Adele Tamboli, NREL
Joel Ager, Lawrence Berkeley National Laboratory / University of California, Berkeley
David Scanlon, University College London
Lydia Wong, Nanyang Technological University
EN19.01: High–Throughput Techniques
Tuesday AM, April 03, 2018
PCC North, 100 Level, Room 124 B
10:30 AM - EN19.01.01
Materials by Design for Energy Applications
National Renewable Energy Laboratory1Show Abstract
The United States’ Materials Genome Initiative (MGI), announced in 2011, helped launch the era of Materials-by-Design which combines theory, computation, experiment and data sciences to accelerate materials development and deployment. So far, Materials-by-Design research has primarily focused on the intrinsic properties of equilibrium materials. However, many technologically relevant materials are metastable and, further, roughly 1/3 or more of new materials discovered each year are metastable rather than equilibrium materials. Therefore, for new materials to help in addressing energy needs, we must directly address the research challenges of both including metastable materials and moving from Materials-by-Design to Solutions-(or Devices)-by-design. Our approach to and progress towards this challenge will be demonstrated by examples taken from current research on opto-electronic semiconductors and transparent conductors.
11:00 AM - EN19.01.02
Earth-Abundant Bismuth-Based Semiconductors as Novel Photovoltaics
Alex Ganose1,2,Keith Butler3,Aron Walsh4,5,David Scanlon1,2
University College London1,Diamond Light Source2,University of Bath3,Imperial College London4,Yonsei University5Show Abstract
Bismuth-based solar absorbers are of interest due to similarities in the chemical properties of bismuth halides and the exceptionally efficient lead halide hybrid perovskites. Both Pb2+ and Bi3+ possess a similar soft polarisability and form a wide range of compounds with rich structural diversity, such as BiX6 clusters, 1D ribbons, and layered perovskite type structures. Whilst both are composed of earth-abundant materials and experience the same beneficial relativistic effects acting to increase the width of the conduction band, bismuth is non-toxic and non-bioaccumulating, meaning the impact of environmental contamination is greatly reduced.
Here, we use hybrid density functional theory, with the addition of spin orbit coupling (SOC), to examine a range of bismuth containing V-VI-VII candidate photovoltaic (PV) absorbers.[2-5] We show that BiSI and BiSeI possess electronic structures suitable for photovoltaic applications. Furthermore, we calculate band alignments against commonly used hole transporting and buffer layers, which indicate band misalignments are likely to be the source of the poor efficiencies reported for devices containing these materials. Based on this, we have suggested alternative device architectures expected to result in improved power conversion efficiencies. Lastly, we explore the defect properties of BiSI and suggest ideal growth conditions for optimised film properties.
 A. M. Ganose, C. N. Savory and D. O. Scanlon, Chem. Commun. 53, 20–44 (2017)
 K. T. Butler, J. M. Frost, and A. Walsh, Energy Environ. Sci. 8, 838 (2015)
 A. M. Ganose, K. T. Butler, A. Walsh, and D. O. Scanlon, J. Mater. Chem. A 4, 2060 (2016).
 A. M. Ganose, M. Cuff, K. T. Butler, A. Walsh and D. O. Scanlon, Chem. Mater. 28, 1980 (2016)
 D. S. Bhachu et. al., Chem. Sci. DOI: 10.1039/C6SC00389C (2016)
11:15 AM - EN19.01.03
High Throughput Screening of P-Type Transparent Chalcogenide Candidates
Rachel Woods-Robinson1,2,3,Shyam Dwaraknath2,Andriy Zakutayev3,Kristin Persson1,2
University of California, Berkeley1,Lawrence Berkeley National Lab2,National Renewable Energy Laboratory3Show Abstract
Given rapid advances in photovoltaics, transparent electronics, and other emerging energy technologies, the development of p-type transparent conductors (TCs) has been a relatively slow-moving front. Both n-type and p-type TCs are conventionally oxides, but there is increasing evidence that chalcogenide (S, Se, Te) semiconductors should have higher hole transport and probability of p-type doping than oxides (in exchange for lower transparency). We use a high throughput computational framework to screen a large database of chalcogenide compounds likely to have a high hole conductivity and high optical transparency. Our ultimate goal is to synthesize promising compounds in the laboratory for use in stable devices. To this effect, we investigate potential metrics for TC performance including various weighting methods for Boltzmann transport calculations, defect formation energies, dopant selection criteria and global thermodynamic stability. From these criteria, we discover over one hundred computationally stable multi-anionic compounds with indirect computed gaps EG > 1.5 eV (since PBE underestimates the gap) and average effective masses 0 < mh* < 1 by screening the Materials Project database. Several compounds studied previously as TCs emerge from our screening, including ZnS and sulvanites TaCu3X4 (X = S, Se, Te). We further pare down this list for synthesis by selecting only single anionic compounds, removing compounds with toxic and highly reactive elements, and estimating p-type dopability. A refined list of ten top experimentally-favorable candidates emerges and includes spinel ZnAl2S4, distorted rocksalt BaSnS2, and several other rocksalt structures. We will also present our initial attempts to combinatorially synthesize and characterize a few of the candidates, and lay out a roadmap for a future high-throughput screening, synthesis, characterization closed loop to enable new device paradigms using transparent conducting chalcogenides.
