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 PM, 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 analytical techniques has enabled us in fabricating better quality films. This presentation will exclusively focus on fabrication of such two semiconducting thin film materials viz. CISe and Se-Te alloy. Amongst which, CISe is a low bandgap material and is of great interest for its application in fabrication of tandem solar architectures with high band gap materials like perovskite. We will discuss the effect of various precursor selection and various selenization conditions on CISe film quality and also will demonstrate the fabrication of working device from CISe system with more than 11% efficiency.
Along with novel approach for fabricating conventional material like CISe, we will also discuss our work in synthesizing a novel thin film alloy of Se and Te for photovoltaic application. While people have fabricated working devices with selenium as an absorber material for indoor applications, we will demonstrate an alloying process of Se with Te to tune the bandgap of the absorber material. Our understanding of chalcogen dissolution mechanism in amine-thiol system has made it possible to alloy two chalcogens to form crystalline phase material. This presentation will include synthesis as well as thin film fabrication from Se-Te alloy followed by primary attempt of making a working photovoltaic device with various architectures.
In conclusion, we have come up with better understanding of amine-thiol system which enabled us to develop novel approach for its use in better quality semiconducting films. The results of which are applicable to the synthesis of nanoparticles and films of a vast array of chalcogenides including Cu2S, WSe2, SnS, SnSe, FeS2, CZTS and other Kesterites.
5:00 PM - EN19.04.06
Bismuth Silver Oxysulfide for Photoconversion Applications—Structural and Optoelectronic Properties
Amal BaQais1,2,Antton Curutchet3,Ahmed Ziani1,Hassan Ait Ahsaine1,Philippe Sautet4,Kazuhiro Takanabe1,Tangui Le Bahers3
King Abdullah University of Science and Technology (KAUST)1,Princess Nourah bint Abdulrahman University (PNU)2,Universite Lyon, ENS de Lyon, CNRS, Universite Claude Bernard Lyon 13,University of California4Show Abstract
Solar energy is an abundant, clean and free access resource, but it require harvesting and storage for sustainable future. Photovoltaics or photocatalysis technologies dedicated to sun light conversions frequently involve photo-visible-responsive semiconductors , such us materials with formula BiMOS (M; Cu  or Ag). In this study, we applied a strategy of substitution of Cu by Ag to produce a new family of oxysulfide BiAgOS . We were interested to address how the total substitution of Cu by Ag in BiCuOS system affect its crystal structure, optical and electronic properties by using experimental characterizations and theoretical calculations. Single-phase bismuth silver oxysulfide BiAgOS was prepared by a hydrothermal method. Its structural, morphological, and optoelectronic properties were investigated and compared with those of BiCuOS. Rietveld refinement of the powder X-ray diffraction confirmed that BiAgOS has the same crystal structure as BiCuOS. The diffraction peak positions of BiAgOS, relative to those of BiCuOS, are shifted toward lower angles, indicating an increase in the cell parameters. Combined with experimental measurements, density functional theory calculations employing the range-separated hybrid HSE06 exchange-correlation functional with spin–orbit coupling quantitatively elucidated photophysical properties such as absorption coefficients, effective masses, and dielectric constants. BiCuOS and BiAgOS were found to have indirect bandgaps of 1.1 and 1.5 eV, respectively. The difference in the bandgap results from the difference in the valence band compositions. The hybrid level of the S and Ag orbitals is located at a more positive potential than that of S and Cu leads to widening band gap. Both materials possess high dielectric constants and low electron and hole effective masses. BiAgOS has dielectric constant larger than BiCuOS making it very interesting for photoconversion applications because the material could efficiently screen photogenerated charges. By combining the UV-Vis, Mott-Schottky, and photoelectron spectroscopy in air measurements, the relatively low bandgap of and their p-type character, BiCuOS and BiAgOS can be considered as interesting starting compositions for the development of new semiconductors for photovoltaics or Z-scheme photocatalytic applications. Moreover, this study opened the window toward new oxychalcogenides materials BiAgOCh (Ch; Se, Te) which are not yet synthesis or investigate.
 P. Hoffmann, Tomorrow’s Energy; MIT Press: Cambridge, MA, 701, 2004; Vol. 1.
 W.C. Sheets, et al., Inorg. Chem., 2007, 46, 10741.
 A. BaQais, et al., Chem. Mater., 2017, 29 (20), 8679.
5:00 PM - EN19.04.07
Spontaneous Etching of Oxide and Sulfide Underlayers by Cu2-xS Atomic Layer Deposition
Raphael Agbenyeke1,2,Bo Keun Park1,2,Gun Hwan Kim1,Taek-Mo Chung1,2,Chang Gyoun Kim1,2,Jeong Hwan Han3
Korea Research Institute of Chemical Technology1,Korea University of Science and Technology2,Seoul National University of Science and Technology3Show Abstract
The high diffusivity of Cu+ ions in the hexagonal-close-packed structure of Cu2-xS often leads to interesting and unexpected observations. Herein, we present the etching of oxide and sulfide thin film underlayers during the atomic layer deposition of Cu2-xS thin films. The infiltration of the underlayers by Cu+ ions is an essential step that precedes the etching process. However, it is suspected that the eventual etching of the underlayer, and the etch rate strongly depend on the lattice (bond dissociation energy) of the underlayer material. Thin films of ZnS, ZnO, SnS, and SnO were etched to different degrees during the deposition of Cu2-xS while SnO2 exhibited a high resistance to etching. Interestingly, a selective removal of Zn2+ was observed when a ternary Zn1-xSnxO film was used as underlayer. Based on XPS results and findings from other supplementary experiments, a possible reaction mechanism was proposed for the etching process. Finally, the observation was extended to the synthesis of Cu2-xS nanowires that can be used as effective absorbers for photovoltaic cells. Details of the findings of this work will be presented at the conference.
