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In recent years there has been an increasing interest to integrate optical sensing in mobile displays, for instance, for biometric fingerprint scanning functionality. There are several routes to incorporate optical fingerprint functionality within the full display area, each with their own benefits and challenges. Here we investigate the different integration routes using large-area, ultra-thin imagers based on organic photodiodes.

Solution-processed photodetectors could be of use in large-area light-sensing applications because they can be fabricated at low cost on plastic substrates and their absorption spectra can be tuned by chemical design. However, fabricating photodetectors with low dark currents and integrating them into high-resolution backplanes remains challenging. Here we show that solution-processed metal halide perovskite photodiodes on top of an amorphous indium gallium zinc oxide transistor backplane can be used to create a flexible image sensor that is ~100 μm thick and has a resolution of 508 pixels per inch. We have developed a pixel edge cover layer for the system that reduces electrode current leakage and thus dark current density. The low noise current in combination with high external quantum efficiency results in high photodetectivity at wavelengths from 550 nm to 770 nm. We show that our imager can be used for document scanning and biometric fingerprinting and that it can be wrapped around objects with radii as small as 0.6 cm. Low-dark-current perovskite photodetectors can be integrated with an oxide transistor backplane to create a high-resolution optical scanning array capable of imaging flat and curved surfaces.

Sub-micrometer thin-film transistors (TFTs) are realized using multi-level imprint lithography. Amorphous indium gallium zinc oxide ( $\alpha $ -IGZO) TFTs with channel lengths as small as $0.7~\mu \text{m}$ , field-effect mobility of 10 cm2V−1s−1 and on/off ratio of circa 107 were integrated into a 1000-pixels per inch (ppi) TFT backplane array. The reduction of the number of patterning steps and the inherent self-registration of the most critical transistor layers on top of each other offer a cost-effective high-throughput fabrication route for high-resolution TFT arrays.

Thin film transistors (TFTs) are the basis for current AMOLED display arrays. For next-generation displays, higher resolution and cost-effective manufacturing of panels is adamant. The current benchmark patterning method in the display industry is photolithography. Here, we propose the use of a hybrid approach of nanoimprint lithography and conventional FPD processing for the realization of high-resolution display backplanes. We demonstrate the realization of sub-micron amorphous oxide semiconductor TFTs with multi-level nanoimprint lithography in order to decrease the number of patterning steps in display manufacturing. Top-gate self-aligned a-IGZO TFTs are realized with performance comparable to benchmark photolithography-based TFTs.

We combine a low dark current and high-detectivity near-infrared (NIR)-sensitive organic photodetector with a high-resolution 508 pixels per inch (ppi) oxide thin-film transistor (TFT) backplane to create a large-area thin NIR detector, using processes that are compatible with flat-panel display fabrication. The detector is characterized showing high uniformity and linearity. With the use of a NIR light source, the detector is capable of imaging the (pattern of) veins under the skin in reflection, leading to improved biometric authentication.

Quality Assurance (QA) in hadron therapy is crucial to ensure safe and accurate dose delivery to patients. This can be achieved with fast, reliable and high-resolution detectors. In this paper, we present a novel solution that combines a triple Gas Electron Multiplier (GEM) and a highly pixelated readout based on a matrix of organic photodiodes fabricated on top of an oxide-based thin-film transistor backplane. The first LaGEMPix prototype with an active area of 60 × 80 mm2 was developed and characterized using low energy X-rays. The detector comprises a drift gap of 3.5 mm, a triple-GEM stack for electron amplification, and a readout featuring 480 × 640 pixels at a 126 µm pitch. Here, we describe the measurements and results in terms of spatial resolution for various experimental configurations. A comparison with GAFCHROMIC® films and the GEMPix detector used in the charge readout mode was performed to better understand the contribution to the spatial resolution from both the electron diffusion and the isotropic emission of photons. The measurements were compared to Monte Carlo simulations, using the FLUKA code. The simulation predictions are in good agreement with the GEMPix results. Future plans with respect to applications in hadron therapy are discussed.