11:30 AM - EN19.01.04
High Throughput Experimental Database for Optoelectronic Materials
Andriy Zakutayev1,Nick Wunder1,Marcus Schwarting1,John Perkins1,Robert White1,Kristin Munch1,William Tumas1,Caleb Phillips1
National Renewable Energy Laboratory1Show Abstract
The use of advanced machine learning algorithms for design and discovery of novel inorganic semiconductors for optoelectronics energy conversion applications requires large datasets amenable to data mining. Whereas a number of computational materials property databases exists (e.g. materialsproject.org, materials.nrel.gov), the machine learning based on experimental data is limited by the lack of large and diverse data resources. Here, we report on our progress towards a publicly open High Throughput Experimental Materials (HTEM) Database (htem.nrel.gov). Presently, this database contains 130,000 sample entries, characterized by structural (100,000), chemical (70,000), optical (50,000) and electrical (40,000) properties of novel inorganic thin film semiconductor materials, grouped in >4,000 sample entries across >100 materials systems. More than a half of these data are publicly available. In addition to showing how HTEM database may enable scientists to explore materials by browsing web-based user interface, this presentation will discuss the underlying laboratory information management system (LIMS). Also, this presentation will illustrate how advanced machine learning algorithms can be adopted to materials science problems of predicting materials conductivity using random forest methods, and clustering unrelated samples into groups based on composition similarity.
11:45 AM - EN19.01.05
Combining Chemical Heuristics, Machine Learning and First Principles Calculations for Rapid Materials Screening
Daniel Davies1,Keith Butler1,Olexandr Isayev2,Aron Walsh3
University of Bath1,University of North Carolina2,Imperial College London3Show Abstract
The discovery of earth abundant, functional materials is critical for sustainable technological advancement. There is a concerted global effort to reduce the time it takes to realize such materials via databases, high-throughput screening, informatics, and mapping out the ‘‘materials genome.’’ But what fraction of theoretical chemical space is represented by the number of known compounds that have been thoroughly characterized to date? Forming a four-component compound from the first 103 elements results in excess of 1012 potential combinations. Such a search space is intractable to high-throughput experiment or first principles calculations.
We present a hierarchical screening approach that is capable of dealing with such a search space, consisting of three key stages: First, we employ an arsenal of simple chemical rules that are the product of centuries of research, in order to filter out chemically implausible element compositions. This is implemented using the open-source SMACT package.1 Second, we use supervised machine learning and data mining to rapidly filter for target properties and suggest likely structures of leading candidates. Finally, we apply density functional theory (DFT) calculations in order to verify stability, structure and target properties. At each stage, the size of the search space is drastically reduced, ensuring that as computational cost increases, the number of candidate materials remains feasible.
We demonstrate the power of this approach by discovering new quaternary oxide materials for solar energy applications. SMACT is used to reduce the search space of billions to ~1 million chemically sensible compositions. A gradient boosting regression machine learning model is trained on a database2 of high-quality bandgap calculations, then used to identify ~20,000 oxide compositions that are most likely to have useful bandgaps. A recent statistics-based approach to structure prediction using a probabilistic model proposed by Hautier et al.3 is employed to suggest ~500,000 likely crystal structures, which are then fed to the AFLOW-ML4 model to predict thermodynamic stability via machine-learnt DFT total energies. All of these steps can be carried out in a matter of hours to days, using minimal computing resources. The result is a series of potential new energy materials; our methodology can be applied to materials design in a range of contexts and is an important new tool in the quest for accelerated materials discovery.
1. D. W. Davies, K. T. Butler, A. J. Jackson, A. Morris, J. M. Frost, J. M. Skelton, A. Walsh, Chem, 2016, 1, 617.
2. I. E. Castelli, F. Hüser, M. Pandey, H. Li, K. S. Thygesen, B. Seger, A. Jain, K. A. Persson, G. Ceder, K. W. Jacobsen, Adv. Energy Mater., 2015, 5, 1400915
