Solution-processed semiconductors for next-generation photodetectors

Efficient light detection is central to modern science and technology.Current photodetectors mainly use photodiodes based on crystalline inorganic elementalsemiconductors, such as silicon, or compounds such as III–V semiconductors. Photodetectorsmade of solution-processed semiconductors — which include organic materials, metal-halideperovskites and quantum dots — have recently emerged as candidates for next-generation lightsensing. They combine ease of processing, tailorable optoelectronic properties, facile integrationwith complementary metal–oxide–semiconductors, compatibility with flexible substrates andgood performance. Here, we review the recent advances and the open challenges in the field ofsolution-processed photodetectors, examining the topic from both the materials and the deviceperspective and highlighting the potential of the synergistic combination of materials and deviceengineering. We explore hybrid phototransistorsand their potential to overcome trade-offsin noise, gain and speed, as well as the rapid advances in metal-halide perovskite photodiodesand their recent application in narrowband filterless photodetection

Enhancing Charge Carrier Mobility in Colloidal Quantum Dots For Technological Applications

Colloidal quantum dots (QD) are promising semiconducting materials to engineer photovoltaic and optoelectronic devices due to tunable size-dependent absorption and emission properties. These materials are important as they don’t need complicated equipment and huge setup investment for industrial applications. If formulated into a kind of stable nano-ink, these QDs can be incorporated into devices using the most economical processing technologies, spray or roll to roll printing. More importantly, these are compatible with thin-film stacked devices and circuitry that can be formed on heat-sensitive and flexible substrates to make flexible wearable devices and sensors that are difficult to achieve with crystalline-based material in existence. QD processing technology consists of three main steps (I) Synthesis (II) Purification (III) and ligand exchange and device fabrication. With well-established synthetic procedures, great effort has been done on ligand exchange and device fabrication but there has been negligible attention given to purification strategies. Another major hurdle for their industrial applications is very short-lived post-ligand exchange QD solution stability that compromises the QD ink quality even before device fabrication. This thesis is divided into six chapters. First, I will introduce QD chemistry, applications, and post-synthesis purification en route to device fabrication. Second, I’ll introduce Gel Permeation Chromatography (GPC) as a purification technique in parallel to the established precipitation/re-dispersion (PR) method. This section will demonstrate the effectiveness of GPC in removing byproducts and unbound ligands from PbS QDs, and the subsequent applicability of the GPC-purified QDs in optoelectronic devices. In the third and fourth chapters, I will present highly stable 3-mercaptopropionic acid (MPA) capped and halide capped PbS QDs, dispersed in a single non-coordinating organic solvent, to form printable p-type and n-type nano-inks. These inks are stable and suitable for making standalone, heterojunction, and p-n junction solar cell and photodetection devices. These inks should make QDs a viable option for industrial-scale manufacturing of QD devices through spray, or roll-to-roll printing processes. Chapter 5 will introduce AgBiS2 based QDs ink as environment friendly alternative to eliminate toxicity concerns associated with the current state of the art PbS QD system. This ink has been utilized to fabricate flexible photodetectors to show its broad applicability in sensitive areas such as food processing and biomedical applications. Lastly, Chapter 6 of this thesis demonstrates scanning photocurrent microscopy (SPCM) as a diagnostic technique to characterize III-nitride (GaN/AlGaN) based high electron mobility transistor (HEMT) structures for growth defects and current conduction mechanisms via sub-bandgap excitation

Solution Processable PbS QD-solid Photodetectors for telecommunication application