5:00 PM - EN19.04.09
Splitting Photons—Singlet Fission in a Hybrid System Using PbS Nanocrystals and Functionalized Diphenylhexatriene
Daryl Hawkes1,2,Beverly Ru1,2,Xin Li1,2,MingLee Tang1,2
University of California, Riverside1,Tang Lab2Show Abstract
Solar cell efficiency could potentially be increased by exceeding the Shockley-Queisser limit through singlet fission. The Shockley-Queisser limit on photovoltaic efficiency is the theoretical maximum efficiency of a p-n junction. It states that nearly two-thirds of the light energy incident on a conventional photovoltaic material is not converted to electrical energy. Some of this lost energy could be harvested through singlet fission. Via the Dexter process, inorganic colloidal PbS nanocrystals are used to harvest the energy from triplet excitons generated by organic 1,6-diphenyl-1,3,5-hexatriene (DPH). Singlet fission (SF) is a spin-allowed process in which a high energy photon creates a singlet state in one chromophore, moiety of organic molecule responsible for absorption and emission of light. The singlet state splits into two triplet states—one in the original chromophore and one in a neighboring chromophore that is correctly orientated for electronic coupling. The Dexter process is a simultaneous, correlated transfer of an excited electron on one molecule (donor) to another molecule (acceptor) via a non-radiative pathway which depends on the wavefunction overlap between donor and acceptor. SF material has previously been made from organics, specifically polyacenes such as tetracene and pentacene. Polyacenes have been observed to have shorter triplet lifetimes and energy triplets compared to DPH. This makes DPH a prominent candidate to produce a more efficient SF material with a higher SF yield. The Wittig Reaction is utilized to synthesize DPH from derivatives with specific functional groups that allow DPH to bind to PbS. Energy transfer from DPH (donor) to PbS (acceptor) is characterized by absorbance and emission measurements.
5:00 PM - EN19.04.11
ReS2/ReSe2 Heterojunction-Based Infrared Photodetector
Hae Won Lee1,Seo-Hyeon Jo1,Jin-Hong Park1
Sungkyunkwan University1Show Abstract
In past few years, transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), tungsten diselenide (WSe2), have attracted great research interests due to their superior electronic, optical, and mechanical properties. In particular, excellent absorbance (5-10 %) and high quantum efficiency (~104 %) have allowed TMDs to be exploited as channel materials in high-performance photodetectors with high responsivity and high detectivity. However, most TMD photodetectors operated in the limited detection range from ultraviolet to visible region (10 ~ 780 nm) because TMDs generally have an energy bandgap between 1 and 2 eV. Although it was possible to detect 1064 nm and 980 nm lights by utilizing rhenium diselenide (ReSe2) and multi-layer MoS2 with relatively small bandgap (1 and 1.3 eV, respectively), no further researches to detect lights with longer wavelength by using TMD materials were reported yet.
Here, we demonstrate a highly responsible IR photodetector operating based on the interlayer optical transition phenomenon. This IR photodetector formed on ReS2/ReSe2 heterostructure not only extended the detection range up to IR region (1310 nm) that individual ReS2- and ReSe2- photodetectors cannot cover, but also maintained high responsivity even under IR region (3.64 × 105 A/W at λ = 980 nm and 1.58 × 105 A/W at λ = 1310 nm). The small interlayer bandgap of 0.62 eV formed at the ReS2/ReSe2 heterojunction interface lead to the interlayer optical transition phenomenon, consequently enabling photocarriers to be easily generated with less energy than the band-to-band transition.
 J. Shim, H.-Y. Park, D.-H. Kang, J.-O. Kim, S.-H. Jo, Y. Park, J.-H. Park, Adv. Mater. 29 39 (2017)
 S. Yang, S. Tongay, Q. Yue, Y. Li, B. Li, F. Lu, Sci. Rep. 4 5442 (2014)
 S.-H. Jo, H.-Y. Park, D.-H. Kang, J. Shim, J. Jeon, S. Choi, M. Kim, Y. Park, J. Lee, Y. J. Song, S. Lee, J.-H. Park, Adv. Mater. 28 6711 (2016)
5:00 PM - EN19.04.12
Modifying the Hybridization of Transition Metal d Orbitals with Weak External Fields
Pragathi Darapaneni1,Alexander Meyer1,Orhan Kizilkaya2,Kenneth Lopata1,James Dorman1
Louisiana State University1,Center for Advanced Microstructures and Devices2Show Abstract
Rare earth (RE) elements are frequently employed in many applications, such as catalysis, magnetism, phosphor lighting etc., due to their unique properties stemming from the steady 4f energy levels. Recently, there has been a push to replace these RE elements due to increased economic and national security issues. One proposed family of alternatives; are transition metal (TM) dopants, but are typically avoided because of their field dependent properties. For example, the strong overlap of the TM d orbitals with surrounding ligand is often viewed as an unfavourable phenomenon for luminescent or magnetic applications. In the current project, the field dependent hybridization of TM is utilized to engineer the d energy levels, allowing for controlled optoelectronic properties of TM ion.
In this work, thin films of TiO2:Ni (~ 50 nm) were deposited onto Si substrates by employing sol-gel chemistry and spin-coating techniques. Weak localized external fields were created by surface functionalization of these films with benzoic acid ligands. Initial structural and optical characterization on TiO2:Ni was performed to determine the crystal structure and crystal field splitting. X-ray photoelectron and soft X-ray absorption spectroscopy studies were used to investigate the ability to manipulate Ni core and valence levels without affecting the oxidation states or crystal structures. Additionally, the band structure of surface modified TiO2:Ni, investigated by a combination of ultraviolet photoelectron and X-ray emission spectroscopy techniques, revealed that the interband Ni 3d states suffered a shift towards/away from the valence band depending upon the electron withdrawing/donating nature of the ligand. Furthermore, this phenomenon was theoretically supported by the ligand field multiplet calculations and time-dependent density functional theory simulations on bulk-mimicking TiO2:Ni2+ clusters. These preliminary results on TiO2:Ni demonstrate that the reversible tuning of TM d energy levels can be exploited to as the first step to the substitution of RE elements in phosphors that can have controlled luminescent properties.