3. G. Hautier, C. Fischer, V. Ehrlacher, A. Jain, G. Ceder, Inorg. Chem., 2011, 50, 656.
4. O. Isayev, C. Oses, C. Toher, E. Gossett, S. Curtarolo, A. Tropsha, Nat. Commun., 2017, 8, 15679.
EN19.02: Novel Synthesis Techniques
Tuesday PM, April 03, 2018
PCC North, 100 Level, Room 124 B
1:30 PM - EN19.02.01
Formation, Doping and Grain Growth of Inorganic Chalcopyrite, Kesterite and Bismuth Halide Semiconductors via Solution Processing
Univ of Washington1Show Abstract
Potentially low-cost high-throughput approaches have been demonstrated that form inorganic semiconductor films directly from nanoparticle or molecular inks. The highest efficiency devices have been prepared with CuInGaSe2 utilizing hydrazine as a solvent and complexing agent. Here, we present our progress to develop of a class of solution-phase routes to Cu2ZnSn(S,Se)4, Cu(In,Ga)(S,Se)2 and most recently bismuth halides that do not use suspensions of nanoparticles or hydrazine. We have discovered new effects of alloying and doping using a combinatorial ultrasonic spray coater and high-throughput screening method to map the optoelectronic properties of absorber layers. The presentation will focus on: (i) the formation of films and elimination of deleterious elements, (ii) incorporation of dopants and their effects of absorber properties and grain boundaries, (iii) alloying to form record high open circuit voltage relative to the maximum theoretical open circuit voltage for the bandgap, (iv) a new understanding of grain growth and impurity removal in kesterites, and recent results to yield tandem solar cells with hybrid perovskites.
 Xin, H., Vorpahl, S.M., Collord, A.D., Braly, I.L., Uhl, A.R., Krueger, B.W., Ginger, D.S., Hillhouse, H.W., Phys. Chem. Chem. Phys. 17, 23859-23866 (2015).
 Uhl, A.R., Katahara, J.K., Hillhouse, H.W., Energy & Environmental Science 9, 130-134 (2016).
 Collord, A.D., Hillhouse, H.W., Chem. Mater. 28, 7, 2067–2073 (2016).
 Clark, J.A., Uhl, A.R., Martin, T., Hillhouse, H.W., Chem. Mater. ASAP, DOI: 10.1021/acs.chemmater.7b03313, (2017).
 Uhl, A.R., Yang, Z., Jen, A.K.-Y, Hillhouse, H.W., J. Mater. Chem. A 5, 3214-3220 (2017).
 Williamson, B.W., Eickemeyer, F.T., Hillhouse, H.W., Submitted.
2:00 PM - EN19.02.02
ZnGeP2—A Promising Material for Integration with Silicon-Based Optoelectronics
Rekha Schnepf1,2,Aaron Martinez1,2,John Mangum1,2,Noemi Leick1,Paul Ndione1,Elisa Miller-Link1,Pauls Stradins1,2,Eric Toberer1,2,Adele Tamboli1,2
National Renewable Energy Laboratory1,Colorado School of Mines2Show Abstract
In this work, we present the effects of annealing on the structural and optical properties of polycrystalline ZnGeP2 films on Si substrates. As structural analogs to III-V materials, II-IV-V2 materials have the potential for optoelectronic applications beyond the present III-V materials. Currently, III-V optoelectronic devices enable fiber communications, solid-state lasers, light emitting diodes and high efficiency photovoltaics, but they rely on epitaxial heterostructures limited by lattice matching considerations. In contrast, II-IV-V2 materials can be lattice matched to silicon and have the potential for tunable electronic properties for fixed composition through control of cation ordering. Therefore, implementation of a material with similar properties to the III-Vs and lattice matched with silicon could be transformative for tandem photovoltaics. ZnGeP2 is one such material with a lattice matching within 1% of silicon and a band gap of 2.1 eV.
In order to control crystallinity and ordering independently of composition, stoichiometric amorphous ZnGeP2 films were grown and then annealed ex-situ. The crystallinity and ordering of the annealed films was studied with x-ray diffraction and transmission electron microscopy. To gain an understanding of how optical properties change with structure, spectroscopic ellipsometery was used to determine the optical constants and absorption coefficient of the films. Using these characterization techniques, we have confirmed the ability to control crystallinity and ordering of the films as a function of anneal temperature and time. Subsequently, with changes in crystallinity and ordering we observed variations in the optical properties of the films. From our results we can conclude that ZnGeP2 shows large potential for integration in Si-based devices.
2:15 PM - EN19.02.03
Defects and Defect Dynamics in Amorphous Oxide Semiconductors
Missouri University of Science and Technology1Show Abstract
Tunable electrical conductivity – the ability to change carrier concentration over a wide range of useful values while maintaining superior mobility – is arguably the central technological advantage of an Amorphous Oxide Semiconductor (AOS) such as ternary or quaternary oxides of post-transition metals, for example, In-Sn-O, Zn-Sn-O, or In-Ga-Zn-O. Compared to the crystalline counterparts, where the electron mobility is governed primarily by scattering on ionized impurities, phonons, and grain boundaries, the nature of and the relationship between the carrier generation and transport in AOSs are more complex. Although amorphous materials lack grain boundaries, the strong local distortions in the Metal-Oxygen (M-O) polyhedra associated with a weak ionic M-O bonding as well as long-range structural correlations in the disordered system give rise to entangled transport phenomena at different length scales [1-3]. Given the many degrees of freedom in the amorphous structure, the long-range structural characteristics and the electronic properties of the donor defects in AOSs differ fundamentally from those in the crystalline transparent conducting oxides. Therefore, defects in AOSs must be considered along with the structural evolution of the disordered system.
