| FUNDED RESEARCH PROJECTS |
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Theoretical Materials Physics
[top of the page] Professor in Theoreticals Materials Physics, Department of Physics, Faculty of Mathematics and Natural Sciences, University of Oslo. The academic postition is funded by the University of Oslo. |
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Functionalizing defects in advanced semiconductors
[top of the page] Defects define, to a large extent, the functionalities of a semiconductor - they are the catalyst for electrical conduction and optical interaction, but can at the same time can be detrimental to device performance. Based on the foundation (in Norwegian: 'fundament') of a holistic understanding of defects in solids, a key approach of FUNDAMeNT is functionalizing semiconducting materials using defects as enablers. The basis of the project is formed around topics where a profound understanding of defects in semiconductors holds the key to radical advances; (i) the development of an ideal single-photon source may reveal novel quantum mechanical phenomena, (ii) overcoming the doping asymmetry in wide band gap semiconductors may drastically improve energy efficiency in, e.g., solid state lighting, photovoltaics and power electronics, (iii) functionalizing defects in nanostructures may open up novel possibilities for radical advances in nanoscience- and technology. FUNDAMeNT relies heavily on ab initio modeling and several novel characterization techniques that expand defect characterization beyond state-of-the-art where ground-breaking results are expected, including online junction spectroscopy, temperature-variable scanning probe microscopy and synchrotron based deep level transient spectroscopy. http://www.mn.uio.no/smn/english/research/projects/physics/ This is a Top-Research Project ("ToppForsk") funded by the Research Council of Norway; 251131; 2016-2021. |
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Longer lifetime and higher efficiency of CZTS thin-film solar cells
[top of the page] Thin-film solar cells based on Cu2ZnSn(S,Se)4 and Cu2Zn(Sn,Ge,Si)S4 are developed for affordable sunlight harvesting with sufficiently high efficiency, long lifetime, and low exploitation of material resources. We optimize the solar cell efficiency with the profile of S/Se, Sn/Ge and Sn/Si gradients in depth, and we investigate the degradation processes, as well as the positive or detrimental impacts on the device performance due to structural disordering and local defect formations. Four scientifically intertwined work packages comprise the project objectives that combine synthesis and device prototyping with structural, electrical and optical characterization together with supporting calculations/simulations. This involves temperature dependent IV coupled to device modeling, with characterization of films by transmittance, reflectance, ellipsometry, and PL. Compositional and impurity profiling obtained by RBS and SIMS, phase analysis by XRD and Raman scattering, and analysis of open volume defects by positron annihilation spectroscopy. Defect calculations and diffusion modeling are performed by means of density functional and molecular dynamics. The project outcome will be a fundamental understanding of material/defects physics, degradation mechanisms, and stability of solar cell performance. The project is carried out in two research groups at the Dept of Physics, Univ of Oslo, linked to the Centre for Materials Science and Nanotechnology, and also at the Angstrom Lab, as well as national high-performance computing centers through NOTUR. http://www.mn.uio.no/fysikk/english/research/projects/czts/index.html This project is funded by the Research Council of Norway within ENERGIX programme; 243642; 2015-2018 |
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Gradient control in thin film solar cells
[top of the page] This project aims at the development of annealing processes in sulfur, selenium or other vapors, for higher efficiency thin film solar cells based on Cu(In,Ga)Se2 and Cu2ZnSnS4. The specific aim is to make band gap graded films both at the back- and front interfaces for reduced solar cell losses. The starting material can be either graded or non-graded, and is deposited by co-sputtering or co-evaporation. A custom-built furnace, with three sources, where temperature and partial pressure can be separately controlled, is used. The third source can for example be used for Na- or Ka-based compounds for surface modifications. The process development, including fabrication, analysis and modeling of solar cell devices, is accompanied by fundamental studies of inter-diffusion, phase segregation, homogeneity and grain growth in these material systems. Large area depth profiling measurements are complemented by local characterization by atom probe tomography. Theoretical analysis by first-principles based methods is used to study properties such as phase stability, electronic structure, defect and cluster formation and grain boundary properties. Diffusion studies are supported by molecular dynamics simulations. The expected outcome is processes for increased device efficiency for these solar cells, or maintained efficiency with processes better suited for industrial processing. This is based on better understanding of fundamental limitations set by thermodynamic and kinetic concerns. This project is funded by the Swedish Foundation for Strategic Research (SSF); 2016-2021 |
