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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 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.

Methylammonium lead iodide (CH3NH3PbI3 or MAPI) is an organohalide lead perovskite, a promising material for optoelectronic application, e.g. in solar cells and photodetectors. It has a number of advantages over more traditional photovoltaics materials such as CIGS, GaAs, and even silicon due to facile solution processing, direct bandgap, high optical absorption, sufficiently high, balanced carrier mobilities, shallow trap defects and low-cost synthesis. At present moment there are number of article which reported measurement of second-harmonic generation in MAPI single crystals and presented contradictory results demonstrating the presence or absence of second-harmonic signal from MAPI. In this work, the second-harmonic generation (SHG) is observed in MAPI thin film that can be used as a base of high efficient solar sell. The SHG intensity from MAPI is measured to be at least three orders of magnitude more than one from substrate and encapsulation glasses. The MAPI nonlinear second-order susceptibility is estimated as compared to reference sample of y-cut quartz. In case of bulk contribution, the MAPI second-order susceptibility of 10-15 m/V is achieved. The polarization of SHG signal is experimentally studied in MAPI samples. The fraction of linearly polarized SHG signal is different for p- and s-polarization of fundamental wave and increases in case of p-polarized pump. The large non-polarized background of SHG signal is partly corresponded to SHG hyper-Rayleigh scattering. The hyper-Rayleigh scattering angle is experimentally estimated using the diaphragm behind the collimating objective lens and found to be less then 11 degrees.

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.

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).

We investigate various electrode combinations of bottom and top contacts for organic photovoltaic (OPV) cells. Silver (Ag), indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and silver nanowires (AgNW) are used as bottom electrodes. As top electrodes, thin silver layers (t-Ag) and free-standing carbon nanotube (f-CNT) sheets are employed. The manufactured zinc phthalocyanine (ZnPc): fullerene C 60 small molecule bulk heterojunction OPV cells with different kinds of bottom electrodes show efficiencies of 1.9∼2.2% and 1.1∼1.5%, when comprised of t-Ag and f-CNT top contacts, respectively. We demonstrate alternative electrodes beyond ITO, silver, and aluminum, which can be readily used for organic photovoltaics technology.

By placing a thin silver grating inside a microcavity comprised of an organic semiconductor and two dielectric mirrors, researchers show that coherent emission can be selectively stimulated between in- and out-of-phase-locked arrays at room temperature. This work demonstrates that incorporating a lossy metal into a cavity does not suppress lasing.

Direct structuring techniques are an indispensable need for future low-cost applications of organic semiconductor materials in e.g. active matrix displays or integrated circuits. We demonstrate direct structuring of a small molecule organic semiconductor by a photo-lithography lift off process under ambient conditions. To show compatibility of this process, we fabricate organic thin film transistors (OTFT) containing the benchmark electron transporting semiconductor C60 as active material in a top-contact geometry. C60 as electron transporting semiconductor serves as good indicator for contamination and degradation caused by the structuring procedure. To disclose influences of structuring, we discuss the OTFT performance for different channel lengths from 100 μm down to 2.7 μm. In particular, we show that lithography processing gives rise to increased contact resistances. Apart from that, mobility of C60 as material parameter is only weakly affected which underlines the compatibility of the suggested structuring procedure. The potential of this structuring procedure for future integration of driving transistors in active matrix displays is demonstrated.

A light-emitting memristor (LEM) is reported based on a metal/mixed conductor/metal structure, where the mixed conductor is the ionic transition metal complex ruthenium(II) tris(bipyridine) with hexafluorophosphate counter ions. The device shows memory effects upon the application of an ac bias, in both current and electroluminescence intensity. The observation of memory in light emission offers the potential for optical read-out of the state of memristive devices.