La detección óptica y la conversión de luz en electricidad (conversión fotovoltaica), en la que los dispositivos optoelectrónicos encuentran su campo de aplicación más importante, han sido importantes campo de aplicación durante décadas de los semiconductores monocristalinos como materiales fotoactivos (absorción de luz y fotogeneración de portadores). Su éxito se debe a los excelentes resultados alcanzados, aunque viene acompañado de inconvenientes, como el elevado coste de producción, la complejidad de fabricación y su incompatibilidad con substratos flexibles. Durante la última década, el advenimiento de los materiales optoelectrónicos procesados en disolución, como los puntos cuánticos (QDs) coloidales, ha abierto nuevas posibilidades para aplicaciones optoelectrónicas. Este nuevo enfoque aplicado al desarrollo de detectores ópticos reside en el alto grado de control que ofrece la ingeniería de materiales a la nanoescala. Además, la procesabilidad de dichos nanomateriales ha permitido su integración en muchos tipos de substratos comerciales de bajo coste, en algunos casos logrando fotodispositivos con figuras de mérito comparables a las de los fotodetectores convencionales. Por ejemplo, en el grupo del Prof. Edward H. Sargent (Universidad de Toronto, Canadá) han medido detectividades de hasta 1013 jones en la segunda ventana de telecomunicaciones (1.3 μm) en fotodetectores basados en QDs de Pbs, un orden de magnitud mayor que la detectividad lograda en fotodetectores basados en películas delgadas de InGaAs crecidas epitaxialmente. En esta tesis hemos pretendido y conseguido desarrollar fotodetectores basados en QDs de PbS estables en aire y procesados en disolución, con una alta sensibilidad en longitudes de onda del infrarrojo próximo (hasta unos 1600 nm), esto es, útiles para la detección óptica en la tercera ventana de telecomunicaciones. Para conseguir este objetivo principal, primero se ha dedicado el esfuerzo necesario para optimizar la síntesis química de los QDs de PbS con el tamaño adecuado para obtener su borde de absorción en las longitudes de onda señaladas, como base para la producción de películas delgadas con un enfoque ascendente (bottom-up). En segundo lugar, hemos conseguido “nanotintas” estables en aire con estos QDs de PbS, adecuadamente formuladas para su compatibilidad (propiedades reológicas adecuadas) con un método eficiente de deposición en área extensa, que permita la formación de una película delgada de QDs de PbS libre de defectos estructurales (ligados al apilamiento vertical de los QDs, que no son más que nanocristales aproximadamente esféricos de unos 6-7 nm de diámetro). La técnica de deposición elegida ha sido “doctor blading”, donde una cuchilla extiende la nanotinta sobre un substrato dado, en nuestro caso vidrio/ITO (ITO: Indium Tin Oxide) o Si/SiO2, para la fabricación de fotodiodos y fotoconductores, respectivamente. También hemos estudiado la influencia de la química superficial de los QDs de PbS en las propiedades de las películas delgadas producidas con éstos y los dispositivos fabricados con estas capas. De hecho, el paso clave en la formación de películas conductoras con QDs de PbS es el procedimiento de intercambio de ligandos en estado sólido, en el que se reemplazan las moléculas largas (Oleilamina) utilizadas para la síntesis de los QDs de PbS con otras más cortas, con el fin de reducir la distancia entre los QDs, lo que aumentaría la movilidad de los portadores en el sólido de QDs resultante debido a un fuerte acoplamiento electrónico. Específicamente, hemos utilizado el ácido 3-Mercapto-Propiónico (MPA) y el Ioduro de Tetra-butil-amonio (TBAI), lo que ciertamente influye en la superficie de los QDs de PbS y en el rendimiento de los fotodetectores basados en PbS. Encontramos que, en ambos casos, el mecanismo responsable de la fotoconductividad está relacionado con la sensibilización de sólido de QDs por trampas, origen de la alta responsividad observada y la lenta respuesta temporal de los dispositivos fotoconductores. Se ha identificado la importante influencia de los niveles de trampas sobre la dinámica de los portadores y la eficiencia final del dispositivo, además de evidenciar el compromiso entre la velocidad del dispositivo y la alta responsividad de los dispositivos fotoconductores. El intercambio de ligandos en el sólido de QDs con MPA, mientras pasiva eficazmente su superficie, produce un rendimiento superior del dispositivo (mayor foto-sensibilidad y detectividad), lo cual se debe a una menor corriente oscura y menor nivel de ruido en comparación con el caso de los sólidos de QDs tratados con TBAI. Además, el intercambio de ligandos con MPA confiere una excelente estabilidad en aire a los sólidos de QDs, reduciendo la oxidación del PbS, como se