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Thermoelectric materials: nanostructuring for improving the energy efficiency of
thermoelectric generators and heat-pumps
[top of the page] Nationally Coordinated Research Project within the NANO2021 programme, 228854 THELMA "Thermoelectric materials: nanostructuring for improving the energy efficiency of thermoelectric generators and heat-pumps", 2013-2017. Partners: the universities in Oslo (UiO), Trondheim (NTNU), Stavanger (UiS), Agder (UiA), The Foundation for Scientific and Industrial Research (SINTEF), Norwegian Defense Research Establishment (FFI), and Institute for Energy Technology (IFE). Nanostructuring of thermoelectric materials is expected to result in improved efficiency of the thermoelectric generator, which in turn will lead to more energy efficient thermoelectric heat-pumps (TEHP) and thermoelectric electricity generators (TEG). The thermoelectric efficiency, material stability, and contacting properties are determined at the nano-scale and precise knowledge of these parameters are a necessity. Succeeding in developing stable, non-degrading and energy efficient TEPH and TEG will yield increased opportunities to harvest waste-heat from industry, the transportation fleet, and waste heat sources in general. In addition new installations for extraction of geothermal and solar heat can be established, and more efficient and modular heat-pumps for residential and office heating can be developed. Realization of such prospects can play an important role in our current challenges to reduce our dependence on fossil fuels and cut greenhouse gas emissions. THELMA is a nationally coordinated project which combines expertise from multiple fields in order to address these issues at the nano-scale. Thermoelectric modules are emerging as a highly promising and novel alternative to heat pumps and heat engines. The conversion between heat and electricity (or vice versa) is an intrinsic solid-state property, without any moving parts. A thermoelectric module can operate in a broad range of temperatures, from room temperature to 1200 K. With its low maintenance, scalability, portability, low-noise operation and potentially unmatched flexibility it is ideal for harvesting waste heat and heat in general from sources where no alternatives exist. Norwegian research and development on thermoelectrics has been steadily growing in the last years, building up a solid infrastructure of knowledge and equipment to study thermoelectric materials and related phenomena. The THELMA project represents a leap in the activity, bringing together a number of new thermoelectric participants in a highly interdisciplinary consortium. This is a coordinated national team which represents virtually all scientific activity relevant to thermoelectrics in Norway. The main objective of the project is to advance thermoelectric technology towards commercial maturity, and by that developing novel heat pump technology as well as a versatile tool for heat recovery and exploitation. This will be accomplished by combining state-of-the-art synthesis and material characterization, thorough performance measurements, and fundamental computations of material properties. The national and international partners will have their focus on four topics: improving conversion efficiency, reducing environmental impact, aiding commercialization and obstructing material degradation over time. This covers all current critical scientific aspects in the field of thermoelectrics. The projects main objective is to develop thermoelectricity (TE) in a joint manner towards a more mature technology for heat exploitation and heat pumps by combining the three pillars of modern material science; synthesis, characterization and computations. http://www.mn.uio.no/fysikk/english/research/projects/thelma/ This project is funded by the Research Council of Norway within NANO2021; 228854; 2013-2017 |
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Casimir effect and van der Waals forces in multilayer systems