2009 WILEY-VCH Verlag Gmb Organic electronics is an emerging technology opening new opportunities in the field of large-area electronics. It has received enormous attention as a technology platform that enables flexible, large-scale devices by exploiting the unique properties of organic materials. In addition, organic electronics offer the possibility of affordable manufacture of devices through solution processing of active materials. Before this vision is realized, several challenges must be overcome, in particular the issues of patterning. Patterning of electronic materials enables microscale device structures to be defined for organic light-emitting diode (OLED) displays and organic thin-film transistors (OTFTs). In addition, patterning enhances the device performance by preventing cross-talk, increases transconductance, and prevents high off-currents in transistor arrays or drivers. The patterning of organic materials has been demonstrated by many different methods, including ink-jet printing, vapor deposition through shadow masks, soft and hard imprint lithography, and photolithography. Ink-jet printing boasts continuous roll-to-roll process capabilities and is the patterning technique of choice for polymeric materials. However, the resolution is limited to approximately 10–20mm. Shadow mask deposition is the dominant technique for small-molecule patterning, but also has notable resolution limitations. A shadow mask feature resolution is typically 25–30mm, although special masks have shown resolution down to 5mm. Shadow-mask deposition also requires a high-vacuum environment, which can introduce further limitations. Imprint lithography has demonstrated promising results, showing feature resolution down to 10 nm. However, this technique has shown only limited applicability with respect to materials and device architectures. Furthermore, in all of the aforementioned methods, registration is an issue that renders fabrication of multilayer devices exceptionally challenging. Multilayered device architecture will be essential to achieve fully integrated circuits. Photolithography, in contrast, is a widely applicable patterning method that consistently achieves both high-resolution and registration. Photolithography has the added advantage of being the most developed patterning technology and the patterning method of choice for the current semiconductor industry. In spite of the proven technical advantages, conventional photolithography has not been recognized as a suitable technique for patterning organic electronic materials. It is presently hindered by concerns of chemical deterioration of active organic materials upon exposure to process solvents for lithography. By introducing a new set of benign processes that involve new specially tailored photopolymers, we show that this problem can be circumvented. Recently, we identified hydrofluoroethers (HFEs), a family of nontoxic and environmentally friendly solvents, as being chemically benign to nonfluorinated organic electronic materials. Not only are they benign, but HFEs are orthogonal solvents to many organic compounds, that is, the organic compounds are insoluble and are not swollen in HFEs. This is a particularly useful property in the fabrication of multilevel devices, since new layers can be added without damage to existing ones. Our challenge was, therefore, to develop compatible lithographic materials for these new process solvents. By employing a fluorinated photoresist compatible with HFEs, we demonstrate sub-micrometer photolithographic patterning of organic electronic materials. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is a mechanically flexible, transparent, and highly conductive polymer blend, which has found various applications in organic electronics including serving as the electrode material for plastic substrates, because of its low-temperature processing requirements, and as charge-injection/-extraction layers in OLEDs and photovoltaic devices. However, photolithographic patterning of PEDOT:PSS for device components is not straightforward, because i) PEDOT:PSS films are damaged by aqueous solutions, which are standard developers in conventional photolithography, and ii) acid-sensitive photoresists are adversely affected by the acidic PEDOT:PSS. In this communication,wepresent a unique acid-stable imaging material for PEDOT:PSS and organic electronic materials in general. We report on the sub-micrometer patterning of PEDOT:PSS films and their subsequent application to the fabrication of a field-effect transistor, in which an organic semiconductor material, pentacene, is patterned by the same protocol. In designing a HFE-compatible photoresist, it was most important that the photoresist be soluble in fluorous solvents. In general, fluorous solvents dissolve highly fluorinated materials. The copolymer 3, derived from the highly fluorinated monomer 1 and photolabile monomer 2, was expected to yield a material that exhibits a solubility switch following UV irradiation

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.