In this work, we report the results of computationally intensive ab-initio molecular dynamics (MD) simulations combined with accurate density functional electronic structure calculations for amorphous In-based and Sn-based oxide semiconductors. Novel approaches for non-stoichiometric-melt cooling and time-dependent statistical analysis not only show a significant improvement over the standard oxygen vacancy models but also allow us to simultaneously address the structural morphology, evolution, and the dynamics of defect formation, thereby providing the necessary integral framework to comprehensively understand the fundamental materials properties of AOSs. We demonstrate that the approach provides a statistically complete defect picture of conducting amorphous oxides by capturing the formation of both shallow defects that produce carriers and localized deep defects that limit carrier mobility via electron trapping or scattering. The scheme also allows us to study the long-range structural reconstruction in AOSs and the defect dynamics during quenching or annealing processes providing the necessary information for optimizing the electronic and optical properties of AOSs toward their application in optoelectronic technologies.
 D. Buchholz, Q. Ma, D. Alducin, A. Ponce, M. Jose-Yacaman, R. Khanal, J.E. Medvedeva, and R.P.H. Chang,
Chemistry of Materials, 26, 5401-5411 (2014).
 R. Khanal, D.B. Buchholz, R.P.H. Chang, and J.E. Medvedeva, Physical Review B, 91, 205203 (2015).
 J.E. Medvedeva, D.B. Buchholz, and R.P.H. Chang, Advanced Electronic Materials, 3, 1700082 (2017).
EN19.03: Oxides and Perovskites for Solar Energy
Tuesday PM, April 03, 2018
PCC North, 100 Level, Room 124 B
3:30 PM - EN19.03.01
Recent Insights in Bulk and Interfacial Properties of Ternary Metal Oxide Photoelectrodes for Water Splitting
Roel Van de Krol1
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH1Show Abstract
Ternary oxides represent a large but relatively little-explored class of semiconductors that are of interest for photoelectrochemical energy conversion applications. One of the best performing candidates thus far is BiVO4, an n-type photoanode with a bandgap of 2.4 eV. One well known ‘trick’ to improve this material is doping. Donor doping with tungsten (W) supposedly improves the photocurrent by increasing the conductivity, but is also found to reduce the magnitude as well as the lifetime of the photoconductivity. Another dopant that was recently found to greatly improve the photocurrent of BiVO4 is hydrogen . We found that in contrast to tungsten, hydrogen actually increases the carrier lifetime and diffusion length in BiVO4 . This is due to the passivation of trap states, the possible origins of which will be discussed. A second ‘trick’ to improve the performance of BiVO4 is by deposition of a cobalt phosphate (CoPi) co-catalyst on the surface. We recently showed that the main role of this ‘co-catalyst’ is not to enhance the charge transfer kinetics, but to passivate surface defects on BiVO4 . To get a better understanding of the chemical nature of these states, we employed ambient pressure resonant photoemission (AP-resPES) and operando XPS methods. These experiments revealed the presence of two separate states in the bandgap of BiVO4 and a redistribution of the phosphate species in the electrolyte under illumination . These initial results are the first steps towards a molecular-level understanding of the BiVO4/electrolyte interface that may eventually help to design efficient solar fuel generators. In the last part of the talk I will discuss recent results on CuBi2O4, a photocathode material with a bandgap of ~1.7 eV. We developed a modified solution chemistry that enables us to spray highly homogeneous films that show photocurrent densities up to 2 mA/cm2 . To enhance the charge separation in these films, we introduced a gradient of copper vacancies across the film thickness. This results in the formation of a homojunction in CuBi2O4 that increases the carrier diffusion length and reduces charge recombination . The resulting films showed AM1.5 photocurrent densities of up to 2.5 mA/cm2 at +0.6 V vs. RHE in the presence of a hole scavenger, which is a new benchmark for this material.
