Group III-V Quantum Dot Solar Cells

This thesis aims to use quantum dot solar cells (QDSC) to realise the IBSC and, hence, achieve the 63.2% ultra-high power conversion efficiency. The approach is to epitaxially grow the QDSC using the most advanced epitaxial growth technique—molecular beam epitaxy (MBE). The nature of quantum dots (QDs) exhibits 3-dimensional (3D) carrier confinement and have discrete energy levels, a perfect candidate to implement the intermediate energy levels required for IBSCs. In this thesis, the practical implementations, physical limitations and technical difficulties are addressed and reviewed in detail

Development of High Efficiency III/V Photovoltaic Devices

Developments of photovoltaic (PV) devices are driven by increasing needs for economically competitive renewable energy conversion. To improve the efficiency of PV devices for outdoor applications, the concept of intermediate band solar cell (IBSC) has been proposed to boost the conversation efficiency to 63% under concentrated suns illumination, which requires two-step photon absorption (TSPA) dominates among other competing processes: carrier thermal escape, tunneling and recombination. To optimize the design of III-V QD-IBSCs, first, the effect of electric field on band structure and carrier dynamics and device performances were quantitative investigated via simulation and experiments. Second, to experimentally increase TSPA at room temperature, novel QD systems related QD-IBSCs were designed, fabricated and characterized. The InAs/Al0.3GaAs QD-IBSC shows high TSPA working temperature towards 110K, promising for a room temperature IBSC under concentrated sunlight. Alternative QD systems including GaSb/GaAs and type II InP/InGaP were also investigated via band structure simulations. Meanwhile, developments of PV devices under indoor low intensity light (0.1 µW/cm2-1 mW/cm2) illumination not only enable long lifetime radio-isotope based batteries, but also, more important for the daily life, have the potential to promote an emerging market of internet of things by efficiently powering wireless sensors. Single junction InGaP PV devices were optimized for low intensity light sources using via simulations and statistical control. To reduce the dark current and increase the absorption at longer wavelengths (\u3e550 nm), several parameters including doping and thickness were evaluated. The experimental results on the devices show higher conversion efficiencies than other commercial PVs under varied indoor light sources: 29% under 1µW/cm2 phosphor spectrum and over 30% efficiency under LEDs illumination. In addition, the work includes developments of InAs nanowires epi-growth for PV applications. Several marks for selective area growth were successfully made

Multiscale in modelling and validation for solar photovoltaics

Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges.</p

Quantum Dots for Intermediate Band in Solar Cells

The commercially available solar cells suffer from low efficiency and high cost. This would avoid presence of solar cells as a secure energy resource in the market. Problems stem from two facts. Firstly, band gap of materials deployed for cell fabrication do not match the solar spectrum. Secondly, harvesting all the generated electrons is imperfect due to presence of many non-radiative recombination processes and, thermalization of electrons. To transcend these deficiencies, third generation of solar has been introduced. This new generation renders a whole new concept both in design and materials of solar cells scope. One of new introduction to solar cell field is Quantum Dot (QD). QD offers a broad range of tunability. The optical and electrical properties of QDs can be altered by choice of material, size and shape; therefore; they have great potential for high efficiency cell fabrication. QDs are mainly grown via MBE or synthesized via Colloidal solutions. QDs could be integrated as a part of one of new and promising third generation cells, named Intermediate Band Solar Cells. QDs could be employed as the intermediate level. If MBE is the selected method for cell fabrication, QDs would grow in a matrix of barrier material accompanied with a wetting layer. Wetting layer would disturb the ideal condition predicted in theory for gaining the high efficiency. To study how wetting layer would affect IB performance two sets of simulations have been carried out. One part is done with COSMOL. In this part different number of QDs layers have been simulated with and without wetting layer. The result showed that parasitic effect of wetting layer could not be eliminated large stacks of QD are stacked together, to achieve the promised efficient wetting layer should be eliminated from the system. In MATLAB part QDs have been approximated with simple cuboid. The main aim in this part was to compare how the result of taking into the account the real shape differs from a simple approach which has been the most reported the most in literature. If all the restrains on achieving high efficiency of IBSC are met, still one major draw- back remains and, that is high cost of MBE process. This would hinder mass production of IB cell. One possible potential method to gradually replace MBE can be Colloidal QDs. Colloidal QDs are fairly low cost and easy to fabricate. In this work, colloidal crystal growth was examined. The best condition for monolayer deposition was obtained and, the feasibility of crystal growth was demonstrated. additionally, There was an attempt to grow more than one layer and investigate result of embedding QDs in a barrier of another material