[top of the page] Quantum electrodynamic fluctuation forces, which generalize the idealized force predicted by Casimir in 1948, have moved to an ever more central position in our understanding of mesoscopic forces between surfaces and between molecules and surfaces. Casimir-Lifshitz and Casimir-Polder forces generalize and transform the notion of van der Waals forces. Advances in experimental technique and development of new theoretical tools have enabled this transformation. There remain highly controversial issues to be resolved, such as the nature of thermal corrections, the form of Casimir friction, and whether it is realistic to achieve Casimir repulsion and levitation. The driver, however, is the potential applications of these phenomena to physical systems. Here, we want to link developments in theory to what is needed in practical situations, such as the interaction of molecular gases with oil shale. The project will be carried out and completed at the Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU) together with researchers from the Centre for Materials Science and Nanotechnology at the University of Oslo (UiO). Joint subprojects with the co-applicants from the USA, Germany, Brazil, Sweden and Australia will involve short and long-term research visits. We will also arrange two workshops (one at UiO and one at NTNU). A postdoc will be hired and advanced researchers will be supported by the grant. A summer school will be given the 3rd year (in connection with the second workshop) at NTNU. Collaboration via the internet will facilitate progress and coordination of research activities. Strong interaction with our research partners and collaborators will facilitate rapid progress in research and serve for networking, exchanging knowledge, and scientific visibility. Long-term international research visits by the postdoc and researchers will increase scientific productivity and strengthen the collaborations. http://www.mn.uio.no/fysikk/english/research/projects/casimir/ This project is funded by the Research Council of Norway within FRIPRO; programme; 250346; 2016-2019. |
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Cross-scale modeling of CO2/hydrocarbon conversion in hydrofractured shale
[top of the page] In shale gas systems, natural gas is produced directly from organic-rich shales through drilling and hydrofracturing. The methane is adsorbed at interfaces, absorbed by the kerogen shale, and contained as free gas and/or gas dissolved in water in pore volumes and fracture apertures. Interestingly, the affinity for carbon dioxide is stronger than the affinity for methane, and carbon dioxide may therefore be used to enhance gas production. This project involves fundamental research on the molecular physisorption/chemisorption and the meso-scale gas transport processes in nanostructured shales. The project strategy is to combine our knowledge from condensed matter physics, forces theories for molecules, and continuous transport models to explore details in the physical properties and processes of carbon dioxide and methane in water-rich shale nanostructures [1,2]. This will be realized by four scientifically intertwined work plans. WP1: Analyses of crystal structure and surface reconstruction. WP2: Atomistic simulation of surface of adsorption and desorption. WP3: Modeling of physisorption and chemisorption to analyze surface-gas-water interactions, molecule formation, and stiction. WP4: Explore the surface-near transport and deformation in shale systems. The project will be carried out and completed at the Department of Physics at University of Oslo by the research teams at Geophysics, linked to Center for the Physics of Geological Processes, and at Structure Physics, linked to the Centre for Materials Science and Nanotechnology. Strong international collaboration will serve for networking, exchange of knowledge, and scientific visibility. Long-term international research visits by the PhD student and the postdoctor will strengthen these contacts. http://www.mn.uio.no/smn/english/research/projects/physics/cross-scale/index.html This project is funded by the Research Council of Norway within FRIPRO; 221469; 2013-2017. [1] C. Persson and A. Zunger, Phys. Rev. Lett. 91, 266401 (2003); ibid, Appl. Phys. Lett. 87, 211904 (2005). [2] H. Svensen, et al., Nature 429, 542 (2004). |
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Novel semiconducting alloys in energy technology
[top of the page] Materials with radically enhanced performance for harvesting, conversion and transmission of energy are essential for a sustainable planet, where semiconducting oxides offer an attractive and wide span of functional properties. SALIENT explores such new environmentally friendly materials for harvesting, conversion and transmission of energy. Oxide semiconductors have become a vibrant field of research and development . This holdsespecially for zinc oxide (ZnO) which has undergone a spectacular revival during the past 15 years, being for a period since 2007 the second most studied semiconductor after silicon. However, for oxide semiconductors in general, and for ZnO in particular, it is challenging to exploit all their attractive inherent properties and full use in solid state devices. On the other hand, combined with a more mature technology, like gallium nitride (GaN), ZnO can overcome several of its major challenges, like a stable p-type material. In fact, the tuning possibility of functional properties may exceed that of the parent semiconductors by unconventional alloying. Hence, the research proposal SALIENT aims at investigating the physics of unconventional alloying of ZnO (ZnO-X), for enhanced functional properties. http://www.mn.uio.no/smn/english/research/projects/physics/salient/ This project is funded by the Research Council of Norway within FRIPRO; 239895; 2016-2019. [1] M. Dou and C. Persson, Cryst. Growth Des. 14, 4937 (2014). [2] M. Dou, G. Baldissera, and C. Persson, Int. J. Hydrogen Energy 38, 16727 (2013); ibid, J. Cryst. Growth 350, 17 (2012). |
