Acousto-Electrically Driven Charge Carrier Dynamics in Metal Halide Perovskites: From Fundamental Studies to Sensor Applications

In recent research on solar cells that can surpass with the efficient conventional modules on the basis of silicon, ever more new materials are synthesized and tested. One class of these materials which is highly promising for the application in solar cells is in perovskite structure. After a few years of research it was already shown to reach efficiencies as high as commercial cells. This crystal structure can be formed with various different material combinations. This thesis focuses on the group of metal halide perovskites as they have very promising optoelectronic properties. Beside variations in the composition, structures with lower dimensionality have also gained more interest. Their very flexible and easy fabrication process paired with their very interesting and exceptionally good electronic properties makes them exciting objects to study. The high absorption and photoluminescence quantum yield in the perovskite nanostructures opens up a whole field of possible studies to conduct. This thesis deals with electrical characterizations and manipulations of the charge carrier dynamics in nanowires of metal halide perovskites. In order to investigate the mechanisms that happen during transport of electrons and holes in such small systems, the contact-free detection method of acousto-optoelectric spectroscopy via surface acoustic wave application is chosen. This method is especially helpful in the examination of these small structures because it lacks the necessity of precisely defining metal contacts on the sample. Instead, the electric field accompanying the mechanical wave is exploited to act upon the charge carriers in a quasi-static approach as the field is adapted to the velocity of the sound wave which is much slower than the speed of light in the medium. In this way the electrons and holes react to the wave and can follow its movement and thus, be transported. In CsPbI3 perovskite nanowires the effect on excitons, bound electron hole pairs, is observed. Time-resolved measurements show that both charge carriers have similar mobilities and that both are moved. The effect of the surface acoustic wave on an exciton can be described by two phenomena: a dissociation and a polarization of the quasi-particle. This is validated in this thesis by the application of a phenomenological model that mimics exactly those two processes. A subsequent numerical calculation of the drift and diffusion equations applied to the system reveals more insights into the dynamics that happen during the application of the surface acoustic wave and confirms that the mobilities of electrons and holes are indeed equal and can be quantified to around 3 cm2/Vs. Low temperature studies confirm the findings and provide a deeper understanding of the system. A final repetition of the time-resolved experiment on the perovskite structure CsPbBr3 corroborates the great influence of the exciton binding energy on the strength of the observed phenomena. Additionally, the easily bandgap-tunable nanowires are perfect sensing materials via the effect of photoconductivity. In a tapered transducer design, the spatially resolved detection of light absorption via the nanostructures is realized. Through the photo-induced charge carrier creation in material deposited in the travelling path, the transmission of a surface acoustic wave can be altered by the reciprocal impact of charge carriers and electric field. This is exploited in the construction of a wavelength and position resolved detector. In the measurements, the recorded absorption edge of several materials nicely reproduces optical absorption measurements and matches the energy of the photoluminescence of the nanowires. The addition of a perpendicular pair of surface acoustic wave transmitter and receiver enables a full two dimensional position detection. The switch from a simple delay line to a resonator-based design shows that the surface acoustic wave can even be used as a mass load detector with high sensitivity through the shifts in resonance frequency induced by the mass. Through the stiffness changes, this works even for rather light material and small amounts. Furthermore, this design is also capable of sensing the conductivity of deposited material analogously to the aforementioned system. As a proof of principle, the absorption edge of one exemplary material is presented for this chip as well, which coincides nicely with the previous investigations.In der aktuellen Forschung zu Solarzellen, die mit den effizienten Modulen aus industriellem Silizium mithalten können, werden