5:00 PM - EN19.04.13
Unique Nanocrystalline Bulk pn Homojunctions for Opto-Electronic Devices
Shalini Menezes1,Anura Samantilleke2
InterPhases Solar1,Universidade do Minho2Show Abstract
Inorganic semiconductor based pn junctions constitute an integral part of most optoelectronic devices, such as photovoltaic (PV) solar cells and light emitting diodes (LEDs). Although the inorganic materials offer higher stability and superior electro-optic properties relative to organics, they present exorbitant cost and scale-up challenges. This paper presents a radically different approach to circumvent such challenges. It takes advantage of naturally formed nanocrystalline pn junctions, resulting from judicious coupling of semiconductor materials and electrochemical processes. Certain combinations can lead to highly practical inorganic material systems, comprising process-induced bulk pn junction nanostructures. The paper will illustrate an exemplary material system, based on single-step electrodeposited copper-indium-selenide (CISe) ordered defect chalcopyrite compounds. Thermodynamically driven reactions enable the electrodeposition of highly-ordered, interlinked, space-filling CISe films, in a single step. This approach creates a low-cost processing platform to produce nanocrystalline films, with all the attributes necessary for efficient bulk homojunction (BHJ) operation. Surface analytical microscopies and spectroscopies reveal unusual phenomena and extraordinary electro-optical properties that could potentially maximize spectral absorption and reduce recombination loss. They support the fortuitous formation of surprisingly ordered, sharp, abrupt 3-dimensional, nanoscale CISe pn BHJs. The specific BHJ structure enables efficient separation and transport of free carriers and essentially performs the same functions as planar pn junctions. The CISe nanocrystals are very different from colloidal nanocrystals used in state-of-the-art BHJs; they exhibit mixed conductivity and high doping densities. These distinctive innate attributes of CISe films naturally create ordered nanoscale morphology and facilitate interconnections between the nanocrystals to form the BHJ structure. This totality manifests a highly significant advance in semiconductor processing as it creates an accessible, low cost solution-based method to fabricate high quality pn BHJ nanocrystalline material systems that can be directly used in PV or LED devices. Furthermore, with the incorporation of finely band-aligned contact electrode materials, the CISe BHJ film can transition into high performance flexible devices and roll-to-roll processing in simple thin-film form factor for easy scale-up.
5:00 PM - EN19.04.14
Mid-Infrared Photo-Detecting Devices Using InSb Grown on Si Substrate
Kian Hua Tan1,Bowen Jia1,Wan Khai Loke1,Satrio Wicaksono1,Soon Fatt Yoon1,Kwang Hong Lee2
Nanyang Technological University1,Singapore MIT Alliance for Research and Technology Centre2Show Abstract
Mid-infrared detection devices, which are employed in medical imaging, gas sensing, security surveillance and navigation sensing system applications, are essential components in an internet of things platform. InSb is one of the promising candidates for mid-infrared detection devices. It has a room temperature bandgap energy of 0.17 eV and is capable of detecting photon with a wavelength up to 7.3 µm. Growth of InSb on Si overcomes the restrictions of InSb substrate, which are expansive, small size (< 4 inches in diameter) and low ruggedness . Hetero-epitaxy growth of InSb devices on Si is also one of approaches to realize the integration of mid-infrared InSb with Si-based electronic devices on a single wafer, without requiring any wafer bonding process. However, the growth of InSb on (100) Si substrate is challenging due to their large lattice mismatch (19.3 %) and different lattice structures (zinc blende vs. diamond), resulting in high density of defects. Furthermore, direct growth of InSb on (100) Si surface is prohibited because of the formation of In metallic islands on Sb-terminated Si surface instead of InSb film . Therefore, an intermediate buffer layer between InSb layer and Si substrate is needed. In this report, we demonstrated two different intermediate buffer: Ge/GaAs buffer and AlSb/GaSb buffer.
Growth of InSb on a 6° offcut Si substrate using an AlSb/GaSb intermediate buffer was carried out using a molecular beam epitaxy (MBE) system. A 5nm AlSb island was firstly grown, followed by a 100 nm GaSb layer. Subsequently, a 50 nm AlSb layer was grown. Finally, a 0.8 µm InSb was grown on the AlSb surface using interfacial dislocation array to accommodate lattice mismatch. At the initial growth stage of InSb, a spotty RHEED pattern was observed and changed to a clear (1×3) after ~1 minute of growth. Using this InSb layer, an InSb photoconductor was fabricated and its photo-response was measured.
In Ge/GaAs buffer, growth of Ge on Si substrate was carried using a MOCVD system. Growth of GaAs and InSb was carried using a MBE system. Lattice-mismatch (14.6%) strain between GaAs and InSb was accommodated by an interfacial misfit (IMF) array formed at InSb/GaAs interface, which consisted of uniformly distributed 90° misfit dislocations . TEM observation exhibited a low defect density in InSb layer. An InSb p-i-n photo-detector structure was grown. Spectral response of the InSb photodetector with the detector area of 0.0285 mm2 was measured using a Fourier Transform Infrared (FTIR) spectrometer with a KBr beam splitter from 80 K to 200 K.
 J.I. Chyi, D. Biswas, S. Iyer, N. Kumar, H. Morkoc, R. Bean, K. Zanio, H.Y. Lee, H. Chen, Appl. Phys. Lett. 54(11) (1989) 1016-1018.
 G. Franklin, D. Rich, H. Hong, T. Miller, T.-C. Chiang, Phys. Rev. B 45(7) (1992) 3426.
 Jia, B. W.; Tan, K. H.; Loke, W. K.; Wicaksono, S.; Yoon, S. F., Mater. Lett. 2015, 158, 258-261.
5:00 PM - EN19.04.15
Synthesis of P-Type ZnO Nanorods by Self-Doping of Ag Electrode and Application of Homojunction Light-Emitting Diodes
Do-kyun Kwon1,Yoann Porte1,Jae-Min Myoung1
Yonsei University1Show Abstract
Zinc oxide (ZnO) nanorods (NRs) have attracted lots of attention as a potential material for light-emitting diode (LED) applications, due to its outstanding characteristics such as large direct band gap and high exciton binding energy. However, despite these advantages, ZnO NRs are not used in LEDs due to several problems. Since ZnO NRs have naturally n-type conductivity owing to donor type defects in the lattice, the achievement of p-type conductivity is difficult. As a result, most studies have employed heterojunctions which use alternative p-type materials to fabricate LEDs using ZnO NRs. But, heterojunction LEDs generally exhibit poor luminescent efficiency due to lattice mismatch at the p-n junction interface. Another difficulty in fabricating ZnO NRs-based LEDs is the initial growth control of vertical ZnO NR on conventional electrodes, which is due to different crystal structure between the ZnO NR and the electrodes.
