Ionic-electronic interaction in optoelectronic and sensing devices

En este trabajo se estudian las interacciones entre cargas electrónicas e iónicas y sus aplicaciones en sensores y dispositivos optoelectrónicos. El mecanismo de funcionamiento de los dispositivos optoelectrónicos actualmente más comunes, como los transistores de efecto de campo de capa fina (TFTs), los diodos emisores de luz (LEDs) y las células solares se basa solamente en procesos electrónicos, lo que significa que los efectos de cargas iónicas están ausentes, son irrelevantes o incluso perjudiciales para el propósito de dichos dispositivos. Sin embargo, muchas aplicaciones se benefician de la presencia de iones en la estructura de un dispositivo. Un ejemplo típico en el campo de investigación para la búsqueda de fuentes de luz eficientes y baratas son las células electroquémicas emisoras de luz (en inglés light-emitting electrochemical cells, con acrónimo LECs). Estos dispositivos están basados en una única capa activa de un material electroluminiscente iónico intercalada entre dos electrodos, uno de los cuales es transparente. En los p-LECs (basados en polémeros electroluminiscentes) los iones en la capa activa se añaden en forma de sales, mientras que en los iTMC-LECs (basados en compuesto iónicos de metales de transición, ionic transition metal complex, iTMCs) forman parte del complejo electroluminiscente. A menudo, al complejo electroluminiscente se añaden también líquidos iónicos con el fin de mejorar las prestaciones del dispositivo. Cuando se aplica un voltaje Vbias a los contactos externos del dispositivo, los iones en la capa activa migran hacia los electrodos, donde generan un campo interfacial que disminuye labarrera de inyección de cargas electrónicas desde los mismos electrodos. Como consecuencia, electronesy huecos son inyectados en los niveles LUMO y HOMO del polímero o del complejo electroluminiscente (HOMO es el acrónimo de highest occupied molecular level, e indica el nivel de energía molecular ocupado más elevado, el análogo para los semiconductores orgánicos de la banda de valencia de los semiconductores inorgánicos; asimismo, LUMO, acrónimo de lowest unoccupied molecular orbitalindica el nivel energético molecular libre más bajo, el análogo para conductores orgánicos de la banda de conducción de los semiconductores inorgánicos). Las cargas inyectadas difunden en el volumen de la capa activa, donde se recombinan radiativamente emitiendo fotones (luz). La inyección de carga asistida por iones se autoregula, permitiendo la fabricación de un dispositivo emisor de luz sencillo y potencialmente barato, cuyo comportamiento es prácticamente independiente de la función de trabajo de los electrodos. El equivalente "no iónico" de los LECs son los más conocidos, y ya comercializados, diodos orgánicos emisores de luz (organic light-emitting diodes, OLEDs). Los OLEDs actualmente más avanzados son dispositivos multicapa cuya fabricación requiere procesos de deposición en múltiples pasos por medio de técnicas de vacío. Estas técnicas son lentas y costosas y limitan la viabilidad comercial de los OLEDs como alternativa económica para el mercado de la iluminación. Las técnicas de deposición por disolución en múltiples pasos son difíciles de poner en práctica, porque el disolvente necesario para depositar una capa puede dañar las capas subyacentes o ser incompatible con ella, generando problemas de uniformidad durante la deposición. En el caso de los OLEDs, el enfoque multicapa es necesarios, a fin de integrar en la estructura del dispositivo materiales para la inyección y el transporte de electrones y huecos y facilitar su transporte y su confinamiento hasta el interior de la capa electroluminiscente. Además, a diferencia de los LECs, los OLEDs a menudo requieren una encapsulación rigurosa (es decir, el contacto con el aire y la humedad tiene que ser eliminado o reducido al extremo), no sólo para proteger el material orgánico, sino también porque metales muy reactivos como Ba o Ca, con función de trabajo baja, son necesarios para asegurar una eficiente inyección de electrones desde el cátodo. Como mencionado anteriormente, estos requisitos no son indispensables en el caso de los LECs. Por lo tanto, en relación a la facilidad de fabricación y a la necesidad de encapsulación, el diferente principio de funcionamiento de los LECs los favorece respecto a los OLEDs, disminuyendo el coste final del dispositivo. Sin embargo, es importante puntualizar que mientras los LECs son un campo de investigación casi exclusivamente académico, los OLEDs representan una tecnología ya madura que se está usando en aplicaciones comerciales, como por ejemplo pantallas para televisores y teléfonos móviles. Por otro lado, la presencia de iones móviles en los LECs hace que éstos puedan potencialmente competir en