immer mehr neue Materialien hergestellt und getestet. Eine sehr vielversprechende Klasse von solchen Materialien für den Einsatz in Solarzellen ist die der Perowskitstrukturen. Nach nur wenigen Jahren der Forschung konnten diese bereits die gleiche Effizienz erreichen wie kommerzielle Solarzellen. Die besondere Kristallstruktur kann mit vielfältigen Materialkombinationen erreicht werden. Diese Arbeit fokussiert sich dabei auf die Gruppe der Metall-Halogen-Perowskite. Neben den Variationen in der Zusammensetzung des Materials haben auch niederdimensionale Strukturen an Interesse gewonnen. Ihre Flexibiltät und einfache Herstellung gepaart mit ihren sehr interessanten und außergewöhnlich guten elektrischen Eigenschaften, machen sie zu einem spannenden Forschungsobjekt. Die hohe Absorptionsrate und die hohe Quantenausbeute in der Photolumineszenz eröffnen ein ganz neues Feld möglicher Studien. Diese Arbeit beschäftigt sich mit der elektronischen Charakterisierung und der Manipulation der Ladungsträger in Nanodrähten aus Metall-Halogen-Perowskit. Um die Mechanismen während des Ladungsträgertransportes in so kleinen Systemen untersuchen zu können, wurde die Methode der akusto-optoelektrischen Spektroskopie mithilfe von Oberflächenwellen gewählt. Diese Messmethode ist besonders dann hilfreich, wenn ultrakleine Strukturen untersucht werden sollen, da keine Metallkontakte hochpräzise auf der Probe aufgebracht werden müssen. Stattdessen wird das elektrische Feld, welches die mechanische Wellen auf einem piezoelektrischen Substrat begleitet, ausgenutzt, um die Ladungsträger zu beeinflussen. Dies geschieht quasi-statisch, da die Feldgeschwindigkeit an die Schallgeschwindigkeit der akustischen Welle angepasst ist und sich nicht mit der Lichtgeschwindigkeit im Substrat ausbreitet. Dadurch haben die Elektronen und Löcher genug Zeit auf das Feld zu reagieren und können der Bewegung der Welle folgen, wodurch ein Transport stattfindet. In den CsPbI3-Nanodrähten wird der Effekt der Wellen auf Exzitonen, gebundene Elektron-Loch-Paare, beobachtet. In zeitaufgelösten Messungen zeigt sich, dass beide Ladungsträger die gleiche Mobilität haben und beide von der Welle gleichermaßen bewegt werden. Der Effekt der Oberflächenwelle auf die Exzitonen kann durch zwei Phänomene beschrieben werden: die Aufspaltung und die Polarisation des Quasi-Teilchens. Das wird in der Arbeit durch die Anwendung eines phänomenologischen Models bestätigt, welches genau diese beiden Effekte modelliert. Eine numerische Simulation der Drift- und Diffusionsgleichungen, angewendet auf das zu untersuchende System, bringt tiefere Einblicke in die Dynamik der Ladungsträger innerhalb der Nanostruktur und bestätigt, dass die Mobilitäten von Elektronen und Löchern gleich sind und kann zudem deren Wert auf rund 3 cm2/Vs quantifizieren. Messungen bei tiefen Temperaturen untermauern die Ergebnisse und liefern ein tieferes Verständnis des Systems. Eine Wiederholung der zeitaufgelösten Messung an CsPbBr3-Nanodrähten bestätigt den großen Einfluss der Bindungsenergie der Exzitonen auf die Stärke des beobachteten Effekts. Neben der Untersuchung der Nanodrähte selbst, können diese durch ihre einfach anzupassende Bandlücke über ihre Photoleitfähigkeit auch als Sensormaterial dienen. In einem speziellen, konischen Design der Schallwandler kann eine ortsaufgelöste Detektion realisiert werden. Durch die photoinduzierte Ladungsträgerbildung in dem im Schallpfad deponierten Material kann die Übertragung einer akustischen Oberflächenwelle durch die wechselseitige Wirkung von Ladungsträgern und elektrischem Feld verändert werden. Dies wird bei der Konstruktion eines wellenlängen- und positionsaufgelösten Detektors genutzt. Während der Messungen bildet die aufgenommene Absorptionskante mehrerer Materialien optische Absorptionsmessungen gut nach und passt zur Photolumineszenz der Nanodrähte. Das Hinzufügen eines senkrechten Paares von akustischem Oberflächenwellen-Sender und -Empfänger ermöglicht eine vollständige zweidimensionale Positionserfassung. Der Wechsel von einer einfachen Verzögerungsstrecke zu einem resonatorbasierten Design zeigt, dass die SAW auch als Massendetektor mit hoher Empfindlichkeit eingesetzt werden kann, wobei die Masse Verschiebungen in der Resonanzfrequenz induziert. Durch die Steifigkeitsänderungen funktioniert dies auch bei eher leichtem Material und kleinen Mengen. Darüber hinaus ist diese Anordnung auch in der Lage, zusätzlich die Leitfähigkeit des abgeschiedenen Materials analog zum vorgenannten System zu detektieren. Als Proof-of-principle wird auch für diesen Chip die Absorptionskante eines exemplarischen Materials vorgestellt, die gut mit den bisherigen Studien übereinstimmt