Indium Gallium Zinc Oxide (IGZO) films are deposited using plasma-enhanced spatial Atomic Layer Deposition (sALD) on substrates as large as 32 cm x 35 cm. Excellent uniformity and thickness control leads to high-performing and stable co-planar top-gate self-aligned (SA) thin-film transistors (TFTs), demonstrating the viability of atmospheric spatial ALD as a novel deposition technique for the flat-panel display Industry.

Quality Assurance in hadron therapy is crucial to ensure a safe and accurate dose delivery to the patients. This requires fast and reliable detectors with high spatial resolution. A first LaGEMPix prototype that combines a triple Gas Electron Multiplier and a highly pixelated readout based on a matrix of organic photodiodes coated on an oxide thin film transistor backplane has been built. The first version of the LaGEMPix has proven to have a limited spatial resolution, mainly attributed to the isotropic emission of the scintillation photons within the GEM holes. To improve the spatial resolution and confirm our predictions of the role of the photons, we built a new version of the detector with a reduced gap between the last GEM foil and the readout. Experimental results acquired using different methods and experimental set-ups show that the spatial resolution significantly improved with the new design.

The authors reply to the Comment by Nenashev et al.

Indium gallium zinc oxide (IGZO) is deposited using plasma-enhanced spatial atomic layer deposition (sALD) on substrates as large as 32 × 35 cm 2 . Excellent uniformity and thickness control leads to high-performing and stable coplanar top-gate self-aligned (SA) thin-film transistors (TFTs). The integration of a sALD-deposited aluminum oxide buffer layer into the TFT stack further improves uniformity and stability. The results demonstrate the viability of atmospheric sALD as a novel deposition technique for the flat-panel display industry.

We use sALD to deposit IGZO and Al2O3 layers in top-gated self-aligned TFTs, achieving a low-temperature process flow (d200oC). We attain mobility of 8 cm2/Vs and switch-on voltage of -0.1 V for transistors with channel lengths down to 1 μm, enabling a 200 ppi QVGA display on transparent PEN foil.

Generating electricity from low-frequency mechanical agitations produced by ocean waves, plants, or human motion is emerging as a key, environmentally friendly technology in combating harmful emissions caused by burning fossil fuels. The electric pulses generated by the appropriate transducers, such as triboelectric or piezoelectric generators, have to be rectified and stored in a sustainable external circuit. The bottleneck, however, is the harvesting circuitry, which relies on rather expensive up-conversion oscillation technologies. Such circuits are primarily designed and optimized for frequencies well above the kHz range, much higher than the aforementioned mechanical stimuli, and are therefore energy demanding. Herein, a sustainable energy harvester is developed that alleviates the need for using up-conversion and allows efficient harvesting of energy from low-frequency voltage pulses, such as the ones typically generated by triboelectric or piezoelectric generators. The resonant circuit is designed to match overall response originating from such low-frequency oscillating energy sources. The design enables the harvester to be operable at frequencies as low as 1 Hz. Computer aided design simulations are demonstrated, which are miniaturized harvesters on a printed-circuit board using low-cost components, and scalability of the proposed design is discussed, which paves the way to affordable, efficient, and sustainable low-cost energy solutions.

In this manuscript, the authors investigate the growth of indium zinc oxide, indium zinc oxide (InZnO, IZO) as a channel material for thin-film transistors. IZO is grown at atmospheric pressure and a high deposition rate using spatial atomic layer deposition (S-ALD). By varying the ratio of diethylzinc and trimethylindium vapor, the In/(In + Zn) ratio of the film can be accurately tuned in the entire range from zinc oxide to indium oxide. Thin film transistors with an In to Zn ratio of 2:1 show high field-effect mobility—exceeding 30 cm2/V s—and excellent stability. The authors demonstrate large scale integration in the form of 19-stage ring oscillators operating at 110 kHz. These electrical characteristics, in combination with the intrinsic advantages of atomic layer deposition, demonstrate the great potential of S-ALD for future display production.

We present recent developments on flexible a-IGZO TFT technology scaling in terms of TFT channel length and operating voltage, as well as manufacturing cost optimization with a focus on R2R processing compatibility. We present progress on the relevant technology building blocks; (i) R2R compatible TFT architecture, (ii) (multilevel) nanoimprint lithography and (iii) S-ALD deposited TFT materials. Additionally, we show a developing application that drives scaling of the technology, namely a monolithically integrated flexible AMOLED display with in-display fingerprint detection.

