Accelerated development of CuSbS2 thin film photovoltaic device prototypes

Development of alternative thin film photovoltaic technologies is an important research topic because of the potential of low‐cost, high‐efficiency solar cells to produce terawatt levels of clean power. However, this development of unexplored yet promising absorbers can be hindered by complications that arise during solar cell fabrication. Here, a high‐throughput combinatorial method is applied to accelerate development of photovoltaic devices, in this case, using the novel CuSbS₂ absorber via a newly developed three‐stage self‐regulated growth process to control absorber purity and orientation. Photovoltaic performance of the absorber, using the typical substrate CuIn_xGa_{1 − x}Se₂ (CIGS) device architecture, is explored as a function of absorber quality and thickness using a variety of back contacts. This study yields CuSbS₂ device prototypes with ~1% conversion efficiency, suggesting that the optimal CuSbS₂ device fabrication parameters and contact selection criteria are quite different than for CIGS, despite the similarity of these two absorbers. The CuSbS₂ device efficiency is at present limited by low short‐circuit current because of bulk recombination related to defects, and a small open‐circuit voltage because of a theoretically predicted cliff‐type conduction band offset between CuSbS₂ and CdS. Overall, these results illustrate both the potential and limits of combinatorial methods to accelerate the development of thin film photovoltaic devices using novel absorbers. Copyright © 2016 John Wiley & Sons, Ltd.     

Enhanced Electron Mobility Due to Dopant‐Defect Pairing in Conductive ZnMgO

The increase of the band gap in Zn_1‐xMg_xO alloys with added Mg facilitates tunable control of the conduction band alignment and the Fermi‐level position in oxide‐heterostructures. However, the maximal conductivity achievable by doping decreases considerably at higher Mg compositions, which limits practical application as a wide‐gap transparent conductive oxide. In this work, first‐principles calculations and material synthesis and characterization are combined to show that the leading cause of the conductivity decrease is the increased formation of acceptor‐like compensating intrinsic defects, such as zinc vacancies (V_Zn), which reduce the free electron concentration and decrease the mobility through ionized impurity scattering. Following the expectation that non‐equilibrium deposition techniques should create a more random distribution of oppositely charged dopants and defects compared to the thermodynamic limit, the paring between dopant Ga_Zn and intrinsic defects V_Zn is studied as a means to reduce the ionized impurity scattering. Indeed, the post‐deposition annealing of Ga‐doped Zn_0.7Mg_0.3O films grown by pulsed laser deposition increases the mobility by 50% resulting in a conductivity as high as σ = 475 S cm^‐1.     

Applying Optimisation and Uncertainty Analysis to Help Develop an Integrated Water Resources Plan for South East England

The need to promote opportunities for integration between commercially independent water supply providers is becoming important as pressure on water resources increases in many parts of the world. Water stress in south eastern England is projected to increase over the next 25 years, influenced by factors such as: population growth; demographic change; changes in water use; water requirements for environmental protection; and, the potential effects of climate variability and change. Actions to avoid unacceptable deficits in the supply–demand balance are being planned, involving measures to reduce the demand for water and schemes to increase the sharing of water resources within the region. This paper outlines the application of a modelling framework incorporating optimisation modelling and stochastic simulation to help identify options to improve the integration of water resources systems across the region. The regional modelling framework has helped to identify integrated sets of schemes that can provide a cost-effective regional solution for securing the supply–demand balance at an acceptable level of security of supply. These integrated sets of schemes include shared water storage and transfer options that are broader in scope than those considered by individual water companies in drawing up their own water resources plans. Outcomes to-date have included: (a) the identification of cost-effective opportunities for shared resource development and new infrastructure to transfer water from areas of surplus to areas of deficit; (b) interest from the six commercially independent water companies across the region in looking beyond their own water resource zones to develop a better integrated and more sustainable water supply and distribution network across South East England; and (c) valuable insight which is being applied in further regional modelling work to inform the development of the next round of water resources plans due to be submitted by water companies in 2014.     

Doping Rules and Doping Prototypes in A2BO4 Spinel Oxides

A₂BO₄ spinels constitute one of the largest groups of oxides, with potential applications in many areas of technology, including (transparent) conducting layers in solar cells. However, the electrical properties of most spinel oxides remain unknown and poorly controlled. Indeed, a significant bottleneck hindering widespread use of spinels as advanced electronic materials is the lack of understanding of the key defects rendering them as p‐type or n‐type conductors. By applying first‐principles defect calculations to a large number of spinel oxides the major trends controlling their dopability are uncovered. Anti‐site defects are the main source of electrical conductivity in these compounds. The trends in anti‐sites transition levels are systemized, revealing fundamental “doping rules”, so as to guide practical doping of these oxides. Four distinct doping types (DTs) emerge from a high‐throughput screening of a large number of spinel oxides: i) donor above acceptor, both are in the gap, i.e., both are electrically active and compensated (DT‐1), ii) acceptor above donor, and only acceptor is in the gap, i.e., only acceptor is electrically active (DT‐2), iii) acceptor above donor, and only donor is in the gap, i.e., only donor is electrically active (DT3), and iv) acceptor above donor in the gap, i.e., both donor and acceptor are electrically active, but not compensated (DT‐4). Donors and acceptors in DT‐1 materials compensate each other to a varying degree, and external doping is limited due to Fermi level pinning. Acceptors in DT‐2 and donors in DT‐3 are uncompensated and may ionize and create holes or electrons, and external doping can further enhance their concentration. Donor and acceptor in DT‐4 materials do not compensate each other, and when the net concentration of carriers is small due to deep levels, it can be enhanced by external doping.     

