Ligand Exchange Optimization for Quantum Dot Based Infrared Thin-film Photodetectors

Image in the infrared wavelength range offers several advantages when compared with the visible range. The information that is impossible to acquire with our naked eyes can be used for different industries such as quality control, surveillance, augmented and virtual reality, medical diagnostics, and others. Colloidal quantum dots (QDs) have been gathering increased attention and becoming one of the most promising candidates for infrared optoelectronic devices, being praised for their size-dependent bandgap tunability, low-cost manufacturing when comparing to III-V semiconductors, and suitability for deposition on large and flexible substrates. However, the main challenge to accomplish is precise control over their material properties through surface passivation. The work performed in this thesis is focused on exploring the effect of different strategies of surface ligand treatments to colloidal QDs for further integration as an active layer of thin-film photodetectors. Therefore, thin-films made from solution-phase ligand exchange lead sulfide (PbS) QDs deposited on glass substrates were analyzed in terms of their optical and morphological properties through multiple characterization techniques. Full processing and fabrication of thin-film photodiode detectors were then carried out, pursuing the highest devices performance according to their electrical characterization. In the end, the developed PbS QD-based photodiode stack successfully completed the proposed optimization by reaching dark currents values close to 10-5 A/cm2 at -3 V and external quantum efficiency of 29% at 1450 nm

Charge Transport in Trap-Sensitized Infrared PbS Quantum-Dot-Based Photoconductors: Pros and Cons

Control of quantum-dot (QD) surface chemistry offers a direct approach for the tuning of charge-carrier dynamics in photoconductors based on strongly coupled QD solids. We investigate the effects of altering the surface chemistry of PbS QDs in such QD solids via ligand exchange using 3-mercaptopropionic acid (MPA) and tetrabutylammonium iodide (TBAI). The roll-to-roll compatible doctor-blade technique was used for the fabrication of the QD solid films as the photoactive component in photoconductors and field-effect phototransistors. The ligand exchange of the QD solid film with MPA yields superior device performance with higher photosensitivity and detectivity, which is due to less dark current and lower noise level as compared to ligand exchange with TBAI. In both cases, the mechanism responsible for photoconductivity is related to trap sensitization of the QD solid, in which traps are responsible of high photoconductive gain values, but slow response times under very low incident optical power (100 pW), where traps are filled, both MPA- and TBAI-treated photodevices exhibit similar behavior, characterized by lower responsivity and faster response time, as limited by the mobility in the QD solid

Field effect transistors and phototransistors based upon p-type solution-processed PbS nanowires.

We demonstrate the fabrication of solution processed highly crystalline p-type PbS nanowires via the oriented attachment of nanoparticles. The analysis of single nanowire field effect transistor (FET) devices revealed a hole conduction behaviour with average mobilities greater than 30 cm2 V-1 s-1, which is an order of magnitude higher than that reported to date for p-type PbS colloidal nanowires. We have investigated the response of the FETs to near-infrared light excitation and show herein that the nanowires exhibited gate-dependent photo-conductivities, enabling us to tune the device performances. The responsivity was found to be greater than 104 A W-1 together with a detectivity of 1013 Jones, which benefits from a photogating effect occurring at negative gate voltages. These encouraging detection parameters are accompanied by relatively short switching times of 15 ms at positive gate voltages, resulting from a combination of the standard photoconduction and the high crystallinity of the nanowires. Collectively, these results indicate that solution-processed PbS nanowires are promising nanomaterials for infrared photodetectors as well as p-type nanowire FETs

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

Colloidal quantum dot (CQD) based mid-wavelength infrared optoelectronics

Colloidal quantum dot (CQD) photodetectors are a rapidly emerging technology with a potential to significantly impact today’s infrared sensing and imaging technologies. To date, CQD photodetector research is primarily focused on lead-chalcogenide semiconductor CQDs which have spectral response fundamentally limited by the bulk bandgap of the constituent material, confining their applications to near-infrared (NIR, 0.7-1.0 um) and short-wavelength infrared (SWIR, 1-2.5 um) spectral regions. The overall goal of this dissertation is to investigate a new generation of CQD materials and devices that advances the current CQD photodetector research toward the technologically important thermal infrared region of 3-5 ?m, known as mid-wavelength infrared (MWIR). In this dissertation, electronic and optoelectronic characteristics of Ag2Se CQD based devices are analyzed by different device architectures with detailed analysis of detector performance parameters. The first part of the dissertation includes the report on the fabrication of solution-processed lateral photoconductive photodetectors. Significant photoresponse is demonstrated in MWIR with the lateral photoconductor at room temperature. The detailed analysis on the effect of ligand exchange as well as temperature and spectral dependent photoresponses is presented. In the second device structure, vertically stacked quantum dot devices are demonstrated. In this device architecture, a barrier QD layer is placed in between mid-wavelength absorber intraband Ag2Se QD layer. The insertion of barrier layer reduces dark current significantly since 1Se Ag2Se QD-1Se PbS QD conduction offset serves as a potential barrier, blocking the transport of thermally generated electrons and holes. In addition, vertical device design improves detector performance parameters significantly at room temperature. At the last part of the dissertation, development of p-n heterojunction diode devices is presented as third device structure. High performance detectors can be realized using a traditional p-n junction device design, however, the heavily-doped nature of intraband quantum dots present a new challenge in realizing diode devices. To address this challenge, an unique trait of blending two different QDs is employed to control electrical property. The fabricated p-n junction devices demonstrate reduced noise current density due to reverse bias operation, which shows improvement in the specific detectivity of the detector at room temperature. Consequently, this dissertation presents the feasibility of uncooled, room-temperature photodetection in the MWIR with intraband silver selenide quantum dots that has the potential to impact numerous applications ranging from all-weather night vision, machine vision, biomedical imaging, to free-space optical communication

Design, Fabrication, and Characterization of Novel Optoelectronic Devices for Near-infrared Detection

Investigating semiconductor materials and devices at the nanoscale has become crucial in order to maintain the exponential development in today’s technology. There is a critical need for making devices lower in power consumption and smaller in size. Nanoscale semiconductor materials provide a powerful platform for optoelectronic device engineers. They own interesting properties which include enhanced photoconductivity and size-tunable interband transitions. In this research, different types of nanostructures were investigated for optoelectronic devices: nanocrystals, nanowires, and thin-films. First, lead selenide nanocrystals with narrow bandgap were synthesized, size-tailored, and functionalized with molecular ligands for the application of uncooled near-infrared photodetectors. The devices showed strong room-temperature responsivity that is covering the entire near-infrared spectral region. In the second investigation self-powered devices based on asymmetric Schottky contacts were designed and fabricated to efficiently detect near-infrared radiations without external biasing. The dimensions and the type of the metal contacts were optimized in order to improve on the device performance. Then silicon nanowires were integrated with the asymmetric contacts to further enhance the performance of the self-powered detectors by increasing the light absorption. Third, an array of gold thin-films was designed to enhance the photocurrent in the near-infrared through the internal photoemission of hot electrons. The photocurrent enhancement was studied as function of thickness and type of the metal thin-film. Overall, those investigations provided important design considerations for future optoelectronic devices based on nanostructures. Moreover, the implementation of nanostructures with the devices showed superior performance as compared to the bulk

High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction.

Hybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency