Recent advances in electronic and optoelectronic Devices Based on Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides (2D TMDCs) offer several attractive features for use in next-generation electronic and optoelectronic devices. Device applications of TMDCs have gained much research interest, and significant advancement has been recorded. In this review, the overall research advancement in electronic and optoelectronic devices based on TMDCs are summarized and discussed. In particular, we focus on evaluating field effect transistors (FETs), photovoltaic cells, light-emitting diodes (LEDs), photodetectors, lasers, and integrated circuits (ICs) using TMDCs

Photodetectors based on low-dimensional materials and hybrid systems

Premi extraordinari doctorat UPC curs 2015-2016, àmbit de CiènciesIn the last decade, two-dimensional (2D) materials have attracted attention both in the nascent field of flexible nanotechnology as well as in more conventional semiconductor technol-ogies. Within the rapidly expanding portfolio of 2D materials, the group of semiconducting transition metal dichalcogenides (TMDCs) has emerged as an intriguing candidate for various optoelectronic applications. The atomically thin profile, favorable bandgap and outstanding electronic properties of TMDCs are unique features that can be explored and applied in novel photodetecting platforms. This thesis presents highly sensitive two-dimensional phototransistors made of sub-nanometre thick TMDC channels. Firstly, an encapsulation route is developed to address the detrimental and, to date, uncontrollable impact of atmospheric adsorbates, which severely deteriorate detector performance. The passivation scheme improves the transport properties of TMDCs, leading to high photoconductive gain with gate dependent responsivity of 10 -10^4 A/W throughout the visible, and temporal response down to 10 ms, which is suitable for imaging applications. The atomic device thickness yields ultra-low dark current operation and record detectivity of 10^11 - 10^12 Jones for TMDC-based detectors is achieved. The use of monolayer TMDCs, however, has disadvantages like limited spectral absorption due to the bandgap and limited absorption efficiency. In order to increase the absorption and to extend the spectral coverage, TMDC channels are covered with colloidal quantum dots to make hybrid phototransistors. This compelling synergy combines strong and size-tunable light absorption within the QD film, efficient charge separation at the TMDC-QD interface and fast carrier transport through the 2D channel. This results in large gain of 10^6 electrons per absorbed photon and creates the basis for extremely sensitive light sensing. Colloidal quan-tum dots are an ideal sensitizer, because their solution-processing and facile implementation on arbitrary substrates allows for low-cost fabrication of hybrid TMDC-QD devices. Moreover, the custom tailored bandgap of quantum dots provides the photodetector with wide spectral tunability. For photodetection in the spectral window of NIR/SWIR, which is still dominated by expensive and complex epitaxy-based technologies, these hybrid detectors have the potential to favorably compete with commercially available systems. The interface of the TMDC-QD hybrid is of paramount importance for sensitive detector operation. A high density of trap states at the interface is shown to be responsible for inefficient gate-control over channel conductivity, which leads to high dark currents. To maintain the unique electrical field-effect modulation in TMDCs upon deposition of colloidal quantum dots, a passivation route of the interface with semiconducting metal-oxide films is developed. The buffer-layer material is selected such that charge transfer from QDs into the channel is favored. The retained field-effect modulation with a large on/off ratio allows operation of the phototransistor at significantly lower dark currents than non-passivated hybrids. A TMDC-QD phototransistor with an engineered interface that exhibits detectivity of 10^12 - 10^13 Jones and response times of 12 ms and less is reported. In summary, this work showcases prototype photodetectors made of encapsulated 2D TMDCs and TMDC-QD hybrids. Plain TMDC-detectors have potential for application as flexible and semi-transparent detector platforms with high sensitivity in the visible. The hybrid TMDC-QD device increases its spectral selectivity to the NIR/SWIR due to the variable absorption of the sensitizing quantum dots and reaches compelling performance thanks to im-proved light-matter interaction and optimized photocarrier generation.En la última década ha surgido un gran interés por los materiales bidimensionales (2D) tanto para las tecnologías emergentes de dispositivos flexibles, como para las tecnologías de semiconductores tradicionales. Dentro del creciente catálogo de materiales 2D, los semiconductores basados en dicalcogenuros de metales de transición (DCMTs) han surgido como candidatos para aplicaciones optoelectrónicas. Sus características únicas, tales como grosor atómico, banda prohibida y propiedades electrónicas pueden ser examinadas y aplicadas en nuevas plataformas de fotodetección. En esta tesis se presentan nuevos fototransistores bidimensionales ultrasensibles basados en canales de DCMTs subnanométricos. Se presenta una ruta de encapsulación para intentar solucionar el impacto negativo, e incontrolable hasta la fecha, producido por la adsorción de sustancias atmosféricas que degradan el funcionamiento de los detectores. Este proceso mejora el transporte en los DCMTs dando lugar a una gran ganancia fotoconductora, una respuesta, dependiente de la tensión aplicada en el gate, de 10-10^4 A/W en el visible y una respuesta temporal de tan solo 10 ms, todo ello adecuado para aplicaciones de imagen. El grosor atómico de los dispositivos da lugar a corrientes de oscuridad muy bajas y una detectividad de 10^11-10^12 Jones. Sin embargo, el uso de monocapas de DCMTs presenta ciertas desventajas como por ejem-plo una eficiencia en la absorción baja. Con el fin de mejorar la absorción, los canales de DCMTs se han recubierto con puntos cuánticos (QDs) para fabricar fototransistores híbridos. Esta sinergia combina la alta absorción de los QDs, una eficiente separación de cargas en la interfaz DCMT-QD y un rápido transporte de cargas a través del canal 2D. Todo esto resulta en una ganancia de 10^6 electrones por fotón absorbido y crea la base para sensores de luz extremadamente sensibles. Los puntos cuánticos coloidales son sensibizadores ideales ya que su procesado en disolución y su fácil incorporación sobre cualquier sustrato permiten la fabricación de sistemas híbridos DCMT-QD a bajo coste. Además, la posibilidad de modifi-car la banda prohibida, ofrecida por los QDs, proporciona al fotodetector una amplia respuesta espectral. Para fotodetección en la ventana espectral del infrarrojo cercano (NIR/SWIR), estos detectores híbridos presentan el potencial de competir favorablemente con los sistemas comerciales disponibles. La interfaz entre el híbrido DCMT-QD es de la mayor importancia para la sensibilidad del detector. Se ha demostrado que una alta densidad de trampas en la interfaz es la responsable del ineficiente control mediante el gate de la conductividad del canal, dando lugar a corrientes de oscuridad muy altas. Para mantener la excepcional modulación de efecto campo aún después de la deposición de los QDs, se ha desarrollado una ruta de pasivación de la interfaz con óxidos metálicos semiconductores. El material de esta capa amortiguadora (buffer) es seleccionado de tal manera que permita la transferencia de cargas desde los puntos cuánticos hasta el canal DCMT. Esto retiene la modulación de efecto campo con una relación encendido/apagado muy alta, permitiendo el funcionamiento del fototransistor con corrientes de oscuridad significativamente menores que las de los híbridos sin pasivar. Así, se presenta un fototransistor híbrido DCMT-QD, con una interfaz cuidadosamente diseñada, que exhibe una detectividad de 10^12-10^13 Jones. En resumen, este trabajo presenta unos prototipos de fotodetectores basados en DCMT 2D encapsulados y en híbridos DCMT-QD. Los fotodetectores basados en DCMT simples presentan potencial para su aplicación en detectores flexibles y semitransparentes, con gran sensibilidad en el visible. Los híbridos DCMT-QD amplían la selectividad espectral al infrarrojo cercano gracias a la absorción variable ofrecida por los puntos cuánticos y alcanzan un muy interesante rendimiento gracias a una mejor interacción luz-materia.Award-winningPostprint (published version