Advanced Materials and Technologies in Nanogenerators

This reprint discusses the various applications, new materials, and evolution in the field of nanogenerators. This lays the foundation for the popularization of their broad applications in energy science, environmental protection, wearable electronics, self-powered sensors, medical science, robotics, and artificial intelligence

Applications of MXenes in human-like sensors and actuators

Human beings perceive the world through the senses of sight, hearing, smell, taste, touch, space, and balance. The first five senses are prerequisites for people to live. The sensing organs upload information to the nervous systems, including the brain, for interpreting the surrounding environment. Then, the brain sends commands to muscles reflexively to react to stimuli, including light, gas, chemicals, sound, and pressure. MXene, as an emerging two-dimensional material, has been intensively adopted in the applications of various sensors and actuators. In this review, we update the sensors to mimic five primary senses and actuators for stimulating muscles, which employ MXene-based film, membrane, and composite with other functional materials. First, a brief introduction is delivered for the structure, properties, and synthesis methods of MXenes. Then, we feed the readers the recent reports on the MXene-derived image sensors as artificial retinas, gas sensors, chemical biosensors, acoustic devices, and tactile sensors for electronic skin. Besides, the actuators of MXene-based composite are introduced. Eventually, future opportunities are given to MXene research based on the requirements of artificial intelligence and humanoid robot, which may induce prospects in accompanying healthcare and biomedical engineering applications. [Figure not available: see fulltext.

Research status and prospect of graphene materials in aviation

Among various 2D materials, graphene has received extensive research attention in the past 2-30 years due to its fascinating properties. The discovery of graphene has provided a huge boost and a new dimension to materials research and nanotechnology. Many lightweight materials with good performance have been widely used in the aviation field, which has greatly promoted the development of military and civilian industries and promoted technological innovation. Based on the introduction of the structure and properties of graphene, this paper summarizes the application value of graphene in the aerospace field in three aspects: energy equipment, sensors, and composite materials used outside aircraft.Comment: (22 pages, 23 figures

Piezoelectric Materials

The science and technology in the area of piezoelectric ceramics are extremely progressing, especially the materials research, measurement technique, theory and applications, and furthermore, demanded to fit social technical requests such as environmental problems. While they had been concentrated on piezoelectric ceramics composed of lead-containing compositions, such as lead zirconate titanate (PZT) and lead titanate, at the beginning because of the high piezoelectricity, recently lead water pollution by soluble PZT of our environment must be considered. Therefore, different new compositions of lead-free ceramics in order to replace PZT are needed. Until now, there have been many studies on lead-free ceramics looking for new morphotropic phase boundaries, ceramic microstructure control to realize high ceramic density, including composites and texture developments, and applications to new evaluation techniques to search for high piezoelectricity. The purpose of this book is focused on the latest reports in piezoelectric materials such as lead-free ceramics, single crystals, and thin films from viewpoints of piezoelectric materials, piezoelectric science, and piezoelectric applications

Aligned Silver Nanowire Networks as Transparent Electrodes for High-Performance Optoelectronics and Electronic Devices