Ferroelectric nanostructures offer a promising route for novel integrated electronic devices such as non-volatile memories. Here we present a facile fabrication route for ferroelectric capacitors comprising a linear array of the ferroelectric random copolymer of vinylidenefluoride and trifluoroethylene (P(VDFTrFE)) interdigitated with the electrically insulating polymer polyvinyl alcohol (PVA). Micrometer size line gratings of both polymers were fabricated over large area by solution micromolding, a soft lithography method. The binary linear arrays were realized by backfilling with the second polymer. We investigated in detail the device physics of the patterned capacitors. The electrical equivalent circuit is a linear capacitor of PVA in parallel with a ferroelectric capacitor of P(VDFTrFE). The binary arrays are electrically characterized by both conventional Sawyer–Tower and shunt measurements. The dependence of the remanent polarization on the array topography is explained by numerical simulation of the electric field distribution.

Perovskite solar cells attract a lot attention as alternative energy sources for the future energy market. With the remarkable lab-scale achievements, the investigations into a high-throughput large-scale production of perovskite devices are now on the agenda. The first step towards mass manufacturing should be a replacement of toxic solvents used in the manufacturing of perovskite layer. In this study, a non-toxic and up-scaling compatible solvent system is developed. The impact of the solvent properties on the perovskite crystallization kinetics has been systematically investigated, which is found to be crucial in controlling the surface coverage and layer crystallinity. By optimizing the processing conditions, a stabilized efficiency of 16.0% is achieved with the developed non-toxic solvent system.

In this work, the macroscopic polarization of a ferroelectric capacitor is correlated with the local domain morphology. To this end, a ferroelectric capacitor of the random copolymer poly(vinylidenefluoride-trifluoroethylene) [P(VDF-TrFE)] is poled to a set polarization state in a Sawyer-Tower setup. After chemically removing the top electrode, the exposed ferroelectric is locally probed with piezoresponse force microscopy. The domains without the top electrode are thermodynamically stable for weeks in ambient environment, as proven by comparing the remanent polarization measured before etching away and after re-depositing the top electrode. Out-of-plane PFM phase images show a random distribution of domains with up and down polarity. We unambiguously demonstrate a linear correlation between the mean PFM phase and the macroscopic polarization. As a demonstration of the insights that the global excitation and local probing method can provide, we show how thermal and electrical depoling can result in identical macroscopic polarization yet completely different domain morphologies.

Poly(vinylidene fluoride) (PVDF) has long been regarded as an ideal piezoelectric 'plastic' because it exhibits a large piezoelectric response and a high thermal stability. However, the realization of piezoelectric PVDF elements has proven to be problematic due to, amongst other reasons, the lack of industrially scalable methods to process PVDF into the appropriate polar crystalline forms. Here, we show that fully piezoelectric PVDF films can be produced via a single-step process that exploits the fact that PVDF can be molded at temperatures below its melting temperature, i.e. via solid-state-processing. We demonstrate that we thereby produce δ-PVDF, the piezoelectric charge coefficient of which is comparable to that of biaxially stretched β-PVDF. We expect that the simplicity and scalability of solid-state processing combined with the excellent piezoelectric properties of our PVDF structures will provide new opportunities for this commodity polymer and will open a range of possibilities for future, large-scale, industrial production of plastic piezoelectric films.

Recently reported piezoresponse force microscopy (PFM) measurements have proposed that porcine aortic walls are ferroelectric. This finding may have great implications for understanding biophysical properties of cardiovascular diseases such as arteriosclerosis. However, the complex anatomical structure of the aortic wall with different extracellular matrices appears unlikely to be ferroelectric. The reason is that a prerequisite for ferroelectricity, which is the spontaneous switching of the polarization, is a polar crystal structure of the material. Although the PFM measurements were performed locally, the phase-voltage hysteresis loops could be reproduced at different positions on the tissue, suggesting that the whole aorta is ferroelectric. To corroborate this hypothesis, we analyzed entire pieces of porcine aorta globally, both with electrical and electromechanical measurements. We show that there is no hysteresis in the electric displacement as well as in the longitudinal strain as a function of applied electric field and that the strain depends on the electric field squared. By using the experimentally determined quasi-static permittivity and Young's modulus of the fixated aorta, we show that the strain can quantitatively be explained by Maxwell stress and electrostriction, meaning that the aortic wall is neither piezoelectric nor ferroelectric, but behaves as a regular dielectric material.