Optoelectronic devices based on van der Waals heterostructures

In this thesis we investigate the use of van der Waals heterostructures in optoelec- tronic devices. An improvement in the optical and electronic performance of specific devices can be made by combining two or more atomically thin materials in layered structures. We demonstrate a heterostructure photodetector formed by combining graphene with tungsten disulphide. These photodetectors were found to be highly sensitive to light due to a gain mechanism that produced over a million electrons per photon. This arises from the favourable electrical properties of graphene and the strong light-matter interaction in WS2 . An analysis of the photodetector per- formance shows that these devices are capable of detecting light under moonlight illuminations levels at video-frame-rate speeds with applications in night vision ima- ging envisaged. We also report a novel method for the direct laser writing of a high-k dielectric embedded inside a van der Waals heterostructure. Such structures were shown to be capable of both light-detection and light-emission within the same de- vice architecture, paving the way for future multifunctional optoelectronic devices. Finally we address a more fundamental problem in the properties of aligned grap- hene/hBN heterostructures. Strain distributions are shown to modify the electronic properties of graphene due to a change in the interlayer interaction. We demon- strates a method to engineer these strain patterns by contact geometry design and thermal annealing strategies.Engineering and Physical Sciences Research Council (EPSRC

High-performance self-powered ultraviolet photodetector based on PVK/amorphous-WO3 organic-inorganic heterojunction