 J. Cooper et al., Chem. Mater. 28, 5761 (2016)
 J.W. Jang et al., Adv. Energy Mater. 1701536 (2017)
 C. Zachäus et al., Chem. Sci. 8, 3712 (2017)
 M. Favaro et al., J. Phys. Chem. B (in press, DOI:10.1021/acs.jpcb.7b06942)
 F. Wang et al., J. Mater. Chem. A 5, 12838 (2017)
 F. Wang et al., J. Am. Chem. Soc. 139, 15094 (2017)
4:00 PM - EN19.03.02
Combinatorial Alloying Improves Bismuth Vanadate Photoanodes via Reduced Monoclinic Distortion
Paul Newhouse1,Dan Guevarra1,Mitsu Umehara1,Sebastian Reyes-Lillo2,Lan Zhou1,David Boyd1,Santosh Suram1,Joel Haber1,Jeffrey Neaton2,John Gregoire1
Joint Center for Artificial Photosynthesis, California Institute of Technology1,Joint Center for Artificial Photosynthesis and Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA2Show Abstract
Improving the efficiency of solar-power oxygen evolution is both critical for development of solar fuels technologies and challenging due to the broad set of properties required of a solar fuels photoanode. Bismuth vanadate, in particular the monoclinic clinobisvanite phase, has received substantial attention and has exhibited the highest radiative efficiency among metal oxides with a band gap in the visible range. Efforts to further improve its photoelectrochemical performance have included alloying one or more metals onto the Bi and/or V sites, with progress on this frontier stymied by the difficulty in computational modelling of substitutional alloys and the high dimensionality of co-alloying composition spaces. Since substituional alloying simultaneously changes multiple materials properties, understanding the underlying cause for performance improvements is also challenging, motivating our application of combinatorial materials science techniques to map photoelectrochemical performance of 948 unique bismuth vanadate alloy compositions comprising 0 to 8% alloys of various p-block, alkalai earth, rare earth, and 4d & 5d elements (e.g Bi-V-A) along with a variety of compositions from each pairwise combination (e.g. Bi-V-A-B) of these elements. Upon identification of substantial improvements in the co-alloying Bi-V-A-B spaces, structural mapping was performed to reveal a remarkable correlation between performance and a lowered monoclinic distortion. First-principles density functional theory calculations indicate that the improvements are due to a lowered hole effective mass and hole polaron formation energy, and collectively, our results identify the monoclinic distortion as a critical parameter in the optimization and understanding of bismuth vanadate-based photoanodes.
4:15 PM - EN19.03.03
Main-Group Halide Perovskites—Crystal Structure, Dynamics and Insights for Functional Materials Discovery
Douglas Fabini1,Mitchell Koerner1,Geneva Laurita2,Ram Seshadri1
University of California, Santa Barbara1,Bates College2Show Abstract
Inorganic and hybrid organic–inorganic main-group halides that adopt the perovskite structure combine excellent performance in photovoltaic applications, ease of preparation, and abundant constituent elements, but the origins of their remarkable properties are a matter of debate . Here, we address two unusual aspects of the crystal structure and dynamics of these materials which suggest the primacy of the group IV–halogen sublattice in dictating performance, and apply these insights to discover new optoelectronic materials via high-throughput computational screening.
First, X-ray scattering studies reveal local off-centering of the group 14 cations within their coordination octahedra across the materials class reflecting a preference for lower symmetry coordination than that implied by crystallographic approaches [2,3]. Ab initio calculations, optical measurements, and analogies to existing theory implicate the ns2 lone pair electrons as the origin of this phenomenon, which we propose leads to enhanced defect screening, reduced thermal conductivity, and unusual temperature-dependence of the electronic bandgap . We further demonstrate control of the strength of this phenomenon by chemical substitution on all sites of the perovskite .
Second, solid state nuclear magnetic resonance and dielectric spectroscopies reveal the full temperature-dependent dynamics of molecular reorientation in the high-performance formamidinium lead iodide . Despite markedly different barriers for molecular rotation compared to those in the homologous methylammonium lead iodide, both systems exhibit similar dynamics at room temperature . Together with the vastly different dipole moments for the two molecules, this result sheds light on emerging hypotheses of polaronic transport and transient Rashba–Dresselhaus effects.
Using design criteria based in part on these findings about the unusual electronic structure and lattice polarizability of the halide perovskites, we screen 54,000 compounds in the Materials Project database to identify candidate optoelectronic materials. Subsequent ab initio calculations and experimental preparation and screening are employed to test the validity of these criteria as predictors of high performance.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award number DE-SC-0012541.
1. D. H. Fabini, J. G. Labram, A. J. Lehner, J. S. Bechtel, H. A. Evans, A. Van der Ven, F. Wudl, M. L. Chabinyc, R. Seshadri, Inorg. Chem. 56 (2017).
2. D. H. Fabini, G. Laurita, J. S. Bechtel, C. C. Stoumpos, H. A. Evans, A. G. Kontos, Y. S. Raptis, P. Falaras, A. Van der Ven, M. G. Kanatzidis, R. Seshadri, J. Am. Chem. Soc. 138 (2016).
3. G. Laurita, D. H. Fabini, C. C. Stoumpos, M. G. Kanatzidis, R. Seshadri, Chem. Sci. 8 (2017).
4. D. H. Fabini, T. A. Siaw, C. C. Stoumpos, G. Laurita, D. Olds, K. Page, J. G. Hu, M. G. Kanatzidis, S. Han, R. Seshadri, J. Am. Chem. Soc. 2017, DOI:10.1021/jacs.7b09536.