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Coupled processes in gas-hydrate disassociation
[top of the page] Gas hydrates are crystalline solids consisting of cages of water molecules around a gas molecule such as methane. They are stable at 0-10 C at 300-600 m and are found in arctic regions and along the continental margins. There are more hydrocarbons in gas hydrates than in all other sources of hydrocarbons together. Gas hydrates therefore represent a possible energy game-changer. However, it is currently not economically or technologically feasible or safe to produce hydrocarbons from gas hydrates at large scales. Gas hydrate production can be combined with CO2 storage by CO2 injection, but it is limited by slow exchange rates. This project comprises fundamental research on how fracture and deformation of gas hydrates are related to the disassociation of the hydrates and to the possible replacement of methane by CO2. By understanding the underlying physical processes from atoms to the continuum, we aim to improve the production process and accelerate the conversion process. Our strategy involves three scientifically coupled subprojects that combined will give new insights into the mechanisms of gas hydrate deformation and how it affects disassociation: Molecular dynamics simulations of fracturing during gas hydrate disassociation; Continuum scale simulation of coupled processes during gas hydrate disassociation; and Combining atomic and continuum results to improve production methods. The project will be carried out at the Center for the Physics of Geological Processes, an established cross-disciplinary collaboration between physics and geology lasting more than 15 years with a record of producing research of fundamental importance with practical applications. The center is therefore in a unique position to address the proposed cross-disciplinary project. Through strong international collaborations and extended international stays, we will build a local competence in atomic scale modeling, and open for networking, exchange of knowledge, and scientific visibility. This project is funded by the Research Council of Norway within FRIPRO; 231621; 2014-2017 |
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Advanced ZnO-X alloys with tailored functionalities
[top of the page] ZnO is a functional semiconductor suitable for numerous types of technological applications. Whereas bulk ZnO and its cation alloys are investigated rather extensively, surprisingly little attention has been paid to understand more advanced alloy structures. In this project we will theoretically explore an unconventional type of ZnO-X alloys, where X = GaN, SiC, and Si, employing first-principles atomistic methods (GGA, GW, and ATAT phase stability) within the density functional theory. Our preliminary results [1,2] on ZnO-GaN indicate intriguing properties, and this will be analyzed in details. By exploring these ZnO-X compounds, we aim to realize tailored optoelectronic functionalities of ZnO-based materials. We anticipate finding new (unknown) functionalities that can be utilized in solar-energy technologies, for instance in photovoltaics, solid-state lightening, and photocatalytic hydrogen production. Four work packages define our research strategy: WP1) Crystalline, electronic, and optical properties of the three ZnO-X alloys; WP2) Impact due to formation of nanoclusters and ordered alloy phases; WP3) n- and p-type dopability; WP4) Search for alternative ZnO-X compounds with unique functionalities. The project will serve as a platform for exploring advanced nanostructured semiconductor alloys, and a strong international collaboration with both experimental and theoretical research groups will serve for networking, exchange of knowledge, and scientific visibility. This project is funded by the Swedish Research Council; C0485101; 2012-2015 Guest researcher is funded by NFR in the project "Novel semiconducting alloys in energy technology"; 239895; 2015-2019. [1] M. Dou and C. Persson, Phys. Status Solidi A 209, 75 (2012). [2] M. Dou, G. Baldissera, and C. Persson, J. Cryst. Growth 350, 17 (2012). |
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Multiscale in modelling and validation for solar photovoltaics
