This project aims to develop a new class of semiconductors for photovoltaics (PVs) that can tolerate defects to achieve high efficiencies when manufactured by low capital-intensity and scalable methods. PVs produce clean electricity from sunlight, and their deployment in the UK needs to accelerated by over an order of magnitude so that we can meet our legislated net-zero CO2 emissions target by 2050. New thin film PV materials are urgently needed. Thin film PVs can be used in tandem device structures, in which they are deposited on top of silicon PVs (which dominate the market) or smaller-bandgap thin film PVs. These tandem devices convert a larger fraction of the solar spectrum into electrical energy and can achieve efficiencies surpassing the best single-junction devices, which will be vital for accelerating utility-scale PV deployment. Thin film PVs can also be used as energy-harvesting roof-tiles, windows or cladding to enable sustainable carbon-neutral buildings. But across all applications, it is essential that the materials are efficient when made with by low cost manufacturing methods. The limiting factor is the deleterious role of point defects, such as vacancies. In traditional semiconductors, these point defects introduce energy levels deep within the bandgap and cause irreversible losses in energy. Minimising the density of these defects often requires expensive manufacturing routes. Defect-tolerant semiconductors circumvent these limitations by forming defect levels close to the band-edges (i.e., shallow), where they are less harmful. Such materials were rare until the recent serendipitous discovery of the lead-halide perovskites. Grown cheaply by solution-processing, these polycrystalline materials have over a million times more defects than silicon but are already more efficient in PVs than multi-crystalline silicon. A critical question is whether defect-tolerance can be found in other classes of materials that are free from the toxicity burden of the halide perovskites. This work aims to develop a set of design rules to pinpoint lead-free defect-tolerant semiconductors, and systematically develop these materials into efficient, stable PVs that can be deployed on the terawatt scale. The materials focussed on are ABZ2 compounds, where A is a monovalent cation, B a divalent cation and Z a divalent anion. These materials already show promising signs hinting at defect-tolerance. My approach draws off my experimental strengths in the control of complex thin films. I hypothesise that materials forming shallow traps can be identified through their crystal structure, band-edge orbital composition and degree of cation-anion orbital overlap. I will experimentally elucidate the role of each property by tuning the composition of a small set of ABZ2 materials to vary one property at a time. Defect tolerance will be measured by intentionally inducing vacancies and measuring their effect on charge-carrier lifetime and electronic structure. These design rules will be applied to identify the most promising ABZ2 materials, which will be grown by scalable solution- and vapour-based methods. I will optimise their growth using a fast experimental feedback loop to achieve materials with promising bulk properties for solar absorbers. Such materials will be developed into PVs, drawing off my skills and experience in device engineering. This work is extremely timely and will lead the emerging area of defect-tolerant semiconductors away from toxic perovskites. The new materials can ultimately become commercial contenders for tandem or building-integrated PVs, and therefore impact on the £120B PV industry. These new materials can also have much broader impact and be used, for example, as cheap but efficient materials for clean solar fuel production or biosensors. This project sets the key foundations for achieving these exciting possibilities and will enable me to set-up my group with a cutting-edge programme.