Roadmap on optical energy conversion

For decades, progress in the field of optical (including solar) energy conversion was dominated by advances in the conventional concentrating optics and materials design. In recent years, however, conceptual and technological breakthroughs in the fields of nanophotonics and plasmonics combined with a better understanding of the thermodynamics of the photon energy-conversion processes reshaped the landscape of energy-conversion schemes and devices. Nanostructured devices and materials that make use of size quantization effects to manipulate photon density of states offer a way to overcome the conventional light absorption limits. Novel optical spectrum splitting and photon-recycling schemes reduce the entropy production in the optical energy-conversion platforms and boost their efficiencies. Optical design concepts are rapidly expanding into the infrared energy band, offering new approaches to harvest waste heat, to reduce the thermal emission losses, and to achieve noncontact radiative cooling of solar cells as well as of optical and electronic circuitries. Light–matter interaction enabled by nanophotonics and plasmonics underlie the performance of the third- and fourth-generation energy-conversion devices, including up- and down-conversion of photon energy, near-field radiative energy transfer, and hot electron generation and harvesting. Finally, the increased market penetration of alternative solar energy-conversion technologies amplifies the role of cost-driven and environmental considerations. This roadmap on optical energy conversion provides a snapshot of the state of the art in optical energy conversion, remaining challenges, and most promising approaches to address these challenges. Leading experts authored 19 focused short sections of the roadmap where they share their vision on a specific aspect of this burgeoning research field. The roadmap opens up with a tutorial section, which introduces major concepts and terminology. It is our hope that the roadmap will serve as an important resource for the scientific community, new generations of researchers, funding agencies, industry experts, and investors.United States. Department of Energy (DE-AC36-086038308

Electronic and Optical Properties of Silicon Nanowires: Theory and Modeling

Narrow silicon nanowires host a rich set of physical phenomena. Understanding these phenomena will open new opportunities for applications of silicon nanowires in optoelectronic devices and adds more functionality to silicon especially in those realms that bulk silicon may not operate remarkably. Compatibility of silicon nanowires with the mainstream fabrication technology is also advantageous. The main theme of this thesis is finding the possibility of using silicon nanowires in light sources; laser and light emitting diodes. Using Tight Binding (TB) and ab-initio Density Functional Theory (DFT) methods it was shown that axial strain can induce significant changes in the effective mass, density of states and bandgap of silicon nanowires. Generality of the observed effects was proven by investigating nanowires of different crystallography, diameter and material (e.g. germanium nanowires). The observed direct to indirect bandgap conversion suggests that strain is able to modulate the light emission properties of silicon nanowires. To investigate this possibility, spontaneous emission time was formulated using perturbation theory including Longitudinal Optical (LO) and Acoustic (LA) phonons. It was observed that corresponding to bandgap conversion, the spontaneous emission time can be modulated by more than one order of magnitude. This emanates from bandgap conversion and symmetry change of wave function in response to strain. A mechanism for population inversion was proposed in the thesis which is based on the Ensemble Monte Carlo (EMC) study of carrier statistics in direct and indirect conduction sub bands. By calculating all possible electron-phonon scattering mechanisms which may deplete the already populated indirect subband, it was shown that at different temperatures and under different electric fields there is a factor of 10 difference between the population of indirect and direct sub bands. This suggests that population inversion can be achieved by biasing an already strained nanowire in its indirect bandgap state. The light emission is possible then by releasing or inverting the strain direction. A few ideas of implementing this experiment were proposed as a patent application. Furthermore the photo absorption of silicon nanowires was calculated using TB method and the role of diameter, optical anisotropy and strain were investigated on band-edge absorption

Earth Abundant Thin Film Technology for Next Generation Photovoltaic Modules

With a cumulative generation capacity of over 100 GW, Photovoltaics (PV) technology is uniquely poised to become increasingly popular in the coming decades. Although, several breakthroughs have propelled PV technology, it accounts for only less than 1% of the energy produced worldwide. This aspect of the PV technology is primarily due to the somewhat high cost per watt, which is dependent on the efficiency of the PV cells as well as the cost of manufacturing and installing them. Currently, the efficiency of the PV conversion process is limited to about 25% for commercial terrestrial cells; improving this efficiency can increase the penetration of PV worldwide rapidly. A critical review of all possibilities pursued in the public domain reveals serious shortcomings and manufacturing issues. To make PV generated power a reality in every home, a Multi-Junction Multi-Terminal (MJMT) PV architecture can be employed combining silicon and another earth abundant material. However, forming electronic grade thin films of earth abundant materials is a non-trivial challenge; without solving this, it is impossible to increase the overall PV efficiency. Deposition of Copper (I) Oxide, an earth abundant semiconducting material, was conducted using an optimized Photo assisted Chemical Vapor Deposition process. X-Ray Diffraction, Ellipsometry, Transmission Electron Microscopy, and Profilometry revealed that the films composed of Cu2O of about 90 nm thickness and the grain size was as large as 600 nm. This result shows an improvement in material properties over previously grown thin films of Cu2O. Measurement of I-V characteristics of a diode structure composed of the Cu2O indicates an increase in On/Off ratio to 17,000 from the previous best value of 800. These results suggest that the electronic quality of the thin films deposited using our optimized process to be better than the results reported elsewhere. Using this optimized thin film forming technique, it is now possible to create a complete MJMT structure to improve the terrestrial commercial PV efficiency