un futuro en aplicaciones prácticas en las cuales el coste final sea un factor predominante a la hora de elegir entre las tecnologías disponibles. En muchos casos, la interacción entre los electrones y los iones puede mejorar las prestaciones de dispositivos ya existentes, como en el caso de los transistores de puerta electrolítica (electrolyte-gated transistors, EGTs). Estos dispositivos son los homólogos de los transistores de película delgada de estado sólido (thin- film transistors, TFTs). Los EGTs y los TFTs comparten el mismo principio de funcionamiento, siendo ambos transistores de efecto de campo, pero en los EGTs el aislante de la puerta se substituye por un electrolito líquido o polimérico en contacto directo con el canal semiconductor. Por medio de la aplicación de un voltaje de la puerta, los iones en el electrolitose acumulan en la interfaz con el semiconductor, formando lo que se conoce como una doble capa eléctrica (electrical double layer, EDL). Dado que la capacitancia de dicha EDL es mucho más alta que la de los aislantes comúnmente usados en los TFTs, y teniendo en cuenta que la corriente en un transistor es proporcional a dicha capacitancia, la corriente en los EGTs se puede modular usando voltajes de puerta extremadamente bajos (por lo general, inferior a 3V). Además, la capacitancia del EDL y las prestaciones de un EGT son en principio independientes del grosor del electrolito, simplificando la fabricación del transistor. Lo que es aún más importante, el hecho de que el electrolito esté en contacto directo con el semiconductor implica que éste puede ser modificado o funcionalizado, o que la misma arquitectura del transistor se puede variar, para que el dispositivo funcione come sensor de iones o moléculas en ambiente acuoso o fisiológico. Más recientemente, otra clase de dispositivos cuyo comportamiento está profundamente influenciado por las interacciones entre iones y electrones está recibiendo mucha atención por parte de la comunidad científica. Estos dispositivos son las células solares basadas en perovskitas híbridas orgánicas-inorgánicas (perovskite solar cells, PSCs), materiales con la estructura cristalina de tipo perovskita ycompuestos por cationes orgánicos intercalados en una estructura inorgánica. La perovskita más comúnmente utilizada es la de ioduro de plomo y metilammonio (CH3NH3PbI3 o MAPbI3, donde MA indica el catión orgánico metilamonio). La popularidad de las células solaresbasadas en estos materiales se debe a su extraordinaria eficiencia (que hoy en día supera el 20%), unida a la simplicidad y flexibilidad de los métodos de preparación, que van desde la deposición por técnicas de vacío al procesado por disolución. Por otro lado, pese a sus excepcionales eficiencias, las PSCs son dispositivos peculiares que, a diferencia de los dispositivos fotovoltaicos de estado sólido u orgánicos, a menudo muestran un cierto grado de histéresis en las medidas de densidad de corriente–voltaje (J–V), dependiendo de la velocidad y dirección del barrido. También son frecuentes fenómenos transitorios en la escala de tiempo de decenas o centenares de segundos, como por ejemplo un incremento gradual de la fotocorriente como consecuencia de la exposición a luz. Todo esto complica la obtención unívoca de la efectiva eficiencia de dichos dispositivos, que se determina normalmente a partir de simple barridos J–V . Una de las hipótesis que ha sido barajada para explicar este comportamiento contempla la presencia de iones móviles en la misma perovskita. Estos iones tienen efectos relevantes en el comportamiento de las PSCs y en principio podrían ser usados para mejorar su rendimiento. En este trabajo se demuestra que la interacción entre las cargas electrónicas y las cargas iónicas se puede emplear para desarrollar dispositivos de nueva concepción basados en diseños tradicionales y, además, se muestra que la presencia de determinadas especies iónicas móviles puede influenciar de manera significativa el comportamiento de dispositivos como las células electroquímicas emisoras de luz y las células solares. En particular, se hace referencia a los siguientes casos específicos: - La fabricación de un sensor de iones potasio K+ y de un sensor de glucosa a partir de un EGT basado en nanopartículas de ZnO como semiconductor. El sensor trabaja a bajo voltaje en un medio acuoso. El sensor de iones K+ se fabrica integrando una membrana selectiva de iones en la estructura del transistor, mientras que el sensor de glucosa se fabrica funcionalizando directamente las nanopartículas de ZnO por medio del enzima oxidasa de glucosa. - El estudio del efecto de la adición de sales de litio con diferentes aniones en las prestaciones y el tiempo de vida de iTMC-LECs. Se muestra que el tiempo de encendido y el tiempo de vida de los LECs depende fuertemente del anión de