[top of the page] COST action: An intergovernmental framework for European Cooperation in Science and Technology, allowing the coordination of nationally-funded research on a European level. Nanostructures show unique tunable material properties with major and proven potential for state-of-the-art optoelectronics. Exploiting them for the challenging implementation of next generation solar cell architectures requires novel multiscale modelling and characterization approaches which capture both the peculiar features at nanoscale and their impact on the optoelectronic performance at device levels. To foster progress towards such approaches, Multiscalesolar creates a new network of experts defragmenting knowledge by combining existing research activities to address key issues in in next generation photovoltaics raised by academic and industrial end users. It provides quantum mechanical descriptions of electronic, optical and vibrational properties in order to parametrize mesoscopic models for the dynamics of charge carriers, photons and phonons in nanostructures. This yields effective material parameters for use in macroscopic device level models validated at each step by experiment. This Action combines theoretical and experimental expertise in industry and academia benefitting the European Research Area. The Action actively addresses gender issues, and favours early stage researchers, developing their scientific and management skills. The Action yields, for the first time, validated multiscale understanding of nanostructure properties for optoelectronic applications, with a focus on third generation photovoltaics. http://www.cost.eu/COST_Actions/mpns/Actions/MP1406 The COST action is supported by the Horizon 2020 - European Commission; 2015- |
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High-performance computing allocation
[top of the page] NOTUR project: “Atomistic modeling of oxides for solar-energy applications” HPC allocation for the team. Today, 8 million core-hours/year on Abel (USIT, Norway). Our research team searches for the optimized materials for solar-energy technologies, like next generation solar cells, solar-fuel conversion, light-emitting diodes. Our research also covers energy related research on CO2 storage, power battery, and smart windows. We model, calculate, and analyze materials and material structures in order to understand fundamental material physics, support experimentalists in their work, but also to explore new types of material structures. By modeling the material on atomistic and nanoscale, we study the electronic and optical properties, the stability of the materials, impact of defects or alloying, interfaces between materials. With this knowledge we can tailor make materials for an optimized performance of devices. This HPC allocation is supported through the Norwegian Metacenter for Computational Science (NOTUR), proj NN9180K, 2011-. SNIC project: “Atomistic modeling of unconventional alloys for solar-energy applications” HPC allocation. Today, 2,5 million core-hours/year on Beskow (PDC, Sweden) and 1 million core-hours/year on Triolith (NSC, Sweden). This HPC allocations is supported through the Swedish National Infrastructure for Computing (SNIC), proj SNIC 2016/1-332, 2004-. PRACE project: “Emerging solar cell materials” HPC allocation for 4 partners, 16 million core-hours on MareNostrum (BSC, Spain). In this project, we focus on the further understanding and development of thin film solar cell materials. Since the research groups together involves relatively many researchers, we define and form a project with two connected work packages: WP1 Explore various high-absorbing Cu-based chalcogenides in order to search for alternative environmentally friendly compounds, with potentially advantageous materials properties. WP2 Explore the properties of organic/inorganic hybrid systems and understand the potential of hybrid materials in solar cell application. The first principles studies of such materials require the calculations for systems containing more than 100 atoms as well as the modelling of complex structures (defect complexes, amorphous solids, etc.). Moreover, since traditional density functional theory (DFT) calculations cannot describe band structures of the materials accurately, we will use hybrid functional and GW calculations. In this project, the combinations of different first principles methods as well as our coding and method development experience will allow us to perform a detailed study of emerging solar cell materials. This HPC allocation is supported by the Partnership for Advanced Computing in Europe (PRACE),proj no. 2016143258, 2016-2017. DECI project: “Fundamental optoelectronic properties of ZnO-X alloy” HPC allocation, 3,3 million core-hours on Archer (EPCC, UK). In the project we will further explore the rather unconventional type of ZnO-based materials, that is (ZnO)1-yXy where X is an isovalent alloy compound, for instance X = SiC. By substituting both the cations and anions in ZnO (e.g. SiZn and CO) one can significantly alter and control the material properties, while ensuring relatively small disturbance on the crystalline structure since the binary constituents are isovalent with matching bond lengths. This HPC allocation is supported by the Distributed European Computing Initiative (DECI), 2016-2017. |