4:30 PM - EN19.03.04
A First Principles Study on the Electronic and Optical Properties and Hole Effective Masses of Pure and Mg-Doped CuAlO2 and AgAlO2 Transparent Conducting Oxides
Luisa Scolfaro1,James Shook1,Pablo Borges2
Texas State University1,Universidade Federal de Vicosa2Show Abstract
P-type transparent conducting oxides (TCOs) present many exciting problems for materials scientists due to low conductivity arising from large hole effective masses. The applicability of TCOs to technologies like flat panel displays and photovoltaic cells establishes the need for complementarity between the well documented and commercially available n-type TCOs and p-type TCOs and motivates the search for a means to delocalize the O-2p states that limit shallow acceptors and lead to large hole effective masses. CuAlO2 and AgAlO2 (XAO) show promise as p-type TCOs due to the presence of X-3d/4d states, which hybridize with O-2p states and delocalize the valence states. Additionally, p-doping with Mg may further enhance conductivity of pristine XAO.
XAO exists as three polymorphs: the delafossites 2H and 3R, and an orthorhombic polymorph which is the least stable polymorph energetically. This study is restricted to the 2H polymorph since it is the least studied of the two delafossites. In this work, a theoretical study based on first-principles calculations is presented on pure and Mg-doped (replacing Al) 2H-XAO using density functional theory as implemented in the Vienna Ab initio Simulation Package (VASP) code. Projector augmented wavefunction pseudopotentials with a cutoff energy of 400 eV are employed and the exchange correlation energy is treated using the generalized gradient approximation with the addition of a Coulomb interaction energy (Hubbard correction U) for the Cu-3d and Ag-4d states. Results are also obtained using the hybrid functional (HSE06) approach. Pure 2H-XAO is initially modelled using an 8-atom hexagonal primitive cell, which is used to relax the crystal structure. Band structure, density of states, and the complex dielectric function are obtained. By calculating the total energy per atom as a function of V/V0, where V0 is the relaxed volume, it was observed that the 2H(3R) polymorphs modelled using hexagonal primitive(unit) cell are equal in total energy down to 1 meV/atom at V=V0, with the rhombohedral 3R model 90(37) meV higher for CuAlO2(AgAlO2). This suggests that the 2H polymorph may have a slightly lower total energy. Hole effective masses are determined using parabolic curve fitting of the lower band gap edge around the high symmetry k points of the first Brillouin zone.
Once the electronic structure of the bulk using the primitive cell is well described, 64-atom hexagonal supercells are used to model pristine and Mg-doped 2H-XAO. The electronic structures, densities of states, optical properties and hole effective masses for these systems are presented and discussed in the context of experimental results from literature. A discussion of the effects of Mg-doping on the optical properties and its effectiveness in reducing hole effective masses and increasing conductivity is also presented.
4:45 PM - EN19.03.05
Amorphous Zinc Tin Oxide Using High Power Impulse Magnetron Sputtering—Characterisation, Doping and Device Applications
Hiep Tran1,Billy Murdoch1,Dougal McCulloch1,David McKenzie2,Marcela Bilek2,Anthony Holland1,Jim Partridge1
RMIT University1,The University of Sydney2Show Abstract
Indium tin oxide (ITO) is the most commonly used transparent conducting oxide (TCO) due to its high carrier mobility (up to ~100 cm2 V−1 s−1) and low absorption of visible-light. However, In-free TCOs have been sought for some time due to the cost and relative scarcity of In. Among these, alloys of ZnO and SnO2 (commonly referred to as ZTO) offer acceptable electron mobility (>10 cm2 V−1 s−1) and high thermodynamic stability . Importantly, the large diameter spherical orbitals of the Zn2+ and Sn4+ cations in ZTO cause the carrier mobility to be insensitive to structural disorder . Amorphous ZTO (a-ZTO), produced with low growth temperatures, therefore exhibits similar carrier mobility to its polycrystalline counterpart.
Here, we investigate the characteristics of a-ZTO deposited energetically using high power impulse magnetron sputtering (HiPIMS) . This technique has proved capable of producing high quality metal oxide layers suitable for electronic devices [3, 4]. Reactive co-deposition from Zn (HiPIMS mode) and Sn (DC magnetron sputtering mode) targets yielded a-ZTO with varying Zn:Sn composition across a 4-inch diameter sapphire substrate. The electrical and optical properties of this film were then studied as a function of composition. As-deposited, the films were amorphous, transparent and semi-insulating. Hydrogen (a known n-type dopant in ZnO and SnO2) was introduced into the a-ZTO by post-deposition annealing (1 h, 500 °C, 100 mTorr H2) and resulted in significantly increased conductivity with no measurable structural alterations. After annealing, Hall effect measurements revealed n-type carrier concentrations of ~1 × 1017 cm−3 and mobilities up to 15 cm2 V−1 s–1. These characteristics proved both stable and suitable for device applications. Results from transistors and memristors based on energetically deposited a-ZTO will be presented. These suggest that HiPIMS can produce dense, high quality a-ZTO suitable for electronic applications.