deduce de medidas de XPS. Sobre la base de estos hallazgos, finalmente hemos desarrollado un fotodetector estable en aire y altamente sensible (1011 Jones), basado en un fotodiodo de arquitectura Schottky con una eficiencia cuántica interna superior al 30% a 1500 nm y con una respuesta temporal de unos 135 s.Optical sensing and conversion of light into electricity (photovoltaics), on which optoelectronic devices find their more important application field, have been for decades important fields of application for single-crystal semiconductors as photoactive (light absorption and carrier photogeneration) materials. Their success relay on the excellent performances achieved in these devices, which indeed are accompanied by several drawbacks, as high production cost, manufacture complexity and incompatibility with flexible substrates. During the last decade the advent of solution-processed optoelectronic materials, such as colloidal quantum dots (QDs), has opened new prospective for optoelectronic applications. This new approach applied to develop optical detectors lies in the high degree of control offered by nanoscale materials engineering. Moreover, solution-processability of these nanomaterials has enabled their low-cost integration over many commercial substrates, in some cases achieving photodevices with figures of merit comparable to those of conventional photodetectors. For instance, the group of Prof. Edward H. Sargent (Toronto University, Canada) reported detectivities of about 1013 jones in the second telecom window (1.3 μm) for PbS QD-based photodetectors. This is one order of magnitude higher than the detectivity achieved for photodetectors based on epitaxially grown InGaAs thin films. In this thesis we have tried and succeeded in developing photodetectors based on PbS QDs stable in air and processed in solution, with a high sensitivity in near-infrared wavelengths (up to about 1600 nm), that is, useful for optical detection in the third telecommunications window. To achieve this main objective, we have undertaken the necessary efforts to optimize the chemical synthesis of the PbS QDs with the appropriate size to obtain their absorption edge at the indicated wavelengths, these QDs were the basis to produce thin films within a bottom-up approach. Secondly, we have achieved "nanoinks" with these PbS QDs that are stable in air and suitably formulated for their compatibility (adequate rheological properties) with an efficient method of deposition for large areas that allows the formation of a thin film of PbS QDs free of structural defects (associated to the 3D stacking of the QDs, which are approximately spherical nanocrystals of about 6-7 nm in diameter). The chosen large-area deposition technique has been "doctor blading", where a blade extends the ink on a given substrate, in our case glass/ITO (ITO: Indium Tin Oxide) or Si/SiO2, for the fabrication of photodiodes and photoconductors, respectively. We have also studied the influence of the QD surface chemistry on the properties of their produced thin films and fabricated photodevices. In fact, the key step in the formation of conductive PbS QD films is the solid-state ligand exchange procedure by replacing long isolating molecules (Oleylamine) used for the QD synthesis with shorter ones, in order to reduce the interparticle distance, which would increase the carrier mobility in the resulting strongly-coupled QD-solid. Specifically, we have used 3-mercaptopropionic acid (MPA) and tetrabutylammonium iodide (TBAI), which certainly influences the surface of the PbS QDs and the performances of PbS-based photodetectors. We did find that, in both cases, the mechanism responsible of photoconductivity is related to trap sensitization of the QD-solid, which is responsible of the observed high responsivity and low time response of photoconductor devices. The important influence of trap states over the carrier dynamics and the final device performances have been identified, and evidenced the trade-off between device speed and photon detection responsivity of the device. The ligand exchange of the QD-solid film with MPA, while efficiently passivating the PbS QDs, yields a superior device performance (photo-sensitivity and detectivity), which is due to a smaller dark current and lower noise level as compared to the case of PbS QD-solids treated with TBAI. Furthermore, MPA ligand exchange confers excellent long-term air-stability to the QD-solids reducing the oxidation of PbS QDs, as deduced from XPS measurements. Based on these findings, we have finally developed an air-stable highly sensitive (1012 Jones) photodetector based on a simple Schottky photodiode architecture with internal quantum efficiency higher than 30% at 1500 nm and time response of about 135 s

