Transient optical studies of photoinduced charge transfer in semiconductor quantum dot solar cells

Semiconductor quantum dots (also referred to as 'nanocrystals‘) are well suited as light-harvesting agents in solar cells because they are robust, have tuneable effective band gaps, and are easy to process. The research presented in this thesis is targeted towards the study of excitonic solar cells employing semiconductor nanocrystals as a light harvesting component. Gaining control of the interfacial charge transfer processes in operation in these devices forms a crucial part of any attempt to optimise their performance. In particular, the use of transient spectroscopic techniques reveals how efficient and long-lived charge separation can be achieved in these solar cell architectures. The primary focus of this research is to investigate the parameters influencing charge transfer in dye-sensitised solar cells (DSSCs) using colloidal quantum dots as light-absorbers. One aim is to study the impact of varying the thermodynamic driving forces provided for interfacial electron transfer on the yield of both the electron injection and hole regeneration reactions occurring within the DSSC; this can be achieved by varying the energetics of each component of the system (metal oxide, quantum dot and hole conductor) in turn. In addition, the interfacial morphology can be modulated by changing the passivating ligands present at the QD surface, and by modifying the structure of the redox mediator (or hole conductor). In doing so, we also attempt to improve our understanding of how charge carrier trapping in quantum dots impacts upon solar cell performance. Furthermore, new strategies towards solar cell design are presented, which show great potential as a result of their favourable photophysical properties. One of these approaches (presented in the final chapter) is to effect the in situ growth of CdS nanocrystals in a conducting polymer, a method which circumvents many of the processing issues associated with the use of nanocrystals in polymer blend solar cell architectures. It is hoped that the work presented in this thesis is used to develop design rules for the construction of semiconductor nanocrystal-based excitonic solar cells. By identifying which key parameters control the rates and yields of electron transfer at the nanocrystal interface, improvements in device efficiency can be realised. It is believed that these studies fill an important gap in our current understanding, and highlight some of the potential benefits and shortcomings of using semiconductor nanocrystals in cheap, solution-processed solar cells

量子ドット太陽電池とペロブスカイト太陽電池における界面エンジニアリングと界面電荷移動ダイナミクス

電気通信大学201

Alternative gate dielectrics and application in nanocrystal memory

Ph.DDOCTOR OF PHILOSOPH

Nanoparticles as a charge trapping layer in Metal-Insulator-Semiconductor structures

Memories with floating gate structures are the main device architecture used in current non-volatile memories. Different films for floating gate based devices have been studied to substitute poly-crystalline silicon as the main material in floating gate structures. In current technology tunneling oxides are required to have thicknesses around 30 nm reducing device performance. Nanocrystals and nanoparticles have been emerging as a possible replacement for those films since better retention times and faster devices can be obtained. In this work the study of nanoparticles as the Charge trapping layer was executed. Study of the nanoparticles was made in a MIS structure. Hysteresis loops on C-V curves showing charge trapping was expected. Molybdenum film in the charge trapping layer was characterized as a comparison term for the nanoparticles in the CT-layer. Results of this work detail the importance of the interfacial layers, as well as defects across the oxides, on the electrical characterization of this structures. Hole trapping was achieved with nanoparticles as a charge trapping layer. Data obtained demonstrated the effect of interfacial defects in C-V curves as well as charging behavior in gold nanoparticles and Molybdenum films

Interface Engineering to Control Charge Transport in Colloidal Semiconductor Nanowires and Nanocrystals

Colloidal semiconductor nanocrystals (NCs) are a class of materials that has rapidly gained prominence and has shown the potential for large area electronics. These materials can be synthesized cheaply and easily made in high quality, with tunable electronic properties. However, evaluating if colloidal nanostructures can be used as a viable semiconducting material for large area electronics and more complex integrated circuits has been a long standing question in the field. When these materials are integrated into solid-state electronics, multiple interfaces need to be carefully considered to control charge transport, these interfaces are the: metal contact/semiconductor, dielectric/semiconductor and the nanocrystal surface. Here, we use colloidal nanowire (NW) field-effect transistors (FETs) as a model system to understand doping and hysteresis. Through controllable doping, we fabricated PbSe NW inverters that exhibit amplification and demonstrate that these nanostructured materials could be used in more complex integrated circuits. By manipulating the dielectric interface, we are able to reduce the hysteresis and make low-voltage, low-hysteresis PbSe NW FETs on flexible plastic, showing the promise of colloidal nanostructures in large area flexible electronics. In collaboration, we are able to fabricate high-performance CdSe NC FETs through the use of a novel ligand, ammonium thiocyanate to enhance electronic coupling, and extrinsic atom in indium to dope and passivate surface traps, to yield mobilities exceeding 15 cm2V-1s-1. Combining high-mobility CdSe NC FETs with our low-voltage plastic platform, we were able to translate the exceptional devices performances on flexible substrates. This enables us to construct, for the first time, nanocrystal integrated circuits (NCICs) constructed from multiple well-behaved, high-performance NC-FETs. These transistors operate with small variations in device parameters over large area in concert, enabling us to fabricate NCIC inverters, amplifiers and ring oscillators. Device performance is comparable to other emerging solution-processable materials, demonstrating that this class of colloidal NCs as a viable semiconducting material for large area electronic applications

Characteristics of nanocomposites and semiconductor heterostructure wafers using THz spectroscopy