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Modeling of Functional Materials in Clean Energy Engineering
[top of the page] First-principles theoretical models are developed to study electronic, optical, and carrier transports properties of functional semiconductors/oxides and basic nanostructured devices. We combine density-functional studies with statistics, Boltzmann transport formulae, group theory, and modeled Hamiltonians. Defects/doping and scattering/recombination processes are considered. The models are generic, involve electronic and chemical non-equilibrium, interfaces, consider time-dependency (picoseconds to seconds) and are applicable to different nano- and micro-structures, thereby maintaining high research flexibility. We employ the models on various functional semiconductor and oxide nanostructures, with special focus on sustainable energy, optoelectronics, and biotechnologies, which are recognized experimental efforts at KTH. The theoretical studies will provide basic material and device parameters useful in advanced device simulations and measurements. A major goal is to propose improvements of clean energy photovoltaic, light-emitting solids, microstructured exhaust catalyst materials, and bioelectronics, but also to design new devices structures for these technologies. Additionally, all implemented models can be utilized to study a broad range of low-scale nanoelectronics and bio/electrochemical devices. This project is funded by the Swedish Energy Agency STEM 30952-1 (VR no 2007-4822), and by STEM 34138-1 (VR no 2010-4306). |
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Grain boundary physics in ZnO nanostructures
[top of the page] Nanostructured ZnO have been proposed as components in various types of applications, but only a detailed study of the nanostructured surfaces/interfaces allows for new concepts of functionality. This project involves design of sophisticated material compositions at the grain boundaries and interfaces of ZnO in order to control the bond character, charge density, and electron transport there. We propose that the interfacial structures can be optimized, and even reveal new functionalities by doping in combination with isovalent alloying. Modifying the atomic bonds will distort the charge distribution locally, whereby one can control the electronic transport paths as well as the adsorption/ desorption. The theoretical analysis is based on the density functional theory, involving optimization of interfacial structures and calculation of dopant/alloying formation energies, as well as calculations of local electronic and optical properties of grain boundaries and interfaces. We have already shown [1,2] that the proposed methods of analysis can provide new fundamental understanding of materials physics, which thereby open for new types of functional devices and we intend to continue our research in the same spirit. This project is funded by the Swedish Research Council (VR); 2009-4994 [1] C. Persson et al, Phys. Rev. Lett. 97, 146403 (2006). [2] C. Persson and A. Zunger, Phys. Rev. Lett. 91, 266401 (2003). |
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Biomass Thermochemical Conversion using Density Function Theory
[top of the page] Thermochemical conversion of Biomass is one of the promising technologies to supply renewable energy. It shall be attractive to understand of conversion micro-mechanics at the level of molecular in order to develop new concepts and technologies, solutions and products of biomass application for a selectivity and formation of undesirable production. In this project, the micro-reaction mechanism of lignocellulosic biomass gasification using ultra high temperature steam including pyrolysis and gasification will be investigated on molecular level using density functional theory (DFT) basing on quantum mechanism. Expect results include an optimized super-molecular structure of lignocellulosic biomass, thermal decomposition temperature, kinetics o f decomposition process of biomass super-molecule, micro-reaction mechanisms of biomass high temperature steam gasification. This project is funded by the Swedish Research Council (VR); 2010-4141. |
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Cu Vacancies as Donor Electron Compensator in Highly Off-Stoichiometric Polycrystalline Cu(In,Ga)Se2