In this report, to overcome the problems of ZnO NRs, an Ag bottom electrode was employed as a multiple role implementing layer, which acted as a dopant source for p-type ZnO NRs and a growth template for ZnO NRs. During growth of ZnO NRs on the Ag bottom electrode without seed layer, Ag+ ions are dissolved from the Ag bottom electrode and then incorporated into the ZnO lattice. Then, the n-type ZnO NRs were homoepitaxially grown on these Ag-doped p-type ZnO NRs to fabricate a highly efficient p-n homojunction LED with minimum interface defects. These ZnO NRs p-n homojunction LEDs showed a typical rectifying behavior with a turn-on voltage of 3.5 V and a high rectifying ratio of 1.5 × 105 at 5 V. Furthermore, under a forward bias of 9 V, the LED exhibited a wide yellow EL emission centered at 645 nm, which was attributed to the various emission sites of ZnO deep-level defects.
Keywords: Ag doping, p-type ZnO NRs, Homoepitaxial p-n junction, Solution process, Light-emitting diode.
5:00 PM - EN19.04.16
Atomic Layer Deposited TiO2-IrOx Alloy as a Hole Transport Layer for Perovskite Solar Cells
Wanliang Tan1,Olivia Hendricks1,Andrew Meng1,Michael Braun1,Michael McGehee1,Chris Chidsey1,Paul McIntyre1
Stanford University1Show Abstract
A typical perovskite solar cell consists of a hole transporting layer (HTL), a perovskite light absorber and an electron transporting layer (ETL). The most commonly studied HTL’s are organic/polymeric materials such as Spiro-OMeTAD(2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene ) or PTAA(poly(triaryl amine)), which are expensive and unstable in the presence of water vapor.
This presentation describes the application to perovskite solar cells of a new kind of inorganic HTL synthesized using atomic layer deposition (ALD). By alloying TiO2 with IrOx in a super-cycle ALD process, we found that the electron transporting material TiO2 becomes an effective HTL. Atomic layer deposition offers the prospect of depositing such hole contact materials over a wide variety of substrate materials and geometries, and with excellent control of layer thickness and composition. Previous research  on ALD-grown alloys of TiO2 and RuO2 showed that a large work function (~ 5.2 eV) can be achieved as result of a ~ 10 mol% RuO2 addition. In the current work, a Cs0.17FA0.83Pb(Br0.17I0.83)3 PSC including a TiO2-IrOx HTL of composition 20mol% IrOx and ~ 10 nm thickness achieved a high power conversion efficiency of 13.4% with 1.004 V open circuit voltage, which is 100 mV higher than the Voc of the reference device using a PEDOT:PSS. These results indicate that ALD-grown TiO2-transition metal oxide alloys are promising HTL’s for perovskite photovoltaics.
 O.L. Hendricks, A.G. Scheuermann, M. Schmidt, P.K. Hurley, P.C. McIntyre, and C.E.D. Chidsey, “Isolating the Photovoltaic Junction: Atomic Layer Deposited TiO2–RuO2 Alloy Schottky Contacts for Silicon Photoanodes,” ACS Appl. Mater. Interfaces 8, 23763-73 (2016).
5:00 PM - EN19.04.17
Predicting Defect Concentrations in MoO3-x with DFT Calculations
Daniel Lambert1,Alison Lennon1,Patrick Burr1
University of New South Wales1Show Abstract
In the search for low-cost, high efficiency solar cells, researchers have been investigating new materials to use as carrier-selective contact layers. Sub-stoichiometric molybdenum trioxide (MoO3-x ) is a promising material for interface contacts due to its hole selectivity and low parasitic absorption. It has been investigated as a contact for a number of different photovoltaic devices using different absorber materials, including ZnO/PbS quantum dot, CdS-CdTe nanotubes, CuInSe, and kesterite, as well as organic materials, perovskites, and crystalline silicon.
The performance of these contacts depends on the electrical and chemical properties of MoO3-x. Of particular interest are the bandgap, electron affinity, and the presence of a defect band, the latter imparting n-type semiconducting properties to the metal oxide. These material properties have been found to be sensitive to the presence of intrinsic defects, especially oxygen vacancies. An understanding of how the defect chemistry of MoO3-x varies with preparation conditions could allow the material properties of MoO3-x to be optimised for interfaces with different absorber materials.
This paper reports on the use of density functional theory (DFT) simulations to predict defect concentrations as a function of temperature and oxygen partial pressure for crystalline MoO3-x, by constructing Brouwer diagrams. Additionally it is shown that samples prepared in contact with silicon may be prone to contamination under common preparation conditions, and this contamination can affect the electronic structure of the material. It is therefore reasonable to assume that contamination of MoO3-x may also occur for other absorber materials and the implications of this contamination in terms of device performance and durability needs to be considered. We then extend this analysis into the properties of amorphous MoO3-x by means of reverse Monte Carlo modelling based on experimental thin film diffraction data. This extension is of particular value because most commonly-used preparation methods typically result in an amorphous metal oxide. This study provides a critical theoretical contribution to our understanding of the role of defects in transition metal oxide functionality as a carrier-selective contact for photovoltaic devices.
5:00 PM - EN19.04.18
High Resolution 3D Chemical Characterisation of Novel Cadmium Telluride Solar Cell Architectures
Thomas Fiducia1,Kexue Li2,Chris Grovenor2,John Walls1
Loughborough University1,University of Oxford2Show Abstract
Thin-film solar cells provide an alternative to conventional silicon-based photovoltaics, and modules based on thin film cadmium telluride (CdTe) are cost-competitive with silicon. Device efficiency has increased from 16% to 21% in the last 5 years. Key factors enabling the improvements have been alterations in the device structure, including replacement of the traditional cadmium sulphide (CdS) window layer with higher band-gap alternatives like magnesium-doped zinc-oxide (MZO), and grading/alloying of the near-interface region in the CdTe absorber layer with selenium. However, research on devices incorporating these changes is limited. Little high resolution microstructural/chemical/electronic characterisation has been published. Improved characterisation, and hence understanding, of these devices, can lead to improved cell design/processing for enhanced performance, taking record cell efficiencies closer to the thermodynamic limit of ~30%.
In this work, devices with three architectures are characterised by high resolution dynamic SIMS (‘NanoSIMS’) giving 3-Dimensional chemical maps of the cells at nanometre resolution. This is supplemented by correlative high resolution EBSD and cathodoluminescence measurements, to establish the effects of observed microstructural and chemical features on performance. The three absorber/buffer layer architectures are: 1) traditional CdS/CdTe; 2) MZO/CdTe; and 3) high efficiency, MZO/CdSeTe/CdTe, selenium-graded devices. This set spans the evolution from more conventional device structures to the newer, high-efficiency structures.
The advantage of NanoSIMS is that it’s high sensitivity, combined with high resolution, can directly detect the behaviour of chlorine in grain interiors. This means that, where present, variations in chlorine concentrations can be observed, both between grains and within grains. For instance, data shows concentration gradients within grains that indicate ingress of chlorine from grain interiors to the grain bulk. These local variations in grain interior chlorine concentration can then be correlated to the local luminescence intensity, to ascertain whether chlorine effects carrier recombination activity in the grain bulk. Another interesting behaviour observed is the segregation of chlorine to linear or ribbon-shaped features within many of the grain interiors in all three device types. The 3D nature of the data enables behaviour of chlorine at the three different interfaces to be tracked.
In addition to chlorine, the chemical maps show the three-dimensional behaviour of sulphur and selenium alloying in each of the device types.
5:00 PM - EN19.04.19
Intrinsic Carrier Transport Dynamics in High Speed Black Phosphorus Photodetectors
Jianbo Gao1,Huili Liu2,Apparao Rao1
Clemson University1,University of California, Berkeley2Show Abstract
A fundamental understanding of carrier transport is imperative for efficient semiconductor electronics and optoelectronics. Here, we use high speed black phosphorus photodetectors with sub-40 ps response time to elucidate carrier transport dynamics along its armchair and zigzag directions. Here we report a direct observation of carrier transport transition dynamics from phonon scattering transport to multiple trapping and release transport mechanism along the armchair direction, resulting from the relaxation of free carriers above the band edge to the band-tail states. We identified that the suppression of phonon scattering effects, a characteristic by Hall and field effect transistor measurements, is due to carrier transport in band tail states. Along the zigzag direction, only multiple trapping and release transport in band-tail states is observed, which might be due to low carrier mobility.
5:00 PM - EN19.04.20
Direct Vapor Phase Growth and Optoelectronic Application of Large Band Offset SnS2/MoS2 Vertical Bilayer Heterostructures with High Lattice Mismatch
Hunan University1Show Abstract
2D van der Waals heterostructures with different types of band alignment have recently attracted great attention due to their unique optical and electrical properties. Most 2D heterostructures are formed by transfer-stacking two monolayers together, but the interfacial quality and controllable orientation of such artifcial structures are inferior to those epitaxial grown heterostructures. Herein, for the frst time, a direct vapor phase growth of high-quality vertically stacked heterostructure of SnS2/MoS2 monolayers is reported. An extremely Type II band alignment exists in this 2D heterostructure, with band offset larger than any other reported. Consistent with the large band offset, distinctive optical properties including strong photoluminescence quenching in the heterostructure area are observed in the heterostructure. The SnS2/MoS2 heterostructures also exhibit well-aligned lattice orientation between the two layers, forming a periodic Moiré superlattice pattern with high lattice mismatch. Electrical transport and photoresponsive studies demonstrate that the SnS2/MoS2 heterostructures exhibit an obvious photovoltaic effect and possess high on/off ratio (>106), high mobility (27.6 cm2 V-1 s-1) and high photoresponsivity (1.36 A W-1). Effcient synthesis of such vertical heterostructure may open up new realms in 2D functional electronics and optoelectronics.
5:00 PM - EN19.04.21
Large-Area WS2 Film with Big Single Domains Grown by Chemical Vapor Deposition
Jie Xing1,Pengyu Liu1,Luo Tao1,Hong Xu1,Huiying Hao1,Hao Liu1,Jingjing Dong1
China University of Geosciences1Show Abstract
High quality WS2 film with the single domain size up to 400 microns was grown on Si/SiO2 wafer by atmospheric pressure chemical vapor deposition. The effects of some important fabrication parameters on the controlled growth of WS2 film have been investigated in detail, including the choice of precursors, tube pressure, growing temperature, holding time, the amount of sulfur powder, gas flow rate and so on. By optimizing the growth conditions at one atmospheric pressure, we obtained tungsten disulfide single-domains with an average size over 100 microns. Raman spectra, atomic force microscopy and transmission electron microscopy provided direct evidence that the WS2 film had an atomic-layer thickness and a single-domain hexagonal structure with a high crystal quality. And the photoluminescence spectra indicated that the tungsten disulfide films showed an evident layer-number-dependent fluorescence efficiency, depending on their energy band structure. Our study provides an important experimental basis for large-area controllable preparation of atomic-thickness tungsten disulfide thin film, and can also expedite the development of scalable high-performance optoelectronic devices based on WS2 film.