Solution type metal-oxide semiconductor was processed on mass-production ready equipment and integrated in a backplane with ESL architecture TFTs. Excellent thickness uniformity of the semiconductor layer was obtained over the complete Gen1 glass substrate (320 mm x 352 mm), resulting in homogeneous TFT performance and bias stress reliability. An 85-ppi QVGA AMOLED display is demonstrated.

High permittivity (high-k) materials have received considerable attention as alternatives to SiO2 for CMOS and low-power flexible electronics applications. In this study, we have grown high-quality ZrO2 by using atmospheric-pressure plasma-enhanced spatial ALD (PE-sALD), which, compared to temporal ALD, offers higher effective deposition rates and uses atmospheric-pressure plasma to activate surface reactions at lower temperatures. We used tetrakis(ethylmethylamino)zirconium (TEMAZ) as precursor and O2 plasma as co-reactant at temperatures between 150 and 250◦C. Deposition rates as high as 0.17 nm/cycle were achieved with N- and C- contents as low as 0.4% and 1.5%, respectively. Growth rate, film crystallinity and impurity contents in the films were found to improve with increasing deposition temperature. The measured relative permittivity lying between 18 and 28 with leakage currents in the order of 5 × 10−8 A/cm2 demonstrates that atmospheric PE-sALD is a powerful technique to deposit ultrathin, high-quality dielectrics for low-temperature, large-scale microelectronic applications.

The huge impact of nitrogen dioxide (NO2) emission on public health and the environment is motivating extensive scientific and technological research in the field of NO2 sensing. Field-effect transistors have emerged as a sensitive and reliable technology for monitoring air quality, due to their amplified sensor response. In this review article, the NO2 detection mechanism with field-effect transistors is discussed. The origin is charge trapping at the gate dielectric, yielding a threshold voltage shift. The dynamic response can be described by an analytical model. Implementation in a sensor protocol allows for the fabrication of a functional demonstrator. The sensor, based on a ZnO field-effect transistor, is capable of detecting concentrations as low as 40 ppb of NO2 in real-time. The sensor operates in ambient air, and apart from drying, no further precautions are taken, showing that the fabricated sensor is selective for NO2. The results reviewed in this paper set a precedent for sensing with field-ef...

Charge transport in poly(2-methoxy, 5-(2'-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV)-based hole-only diodes is investigated at high electric fields and low temperatures using a novel diode architecture. Charge carrier densities that are in the range of those in a field-effect transistor are achieved, bridging the gap in the mobility versus charge carrier density plot between polymer-based light-emitting diodes and field-effect transistors. The extended field range that is accessed allows us to discuss the applicability of current theoretical models of charge transport, using numerical simulations. Finally, within a simple approximation, we extract the hopping length for holes in MEH-PPV directly from the experimental data at high fields, and we derive a value of 1.0 +/- 0.1 nm. (C) 2013 Elsevier B. V. All rights reserved.

Ferroelectric poly(vinylidene-fluoride) (PVDF) has, in the past, been proposed as an ideal candidate for data storage applications as it exhibits a bistable, remanent, polarization that can repeatedly be switched by an electric field. However, fabrication of smooth ferroelectric PVDF thin films, as required for microelectronic applications, is a long-standing problem. At present, the copolymer of PVDF with trifluoroethylene P(VDF-TrFE) is used, but the stack integrity and the limited thermal stability of its remanent polarization hamper large-scale integration. Here we show that smooth neat PVDF films can be made at elevated substrate temperature. On applying a short electrical pulse the ferroelectric polar delta-phase is formed, an overlooked polymorph of PVDF proposed 30 years ago, but never experimentally verified. The remanent polarization and coercive field are comparable to those of the copolymer. The enhanced thermal stability of the polarization is directly related to the high Curie temperature, whereas the ferroelectric properties are related to the molecular packing as derived from the refined crystal structure. The replacement of P(VDF-TrFE) by the commodity polymer PVDF may boost large-scale industrial applications.