 H. Chiang, J. Wager , R. Hoffman, J. Jeong and D. A. Keszler, High mobility transparent thin-film transistors with amorphous zinc tin oxide channel layers, Appl. Phys. Lett. 86 (2005) 013503
 V. Kouznetsov, K. Macák, J. M. Schneider, U. Helmersson and I. Petrov, A novel pulsed magnetron sputter technique utilizing very high target power densities, Surface and Coatings Technology 122 (1999) 290–293
 J. G. Partridge, E. L. H. Mayes, N. L. McDougall, M. M. M. Bilek, D. G. McCulloch Characterization and device applications of ZnO films deposited by high power impulse magnetron sputtering (HiPIMS) , Journal of Physics D: Applied Physics 46 (16), 165105 (2013)
 B. J. Murdoch, D. G. McCulloch, R. Ganesan, D. R. McKenzie, M. M. M. Bilek, and J. G. Partridge, Memristor and selector devices fabricated from HfO2-xNx, Appl. Phys. Lett. 108, 143504 (2016)
EN19.04: Poster Session I
Tuesday PM, April 03, 2018
PCC North, 300 Level, Exhibit Hall C-E
5:00 PM - EN19.04.01
Electron Transport and Visible Light Absorption in a Plasmonic Photocatalyst Based on Strontium Niobate
Dongyang Wan1,Teguh Asmara2,Bixing Yan1,2,Mallikarjuna Motapothula1,Thirumalai Venkatesan1,2
NUSNNI-Nanocore1,National University of Singapore2Show Abstract
Semiconductor compounds are widely used for photocatalytic hydrogen production applications, where photogenerated electron-hole pairs are exploited to induce catalysis. Recently, powders of a metallic oxide (Sr1-xNbO3, 0.03 < x < 0.20) were reported to show competitive photocatalytic efficiencies under visible light which was attributed to interband absorption. This discovery expanded the range of materials available for optimized performance as photocatalysts. Here we have studied epitaxial thin films of SrNbO3+δ and found that their bandgaps are ~4.1 eV. Surprisingly the carrier density of the conducting phase exceeds 1022 cm-3 and the carrier mobility is only 2.47 cm2 V-1 s-1. Contrary to earlier reports, the visible light absorption at 1.8 eV (~688nm) is due to the plasmon resonance, arising from the large carrier density. We propose that the hot electron and hole carriers excited via Landau damping (during the plasmon decay) are responsible for the photocatalytic property of this material under visible light irradiation.
1. D.Y. Wan, Y.L. Zhao, Y. Cai, et al., Nature Communications 8, 15070 (2017).
2. T.C. Asmara, D.Y. Wan, Y.L. Zhao, et al., Nature Communications 8, 15271 (2017).
5:00 PM - EN19.04.02
Flexible CZTSSe Solar Cell Characteristics from Sputtering Precursors
The development of flexible solar cells is necessary for achieving market competitiveness through the implementation of low cost solar cells and for applying customized business models, such as Building Integrated Photovoltaics (BIPV). Solar cells with the CZTS-based (Cu2ZnSnS4, Cu2ZnSnSe4, Cu2ZnSn(S,Se)4) absorbers are advantageous for lowing the cost, but the development of solar cells based on flexible substrates has been relatively unexplored. In this work, a flexible CZTSSe solar cell applying the Mo foil substrate was developed. The characteristics of solar cells were examined by applying the sputtering process for four precursor stacking orders (Cu/Sn/Zn/Mo foil, Cu/Zn/Sn/Mo foil, Zn/Sn/Cu/Mo foil, and Sn/Cu/Zn/Mo foil). In addition, NaF was deposited on the Mo foil, and the effects of Na were investigated. The samples were characterized by means of scanning transmission electron microscopy-energy dispersive spectrometry (STEM-EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), a solar simulator, external quantum efficiency (EQE) measurements, depth resolved Raman spectroscopy, and admittance spectroscopy (AS).
Acknowledge: This work was supported by by the DGIST R&D Program of the Ministry of Science and ICT (18-BD-05) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20173010012980).
5:00 PM - EN19.04.03
Semiconductor Nanostructures for Photon Upconversion—Tailoring Performance Metrics Through Bandgap Engineering
Christopher Milleville1,Eric Chen1,Zhuohui Li1,Kyle Lennon1,Diane Sellers1,Joshua Zide1,Matthew Doty1
University of Delaware1Show Abstract
Photon upconversion is a promising strategy to boost the conversion efficiencies of commercially available photovoltaics beyond the Shockley-Queisser limit by harvesting photons with energy below the band gap of a host cell and converting pairs of such low-energy photons into single high-energy photons that can be returned to and absorbed by the host cell. Existing upconversion materials based on lanthanide-doped nanocrystals and sensitized triplet-triplet annihilation have limited potential benefit for solar energy harvesting applications because of their narrow absorption bandwidth and low quantum efficiency.1 II-VI based semiconductor nanostructures offer a new paradigm for photon upconversion in which bandgap, relative band alignment, and carrier dynamics can be tailored through material composition, size, and morphology to achieve high quantum yield at desired wavelengths with broad spectral absorption.2 Recent advances in the synthetic capabilities of semiconductor nanoparticles has enabled the fabrication of complex heterostructures composed of various materials. One class of new semiconductor heterostructures are colloidal double quantum dots (QDs), where two spatially separated semiconductor nanoparticles are electronically coupled.3 Colloidal synthesis of such semiconductor heterostructures offers a route to solution-compatible, low-cost processing. We report a semiconductor nanostructure upconverting platform consisting of coupled quantum dots (QDs)3 that efficiently upconverts low-energy photons under continuous-wave illumination. The nanostructure consists of a narrower bandgap tellurium-doped cadmium selenide QD absorber and a wider bandgap cadmium selenide emitter spatially separated by cadmium sulfide nanorod. The double quantum dot nanostructure is designed such that electrons promoted by a first photon absorption in the Te-doped CdSe absorber are delocalized over the entire structure but the holes remain confined. A second low-energy photon excites the hole via intraband absorption, allowing it to escape into the cadmium sulfide and ultimately relax into the emitter QD. We will present synthesis conditions, structural characterization, and detailed optical spectroscopy of the upconversion photophysics that collectively demonstrate control over the photon energies absorbed and the corresponding photon energy gain through control over the composition and size of the absorbing QDs.