Ultraviolet (UV) photodetectors have found wide-ranging applications, ranging from optical communications to chemical detection. High performance UV photodetectors that can be self-powered are highly desirable in many applications as they can minimize energy consumption during operation. Herein, a self-powered UV photodetector, which consisted of poly(9-vinylcarbazole) (PVK)/amorphous-WO3 organic-inorganic heterojunction with PEDOT:PSS as a hole transport layer, was fabricated using a two-step method at low temperature. The effect of WO3, PVK and PEDOT:PSS films on the performances of the photodetector was also investigated. Under optimized parameters, the PEDOT:PSS/PVK/WO3 photodetector exhibited a maximum responsivity of 12.41 AW−1, specific detectivity of 1.80 × 1013 Jones, photo-dark current ratio of 103 at reverse bias and typical rectification characteristic when exposed to 365 nm light irradiation. The photoelectric conversion mechanism of this novel PVK/WO3 heterojunction is discussed using energy band diagrams. This work presents a method to produce a high performance WO3-based heterostructure at low temperature, which has the potential for UV imaging

Exciton Transport in Organic Semiconductors

University of Minnesota Ph.D. dissertation. June 2015. Major: Material Science and Engineering. Advisor: Russell Holmes. 1 computer file (PDF); xvi, 240 pages.Photovoltaic cells based on organic semiconductors are attractive for their use as a renewable energy source owing to their abundant feedstock and compatibility with low-cost coating techniques on flexible substrates. In contrast to photovoltaic cells based traditional inorganic semiconductors, photon absorption in an organic semiconductor results in the formation of a coulombically bound electron-hole pair, or exciton. The transport of excitons, consequently, is of critical importance as excitons mediate the interaction between charge and light in organic photovoltaic cells (OPVs). In this dissertation, a strong connection between the fundamental photophysical parameters that control nanoscopic exciton energy transfer and the mesoscopic exciton transport is established. With this connection in place, strategies for enhancing the typically short length scale for exciton diffusion (LD) can be developed. Dilution of the organic semiconductor boron subphthalocyanine chloride (SubPc) is found to increase the LD for SubPc by 50%. In turn, OPVs based on dilute layers of SubPc exhibit a 30% enhancement in power conversion efficiency. The enhancement in power conversion efficiency is realized via enhancements in LD, optimized optical spacing, and directed exciton transport at an exciton permeable interface. The role of spin, energetic disorder, and thermal activation on LD are also addressed. Organic semiconductors that exhibit thermally activated delayed fluorescence and efficient intersystem and reverse intersystem crossing highlight the balance between singlet and triplet exciton energy transfer and diffusion. Temperature dependent measurements for LD provide insight into the inhomogeneously broadened exciton density of states and the thermal nature of exciton energy transfer. Additional topics include energy-cascade OPV architectures and broadband, spectrally tunable photodetectors based on organic semiconductors

Optical Properties of Solar Cells Based on Zinc(hydr)oxide and its Composite with Graphite oxide Sensitized by Quantum Dots

This thesis research focuses on developing a hybrid form of solar cell based on zinc(hydr)oxide and its composites with graphite oxide, TiO2, quantum dot, electrolyte and . This work expands upon the Gratzel solar cell with a dye in TiO2. Due to various structural and optical characteristics, zinc(hydr)oxide (Zn(OH)2) and its porous composites with 2% and 5% graphite oxide(GO) can be used for various applications including the manufacture of various , protective elements in electric and electronic appliances, as gas-sensors, catalysts, in cosmetics a UV light absorber, and solar cells. In this research, both TiO2 and zinc(hydr)oxide (refer as Zn(OH)2 ) and its porous composites with 2% and 5% graphite oxide(GO) (refer as ZnGO-2 and ZnGO-5 respectively) have been studied with photoactive quantum dots (QDs). The goal of this research was to understand the optical properties of Zn (OH)2, ZnGO-2 and ZnGO-5 (i.e. We determined the band gap of these three materials using absorptions, photoluminescence and photo-conductivity), structural characterizations of these three samples (i.e.We determined critical points transitions), time resolved fluorescence of these three materials, (i.e. We determined the lifetime of the carrier, the rise time of the carrier, relaxation, and carrier density using new model) and application in the Gratzel like quantum dots (QDs) sensitized hybrid Zn(OH)2/ZnGO solar cells compared with TiO2 which made with QD(CdSe and PbS ).The main objective of this thesis was to develop hybrid solar cell with Zn(OH)/ZnGO material and TiO2 to make alternative designs for the fabrication of quantum dots sensitized Zn(OH)/ZnGO hybrid solar cells that can enhance the efficiencies as well as reduce the cost by making it more amenable to large scale production. In hybrid solar cells, the role of the conductive glass substrate with the performance of Zn(OH)2/ZnGO/QD(CdSe and/or PbS) /Electrolyte or Perovskite based photovoltaic devices, as well as fabricate new photovoltaic devices, to improve the efficiencies in power. The focus of this doctoral thesis was to introduce new composite material such as Zn(OH)2 with QDs CdSe(visible), PbS(UV and NIR) for their applications in new solar cells which incorporate energy transfer processes in order to improve light harvesting.The basic idea to obtain maximum energy efficiency is to absorb most of the solar spectrum in the 3 zones: UV, visible and NIR light. The photoexcitation and processes involved with carriers excitons and multiexcition generations in uv