For its application as transparent conductor in light-emitting diodes and photovoltaic cells, both the in-plane and out-of-plane conductivity of PEDOT:PSS are important. However, studies into the conductivity of PEDOT:PSS rarely address the out-of-plane conductivity and those that do, report widely varying results. Here a systematic study of the out-of-plane charge transport in thin films of PEDOT:PSS with varying PSS content is presented. To this end, the PEDOT:PSS is enclosed in small interconnects between metallic contacts. An unexpected, but strong dependence of the conductivity on interconnect diameter is observed for PEDOT:PSS formulations without high boiling solvent. The change in conductivity correlates with a diameter dependent change in PEDOT:PSS layer thickness. It is suggested that the order of magnitude variation in out-of-plane conductivity with only a 3-4-fold layer thickness variation can quantitatively be explained on basis of a percolating cluster model. Reported measurements of the anisotropy in the conductivity of spin coated films of PEDOT:PSS vary widely. A systematic study of the out-of-plane charge transport in thin films of PEDOT:PSS with varying PSS content is presented. It is suggested that the orders of magnitude change in anisotropy can be quantitatively explained on the basis of a percolating cluster model. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We investigate the nature of charge transport in deoxyribonucleic acid (DNA) using self-assembled layers of DNA in large-area molecular junctions. A protocol was developed that yields dense monolayers where the DNA molecules are not standing upright, but are lying flat on the substrate. As a result the charge transport is measured not along the DNA molecules but in the transverse direction, across their diameter. The electrical transport data are consistent with the derived morphology. We demonstrate that the charge transport mechanism through DNA is identical to non-resonant tunneling through alkanethiols with identical length, classifying DNA as a dielectric.

The current density-voltage (J-V) characteristics of hole-only diodes based on poly(2-methoxy, 5-(2′ ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) were measured at a wide temperature and field range. At high electric fields the temperature dependence of the transport vanishes, and all J-V sweeps converge to a power law. Nuclear tunneling theory predicts a power law at high fields that scales with the Kondo parameter. To model the J-V characteristics we have performed master-equation calculations to determine the dependence of charge carrier mobility on electric field, charge carrier density, temperature, and Kondo parameter, using nuclear tunneling transfer rates. We demonstrate that nuclear tunneling, unlike other semiclassical models, provides a consistent description of the charge transport for a large bias, temperature, and carrier density range. cop. 2016 authors. Published by the American Physical Society.

A homogeneous ferroelectric single crystal exhibits only two remanent polarization states that are stable over time, whereas intermediate, or unsaturated, polarization states are thermodynamically instable. Commonly used ferroelectric materials however, are inhomogeneous polycrystalline thin films or ceramics. To investigate the stability of intermediate polarization states, formed upon incomplete, or partial, switching, we have systematically studied their retention in capacitors comprising two classic ferroelectric materials, viz. random copolymer of vinylidene fluoride with trifluoroethylene, P(VDF-TrFE), and Pb(Zr,Ti)O3. Each experiment started from a discharged and electrically depolarized ferroelectric capacitor. Voltage pulses were applied to set the given polarization states. The retention was measured as a function of time at various temperatures. The intermediate polarization states are stable over time, up to the Curie temperature. We argue that the remarkable stability originates from the coexistence of effectively independent domains, with different values of polarization and coercive field. A domain growth model is derived quantitatively describing deterministic switching between the intermediate polarization states. We show that by using well-defined voltage pulses, the polarization can be set to any arbitrary value, allowing arithmetic programming. The feasibility of arithmetic programming along with the inherent stability of intermediate polarization states makes ferroelectric materials ideal candidates for multibit data storage.