Department of Energy EngineeringFlexible transparent electrode is an essential component for several kinds of electronic and optoelectronic applications, such as organic solar cells (OSCs), perovskite solar cells (PSCs), organic light-emitting diodes (OLEDs), touch sensors, and electronic skins (E-skins). Although conventional indium tin oxide (ITO) has been widely used in commercial transparent electrodes, it still shows a limitation in the fabrication of flexible transparent electrodes for applications in flexible/wearable electronic devices because of their inherent brittleness. Among various alternatives of ITO, silver nanowire (AgNW) network has been considered as promising conductive nano-material due to their high electrical conductivity, excellent transmittance, and mechanical flexibility that can be readily deposited by cost-effective and large-scale solution process. However, random AgNW networks prepared by solution processing have several drawbacks, such as high junction resistance between nanowires (NWs), low transmittance, haze issues, and rough surface morphologies, resulting in a degradation of the device performance. Electrical and optical properties of random AgNW networks can be strongly affected by controlling NW density, electrical current path, and junction resistance related to conductive percolated networks. Therefore, manipulating the assembled structure of AgNW network can provide powerful platforms to realize ideal flexible transparent electrodes with high electrical conductivity, superior transmittance, and smooth surface morphologies for achieving high-performance electronic and optoelectronic device. In this thesis, we introduce aligned AgNW transparent electrodes and their applications in flexible optoelectronic and functional electronic devices. Firstly, Chapter 1 introduces the research tends in transparent electrodes and several issues of AgNW networks that should be carefully considered in the fabrication for their potential device applications. In chapter 2, we demonstrate the capillary printing technique to make highly aligned AgNW network to fabricate high-performance transparent electrodes for improving device efficiency of optoelectronic devices including OSCs and OLEDs. In Chapter 3, we demonstrate the fabrication of nanoparticle (NP)-enhanced plasmonic AgNW electrode for high-performance optoelectronic devices in which the NP-NW hybrid plasmonic system generates gap plasmonic coupling which induces a large electric field enhancement, resulting in an improvement of the device efficiency in both OSC and OLED devices. In Chapter 4, we demonstrate the fabrication of ultrathin and flexible perovskite solar cell foils with orthogonal AgNW electrodes, which exhibits high power-per-weight performance as well as a conformal contact capability to curvilinear surface. In Chapter 5, we introduce a large-scale assembly technique to uniformly align AgNW arrays for the fabrication of large area transparent electrodes, where cross-aligned AgNW network shows better electrical and optical properties as well as large-scale uniformity than random AgNW network. For the proof of the concept demonstration, we fabricated a flexible force-sensitive touch screen panel integrated with a mechanochromic polymer film. Finally, we introduce a transparent and conductive nano-membrane (NM) incorporated with orthogonal AgNW arrays in Chapter 6, which exhibits enhanced electrical and mechanical properties than pure polymeric NMs. To show the unique properties of these hybrid NMs for potential device applications, we demonstrate skin-attachable thermoacoustic-based NM loudspeaker and wearable NM microphone, both of which show much improved device performances compared to conventional thin film-based devices. In this thesis, studies on aligned AgNW transparent electrodes and their device applications could be further expanded for diverse flexible and wearable optoelectronic and electronic applications, such as conformal wearable sensors, healthcare monitoring devices, and wearable plasmonic devices.clos

Semiconductor Infrared Devices and Applications

Infrared (IR) technologies—from Herschel’s initial experiment in the 1800s to thermal detector development in the 1900s, followed by defense-focused developments using HgCdTe—have now incorporated a myriad of novel materials for a wide variety of applications in numerous high-impact fields. These include astronomy applications; composition identifications; toxic gas and explosive detection; medical diagnostics; and industrial, commercial, imaging, and security applications. Various types of semiconductor-based (including quantum well, dot, ring, wire, dot in well, hetero and/or homo junction, Type II super lattice, and Schottky) IR (photon) detectors, based on various materials (type IV, III-V, and II-VI), have been developed to satisfy these needs. Currently, room temperature detectors operating over a wide wavelength range from near IR to terahertz are available in various forms, including focal plane array cameras. Recent advances include performance enhancements by using surface Plasmon and ultrafast, high-sensitivity 2D materials for infrared sensing. Specialized detectors with features such as multiband, selectable wavelength, polarization sensitive, high operating temperature, and high performance (including but not limited to very low dark currents) are also being developed. This Special Issue highlights advances in these various types of infrared detectors based on various material systems