la sal de litio. En particular, un tiempo de vida de casi 2000h se obtiene cuanto litio tetrafluoroborato es usado como aditivo. - El estudio de la electroluminiscencia en diodos basados en la perovskita CH3NH3PbI3. Por medio de la polarización del material, los iones presentes en la perovskita se desplazan y facilitan la inyección de electrones en dispositivos con alta barriera de inyección. Esto conlleva un aumento de la electroluminiscencia y a la disminución de su voltaje de encendido, análogamente a lo que pasa para los LECs. Además, se demuestra la recuperación de la eficiencia de una célula solar degradada debido a la polarización externa, como consecuencia de la mejora en la extracción de cargas.The working mechanism of today’s most common solid-state optoelectronic devices such as field effect thin-film transistors (TFTs), light-emitting diodes (LEDs) and solar cells is based solely on electronic processes, meaning that only electronic charges play a role, while the effects related to ionic charges are either absent, irrelevant or even detrimental for their purposes. On the other hand, many applications do actually benefit from the presence of ions in the structure of a device. One typical example in the research field for inexpensive and efficient lighting sources are light-emitting electrochemical cells (LECs). These are light emitting devices composed by a single ionic electroluminescent layer sandwiched between two electrodes. The ions in the active layer are either added during the fabrication process (LECs based on electroluminescent polymers blended with ionic liquids) or are part of the electroluminescent compound (LECs based on ionic transition metal complexes (iTMCs), even if also in this case a small amount of ionic liquid or other salts is often added to enhance the LECs performances. The deposition of the active layer can be performed by simple solution processing (i.e. spin coating, dip coating, meniscus coating). When a voltage bias is applied at the external contacts, mobile ions in the active layer migrate towards the electrodes and favor the injection of holes and electrons from the electrodes themselves. When the injection barriers are overcome and if the bias is large enough (higher than the band gap of the active material divided by the elementary charge), electrons and hole are injected into the LUMO and the HOMO of the electroluminescent material, respectively (HOMO is the acronym for highest occupied molecular orbital, and is the analogue for organic semiconductors of the top of the valence band of inorganic semiconductors; similarly, LUMO is the acronym for lowest unoccupied molecular orbital, the analogue for organic semiconductors of the bottom of conduction band of inorganic semiconductors). The charge carriers are then driven by diffusion into the bulk of the active layer, where they recombine radiatively. The mechanism by which the charge carrier injection and transport take place has been object of an intense debate (electrochemical model vs. electrodynamical model vs. unified model), nonetheless the main principle is that the ionic-assisted charge injection allows the fabrication of simple and potentially inexpensive light-emitting devices whose performances are only weakly affected by the work function of the chosen electrodes, since the charge injection is self-regulated by the ionic mechanism. The “non-ionic” counterpart of LECs are the widely studied and already commercialized organic light-emitting diodes (OLEDs). State-of-the-art OLEDs are multi-layer devices whose manufacturing requires multi-step vacuum deposition techniques, which are still too expensive to make OLEDs a viable alternative in the mainstream lighting market. In general, multi-layer solution processing is difficult because the solvent needed to deposit a layer has to be carefully chosen to avoid damaging of the underlying stack of layers. This step, moreover, can introduce wetting issues which complicate the uniform deposition of the desired material. In the case of OLEDs, the multilayer approach is used in order to integrate the structure of the device with electron- and hole-injection and transport layers to facilitate the optimal displacement and confinement of charge carriers into the bulk of the electroluminescent material. Moreover, unlike LECs, OLEDs need rigorous encapsulation not only to protect the organic materials from air or moisture, but also because to ensure an efficient electron injection low work function, reactive metals such as Ba or Ca can be used as the cathode. These constraints can be circumvented in LECs, at least partially. It is important to point out that LECs and OLEDs show great differences in their performances such as efficiency, lifetime, and turn-on time. In general, while LECs are still an ongoing research topic, OLEDs