Spectroscopic Study of Triplet Exciton Dynamics at the Hybrid Organic-Inorganic Interface

The control and utilisation of spin-triplet excitons in organic semiconductors is highly sought after for the next generation of Optoelectronic applications. Of particular interest is the utilisation of these photoexcited states in the singlet-fission photon-multiplier and triplet-triplet annihilation upconversion processes. Singlet fission is an exciton multiplication process in organic molecules, in which a photogenerated spin-singlet exciton is rapidly and efficiently converted into two spin-triplet excitons. Conversely, triplet-triplet annihilation is essentially the same reaction operating in reverse. These processes offer two spectral management mechanisms to break the Shockley–Queisser limit by overcoming the thermalisation and absorption losses inherent to all single-junction photovoltaics. Such spectral management technologies have been predicted to increase the maximum possible efficiency of Si-based cells from 32 % to greater than 40 %, breaking the Shockley–Queisser limit. Harnessing these processes would be facilitated if the energy of the triplet exciton could be efficiently interchanged with photons. However, utilising triplet excitons like this poses a significant challenge, as transitions between the ground state and triplet excited states typically have negligible mediation by photons. Transitions such as these are spin forbidden, and have a correspondingly weak oscillator strength. In this thesis, we investigate a promising method to overcome this impasse, using inorganic quantum dots (QDs) to efficiently convert between triplet excitons and photons. We develop a variety of novel hybrid organic-inorganic systems that perform singlet-fission photon-multiplication and triplet-triplet annihilation upconversion. We find that in order to achieve efficient triplet transfer at the hybrid organic-inorganic interface, it is critical to engineer the surface of the QD with a triplet transfer ligand. The triplet transfer ligand facilitates transfer by acting as an intermediate excited state during the transfer process, encouraging the formation of an optimal solid-state morphology, and providing a weak adsorption site for rapid transfer to occur at. Among the many highlights, we develop a solid-state singlet-fission photon-multiplier with an exciton multiplication efficiency of ~190%, showing significant promise for real-world application. Additionally, advanced spectroscopic techniques and mathematic modelling are applied to gather an in-depth understanding of the impact of a variety of photophysical processes on operation under realistic working conditions. These results establish a variety of highly tuneable platforms to understand the triplet transfer process at the organic semiconductor and inorganic QD interface, providing clear design rules for new materials that perform spectral management.Winton Program for the Physics of Sustainability Cambridge Trust St. John's Colleg

Raytrace simulations and experimental studies of luminescent solar concentrators

The luminescent solar concentrator is a planar, non-tracking device. Originally introduced more than three decades ago, it has yet to establish itself as a means of making photovoltaic solar energy more cost effective. Advances in organic luminescent centres, the emergence of inorganic nanocrystals and the development of new light trapping techniques have created promising opportunities for the LSC. This thesis investigates novel geometries and materials for the practical exploitation of LSCs. The research is based on experimental measurements as well as computational simulations using a Raytrace Model. It is shown both experimentally and computationally that a thin- lm structure produces the same effciency as a homogeneously doped LSC. Two building integrated applications are examined. The rst one is a power generating window employing a Lumogen Violet dye that absorbs short wavelength radiation and is mostly transparent in the visible. Annual yields of over 23 kWh/m2 and a conversion effciency of over 1% are predicted for a 50 cm by 50 cm device. The second BIPV application is the light-bar, which is designed to act as the secondary concentrator in a Venetian blind-like system. With linear Fresnel lenses producing a primary concentration factor of 20, an optimised system could generate nearly 60W/m^2 of power at an effciency of nearly 6% using direct sunlight only. Two novel luminescent materials, nanorods and phycobilisomes have been tested for their potential to reduce re-absorption losses. Despite current practical limitations, these materials are found to be promising due to enhanced Stokes shifts. LSCs with optical concentrations of 10 to 20 could be feasible by addressing the key shortcomings in the form of unabsorbed light and escape cone losses. Their versatility with regards to shape, colour and light absorption makes LSCs particularly relevant for building integrated photovoltaics.Imperial Users Onl