5:00 PM - EN19.04.22
Effect of Number of Layers on the Electronic and Optoelectronic Properties of CVD Synthesized MoS2 Domains
Vishakha Kaushik1,Deepak Varandani1,Pintu Das1,Bodh Raj Mehta1
Indian Institute of Technology Delhi1Show Abstract
In the present study, two-dimensional (2D) material molybdenum disulfide (MoS2), deposited using chemical vapour deposition is studied for its interesting electrical, optical and mechanical properties, as a function of number of layers. Raman spectroscopy has been employed to obtain the magnitude of difference between E2g (~385cm-1) and A1g (~404cm-1) peaks of MoS2 which has been used as a signature of the number of layers. The 2D MoS2 is characterized based on the nanoscale variations in the junction properties using Kelvin probe force microscopy (KPFM), conductive atomic force microscopy (CAFM) techniques and macroscale variations in the I-V characteristics, as a function of number of layers. The surface potential values obtained using KPFM technique, gives the effective value of work function for different number of layers of MoS2. For efficient implementation of 2D materials in electronic and optoelecronic devices, it is imperative to form a good metal-semiconductor contact. The metal-semiconductor junction is studied as a function of applied loading force and number of layers of MoS2 using CAFM. The I-V characteristics are obtained with two different AFM metal tips in contact mode, namely, Cobalt (Co) and Platinum (Pt), which should form an Ohmic and Schottky contact with MoS2, theoretically. However, experimental investigation of I-V characteristics using CAFM shows the formation of Schottky barrier and hence a rectifying contact due to Fermi level pinning even for the Co metal electrode contact. The study emphasizes the critical influence of singlelayer nature on the metal contacts in novel 2D materials based devices. Metal – 2D semiconductor contact studied nanoscopically and junction behavior analyzed gives insights for the device formation for photodetector applications. In order to substantiate the layer dependence of MoS2 samples on the optoelectronic properties, photoresponse measurements (I-t characteristics) have been studied macroscopically using Keithley 2400 sourcemeter with a two-electrode configuration. The photoresponse behavior of thin film MoS2 is observed at different wavelengths of light. The photoresponsivity changes from 12.48 for monolayer sample to 4.79 for multilayer sample due to the optically generated carriers for the typical white light wavelength. Photoelectrical measurements on layer dependent, poly-crystalline MoS2 samples show excellent sensitivity, fast photoresponsivity and good reproducibility as a photodetector. The study shows the critical role played by the number of layers on the electronic and optical properties of MoS2 based devices.
5:00 PM - EN19.04.23
Two-Dimensional Lateral Complicated Structure
Hunan Univ1Show Abstract
Two-dimensional layered materials such as garphene, MoS2 and WSe2 have attracted considerable interest in recent times and becoming an important material platform in condensed matter physics and modern electronics and optoelectronics. The studies to date however generally rely on mechanically exfoliated flakes which always be limtited to simple 2D materals. To fully explore the potential of this new class of materials, it is necessary to develop rational synthetic strategies of two dimensional lateral complicated struture,such as lateral heterostructure,multiheterostructure, superlattice,quantum well,etc.
With a relatively small lattice mismatch (∼4%) between MoS2 and MoSe2 or WS2 and WSe2, it is possible to produce coherent MoS2–MoSe2 and WS2–WSe2 heterostructures through a lateral epitaxial process. Our studies indicate that simple sequential growth often fails to produce the desired heterostructures because the edge growth front can be easily passivated after termination of the first growth and exposure to ambient conditions. To this end, we have designed a thermal CVD process that allows in situ switching of the vapour-phase reactants to enable lateral epitaxial growth of single- or few-layer TMD lateral heterostructures. We used this technique to realize the growth of compositionally modulated MoS2–MoSe2 and WS2–WSe2 lateral heterostructures. The WS2–WSe2 lateral heterostuctures with both p- and n-type characteristics can also allow us to construct many other functional devices, for example, a CMOS inverter.
In a typical sequential-growth process for 2D lateral heterostructure, the excessive thermal degradation or uncontrolled nucleation during the temperature swing between sequential growthsteps represents the key obstacle to reliable formation of monolayer r lateral complicated structure.We designed a modified CVD system. A reverse flow from the substrate to the source during the temperature swing between successive growth steps. A forward flow only applied at the exact growth temperature. So, the existing monolayer materials will not exposure to high temperature and chemical vapor source at the tempreture increasing and decreasing steps to minimize thermal degradation and eliminate uncontrolled homogeneous nucleation.We used our approach initially for the general synthesis of a wide range of 2D crystal heterostructures. We also grew more complex compositionally modulated superlattices or multiheterostructures, the number of periods and repeated spacing can be readily varied during growth. HADDF-STEM analysis of the atomic structure of the lateral heterostructures and Multiheterostructures show the atomically sharp interface can be clearly observed.
1.Xidong Duan, Anlian Pan,Ruqin Yu, Xiangfeng Duan,et,al, Nature Nanotechnology 9,2014,1024-1030.
2.Zhengwei Zhang,Xidong Duan, Xiangfeng Duan,et al, Science, 357, 2017,788–792.
5:00 PM - EN19.04.24
Dual Ion Exchange Processes for the Growth of Novel Optoelectronic Structures
Sunay Turkdogan1,2,Seyed Ebrahim Hashemi Amiri2,Cun-Zheng Ning2
University of Yalova1,Arizona State University2Show Abstract
It is a common problem that growing a given composition in a desired morphology is not always possible and thus limiting the application gamut. In this research, we have demonstrated dual ion exchange processes to grow novel optoelectronic structures that are unlikely to be grown directly. In the literature, separate anion or cation exchange has been demonstrated before, but to the best of our knowledge dual ion exchange processes where both anion and cations are partially replaced has been systematically demonstrated for the first time with this research. The scope of this study is to grow any composition of ZnCdSSe quaternary alloys in nanosheet form. It is demonstrated that growing CdS- and CdSe-rich quaternary ZnCdSSe alloys in nanosheet form is easy, but due to very low vapor pressure of wide bandgap materials such as ZnSe and especially ZnS it is almost impossible. The reason is that only VLS mechanism is active under the low vapor pressure and thus leading to grow those materials in 1D nanowire form. On the other hand, materials in Cd, S and Se-rich are easily grown in 2D nanosheet morphologies and therefore they provide a good platform for the morphology transfer process. Here, we have used CdSe-rich materials as a basis and dual ion exchange process was implemented on those materials in a temperature region higher than the one used to grow CdSe nanosheets. As a result of this process we have grown ZnS- and ZnSe-rich ZnCdSSe quaternary alloys in nanosheet form and this paved the way for variety of optoelectronic applications such as white laser, personalized light emitters, full color photodetectors, and so on. This mechanism is not only applicable for ZnCdSSe, but also for any other material systems in which the desired morphology and composition combinations cannot be obtained directly.
5:00 PM - EN19.04.25
Polymer-Assisted Deposition of SrTiO3 Film as Cathode Buffer Layer in Inverted Polymer Solar Cells
Haizhen Wang1,Brian Patterson1,Hongmei Luo1
New Mexico State University1Show Abstract
Polymer solar cells (PSCs) have attracted great attention due to their potential applications as low-cost renewable energy sources. In Conventional PSCs, anode degradation caused by the PEDOT diffusion induced etching of ITO can occur in conventional PSCs, resulting in short lifetimes of the device. In order to enhance the stability of PSCs, PSCs of inverted structures, where the nature of charge collection is reversed, have been employed as an alternative to improve the lifetime of solar cells. To further enhance the device stability, inorganic semiconductor oxides inserted between ITO and the active layer has been implemented as a buffer layer in an inverted device structure to selectively collect electrons as well as avoid the contact of ITO film with PEDOT polymer. The buffer layer works as an electron-collecting electrode and a hole-blocking layer, which is essential for achieving high efficiency PSCs.