Control of the Electrical Properties in Spinel Oxides by Manipulating the Cation Disorder

In this work, the impact of cation disorder on the electrical properties of biaxially textured Co₂ZnO₄ and Co₂NiO₄ thin films grown by pulsed laser deposition are investigated using a combination of experiment and theory. Resonant elastic X‐ray diffraction along with conductivity measurements both before and after post‐deposition annealing show that Co₂ZnO₄ and Co₂NiO₄ exhibit opposite changes of the conductivity with cation disorder, which can be traced back to their different ground‐state atomic structures, being normal and inverse spinel, respectively. Electronic structure calculations identify a self‐doping mechanism as the origin of conductivity. A novel thermodynamic model describes the non‐equilibrium cation disorder in terms of an effective temperature. This work offers a way of controlling the conductivity in spinels in a quantitative manner by controlling the cation disorder and a new design principle whereby non‐equilibrium growth can be used to create beneficial disorder.     

Li‐Doped Cr2MnO4: A New p‐Type Transparent Conducting Oxide by Computational Materials Design

To accelerate the design and discovery of novel functional materials, here, p‐type transparent conducting oxides, an inverse design approach is formulated, integrating three steps: i) articulating the target properties and selecting an initial pool of candidates based on “design principles”, ii) screening this initial pool by calculating the “selection metrics” for each member, and iii) laboratory realization and more‐detailed theoretical validation of the remaining “best‐of‐class” materials. Following a design principle that suggests using d5⁵ cations for good p‐type conductivity in oxides, the Inverse Design approach is applied to the class of ternary Mn(II) oxides, which are usually considered to be insulating materials. As a result, Cr₂MnO₄ is identified as an oxide closely following “selection metrics” of thermodynamic stability, wide‐gap, p‐type dopability, and band‐conduction mechanism for holes (no hole self‐trapping). Lacking an intrinsic hole‐producing acceptor defect, Li is further identified as a suitable dopant. Bulk synthesis of Li‐doped Cr₂MnO₄ exhibits at least five orders of magnitude enhancement of the hole conductivity compared to undoped samples. This novel approach of stating functionality first, then theoretically searching for candidates that merits synthesis and characterization, promises to replace the more traditional non‐systematic approach for the discovery of advanced functional materials.     

Interplay between Composition,Electronic Structure,Disorder, and Doping due to Dual Sublattice Mixing in Nonequilibrium Synthesis of ZnSnN2:O

The opportunity for enhanced functional properties in semiconductor solid solutions has attracted vast scientific interest for a variety of novel applications. However, the functional versatility originating from the additional degrees of freedom due to atomic composition and ordering comes along with new challenges in characterization and modeling. Developing predictive synthesis–structure–property relationships is prerequisite for effective materials design strategies. Here, a first‐principles based model for property prediction in such complex semiconductor materials is presented. This framework incorporates nonequilibrium synthesis, dopants and defects, and the change of the electronic structure with composition and short range order. This approach is applied to ZnSnN₂ (ZTN) which has attracted recent interest for photovoltaics. The unintentional oxygen incorporation and its correlation with the cation stoichiometry leads to the formation of a solid solution with dual sublattice mixing. A nonmonotonic doping behavior as a function of the composition is uncovered. The degenerate doping of near‐stoichiometric ZTN, which is detrimental for potential applications, can be lowered into the 10¹⁷ cm^?3 range in highly off‐stoichiometric material, in quantitative agreement with experiments.     

Selective and Brain‐Permeable Polo‐like Kinase‐2 (Plk‐2) Inhibitors That Reduce α‐Synuclein Phosphorylation in Rat Brain

Polo‐like kinase‐2 (Plk‐2) has been implicated as the dominant kinase involved in the phosphorylation of α‐synuclein in Lewy bodies, which are one of the hallmarks of Parkinson’s disease neuropathology. Potent, selective, brain‐penetrant inhibitors of Plk‐2 were obtained from a structure‐guided drug discovery approach driven by the first reported Plk‐2–inhibitor complexes. The best of these compounds showed excellent isoform and kinome‐wide selectivity, with physicochemical properties sufficient to interrogate the role of Plk‐2 inhibition in vivo. One such compound significantly decreased phosphorylation of α‐synuclein in rat brain upon oral administration and represents a useful probe for future studies of this therapeutic avenue toward the potential treatment of Parkinson’s disease.