Next generation technologies, such as the IoT and 5G technology, are shaping to enhance the standard of life of people by creating a digitally connected world, in which the productivity, health, and communication will be vastly improved. This involves integrating sensors, intelligent circuits and miniature electronic devices into day to day objects around us, including the human body, clothing, buildings, vehicles and streets etc. Such systems become increasingly feasible due to the advancements in low-power electronics and IoT technologies, however, powering these electronics with the required complexity, flexibility, mobility and self-powered capabilities remains one of the key challenges in the modern era. Scavenging power from freely available ambient mechanical energy sources, such as human motion, wind, wave energy and machine vibrations, has been proven to be a viable approach to fulfil such energy and performance requirements. The triboelectric Nanogenerator (TENG) is one of the leading candidates to emerge as a potential energy source for powering autonomous IoT applications. These devices have shown the capability of capturing waste mechanical energy from ambient sources and easily producing a few Watts of output power, with high conversion efficiencies reported. However, knowledge of the electromagnetic behaviour of TENGs and the exact way they operate has been lacking in the past. Consequently, the relationship between the structural, material and motion parameters with the output power has not been adequately studied. This has resulted in non-optimised TENG architectures which suffer from relatively low, instantaneous and irregular output power, along with an impedance mismatch between the TENG and the output applications. Such issues decrease the output power of the TENG and significantly reduce its efficiency. This in turn associates with numerous other issues such as elevated cost, higher carbon footprint, larger device size and unreliable power supply. Recently, we introduced the distance-dependent electric field (DDEF) model, the first analytical theoretical model to fully describe the working principles of TENGs, using Maxwell's equations. This model has been proven to accurately predict the output behaviour of different TENG working modes and has been successfully applied to develop optimisation strategies for simple planar TENGs, significantly reducing most issues described above. In the proposed project, we will use the DDEF model to optimise material, device and motion parameters of TENGs to develop autonomous energy harvesters for IoT applications such as health sensors, wireless communication networks, portable and wearable electronics. We will first assess the energy requirements of IoT devices and design TENGs with suitable efficiencies to capture that energy from ambient sources. These devices are then finetuned to obtain the ideal size, shape, and material type, which will fit the applications while providing optimum electric field distribution, resulting in increased power outputs. We will use commonly available, low cost and flexible triboelectric polymers (eg: nylon, PET) as TENG layers, and further use scalable low-cost manufacturing techniques. Nanotechnology based surface improvements will be conducted to further improve the efficiency of these devices. The suggested improvements will increase the output power by about 100% compared to a non-optimised device, as evident from our simulation and calculation results. To ensure a non-interrupted regular power supply, we will integrate many TENG units with calculated phase differences, which would result in a near DC output current. Finally, we will combine the power management circuits and energy storage units (eg: supercapacitors and flexible batteries) along with the TENG to the IoT module, to assemble the fully integrated self-powered IoT devices.

The development of the theory of quantum mechanics entails one of the most influential achievements in Physics and, arguably, science as a whole. Standing at the basis of the field of condensed matter, one of its earlier triumphs was to shed light on the question why some materials behave as insulators, while others exhibit metallic properties. Upon utilising the wave interpretation of particles, it was readily found that electrons in a periodic potential give rise to energy bands; the spectrum shows bands of continuous energy levels separated by gaps. Hence, filling up the 'Fermi Sea' such that an integer amount of bands are filled ensures that there is an excitation gap, and thus insulating behavior up to that energy scale, whereas filling a band fractionally gives rise to a metallic phase. Although band theory has been extraordinarily successful, it has been reinvigorated the past years due unexpected connections with the mathematical domain of topology. Topology in essence characterises properties of objects that are preserved under smooth deformations. This is usually exemplified by the topological equivalence of a coffee cup and doughnut. Without tearing or poking holes one can be deformed into the other and their general class may be quantified in terms of a so-called invariant, being the integer that counts the number of holes. Rather remarkably, this principle has been found to be of pivotal importance in phases of electronic matter, where the wavefunctions can tie distinctive collective knots, topologically distinguishing different classes of insulators and metals. These topological insulators and metals are not only appealing from a purely theoretical point of view, but also exhibit remarkable physical phenomena such as protected metallic edge states that could shape next-generation electronics, or excitations that can store quantum information, making topological materials a potentially key component of quantum computing platforms. While the impact of topological materials has been underpinned by a vast research interest and a rapid advancement of the field, it was discovered the past year that a whole new class of topological metals exists. These systems feature bands that are gapped everywhere except for special points at which bands pairwise touch. The resulting band nodes furthermore carry exotic kinds of topological charges that can be altered in a highly non-trivial manner. Namely, when such nodes between different sets of bands are braided along each other in momentum space, their charges are converted, inducing specific phase factors in the collective wave function that cannot be untangled. As a result, a new topological structure emerges that can be quantified by a novel type of invariant, known as Euler class. There are however clear indications that these results comprise the tip of the iceberg and that a whole new class of such Euler metals exists, especially when other crystalline symmetries are present that enforce new conditions on the topological classification. This programme aims to exploit these timely indications and investigate these new exciting forms of matter. This articulates around three main pillars that aim to (i) advance the theoretical understanding of these Euler phases, (ii) uncover their physical properties and (iii) design concrete pathways to bring them to the experimental domain. For the latter objective this includes an explicit integration of experimental and ab-initio project partners, with whom we intend to foster long-term alliances, thereby creating a strong programme in the prominent field of topological materials. Given the strong indications that these new Euler phases host exotic physical properties that, apart from their immense scientific potential, could culminate impact future technologies, we anticipate that this programme will generate profound impact, thereby further underpinning the strong research position of the UK.