SrTiO3 with the perovskite structure is a very attractive material for application in microelectronics due to its high charge storage capacity, chemical stability, good insulating properties and excellent optical transparency in the visible region. It has a similar band structure as ZnO but a higher dielectric constant (~104), which can favor charge transfer and thus be used as electron transport layer in PSCs. Here, we report on the inverted PSCs with pure SrTiO3 films as cathode buffer layer for the first time. Uniform SrTiO3 films were fabricated on ITO substrate via the PAD method to work as cathode buffer layer in the inverted PSCs. The results indicate that the power conversion efficiency of the solar cells based on P3HT and PCBM with SrTiO3 film as cathode buffer layer and MoO3 as anode interfacial layer is up to 3.5%, comparable to that of PSCs with ZnO as buffer layer reported previously.
5:00 PM - EN19.04.26
Oxygen Effect on Cu2O Thin Films Obtained from CuO Films Treated Under Argon/Dry-Air Microwave Plasma
Miguel Badillo-Avila1,Rebeca Castanedo-Pérez1,Gerardo Torres-Delgado1,Joaquín Márquez-Marín1,Orlando Zelaya-Ángel1
Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional1Show Abstract
Cu2O is a p-type semiconductor that has promising properties for solar cells, lithium ion batteries, emitting diodes, photo-catalyst for hydrogen production, photo-catalyst for water and air decontamination, sensor for organic molecules, among others. In this work, the conversion of CuO thin films to Cu2O ones, in less than 30 s via an argon/dry-air microwave plasma, is studied. The process is carried out in a lab-made equipment built with low-cost components, such as a commercial microwave oven . Our simple, but reliable processing, is faster, cheaper and easier than other similar plasma treatments (watch video, https://goo.gl/yS7Y1h), this could allow for its application at mass scale production of Cu2O films.
CuO film targets are easily produced by dip-coating on glass substrates from a homogeneous copper acetate solution. CuO targets are annealed in groups at different temperatures, from 350 °C to 550 °C in increments of 50 °C, in open atmosphere. To obtain Cu2O, CuO thin films are treated for 15, 20, 25 or 30 s, under an argon/dry-air microwave plasma. The plasma processing is performed inside a quartz chamber at low vacuum (15 mbar), with small gas fluxes of argon (6 sccm) and dry-air (6 sscm), and with a microwave power of 1500 W.
Interestingly, pure Cu2O is only produced from a metastable form of metallic copper that is obtained after the plasma treatment. The conversion process can take from a couple of minutes, under a controlled flow of oxygen, to some hours, in open atmosphere. This partial oxidation of metallic copper is clearly driven by the oxygen availability right after the plasma treatment, when the sample is still hot. To our knowledge, this phenomenon has not been reported before.
To study the effect of oxygen, argon and dry-air fluxes are shut down immediately after the plasma treatment and a controlled flow of pure oxygen is supplied for 10 minutes. When no oxygen is used, the resulting film looks dark green; XRD patterns show metallic copper as the majority phase. Once pure oxygen is supplied after the treatment, the film acquires a lighter green color; for these films XRD patterns show a mixture of Cu and Cu2O. Pure Cu2O is achieved once a certain minimum amount of oxygen is supplied. The transformative role of oxygen has a profound effect not only in the crystalline phase of the film, but also in its optical and electrical properties. This phenomenon is being studied with the aim of tailoring some of the properties of pure Cu2O.
Depending on different process conditions, like time of plasma, the crystallite size of Cu2O can be increased and controlled. Nonetheless, there is also an optimal annealing temperature, around 400 °C, for which crystallite size can be maximized. By treating the CuO samples for a longer time, bandgap can be decreased from 2.35 to 2.17 eV. The advantages of our plasma processing lie in the simplicity, short time of treatment and, low cost of the built equipment.
5:00 PM - EN19.04.27
Piezo-Phototronic Effect Enhanced Responsivity of Photon Sensor Based on Composition-Tunable Ternary CdSxSe1–x Nanowires
Haiyang Zou1,Guozhang Zhang1,Zhong Lin Wang1
Georgia Institute of Technology1Show Abstract
The piezotronic effect and piezo-phototronic effect on materials and devices have been widely studied in binary semiconductors. Wide-band ternary semiconductors are a great class of materials with potential application in nano/microdevices, because of their continuously tunable physical properties with composition. Here, we first demonstrate the piezo-photronics effect of ternary wurtzite structured nanowires (NWs), opening an innovative materials system. Single-crystal ternary CdSxSe1–x (x = 0.85, 0.60, and 0.38) NWs were synthesized with site-controlled compositions via a chemical vapor deposition process, and high-performance visible photodetectors (PDs) with fast response speed (<2 ms), high photosensitivity, high responsivity, and broadened photoresponse region (than CdS NW) were fabricated based on these ternary materials. By introducing an external tensile strain, the performance of PDs is enhanced by 76.7% upon 0.2 mW/cm2 442 nm light illumination for CdS0.85Se0.15 by the piezo-phototronic effect. The composition effect of materials in ternary materials on light detecting and piezo-phototronics was also first investigated systematically. The results indicate that in the CdSxSe1–x system, as the value of x decreases, the photocurrent and responsivity experience an increase, while the enhancement of the piezo-phototronic effect was weakened. The change in piezoelectric coefficient and carrier screening effect are proposed for the observed phenomenon. This study reports a high-quality ternary CdSxSe1–x NWs system used for high-performance PDs, broadens the family of piezotronic materials, offers an innovative material for high-performance visible PD, and provides a new pathway to modulate the performance of piezo-phototronic devices by tuning the atomic ratios of ternary wurtzite semiconducting materials. This is essential for developing a full understanding of piezotronics on a broader scope, and it also enables the development of the better performance of optoelectronic devices.