Nature materials 15(1), 78 - 84(2015). doi:10.1038/nmat4423, Piezoelectricity describes interconversion between electrical charge and mechanical strain. As expected for lattice ions displaced in an electric field, the proportionality constant is positive for all piezoelectric materials. The exceptions are poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (P(VDF-TrFE)), which exhibit a negative longitudinal piezoelectric coefficient. Reported explanations exclusively consider contraction with applied electric field of either the crystalline or the amorphous part of these semi-crystalline polymers. To distinguish between these conflicting interpretations, we have performed in situ dynamic X-ray diffraction measurements on P(VDF-TrFE) capacitors. We find that the piezoelectric effect is dominated by the change in lattice constant but, surprisingly, it cannot be accounted for by the polarization-biased electrostrictive contribution of the crystalline part alone. Our quantitative analysis shows that an additional contribution is operative, which we argue is due to an electromechanical coupling between the intermixed crystalline lamellae and amorphous regions. Our findings tie the counterintuitive negative piezoelectric response of PVDF and its copolymers to the dynamics of their composite microstructure., Published by Nature Publishing Group, Basingstoke

Switching, i.e., polarization reversal, of ferroelectric P(VDF-TrFE) thin films has been investigated in a wide range of applied electric field and temperature. The measured polarization transients can be quantitatively described by a compressed exponential function as originally formulated by Kolmogorov, Avrami, and Ishibashi (the KAI model). The phenomenological parameters switching time and Avrami index are related to the velocity and morphology of the domain walls, respectively. We show that the switching time depends exponentially on the electric field as described by the Merz law. The experimentally obtained Avrami index is independent of temperature but decreases with applied electric field from 1.55 at low field to 1.0 at 300 MV/m, indicative of an out-of-equilibrium dynamics of the growing domains. Using a random walk model we demonstrate that the observed switching dynamics is in agreement with a domain growth mechanism where domains are circular at low electric field with few nucleation sites, whereas at high field irregular entangled domains are formed. The density of nucleation sites is extracted as 1.5% of the monomeric units in this work. Switching transients on capacitors where the nuclei density has been artificially increased exhibit one-dimensional-like growth, as predicted by our model. cop. 2015 American Physical Society.

Charge transport through alkanes and para-phenylene oligomers is investigated in large-area molecular junctions. The molecules are self-assembled in a monolayer and contacted with a top electrode consisting of poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonic acid) (PEDOT:PSS). The complete set of J(V,T) characteristics of both saturated and π-conjugated molecules can be described quantitatively by a single equation with only two fit parameters. The derived parameters, in combination with a variation of the bulk conductivity of PEDOT:PSS, demonstrate that the absolute junction resistance is factorized with that of PEDOT:PSS. The temperature and bias dependence of charge transport in large-area molecular junctions containing different molecules is investigated. The transport exhibits universal scaling, quantitatively described by a single equation with two fit parameters. The scaling shows that PEDOT:PSS exhibits a dominant role in the electrical transport in these kinds of junctions. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We investigate the electrical transport through mixed self-assembled monolayers of alkanemonothiols and alkanedithiols in large-area molecular junctions. To disentangle the role of the molecular length and the interfacial composition, monothiol-monothiol, dithiol-dithiol, and monothiol-dithiol binary combinations are studied. In all cases, we find that the resistance of these mixed SAMs appears to depend exponentially on the average number of carbon atoms, thus resembling monocomponent SAMs, whose resistance is known to depend exponentially on molecular length. However, in monocomponent SAMs this behavior has a single-molecule tunneling origin, which is not directly relevant for mixtures. Furthermore, in certain mixed SAMs the resistance decreases with increasing average layer thickness (the case of monothiol-dithiol systems). We suggest an explanation for the observed dependence of the resistance in the mixed SAMs on their composition within an equivalent circuit model based on a simple assumption concerning their microdomain structure. The simulated dependence is non-exponential but leads to a good agreement between calculated and measured resistances with only two fit parameters. (C) 2011 Elsevier B.V. All rights reserved.

The on/off current ratio in organic ferroelectric field-effect transistors (FeFETs) is largely determined by the position of the threshold voltage, the value of which can show large device-to-device variations. Here we show that by employing a dual-gate layout for the FeFET, we can gain full control over the on/off ratio. In the resulting dual-gate FeFET the ferroelectric gate provides the memory functionality and the second, non-ferroelectric, control gate is advantageously used to set the threshold voltage. The on/off ratio can thus be maximized at the readout bias. The operation is explained by the quantitative analysis of charge transport in a dual-gate FeFET.