Roadmap on printable electronic materials for next-generation sensors

The dissemination of sensors is key to realizing a sustainable, ‘intelligent’ world, where everyday objects and environments are equipped with sensing capabilities to advance the sustainability and quality of our lives—e.g., via smart homes, smart cities, smart healthcare, smart logistics, Industry 4.0, and precision agriculture. The realization of the full potential of these applications critically depends on the availability of easy-to-make, low-cost sensor technologies. Sensors based on printable electronic materials offer the ideal platform: they can be fabricated through simple methods (e.g., printing and coating) and are compatible with high-throughput roll-to-roll processing. Moreover, printable electronic materials often allow the fabrication of sensors on flexible/stretchable/biodegradable substrates, thereby enabling the deployment of sensors in unconventional settings. Fulfilling the promise of printable electronic materials for sensing will require materials and device innovations to enhance their ability to transduce external stimuli—light, ionizing radiation, pressure, strain, force, temperature, gas, vapours, humidity, and other chemical and biological analytes. This Roadmap brings together the viewpoints of experts in various printable sensing materials—and devices thereof—to provide insights into the status and outlook of the field. Alongside recent materials and device innovations, the roadmap discusses the key outstanding challenges pertaining to each printable sensing technology. Finally, the Roadmap points to promising directions to overcome these challenges and thus enable ubiquitous sensing for a sustainable, ‘intelligent’ world

Novel light-sensitive nanocrystal skins

Ankara : Materials Science and Nanotechnology and the Graduate School of Engineering and Science of Bilkent Univ., 2013.Thesis (Master's) -- Bilkent University, 2013.Includes bibliographical references leaves 89-100.Light sensing devices traditionally made from crystalline or amorphous silicon, operating at the visible and near-infrared wavelengths, have led to a multibillion-dollar annual market. However, silicon faces various limitations including weak detection at long wavelengths (insufficient beyond 1.1 µm) with a cut-off at short wavelengths (in the ultraviolet) and small-area applications. On the other hand, solution-processed semiconductor nanocrystals (NCs), also known as colloidal quantum dots, offer large-area light sensing platforms with strong absorption cross-section. In this thesis we propose and demonstrate a new class of large-area, semi-transparent, light-sensitive nanocrystal skin (LS-NS) devices intended for large-surface applications including smart transparent windows and light-sensitive glass facades of smart buildings. These LS-NS platforms, which are fabricated over areas up to many tens of cm2 using spraycoating and several cm-squares using dip-coating, are operated on the basis of photogenerated potential buildup, as opposed to conventional charge collection. The close interaction of the monolayer NCs of the LS-NS with the top interfacing metal contact results in highly sensitive photodetection in the absence of external bias, while the bottom side is isolated using a high dielectric spacing layer. In operation, electron-hole pairs created in the NCs of the LS-NS are disassociated and separated at the NC monolayer - metal interface due to the difference in the workfunctions. As a result, the proposed LS-NS platforms perform as highly sensitive photosensors, despite using a single NC monolayer, which makes the device semi-transparent and reduces the noise generation Furthermore, because of the band gap tunability, it is possible to construct cascaded NC layers with a designed band gap gradient where the NC diameters monotonically change. Here we present the first account of exciton funneling in an active device, which leads to significant performance improvement in the device. We show highly photosensitive NC skins employing the exciton funneling across the multiple layers of NC film. To further enhance the device photosensitivity performance, we demonstrate embedding plasmonic nanoparticles into the light-sensitive skins of the NCs. In addition, we exhibit the LS-NS device sensitivity enhancement utilizing the device architecture of semi-transparent tandem skins, the addition of TiO2 layer for increased charge carrier dissociation, and the phenomenon of multiexciton generation in infrared NCs. With fully sealed NC monolayers, LS-NS is found to be highly stable under ambient conditions, promising for low-cost large-area UV/visible sensing in windows and facades of smart buildings. We believe the findings presented in this thesis have significant implications for the future design of photosensing platforms and for moving toward next generation large-surface light-sensing platforms.Akhavan, ShahabM.S