are a mature technology that is already being used in commercial applications such as displays. In many cases the interplay between ions and electrons can boost the performances of already existing devices, as in the case of electrolyte-gated transistors (EGTs), that have their counterpart in traditional solid-state TFTs. As will be shown more in detail in the following sections, EGTs share the same working principle of TFTs, meaning that they are fundamentally (organic) field-effect transistors, (O)FETs, in which the gate insulator is replaced by a suitable electrolyte in direct contact with the semiconductor channel. Under the application of a small gate bias (typically below 3V), the ions in the electrolyte accumulate at the interface of the semiconductor, forming a thin electrical double layer (EDL). As the area capacitance of the EDL is much higher compared to that of common insulator, and considering that the current modulation in an (O)FET is proportional to that same capacitance, EGTs can be operated at a very low voltage. Moreover, unlike in classical (O)FETs, the performances of EGTs are in principle independent of the thickness of the electrolyte. This translates in a much simpler fabrication because processes such as drop casting or spin coating can be employed for the deposition of the electrolyte. Also, the organic semiconductor itself can be functionalized or, alternatively, the architecture of the transistor can be modified to make it suitable as a sensor for biologically relevant ions or analytes. More recently, another class of devices whose behavior is influenced by the interaction between ions and electrons has gained popularity among the research community. These are the solar cells based on hybrid organic-inorganic perovskites. Perovskite is the name of the crystalline structure of the photoactive material. The hybrid character of perovskites used in photovoltaics comes from these materials being composed by organic cations intercalated in an inorganic framework. The most commonly used perovskite for photovoltaics is methylammonium lead iodide (CH 3 NH 3 PbI 3 ). The popularity of the solar cells based on this material is due to the extraordinary photovoltaic performances that they can deliver, together with the flexibility of the preparation methods, ranging from vacuum deposition to simple solution processes. Despite their outstanding performances, perovskite solar cells (PSCs) are rather peculiar devices because, unlike classical solid state or even organic solar cells, they often present a certain degree of hysteresis in their current density–voltage measurements (J–V ), depending on the speed and direction of the voltage scan, and also transient phenomena in photocurrent on timescales of the order of hundreds of seconds are often observed. This leads to uncertainties about the “true” performances of these devices, which cannot be inferred confidently by a simple J–V scan. While many attempts have been made to explain and mitigate such a particular behavior, one of the hypotheses involves the presence of mobile ions in the perovskite layer itself. These ions influence the operation of a PSC and as such they can be used to tune or improve the extraction of photogenerated charges. In this work we demonstrate that the interplay between the ionic and the electronic charges can be employed as a tool for the development of new devices based on traditional paradigms, and show how the presence of given mobile ionic species can influence the behavior of devices such as light-emitting electrochemical cells and solar cells. In particular, this has been demonstrated using the following specific cases: - The fabrication and characterization of an EGT based on the zinc oxide semiconductor (ZnO) that works at low voltage in an aqueous environment. The EGT is integrated with an ion-selective membrane to serve as an ion sensor for the selective measurement of K+ ions in an electrolyte. Moreover, the transistor is used as a glucose sensor through the functionalization of the ZnO with the glucose oxidase enzyme. - The influence of lithium salts as substitutes for ionic liquids on the performances and lifetime of iTMC LECs. The behavior of the LECs is shown to be heavily dependent on the lithium counterion. A lifetime close to 2000h is achieved using lithium tetrafluoroborate as the additive. - The electroluminescence of a perovskite solar cell under bias. The effect of the bias on a unipolar device is the onset of an ambipolar charge injection regime and an improved electroluminescence, just like in the case of light- emitting electrochemical cells. Moreover, the efficiency of a degraded device can be partially recovered after biasing it for few minutes. The presence of mobile ions in the perovskite explains the similarities between the processe