1. Zhou, J., Liu, Q., Feng, W., Sun, Y. & Li, F. Upconversion Luminescent Materials: Advances and Applications Jing. Chem. Rev. 115, 395–465 (2015).
2. Sellers, D. G. et al. Novel nanostructures for efficient photon upconversion and high-efficiency photovoltaics. Sol. Energy Mater. Sol. Cells 155, 446–453 (2016).
3. Deutsch, Z., Neeman, L. & Oron, D. Luminescence upconversion in colloidal double quantum dots. Nat. Nanotechnol. 8, 649–653 (2013).
5:00 PM - EN19.04.04
Numerical and Experimental Study of InxGa1-xN Water-Splitting Photoelectrodes
Sophia Haussener1,Yannick Gaudy1,Pavel Aseev2
Ecole Polytechnique Federale de Lausanne, Switzerland1,TU Delft2Show Abstract
Compositionally graded InxGa1-xN layers of high structural and optical quality grown on Si have the potential for cheap and efficient solar harvesting systems1. The theoretical maximum efficiency for tandem photoelectrochemical water-splitting systems could reach over 22.5% with Si as bottom cell and InxGa1-xN as top cell with a bandgap between 1.6-1.8eV, an indium content of 0.37-0.44, respectively1,2. However, the use of InxGa1-xN as a water-splitting photoelectrode (PE) for solar hydrogen production has yet to show promising performance as initial demonstrations of InxGa1-xN on a GaN substrate have exhibited photocurrents below 0.1mA/cm2 at AM1.5G irradiation, i.e. a current even below what has been reported for pure n-GaN3. We developed a numerical model of InxGa1-xN water-splitting PEs which aims at identifying and quantifying the losses in the system. The model included electromagnetic wave propagation calculations for the determination of the locally resolved light absorption and charge generation terms, which built the generation terms in the charge transport and conservation equations solved in the semiconductor. The charge transfer at the semiconductor-electrolyte interface, a boundary condition to our model, accounted for Fermi level pinning and a potential drop in the Helmholtz layer due to surface states. The complete numerical model was validated using linear sweep voltammograms of InxGa1-xN PEs grown by plasma-assisted molecular beam epitaxy with varying indium content (0.095, 16.5, 23.5, 33.3 and 41.4) and under varying light irradiance (1%, 10%, 50% and 100% of AM1.5G). Parametric analyses were performed on optical and electronic properties to identify key performance parameters and evaluate their impact on the performance of compositionally graded InxGa1-xN PEs.
5:00 PM - EN19.04.05
Novel Approach to Utilize Amine-Thiol Chemistry for Synthesis of Conventional as Well as Novel Thin-Film Photovoltaic Semiconductors
Swapnil Deshmukh1,David Rokke1,Rakesh Agrawal1
Purdue University1Show Abstract
Low cost and large-scale production of semiconducting materials requires development of solution processing routes. While developing such solution processing routes for photovoltaic systems, it is also important to achieve sufficient efficiencies of photovoltaic devices which will ultimately reduce the cost of power generation.
Numerous solution processing routes have been tried for fabricating various semiconducting films for solar energy conversion. Amongst which, Hydrazine has shown highest conversion efficiencies for materials like CIGS and CZTS. However, because of its explosive and carcinogenic nature, it is not feasible to use this chemical in any scale up production. On the other hand, amine-thiol system which is safer than Hydrazine has shown promising results in dissolving array of metals, metal salts, oxides, chalcogenides. Even though devices made with amine-thiol system have demonstrated promising efficiencies, the hydrocarbon chains in amine and thiol species results in formation of carbonaceous fine grain layer in the final film which affects the performance of photovoltaic devices.
Herein, we present a novel approach in utilizing amine-thiol chemistry for film fabrication. Understanding amine-thiol chemistry using various analyt