[top of the page] Thin-film photovoltaic (PV) technologies (or "solar cells") are being developed by the means of substantially reducing the cost of the PV systems owing to their reduced material, growth and processing energy, handling, and capital costs. By alloying the chalcopyrite semiconductors CuInSe2 (CIS) with CuGaSe2 (CGS) one can tune the fundamental band-gap energy of CuIn(1-x)Ga(x)Se2 (called, "CIGS") from 1.0 to 1.7 eV, varying the Ga content x = [Ga]/([Ga]+[In]) from 0 to 1. Today's high-quality thin-film CdS/CIGS cells have a record of the solar-cell conversion efficiency of 19% [1,2], using Cu-poor ([Cu] ~ 22.5-24.5%) high off-stoichiometric polycrystalline CIGS absorber with ~1 micrometer size grains with Na-rich and Cu-poor grain boundaries (GB) [1-3]. The best PV cells have a Ga content of ~28% (i.e., x = 0.28). The optical band-gap energy of the CIGS alloys suggests however that the optimum cell performance should be obtained with a much higher Ga content (x = 0.6-0.8), but experimentally one has found that the efficiency drops abruptly at x ~ 0.30 and the cell performance is low in the Ga-rich composition regime (x > 0.30). The reason for this abrupt change in cell efficiency is not known. Moreover, CIGS can easily be grown p-type under Cu-poor conditions. However, it is puzzling that whereas bulk CIS can also be grown n-type under Se-poor/In-rich conditions, CGS seems to resist all kinds of n-type character, both from intrinsic and extrinsic doping. The underlying mechanism for this resistance to n-type dopability in CGS is not fully understood Our preliminary first-principles calculations show [3,4] that the low formation energy dHf of charged Cu vacancies (VCu-) prevents the Fermi level EF to be above the mid-gap energy Eg/2 in CGS. For higher energies, the exothermic behavior of VCu in CGS compensates all kinds of n-type doping, no matter what intrinsic (e.g. InCu++) or extrinsic (e.g. divalent cation Cd-on-Cu or anion halogen Cl-on-Se) donor type is supplied. This exothermic behavior [dHf(VCu-) < 0] cannot be overcome under equilibrium conditions. The theoretical result indicates that any n-type inversion layer in the CIGS absorber will be suppressed by spontaneously formation of VCu- when the Ga-content is increased, and thus, the CIGS alloy may not be the proper wide-gap alloy for a high-efficient PV cell. In this project, we continue the theoretical study of the underlying basic physical mechanisms for the remarkable low formation energy of the electron compensating Cu vacancies in the chalcopyrites. This project is funded by the Swedish Research Council (VR); 2003-3485, and it comprises collaborative research between the theoretical research group at Dept. of Materials Sciences and Engineering, KTH, Stockholm National Renewable Energy Laboratory (NREL), Golden, CO, USA. [1] Clean Electricity from Photovoltaics, ed. by M. D. Archer et al. (Imperical, London, 2001). [2] K. Ramanathan, et al., Prog. Photovolt. Res. Appl. 11, 225 (2003). [3] C. Persson and A. Zunger, Phys. Rev. Lett. 91, 266401 (2003). [4] C. Persson, Y.-J. Zhao, S. Lany. A. Zunger, Phys. Rev. B 72, 035211 (2006). |
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Charge-Neutral Hole Barriers at Grain Boundaries in Polycrystalline CuInSe2 Induced by Cu
Vacancy Reconstruction
[top of the page] Ordering Photovoltaic solar cells and other optoelectronic devices often necessitate the use of (rather expensive) single-crystalline active materials, because the analogues, low-cost polycrystalline substances, tend to exhibit poor carrier transport. A polycrystalline material is made of small crystallites joined at their surfaces via grain boundaries (GB's). These interfaces tend to become sinks for both chemical impurities and structural defects that segregate there from the grain interior (GI) during growth. In polycrystalline Si and GaAs these GB defects form effective recombination centers for the optically generated electrons and holes, thus diminishing and even eliminating carrier transport. Attempts to utilize polycrystalline semiconductors such as Si or GaAs in solar cells thus rest on various schemes for partial chemical passivation of the GB's. However, the device efficiency is always lower than that of the corresponding single-crystalline devices. A notable exception is polycrystalline CuInSe2 solar cells, where todays cell efficiencies (~20%) outperform the best single-crystal devices (~13%), even though no deliberate passivation of the GB's is attempted. Unlike covalent binary semiconductors, the stablest surface of chalcopyrites is the polar (112) surface [1]. This polar surface exhibit universal reconstruction patterns involving rows of Cu vacancies. We show here [2] via first-principles calculations of model CuInSe2 GB's that the Cu vacancy reconstruction always depresses the valence band at the GB, thus impeding holes from entering it. Consequently, the absence of GB holes prevents electrons from recombining at the GB defects. This explains the puzzle of the superiority of polycrystalline CuInSe2 solar cells over their crystalline counterpart. We identify a simple and universal mechanism for the barrier arising from reduced p-d repulsion due to Cu-vacancy surface reconstruction. We conclude that the local Cu-vacancy reconstruction at the GB expels holes, thus creating a ``free zone'' for fast electron transport. Future design of recombination-free zones via engineering of a barrier to one carrier type can open the way to the utilization of polycrystals in high-performance devices. This project is funded by the Swedish Research Council (VR), and it is a collaborative research with the National Renewable Energy Laboratory (NREL), Golden, CO, USA. [1] J. E. Jaffe and A. Zunger, Phys. Rev. B 64, R241304 (2001). [2] C. Persson and A. Zunger, Appl. Rev. Lett, 87, 211904 (2005). |