Efficient vacuum deposited p-i-n and n-i-p perovskite solar cells employing doped charge transport layers

Methylammonium lead halide perovskites have emerged as high performance photovoltaic materials. Most of these solar cells are prepared via solution-processing and record efficiencies (>20%) have been obtained employing perovskites with mixed halides and organic cations on (mesoscopic) metal oxides. Here, we demonstrate fully vacuum deposited planar perovskite solar cells by depositing methylammonium lead iodide in between intrinsic and doped organic charge transport molecules. Two configurations, one inverted with respect to the other, p-i-n and n-i-p, are prepared and optimized leading to planar solar cells without hysteresis and very high efficiencies, 16.5% and 20%, respectively. It is the first time that a direct comparison between these two opposite device configurations has been reported. These fully vacuum deposited solar cells, employing doped organic charge transport layers, validate for the first time vacuum based processing as a real alternative for perovskite solar cell preparation

Phase behaviour of Ag2CrO4 under compression: Structural, vibrational, and optical properties

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/jp401524sWe have performed an experimental study of the crystal structure, lattice dynamics, and optical properties of silver chromate (Ag2CrO4) at ambient temperature and high pressures. In particular, the crystal structure, Raman-active phonons, and electronic band gap have been accurately determined. When the initial orthorhombic Pnma Ag2CrO4 structure (phase I) is compressed up to 4.5 GPa, a previously undetected phase (phase II) has been observed with a 0.95% volume collapse. The structure of phase II can be indexed to a similar orthorhombic cell as phase I, and the transition can be considered to be an isostructural transition. This collapse is mainly due to the drastic contraction of the a axis (1.3%). A second phase transition to phase III occurs at 13 GPa to a structure not yet determined. First-principles calculations have been unable to reproduce the isostructural phase transition, but they propose the stabilization of a spinel-type structure at 11 GPa. This phase is not detected in experiments probably because of the presence of kinetic barriers. Experiments and calculations therefore seem to indicate that a new structural and electronic description is required to model the properties of silver chromate.This study was supported by the Spanish government MEC under grants MAT2010-21270-C04-01/03/04 and CTQ2009-14596-C02-01, by the Comunidad de Madrid and European Social Fund (S2009/PPQ1551 4161893), by the MALTA Consolider Ingenio 2010 project (CSD2007-00045), and by the Vicerrectorado de Investigacion y Desarrollo of the Universidad Politecnica de Valencia (UPV2011-0914 PAID-05-11 and UPV2011-0966 PAID-06-11). A.M. and P.R.-H. acknowledge computing time provided by Red Espanola de Supercomputacion (RES) and MALTA-Cluster. J.A.S. acknowledges Juan de la Cierva Fellowship Program for its financial support. Diamond and ALBA Synchrotron Light Sources are acknowledged for provisions of beam time. We also thank Drs. Peral, Popescu, and Fauth for technical support.Santamaría Pérez, D.; Bandiello, E.; Errandonea, D.; Ruiz-Fuertes, J.; Gomis Hilario, O.; Sans, JÁ.; Manjón Herrera, FJ.... (2013). Phase behaviour of Ag2CrO4 under compression: Structural, vibrational, and optical properties. Journal of Physical Chemistry C. 117(23):12239-12248. https://doi.org/10.1021/jp401524sS12239122481172

Pressure Effects on the Optical Properties of NdVO₄

We report on optical spectroscopic measurements in pure NdVO4 crystals at pressures up to 12 GPa. The influence of pressure on the fundamental absorption band gap and Nd3+ absorption bands has been correlated with structural changes in the crystal. The experiments indicate that a phase transition takes place between 4.7 and 5.4 GPa. We have also determined the pressure dependence of the band-gap and discussed the behavior of the Nd3+ absorption lines under compression. Important changes in the optical properties of NdVO4 occur at the phase transition, which, according to Raman measurements, corresponds to a zircon to monazite phase change. In particular, in these conditions a collapse of the band gap occurs, changing the color of the crystal. The changes are not reversible. The results are analyzed in comparison with those deriving from previous studies on NdVO4 and related vanadates

