue, with beneficial effects on the lifetime of the implant.3, 6 Conducting polymers can also be functionalized with biomolecules that stimulate neural growth and minimize the immune response to the implant.3–5, 7 Other examples are organic electronic ion pumps,8 and ion transistors,9 which are recently invented devices capable of precise delivery of neurotransmitters to neurons. These devices were recently implanted in the ear of a guinea pig and were shown to control its hearing.10 Conducting polymers such as poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS), a material that has been shown to be biocompatible with a variety of different cells,1 have been used for these applications. These examples highlight the main advantages that organic electronic materials bring to the interface with biology, including their “soft” nature, which offers better mechanical compatibility with tissue than traditional electronic materials, and natural compatibility with mechanically flexible substrates, which paves the way for the development of implants that better conform to the non-planar shape of organs. Finally, the ability of organics to transport ionic in addition to electronic charge creates the opportunity to interface with electrically active cells in novel ways, as the work on ion pumps indicates.

By adding photonic wire structures to an organic microcavity, we create an additional confinement and a Bloch-like band structure in the dispersion of periodically structured cavities. We experimentally observe spontaneous and stimulated emission from the ground and different excited discrete modes at room temperature. By changing the spatial gain distribution via a two-beam interference, we are able to directly control the laser emission from both extended and confined modes of such organic photonic wires. Both spatial distribution and dispersion exhibit coherent emission from tunable modes, which we describe with an analytical model and numerical simulations, in agreement with our measurements.

The increased usage of hydrogen as a next generation clean fuel strongly demands the parallel development of room temperature and low power hydrogen sensors for their safety operation. In this work, we report strong evidence for preferential hydrogen adsorption at edge-sites in an edge oriented vertically aligned 3-D network of MoS2 flakes at room temperature. The vertically aligned edge-oriented MoS2 flakes were synthesised by a modified CVD process on a SiO2/Si substrate and confirmed by Scanning Electron Microscopy. Raman spectroscopy and PL spectroscopy reveal the signature of few-layer MoS2 flakes in the sample. The sensor's performance was tested from room temperature to 150 °C for 1% hydrogen concentration. The device shows a fast response of 14.3 s even at room temperature. The sensitivity of the device strongly depends on temperature and increases from ∼1% to ∼11% as temperature increases. A detail hydrogen sensing mechanism was proposed based on the preferential hydrogen adsorption at MoS2 edge ...

We investigate the possible formation of polarons in four organic perovskites (CH3NH3PbI3, CH3NH3PbBr3, CH3NH3PbCl3, and CH3NH3PbI2Cl1) using a density functional theory (DFT) calculations with local potentials and hybrid functionals. We show that DFT+U method with U = 8 eV predicts a correct band-gap and matches the forces on ions from hybrid calculations. We then use the DFT + U approach to study the effect of polarons, i.e. to search the configuration space and locate the lowest energy localized band gap state self-trapped hole (STH). STH configurations were found for three pure halides and one mixed halide system. Spin orbit coupling (SOC) was also taken into account and the results may be found in the supplementary material. This study focuses on the +U method; however, SOC corrections added to the DFT+U calculations also resulted in STH states in all four systems.

The inversion field-effect transistor is the basic device of modern microelectronics and is nowadays used more than a billion times on every state-of-the-art computer chip. In the future, this rigid technology will be complemented by flexible electronics produced at extremely low cost. Organic field-effect transistors have the potential to be the basic device for flexible electronics, but still need much improvement. In particular, despite more than 20 years of research, organic inversion mode transistors have not been reported so far. Here we discuss the first realization of organic inversion transistors and the optimization of organic depletion transistors by our organic doping technology. We show that the transistor parameters—in particular, the threshold voltage and the ON/OFF ratio—can be controlled by the doping concentration and the thickness of the transistor channel. Injection of minority carriers into the doped transistor channel is achieved by doped contacts, which allows forming an inversion layer., Inversion type transistors – which are widely used in silicon-based industries – are thought to not be obtainable in organic devices. Lüssem et al. realize the first inversion organic field-effect transistor by doping at the source and drain contacts without degrading its ON/OFF ratio.

Organohalide lead perovskite (CH3NH3PbI3) is a novel material with promising applicability for visible light photo-detectors. The ability to develop perovskite photo-detector devices using a low temperature solution based process allows straightforward combinations with other materials, including traditional crystalline semiconductors, with minimal contributions to cost and process complexity. There is, however, a need for high-resolution structuring of the perovskite film to minimize cross-talk between neighboring detectors (pixels) for imaging purposes. This work presents a method to develop Ch3NH3PbI3 thin films possessing high-resolution patterning, using lithography processing with hydrofluoroether solvents. The results presented herein confirm that, unlike the majority of traditional solvents utilized in conventional photolithography, hydrofluoroethers do not adversely affect CH3NH3PbI3 films, enabling photolithographic processing. Transfer of the resist pattern is achieved using a SF6 plasma functionalization process which extracts iodine and organic components from the film, converting the perovskite into PbF2. This work also demonstrates that isolation of perovskite photodetecting pixels with a 20 μm-wide stripe of PbF2 leads to a 4.5-fold reduction in the cross-talk between neighboring pixels. It is believed that our method will facilitate simple monolithic integration of perovskite photodiodes to the silicon backplane chip utilized in active-pixel sensor and charge-coupled device applications.