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Optical and Electrical Properties of Transition Metal Oxides
[top of the page] Much work has been devoted to the electronic properties of transition metal oxides as evidenced by the books by Tsuda et al. [1] and Granqvist [2], and references quoted therein. On the experimental side most of the fundamental work has been carried out on crystalline materials [1], thus avoiding many of the complexities of disorder. However, there is today a growing interest in the effects of disorder on the physical properties of materials. In many applications thin films are used and film deposition often leads to polycrystalline and amorphous structures where disorder plays a major role. Indeed, the transition metal oxides we intend to study are of importance for a range of applications. Tungsten and nickel oxide exhibit electrochromism and the coatings can be used in “smart windows”, displays, anti-dazzling rear-view mirrors and variable emittance panels for space applications [2]. Other applications of transition metal oxides are for example as gas sensors [3], as capacitor materials [4] and ferroelectrics [5] for microelectronics or as ion conducting coatings [6]. A thorough understanding of the electronic properties of the oxides is vital for the development of these and other applications. When a single carrier is placed in an empty band of a polar or ionic material it is affected by the coupling to the optical phonons. If this coupling is weak the carrier is still free to move throughout the system. This particle is named a large polaron. When the polar coupling is strong the carrier can be trapped in the attractive potential from the displacement-polarized atoms, induced by the carrier itself. This particle is called a small polaron. If the coupling is strong enough this trapping happens spontaneously even in a perfect crystal, without any defects. In presence of defects or in an amorphous material the trapping happens for weaker coupling and the binding becomes stronger. It is recognized that these effects are of major importance for the optical and electrical properties of many transition metal oxides. The physical properties of tungsten oxide have been extensively studied because of its strong electrochromism and suitability for electrochromic devices. The charge carriers are believed to be large polarons in the case of the crystal [1]. For disordered and amorphous coatings, as well as ion intercalated ones, very little comparison with fundamental theory has been carried out [2]. Polaron-like absorption in disordered coatings can be due to both oxygen vacancies as well as associated with intercalated protons or alkali ions. State-of-the-art band-structure calculations have been carried out both for the crystalline [7] and amorphous [8] structures. However, the states giving rise to the absorption have not been identified. The conduction mechanisms in nickel oxide are believed to be due to hopping processes even in the single crystal, but this picture is quite controversial [1]. An excess of oxygen in the material gives rise to hole conductivity. Disorder and ion intercalation is bound to make the situation more complicated. The optical and electrical properties of intercalated samples appear to be more complex than predicted by simple polaron models [9]. The common underestimate of the band gap by band-structure calculations have so far prevented a detailed comparison with experimental data. This project is funded by the Swedish Research Council (VR), and it is based on a collaboration between Applied Materials Physics, KTH, the Division of Solid State Physics, Department of Materials Science, Uppsala University. This part of the work will benefit very much from interaction with the applied studies carried out within the smart windows project of the Angstrom Solar Center (ASC) [10]. [1] N. Tsuda, K. Nasu, A. Yanase, and K. Siratori, Electronic conduction in oxides (Springer, Berlin 1991). [2] C.G. Granqvist, Handbook of inorganic electrochromic materials (Elsevier, Amsterdam, 1995). [3] J.L. Solis, A. Hoel, L.B. Kish, C.G. Granqvist, S. Saukko and V. Lantto, J. Amer. Ceram. Soc. 84, 1504 (2001). [4] H. Treichel, E. Eckstein and W. Kern: Ceramics Intern. 22, 435 (1996). [5] C.-R. Cho, A. Grishin and B.-M. Moon, Intrgrated Ferroelectrics 31, 35 (2000). [6] AnnaKarin Jonsson, G. Frenning, M. Nilsson, M.S. Mattsson and G.A. Niklasson, IEEE Trans. Dielectr. Electr. Insul. 8, 648 (2001). [7] A. Hjelm, C.G. Granqvist, and J.M. Wills, Phys. Rev. B 54, 2436 (1996). [8] G.A. de Wijs and R.A. de Groot: Phys. Rev. B 60, 16463 (1999). [9] T.M.J. Nilsson and G.A. Niklasson: Proc. SPIE 1272, 129 (1990). [10]C.G. Granqvist, A. Azens, A. Hjelm, L. Kullman, G.A. Niklasson, D. Roennow, M. Stroemme Mattsson, M. Veszelei and G. Vaivars: Solar Energy, 63, 199 (1998). |
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