P–V–T Equation of State of Iridium Up to 80 GPa and 3100 K

In the present study, the high-pressure high-temperature equation of the state of iridium has been determined through a combination of in situ synchrotron X-ray diffraction experiments using laser-heating diamond-anvil cells (up to 48 GPa and 3100 K) and density-functional theory calculations (up to 80 GPa and 3000 K). The melting temperature of iridium at 40 GPa was also determined experimentally as being 4260 (200) K. The results obtained with the two different methods are fully consistent and agree with previous thermal expansion studies performed at ambient pressure. The resulting thermal equation of state can be described using a third-order Birch–Murnaghan formalism with a Berman thermal-expansion model. The present equation of the state of iridium can be used as a reliable primary pressure standard for static experiments up to 80 GPa and 3100 K. A comparison with gold, copper, platinum, niobium, rhenium, tantalum, and osmium is also presented. On top of that, the radial-distribution function of liquid iridium has been determined from experiments and calculations

High-Pressure Structural Behavior and Equation of State of Kagome Staircase Compound, Ni3V2O8

We report on high-pressure synchrotron X-ray diffraction measurements on Ni3V2O8 at room-temperature up to 23 GPa. According to this study, the ambient-pressure orthorhombic structure remains stable up to the highest pressure reached in the experiments. We have also obtained the pressure dependence of the unit-cell parameters, which reveals an anisotropic compression behavior. In addition, a room-temperature pressure–volume third-order Birch–Murnaghan equation of state has been obtained with parameters: V0 = 555.7(2) Å3, K0 = 139(3) GPa, and K0′ = 4.4(3). According to this result, Ni3V2O8 is the least compressible kagome-type vanadate. The changes of the crystal structure under compression have been related to the presence of a chain of edge-sharing NiO6 octahedral units forming kagome staircases interconnected by VO4 rigid tetrahedral units. The reported results are discussed in comparison with high-pressure X-ray diffraction results from isostructural Zn3V2O8 and density-functional theory calculations on several isostructural vanadates

High-Pressure Spectroscopy Study of Zn(IO3)2 Using Far-Infrared Synchrotron Radiation

We report the first high-pressure spectroscopy study on Zn(IO3)2 using synchrotron far-infrared radiation. Spectroscopy was conducted up to pressures of 17 GPa at room temperature. Twenty-five phonons were identified below 600 cm−1 for the initial monoclinic low-pressure polymorph of Zn(IO3)2. The pressure response of the modes with wavenumbers above 150 cm−1 has been characterized, with modes exhibiting non-linear responses and frequency discontinuities that have been proposed to be related to the existence of phase transitions. Analysis of the high-pressure spectra acquired on compression indicates that Zn(IO3)2 undergoes subtle phase transitions around 3 and 8 GPa, followed by a more drastic transition around 13 GPa

Influence of mobile ions on the electroluminescence characteristics of methylammonium lead iodide perovskite diodes

In this work, we study the effect of voltage bias on the optoelectronic behavior of methylammonium lead iodide planar diodes. Upon biasing the diodes with a positive voltage, the turn-on voltage of the electroluminescence diminishes and its intensity substantially increases. This behavior is reminiscent of that observed in light-emitting electrochemical cells (LECs), single-layer electroluminescent devices in which the charge injection is assisted by the accumulation of ions at the electrode interface. Because of this mechanism, performances are largely independent from the work function of the electrodes. The similarities observed between planar perovskite diodes and LECs suggest that mobile ions in the perovskite do play an important rote in device operation. Besides enhanced electroluminescence, biasing these devices can also result in improved photovoltaic performance

Metal-Oxide-Free Methylammonium Lead Iodide Perovskite-Based Solar Cells: the Influence of Organic Charge Transport Layers

Metal-oxide-free methylammonium lead iodide perovskite-based solar cells are prepared using a dual-source thermal evaporation method. This method leads to high quality reproducible films with large crystal domain sizes allowing for an in depth study of the effect of perovskite film thickness and the nature of the electron and hole blocking layers on the device performance. The power conversion efficiency increases from 4.7% for a device with only an organic electron blocking layer to almost 15% when an organic hole blocking layer is also employed. In addition to the in depth study on small area cells, larger area cells (approx. 1 cm(-2)) are prepared and exhibit efficiencies in excess of 10%