The degradation in light output of an Organic Light Emitting Diode (OLED) has been studied extensively and has been explained by different mechanisms, such as formation of chemical defects or electrical traps and by thermally induced inter-diffusion of dopants. However, there is an overlooked type of degradation, where the light output decreases rapidly with time. This catastrophic failure can often be attributed to a hard electrical short due to local defects. Here, we show that this "sudden death" can also occur in the absence of a hard electrical short. We investigate this phenomenon by current-voltage characteristics and small-signal impedance measurements on typical OLEDs with a LiF cathode interlayer. We show that in a short period of time the built-in voltage of the diode vanishes; the J-V characteristics become symmetric. The origin is a dramatic increase in the work-function of the LiF interlayer. The interlayer changes from an electron-injecting contact to a quasi-Ohmic hole-injecting contact. The pristine bipolar diode does not become electrically shorted, but suddenly transforms into a unipolar hole-only diode. Upon applying a high voltage the original diode is restored, manifesting the dynamic switching of the LiF contact. (C) 2015 Elsevier B.V. All rights reserved.

The polarization of the ferroelectric polymer P(VDF-TrFE) decreases upon prolonged cycling. Understanding of this fatigue behavior is of great technological importance for the implementation of P(VDF-TrFE) in random-access memories. However, the origin of fatigue is still ambiguous. Here we investigate fatigue in thin-film capacitors by systematically varying the frequency and amplitude of the driving waveform. We show that the fatigue is due to delamination of the top electrode. The origin is accumulation of gases, expelled from the capacitor, under the impermeable top electrode. The gases are formed by electron-induced phase decomposition of P(VDF-TrFE), similar as reported for inorganic ferroelectric materials. When the gas barrier is removed and the waveform is adapted, a fatigue-free ferroelectric capacitor based on P(VDF-TrFE) is realized. The capacitor can be cycled for more than 108 times, approaching the programming cycle endurance of its inorganic ferroelectric counterparts.

The contact resistance of a transistor using self-assembled monolayer (SAM)-modified source and drain electrodes depends on the SAM tunnel resistance, the height of the injection barrier and the morphology at the contact. To disentangle the different contributions, we have combined here the transmission line measurements in transistors with transport measurements of SAMs in large-area molecular junctions. The tunnel resistance of the SAM has been independently extracted in two-terminal large-area molecular junctions. We show that the tunneling resistance of the SAM can be added linearly to the contact resistance of the transistor with bare Au electrodes, to account for the increased contact resistance in the SAM-modified transistor. The observed agreement is discussed. The manifestation of the SAM in the contact resistance shows that transistors can potentially be used as an experimental test-bed for molecular electronics. (C) 2012 Elsevier B.V. All rights reserved.

A large bias window is required to discriminate between different transport models in large-area molecular junctions. Under continuous DC bias, the junctions irreversibly break down at fields over 9 MV/cm. We show that, by using pulse measurements, we can reach electrical fields of 35 MV/cm before degradation. The breakdown voltage is shown to depend logarithmically on both duty cycle and pulse width. A tentative interpretation is presented based on electrolysis in the polymeric top electrode. Expanding the bias window using pulse measurements unambiguously shows that the electrical transport exhibits not an exponential but a power-law dependence on bias. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3608154]

Electrical transport of a highly doped disordered conducting polymer, viz. poly-3,4-ethylenedioxythiophene stabilized with poly-4-styrenesulphonic acid, is investigated as a function of bias and temperature. The transport shows universal power-law scaling with both bias and temperature. All measurements constitute a single universal curve, and the complete J(V,T) characteristics are described by a single equation. We relate this scaling to dissipative tunneling processes, such as Coulomb blockade. © 2010 The American Physical Society.

Non-volatile memories operating at low voltage are indispensable for flexible micro-electronic applications. To that end, ferroelectric delta-PVDF films were investigated as a function of layer thickness down to 10 nm ultra-thin films. Capacitors were fabricated using PEDOT:PSS as non-reactive electrode. Full polarization reversal was obtained at an unprecedented voltage below 5V. The remanent polarization of 7 mu C/cm(2) and coercive field of 120 MV/m are independent of layer thickness, demonstrating that ferroelectric switching in delta-PVDF is extrinsic, dominated by inhomogeneous nucleation and growth. The ease of processing of delta-PVDF allowed to determine a lower limit of the critical ferroelectric thickness. (C) 2013 AIP Publishing LLC.