Recently, bilayer resist processing combined with development in hydrofluoroether (HFE) solvents has been shown to enable single color structuring of vacuum-deposited state-of-the-art organic light-emitting diodes (OLED). In this work, we focus on further steps required to achieve multicolor structuring of p-i-n OLEDs using a bilayer resist approach. We show that the green phosphorescent OLED stack is undamaged after lift-off in HFEs, which is a necessary step in order to achieve RGB pixel array structured by means of photolithography. Furthermore, we investigate the influence of both, double resist processing on red OLEDs and exposure of the devices to ambient conditions, on the basis of the electrical, optical and lifetime parameters of the devices. Additionally, water vapor transmission rates of single and bilayer system are evaluated with thin Ca film conductance test. We conclude that diffusion of propylene glycol methyl ether acetate (PGMEA) through the fluoropolymer film is the main mechanism behind OLED degradation observed after bilayer processing.

The key active devices of future organic electronic circuits are organic thin film transistors (OTFTs). Reliability of OTFTs remains one of the most challenging obstacles to be overcome for broad commercial applications. In particular, bias stress was identified as the key instability under operation for numerous OTFT devices and interfaces. Despite a multitude of experimental observations, a comprehensive mechanism describing this behavior is still missing. Furthermore, controlled methods to overcome these instabilities are so far lacking. Here, we present the approach to control and significantly alleviate the bias stress effect by using molecular doping at low concentrations. For pentacene and silicon oxide as gate oxide, we are able to reduce the time constant of degradation by three orders of magnitude. The effect of molecular doping on the bias stress behavior is explained in terms of the shift of Fermi Level and, thus, exponentially reduced proton generation at the pentacene/oxide interface. © 2014 AIP Publishing LLC.

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.

We report a family of full-color highly fluorinated light-emitting polymers. The high fluorine content allows the formation of multi-layer devices, which are shown to exhibit emission from the individual layers. A 3 x 3 photoluminescent red-green-blue (RGB) pixel array is also fabricated using standard photolithographic patterning techniques, taking advantage of the robustness of these polymers. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The structurization of organic thin film devices is a challenge, since they are usually incompatible with standard lithography processes. Here, we report on the influence of a fluorinated solvent on the performance and lifetime of small molecule organic light emitting diodes (OLEDs). An initial test right after processing shows only a marginal difference between treated devices and reference device. Luminescence decay curves to 90% brightness (LLT90) data confirm that no long-term damage with respect to the reference sample occurs. LLT90 for treated sample approaches 100,000 hours, which makes this solvent acceptable for wet chemical processing such as photolithography.

We investigate the complex mode structure in microcavities with multidimensional optical confinement. Our active material is composed of the organic blend Alq(3): DCM, embedded into a microcavity containing arrays of photonic wires, facilitating strong lateral confinement. We directly record the energy dispersion for one (k) over right arrow vector component while the second lateral (k) over right arrow component is scanned. Thereby, we obtain a detailed dispersion tomogram of the cavity resonances, showing excellent agreement with our optical model. We are able to exceed the lasing threshold and observe stimulated emission not only from the bottom of the cavity parabola, but also from higher order modes.

The concept of chemical orthogonality has long been practiced in the field of inorganic semiconductor fabrication, where it is necessary to deposit and remove a layer of photoresist without damaging the underlying layers. However, these processes involving light sensitive polymers often damage organic materials, preventing the use of photolithography to pattern organic electronic devices. In this article we show that new photoresist materials that are orthogonal to organics allow the fabrication of complex devices, such as hybrid organic/inorganic circuitry and full-colour organic displays. The examples demonstrate that properly designed photoresists enable the fabrication of organic electronic devices using existing infrastructure.

An acid-sensitive semiperfluoroalkyl resorcinarene has recently been demonstrated as an effective photoresist for the photolithographic patterning of organic semiconductor materials. In this work, we show that it can be used to pattern polymer solar cells fabricated from a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), so as to obtain high open circuit voltages (Voc). An array of 300 solar cells in series, with a period of 50 µm, achieved a Voc of 90 V and a power conversion efficiency (PCE) of 0.3%.