Gas Sensors Based on Electrospun Nanofibers

Nanofibers fabricated via electrospinning have specific surface approximately one to two orders of the magnitude larger than flat films, making them excellent candidates for potential applications in sensors. This review is an attempt to give an overview on gas sensors using electrospun nanofibers comprising polyelectrolytes, conducting polymer composites, and semiconductors based on various sensing techniques such as acoustic wave, resistive, photoelectric, and optical techniques. The results of sensing experiments indicate that the nanofiber-based sensors showed much higher sensitivity and quicker responses to target gases, compared with sensors based on flat films

Perovskites-Based Nanomaterials for Chemical Sensors

The perovskite structure is adopted by many compounds in solid-state chemistry. The sensitivity, selectivity, and stability of many perovskite nanomaterials have been devoted the most attention for chemical sensors. They are capable to sense the level of small molecules such as O2, NO, and CO. This chapter provides a comprehensive overview of perovskite nanoscale materials that concentrate on chemical sensors. The perovskite structure, with two differently sized cations, is amenable to a variety of dopant additions. This flexibility allows for the control of transport and catalytic properties, which are important for improving sensor performance. We devote the most attention on the synthesis, structural information, and sensing mechanism. We will later elaborate on the development mechanism of chemical sensors based on perovskite nanomaterials. We conclude this chapter with the personal perspectives on the directions toward future works on a novelty of nanostructured chemical sensors

High temperature sensor/microphone development for active noise control

The industrial and scientific communities have shown genuine interest in electronic systems which can operate at high temperatures, among which are sensors to monitor noise, vibration, and acoustic emissions. Acoustic sensing can be accomplished by a wide variety of commercially available devices, including: simple piezoelectric sensors, accelerometers, strain gauges, proximity sensors, and fiber optics. Of the several sensing mechanisms investigated, piezoelectrics were found to be the most prevalent, because of their simplicity of design and application and, because of their high sensitivity over broad ranges of frequencies and temperature. Numerous piezoelectric materials are used in acoustic sensors today; but maximum use temperatures are imposed by their transition temperatures (T(sub c)) and by their resistivity. Lithium niobate, in single crystal form, has the highest operating temperature of any commercially available material, 650 C; but that is not high enough for future requirements. Only two piezoelectric materials show potential for use at 1000 C; AlN thin film reported to be piezoactive at 1150 C, and perovskite layer structure (PLS) materials, which possess among the highest T(sub c) (greater than 1500 C) reported for ferroelectrics. A ceramic PLS composition was chosen. The solid solution composition, 80% strontium niobate (SN) and 20% strontium tantalate (STa), with a T(sub c) approximately 1160 C, was hot forged, a process which concurrently sinters and renders the plate-like grains into a highly oriented configuration to enhance piezo properties. Poled samples of this composition showed coupling (k33) approximately 6 and piezoelectric strain constant (d33) approximately 3. Piezoactivity was seen at 1125 C, the highest temperature measurement reported for a ferroelectric ceramic. The high temperature piezoelectric responses of this, and similar PLS materials, opens the possibility of their use in electronic devices operating at temperatures up to 1000 C. Concurrent with the materials study was an effort to define issues involved in the development of a microphone capable of operation at temperatures up to 1000 C; important since microphones capable of operation above 260 C are not generally available. The distinguishing feature of a microphone is its diaphragm which receives sound from the atmosphere: whereas, most other acoustic sensors receive sound through the solid structure on which they are installed. In order to gain an understanding of the potential problems involved in designing and testing a high temperature microphone, a prototype was constructed using a commercially available lithium niobate piezoelectric element in a stainless steel structure. The prototype showed excellent frequency response at room temperature, and responded to acoustic stimulation at 670 C, above which temperature the voltage output rapidly diminished because of decreased resistivity in the element. Samples of the PLS material were also evaluated in a simulated microphone configuration, but their voltage output was found to be a few mV compared to the 10 output of the prototype