Defects and oxide ion migration in the solid oxide fuel cell cathode material LaFeO3

LaFeO3, a mixed ionic electronic conductor, is a promising cathode material for intermediate temperature solid oxide fuel cells (IT-SOFC). Key to understanding the electronic and ion conducting properties is the role of defects. In this study ab initio and static lattice methods have been employed to calculate formation energies of the full range of intrinsic defects—vacancies, interstitials, and antisite defects—under oxygen rich and oxygen poor conditions, to establish which, if any, are likely to occur and the effect these will have on the properties of the material. Under oxygen rich conditions, we find that the defect chemistry favors p-type conductivity, in excellent agreement with experiment, but contrary to previous studies, we find that cation vacancies play a crucial role. In oxygen poor conditions O2– vacancies dominate, leading to n-type conductivity. Finally, static lattice methods and density functional theory were used to calculate activation energies of oxide ion migration through this material. Three pathways were investigated between the two inequivalent oxygen sites, O1 and O2; O2–O2, O1–O2, and O1–O1, with O2–O2 giving the lowest activation energy of 0.58 eV, agreeing well with experimental results and previous computational studies

Point defect formation in M₂AlC (M = Zr,Cr) MAX phases and their tendency to disorder and amorphize

First principles calculations are performed on Zr₂AlC and Cr₂AlC MAX phases to compare their ability to accommodate point defects under irradiation. Interatomic bonding is stronger in Cr₂AlC than Zr₂AlC but contrary to expectation Zr₂AlC exhibits higher vacancy and antisite pair formation energies. However, interstitials and Frenkel defects are generally more difficult to form in Cr₂AlC. The results are attributed to the mixed covalent/ionic/metallic nature of the bonding. Detailed comparison of all the energies suggests that the preferred defects in Zr₂AlC and Cr₂AlC are the V_Al+Al_i Frenkel and Cr_Al+Al_Cr antisite respectively. Thus the potential response of the two phases to irradiation is different and taking account of other competing defects it is suggested that Zr₂AlC is less susceptible to amorphization.The calculations were performed at the Cambridge HPCS and the UK National Supercomputing Service, ARCHER. Access to the latter was obtained via the CaFFE consortium and funded by EPSRC under Grant No. EP/M018768/1

Tuning the electronic band gap of Cu2O via transition metal doping for improved photovoltaic applications

Cu 2 O is a widely known p -type semiconductor with a band-gap value suitable for photovoltaic applications. However, due to its parity-forbidden nature of the first interband transition and high carrier recombination currents, Cu 2 O has failed to reach commercial application. Hybrid density functional theory has been used to study the effect of transition metal dopants on the electronic and optical properties of Cu 2 O . Substitutional transition metal dopants, incorporated on the copper site, allow band-gap tunability by creating a range of defect states in the electronic structure, from shallow levels to deep intermediate bands. The preferred position of the dopants is in the vicinity of copper vacancies, which are naturally found in Cu 2 O and are responsible for its p -type conductivity. Impurity levels created via extrinsic transition metal dopants increase substantially the capacity of Cu 2 O to absorb light, reaching values close to 10%. First row transition metal dopants thus show potential for considerable improvement of the overall photovoltaic performance of Cu 2 O

Automated procedure to determine the thermodynamic stability of a material and the range of chemical potentials necessary for its formation relative to competing phases and compounds

We present a simple and fast algorithm to test the thermodynamic stability and determine the necessary chemical environment for the production of a multiternary material, relative to competing phases and compounds formed from the constituent elements. If the material is found to be stable, the region of stability, in terms of the constituent elemental chemical potentials, is determined from the intersection points of hypersurfaces in an (n-1)-dimensional chemical potential space, where is the number of atomic species in the material. The input required is the free energy of formation of the material itself, and that of all competing phases. Output consists of the result of the test of stability, the intersection points in the chemical potential space and the competing phase to which they relate, and, for two- and three-dimensional spaces, a file which may be used for visualization of the stability region. We specify the use of the program by applying it both to a ternary system and to a quaternary system. The algorithm automates essential analysis of the thermodynamic stability of a material. This analysis consists of a process which is lengthy for ternary materials, and becomes much more complicated when studying materials of four or more constituent elements, which have become of increased interest in recent years for technological applications such as energy harvesting and optoelectronics. The algorithm will therefore be of great benefit to the theoretical and computational study of such materials