All optical, THz-Time Domain Spectroscopic (THz-TDS) methods were employed towards determining the electrical characteristics of Single Walled Carbon Nanotubes, Ion Implanted Si nanoclusters and Si1-xGex HFO2, SiO2 on p-type Si wafers. For the nanoscale composite materials, Visible Pump/THz Probe spectroscopy measurements were performed after observing that the samples were not sensitive to the THz radiation alone. The results suggest that the photoexcited nanotubes exhibit localized transport due to Lorentz-type photo-induced localized states from 0.2 to 0.7THz. The THz transmission is modeled through the photoexcited layer with an effective dielectric constant described by a Drude + Lorentz model and given by Maxwell-Garnett theory. Comparisons are made with other prevalent theories that describe electronic transport. Similar experiments were repeated for ion-implanted, 3-4nm Si nanoclusters in fused silica for which a similar behavior was observed. In addition, a change in reflection from Si1-xGex on Si, 200mm diameter semiconductor heterostructure wafers with 10% or 15% Ge content, was measured using THz-TDS methods. Drude model is utilized for the transmission/reflection measurements and from the reflection data the mobility of each wafer is estimated. Furthermore, the effect of high-K dielectric material (HfO2) on the electrical properties of p-type silicon wafers was characterized by utilizing non-contact, differential (pump-pump off) spectroscopic methods to differ between HfO2 and SiO2 on Si wafers. The measurements are analyzed in two distinct transmission models, where one is an exact representation of the layered structure for each wafer and the other assumed that the response observed from the differential THz transmission was solely due to effects from interfacial traps between the dielectric layer and the substrate. The latter gave a more accurate picture of the carrier dynamics. From these measurements the effect of interfacial defects on transmission and mobility are quantitatively discussed

Reliability Analysis of Hafnium Oxide Dielectric Based Nanoelectronics

With the physical dimensions ever scaling down, the increasing level of sophistication in nano-electronics requires a comprehensive and multidisciplinary reliability investigation. A kind of nano-devices, HfO2-based high-k dielectric films, are studied in the statistical aspect of reliability as well as electrical and physical aspects of reliability characterization, including charge trapping and degradation mechanisms, breakdown modes and bathtub failure rate estimation. This research characterizes charge trapping and investigates degradation mechanisms in high-k dielectrics. Positive charges trapped in both bulk and interface contribute to the interface state generation and flat band voltage shift when electrons are injected from the gate under a negative gate bias condition.A negligible number of defects are generated until the stress voltage increases to a certain level. As results of hot electrons and positive charges trapped in the interface region, the difference in the breakdown sequence is attributed to the physical thickness of the bulk high-k layer and the structure of the interface layer. Time-to-breakdown data collected in the accelerated life tests are modeled with a bathtub failure rate curve by a 3-step Bayesian approach. Rather than individually considering each stress level in accelerating life tests (ALT), this approach derives the change point and the priors for Bayesian analysis from the time-to-failure data under neighborhood stresses, based on the relationship between the lifetime and stress voltage. This method can provide a fast and reliable estimation of failure rate for burn-in optimization when only a small sample of data is available

Fully inorganic oxide-in-oxide ultraviolet nanocrystal light emitting devices

The development of integrated photonics and lab-on-a-chip platforms for environmental and biomedical diagnostics demands ultraviolet electroluminescent materials with high mechanical, chemical and environmental stability and almost complete compatibility with existing silicon technology. Here we report the realization of fully inorganic ultraviolet light-emitting diodes emitting at 390 nm with a maximum external quantum efficiency of ~0.3%, based on SnO(2) nanoparticles embedded in SiO(2) thin films obtained from a solution-processed method. The fabrication involves a single deposition step onto a silicon wafer followed by a thermal treatment in a controlled atmosphere. The fully inorganic architecture ensures superior mechanical robustness and optimal chemical stability in organic solvents and aqueous solutions. The versatility of the fabrication process broadens the possibility of optimizing this strategy and extending it to other nanostructured systems for designed applications, such as active components of wearable health monitors or biomedical devices

Memory and Coupling in Nanocrystal Optoelectronic Devices

Optoelectronic devices incorporating semiconducting nanocrystals are promising for many potential applications. Nanocrystals whose size is below the exciton Bohr radius have optical absorption and emission that is tunable with size, due to the quantum confinement of the charge carriers. However, the same confinement that yields these optical properties also makes electrical conduction in a film of nanocrystals occur via tunneling, due to the high energy barrier between nanocrystals. Hence, the extraction of photo-generated charge carriers presents a significant challenge. Several approaches to optimizing the reliability and efficiency of optoelectronic devices using semiconducting nanocrystals are explored herein. Force microscopy is used to investigate charge behavior in nanocrystal films. Plasmonic structures are lithographically defined to enhance electric field and thus charge collection efficiency in two-electrode nanocrystal devices illuminated at plasmonically resonant wavelengths. Graphene substrates are shown to couple electronically with nanocrystal films, improving device conduction while maintaining carrier quantum confinement within the nanocrystal. And finally, the occupancy of charge carrier traps is shown to both directly impact the temperature-dependent photocurrent behavior, and be tunable using a combination of illumination and electric field treatments. Trap population manipulation is robustly demonstrated and verified using a variety of wavelength, intensity, and time-dependent measurements of photocurrent in nanogap nanocrystal devices, emphasizing the importance of measurement history and the possibility of advanced device behavior tuning based on desired operating conditions. Each of these experiments reveals a path toward understanding and optimizing semiconducting nanocrystal optoelectronic devices