Flexible and substrate-free optoelectronic devices based on III-V semiconductor nanowires

III-V nanowires have been the subject of intense research interest for the past 20 years, as their unique optical and electronic properties, which arise from their nanoscale dimensions and composition, make them particularly suited for high-performance opto-electronic devices. Since epitaxial growth is on expensive, brittle, crystalline substrates, the field of flexible devices has been little explored in the context of III-V nanowires. In order to fully exploit these properties and move away from conventional wafer based electronics to flexible electronics, hybrid devices consisting of organic and inorganic components must be developed to harness the benefits from both materials systems. Embedding high performance vertically aligned III-V nanowires in a flexible matrix enables applications where there is a need for substrate-free, flexible devices. The work in this thesis looks to address this by (1) developing a repeatable method of producing nanowire-polymer thin films and (2) demonstrating how these thin films could be fabricated into different opto-electronic devices. The thin films are made by encapsulating the nanowires in Parylene C, which are then be peeled off from the growth substrate, thus retaining the vertical alignment of the nanowires. These thin films are used to fabricate a THz modulator and a solar cell. Single and multi-layer THz modulators are fabricated from nanowire-Parylene C thin films laminated together. 1,2,4,8, and 14-layer modulators are compared, with the 14-layer modulator displaying the best performance. A high switching speed (<5 ps), modulation depth (-8 dB), extinction (13%) and dynamic range (-9 dB) and broad bandwidth operation (0.1 THz–4 THz) are obtained. This surpasses the performance of several devices in the literature and presents the first THz modulator which combines a large modulation depth, broad bandwidth, picosecond time resolution for THz intensity and phase modulation, which makes it an ideal candidate for ultrafast THz communication. In addition to the THz work, the fabrication process towards a flexible solar cell is also developed. This consists of optimising the dry etching, and annealing-free contacting processes to give nanowire devices that show good ohmic IV characteristics. Following this work, a proof-of-concept Schottky barrier solar cell is fabricated using the knowledge gleaned from this development work. This preliminary device gives a conversion efficiency of 0.02% and a fill factor of 0.3, with scope for device performance improvement by using nanowires that are grown and optimised specifically for solar cell operationEPSRC - Photonics CD

Organic Photodiodes and Their Optoelectronic Applications

Recently, organic photodiodes (OPDs) have been acknowledged as a next-generation device for photovoltaic and image sensor applications due to their advantages of large area process, light weight, mechanical flexibility, and excellent photoresponse. This dissertation targets for the development and understanding of high performance organic photodiodes for their medical and industrial applications for the next-generation. As the first research focus, A dielectric / metal / dielectric (DMD) transparent electrode is proposed for the top-illumination OPDs. The fabricated DMD transparent electrode showed the maximum optical transmittance of 85.7 % with sheet resistance of 6.2 ohm/sq. In the second part of the thesis, a development of novel transfer process which enables the dark current suppression for the inverted OPD devices will be discussed. Through the effort, we demonstrated OPD with high D* of 4.82 x 10^12 Jones at reverse bias of 1.5 V with dark current density (Jdark) of 7.7 nA/cm2 and external quantum efficiency (EQE) of 60 %. Additionally in the third part, we investigate a high performance low-bandgap polymer OPD with broadband spectrum. By utilizing the novel transfer process to introduce charge blocking layers, significant suppression of the dark current is achieved while high EQE of the device is preserved. A low Jdark of 5 nA/cm2 at reverse bias of 0.5 V was achieved resulting in the highest D* of 1.5 x 10^13 Jones. To investigate the benefit for the various OPD applications, we developed a novel 3D printing technique to fabricate OPD on hemispherical concave substrate. The techniques allowed the direct patterning of the OPD devices on hemispherical substrates without excessive strain or deformation. Lastly, a simulation of the OPD stacked a-ITZO TFT active pixel sensor (APS) pixel with external transimpedance amplifier (TIA) readout circuit was performed.PHDElectrical & Computer Eng PhDUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/137168/1/hyunskim_1.pd