Radiation Hardness and Defects Activity in PEA2PbBr4 Single Crystals

Metal halide perovskites (MHPs) are low-temperature processable hybrid semiconductor materials with exceptional performances that are revolutionizing the field of optoelectronic devices. Despite their great potential, commercial deployment is hindered by MHPs lack of stability and durability, mainly attributed to ions migration and chemical interactions with the device electrodes. To address these issues, 2D layered MHPs have been investigated as possible device interlayers or active material substitutes to reduce ion migration and improve stability. Here we consider the 2D perovskite PEA2PbBr4 that was recently discussed as very promising candidate for X-ray direct detection. While the increased resilience of PEA2PbBr4 detectors have already been reported, the physical mechanisms responsible for such improvement compared to the standard "3D" perovskites are not still fully understood. To unravel the charge transport process in PEA2PbBr4 crystals thought to underly the device better performance, we adapted an investigation technique previously used on highly resistive inorganic semiconductors, called photo induced current transient spectroscopy (PICTS). We demonstrate that PICTS can detect three distinct trap states (T1, T2, and T3) with different activation energies, and that the trap states evolution upon X-ray exposure can explain PEA2PbBr4 superior radiation tolerance and reduced aging effects. Overall, our results provide essential insights into the stability and electrical characteristics of 2D perovskites and their potential application as reliable and direct X-ray detectors

Innovative Growth Techniques and Heterogeneous Structures for PbSe-Based MWIR Photodetectors

Lead-chalcogenide semiconductor materials such as PbSe are an attractive class of narrow band-gap semiconductors due to their unique optical and electrical properties, finding their way into numerous mid-wave infrared (MWIR) optoelectronic and topological device applications. While PbSe is a mature material system with research efforts dating back almost a century, the full potential of its physical properties has yet to be realized in fabricated devices. Much of the difficulty lies in the unique bonding nature and crystal structure of PbSe films, which gives both advantageous MWIR optoelectrical properties along with a limited selection of suitable substrates for high-quality growth. Further, PbSe homojunction devices on dissimilar substrates face issues with doping, where diffusion of PbSe dopants through defect channels inhibit the formation of an abrupt junction resulting in degraded device performance. Further, p-n heterojunction formation with PbSe is difficult due to the small bandgap of PbSe, resulting in a shortlist of suitable materials with the corresponding band alignment to form a type-II heterojunction. Even for material systems that share a suitable band alignment, issues often arise with dissimilar crystallinity, lattice constant, and thermal expansion coefficient. The dissimilarity between these films introduces large stress/ strain relations at the interface, resulting in the formation of cracks or dislocations which ruin the interface and bulk electrical properties. For these reasons, PbSe-based MWIR devices have been surpassed by more competitive material systems such as II-VI HgCdTe, and Sb-based type-II superlattices. In this work, new methods for improving PbSe film quality are explored, along with the growth and design of new heterogeneous structures and MWIR photodetectors which may improve the PbSe-based MWIR sensing platform. Presented here will be a new approach for creating heterogeneous material structures with PbSe by molecular beam epitaxy (MBE). Demonstration of a new heterogenous p-n junction structure between mismatched germanium substrates and epitaxial lead selenide thin-films will be introduced utilizing a vicinal growth surface. Extending from this, epitaxial PbSe films with enhanced surface morphology will also be demonstrated on vicinal silicon substrates, showcasing record low surface defect densities compared to traditional growth on nominal silicon. Regarding the former, germanium will also serve as an active layer in the formation of a p-n heterojunction structure due to its suitable type-II band alignment with PbSe. However, large differences in lattice constant and thermal expansion coefficient will need to be addressed to form such a structure. These challenges are tackled by optimizing the surface kinetics of PbSe adatoms through high-temperature surface treatment, along with utilizing misfit accommodation steps induced by the periodic atomic step edges from the high degree of vicinal miscut of the germanium growth surface. Further, different structures and device applications of PbSe-based material systems will be explored, including the growth of a new PbOSe complex oxide thin-film via oxygen-plasma assisted MBE deposition. Fabrication of a single phase (cubic) all-epitaxial n-CdSe/p-PbSe heterojunction structure with room temperature MWIR detection capabilities will also be presented, along with the fabrication of MWIR transparent contacts using cadmium oxide thin-films for enhanced photodetector device design. The combined results of these efforts provide multiple avenues for the development of state-of-the-art MWIR PbSe-based photodetectors, enabling future commercial MWIR sensing capabilities with reduced size, weight, power consumption, and cost