Deeper Understanding of Interstitial Boron-Doped Anatase Thin Films as A Multifunctional Layer Through Theory and Experiment

Thin films of interstitial boron-doped anatase TiO2, with varying B concentrations, were deposited via one-step atmospheric pressure chemical vapor deposition (APCVD) on float glass substrates. The doped films showed a remarkable morphology and enhanced photoactivity when compared to their undoped analogues. The TiO2:B films also presented enhanced conductivity and electron mobility as measured by a Hall effect probe as well as a high adherence to the substrate, stability and extended lifetime. The structure and composition of the different samples of TiO2:B films were studied by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and dynamic secondary ion mass spectrometry (D-SIMS). Hybrid density functional theory was used to explore the defect chemistry of B-doped anatase and to understand the experimental results

Density Functional Studies of Nanomaterials with Applications in Electronic Devices and Hydrogen Storage

We present density functional calculations investigating two different nanomaterials: a titanium carbide nanocluster and few-layered black phosphorus. The titanium carbide nanocluster, Ti₈C₁₂, has properties that are well suited to applications in hydrogen storage, while few-layered black phosphorus has recently been used in the fabrication of novel field effect transistors. Chapter 1 provides some background information regarding hydrogen storage and electronic devices, with Chapter 2 introducing the computational methods used throughout subsequent chapters. In Chapter 3, we investigate the thermodynamic and kinetic profile of H₂ dissociation by Ti₈C₁₂ under a range of conditions. Our results show that that Ti₈C₁₂ is able to reversibly dissociate H₂ with an unusually low activation barrier. In Chapter 4, we introduce few-layered black phosphorus, dubbed phosphorene. The use of black-phosphorus exfoliates in FETs is potentially important given the fast approaching limits of transistor miniaturization using current technologies. Phosphorene appears to have properties necessary for use in next generation FETs, and has therefore attracted enormous experimental and theoretical attention. Our work on phosphorene contributes to an ever growing body of information, with Chapter 5 investigating the effects of deforming monolayer and bilayer phosphorene and Chapter 6 investigating the properties of phosphorene nanoribbons. In Chapter 5, we show that compressing bilayer phosphorene normal to its surface dramatically increases n-type mobility and modulates the band gap. The compressions required to increase n-type mobility by a factor of 10² are modest, meaning that our results are experimentally relevant. We also investigate the effects of bending of phosphorene, which has a highly anisotropic bending modulus. Our work on phosphorene nanoribbons in Chapter 6 shows that in-plane quantum confinement effects lead to a significant increase in the band gap. We replicate this effect by applying periodic boundary conditions to the bulk and derive a formula relating the band gap of phosphorene nanoribbons to phosphorene’s band edge effective masses. We also show that the band gap and mobility of phosphorene nanoribbons can be modified through the application of linear strain. II Chapter 7 concludes the main body of this thesis, summarising its outcomes and giving a direction for future work. We also include a brief investigation into a family of semiconducting quaternary oxynitride compounds in the Appendix. These compounds are of interest given that their band gaps fall within the visible light region

Perovskite-inspired materials for photovoltaics and beyond—from design to devices

Funder: Ministry of Education, TaiwanAbstract: Lead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform industry-standard multi-crystalline silicon solar cells, despite the fact that perovskites are typically grown at low temperature using simple solution-based methods. However, the toxicity of lead and its ready solubility in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temperature, facile fabrication methods. This Review discusses the computational and experimental approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compounds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials in other optoelectronic applications, namely light-emitting diodes, photocatalysts, radiation detectors, thin film transistors and memristors. Finally, the prospects and key challenges faced by the field in advancing the development of perovskite-inspired materials towards realization in commercial devices is discussed

Perovskite-inspired materials for photovoltaics and beyond—from design to devices

Funder: Ministry of Education, TaiwanAbstract: Lead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform industry-standard multi-crystalline silicon solar cells, despite the fact that perovskites are typically grown at low temperature using simple solution-based methods. However, the toxicity of lead and its ready solubility in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temperature, facile fabrication methods. This Review discusses the computational and experimental approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compounds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials in other optoelectronic applications, namely light-emitting diodes, photocatalysts, radiation detectors, thin film transistors and memristors. Finally, the prospects and key challenges faced by the field in advancing the development of perovskite-inspired materials towards realization in commercial devices is discussed