5:00 PM - EN19.04.28
Combinatorial Investigation into the Phase Space of P-Type Transparent Cu-Zn-S
Rachel Woods-Robinson1,2,3,Yanbing Han4,3,Kristin Persson1,2,Andriy Zakutayev3
University of California, Berkeley1,Lawrence Berkeley National Lab2,National Renewable Energy Laboratory3,Fudan University4Show Abstract
Recently, the ternary material system of Cu-Zn-S has shown significant promise as a p-type transparent conductor (TC) for photovoltaic and optoelectronic applications. Previous studies have found evidence of both Cu doping onto the Zn antisite (CuxZn1-xS) and a solid solution of CuyS and ZnS (CuyS:ZnS), depending on the thermodynamics and kinetics of the growth process. Here, we investigate this material system using combinatorial sputtering, high-throughput characterization and percolation theory to explore the phase transitions (both in chemical potential and temperature space) and resulting structure-property relations. Samples are grown with copper concentrations across the entire chemical space, with Cu/(Cu+Zn) ranging from 0 to 1, and films are found to crystallize at room temperature with optimized p-type conductivity and transparency within the range of 0.2 < x < 0.4, in agreement with the “TC regime” of previous studies. We find conductivity to increase monotonically as a function of Cu concentration, which is evidence for a solid solution of amorphous CuyS and ZnS, yet it increases with distinct jumps by an order of magnitude at two different Cu concentrations. Using high spatial resolution synchrotron x-ray diffraction, we find these jumps in conductivity to correlate with structural changes between the wurtzite and zinc-blende crystal structures. This could indicate either (1) greater Cu incorporation into the wurtzite phase than zinc-blende or (2) higher transport in wurtzite due to lower computed effective mass. Additionally, we find conductivity within the “TC regime” to increase to up to 250 S/cm at growth temperatures of 185 - 200 deg. C with no decrease in transparency. At elevated temperature films are found to be solid solutions of Cu2S5 and zinc-blende ZnS, so conductivity is likely due to larger crystal grains and a connected network of Cu2S5 regions within a transparent ZnS matrix. We also present initial results from heterojunction solar cells with combinatorially sputtered Cu-Zn-S and discuss this material’s excellent TC figure of merit in the context of both state-of-the-art p-type TCs and the authors’ recent computational screenings.
5:00 PM - EN19.04.29
Vapor Transfer Deposition Enhanced Sb2Se3 Thin-Film Solar Cells with 7.6% Efficiency
Xixing Wen1,Jiang Tang1
Huazhong University of Science and Technology1Show Abstract
Sb2Se3 possesses a great potential for low-cost and high-efficiency thin film photovoltaic, with a suitable bandgap, high absorption coefficient, benign grain boundaries and earth-abundant constituents. However, the efficiency improvement of Sb2Se3 thin film solar cells was severely restricted by the absorber quality and the trap-assisted recombination caused by the deep defects in the absorber. Herein, we developed a vapor transfer deposition (VTD) technique to fabricate Sb2Se3 films, which highly enhanced the performance of Sb2Se3 solar cells. After the optimization, we significantly enhanced the superstrate CdS/Sb2Se3 solar cells with certified power conversion efficiency of 7.6%, a net 2% improvement over previous 5.6% record of the same device configuration. We analyzed the deep defects in the devices and found that the density of the dominant deep defects was reduced by one order of magnitude using VTD process. Furthermore, the VTD fabricated devices showed fewer interface defects, longer carrier lifetime, and then better performance, compared with the devices fabricated by rapid thermal evaporation. These encouraging results promote the development of Sb2Se3 solar cells in high-efficiency thin film photovoltaic devices.
5:00 PM - EN19.04.30
Sustainable P-Type Copper Selenide Solar Material with Ultra-Large Absorption Coefficient
Pierre Poudeu1,Erica Chen1,Logan Williams1,Emmanouil Kioupakis1
Univ of Michigan1Show Abstract
Photovoltaic research activities over the past decades have focused on the development of low-cost highly efficient materials for application as absorbers in photovoltaic technologies. Popular material systems under consideration in recent years include metal-halide perovskite, organic-inorganic hybrid perovskite, and copper chalcogenide semiconductors such as CuIn1-xGaxSe2 (CIGS). The large absorption coefficient of these materials coupled to the ability to engineer their bandgap through chemical substitutions enable the realization of solar cells devices with power conversion efficiency exceeding 20%. Despite the promise of these material systems, thermal instability associated with hybrid perovskite, restriction on the use of heavy metals (Cd, Pb etc.) and the limitation in supply for In are roadblocks to large scale deployment of the existing leading perovkite and chalcogenide-based technologies. To address these issues, earth abundant copper chalcogenides such as kesterites, Cu2SnZn(S,Se)4 (CZTS), that can be obtained through chemical substitution of In3+ atoms in CuIn(S,Se)2 by Zn2+ and Sn4+, have been investigated. However, the efficiencies of solar cell devices based on these materials remain around 12.6% due to unavoidable anti-site defects such as CuZn and ZnCu. It therefore appears that achieving low-cost, earth abundant copper chalcogenide solar cells with high efficiency requires the development of novel compositions with new crystal structure rather than a simple variation of the chemistry of existing structures. In this work, we report for the first time on the Earth-abundant ternary copper titanium selenide, CTSe, as a promising light-absorbing material for the fabrication of ultra-thin low-cost high efficiency solar cell devices. CTSe is a p-type semiconductor featuring indirect (1.15 eV) and direct (1.34 eV) bandgaps, which are both desirable for ideal solar absorber materials. It crystallizes in a new noncentrosymmetric cubic structure (space group F-43c) in which CuSe4 tetrahedra share edges and corners to form the octahedral anionic clusters, [Cu4Se4]4-, which in turn share corners to build the three dimensional framework, with Ti4+ ions located at tetrahedral interstices within the channels. This unique structural feature results in large density of states (DOS) and relatively flat bands near the band edges, which are believed to be responsible for the ultra-large absorption coefficients (~105 cm-1) observed throughout the visible range for CTSe thin-film. These findings point to CTSe as a promising solar absorber material for scalable low-cost high-efficiency thin-film solar cells.
5:00 PM - EN19.04.31
Zirconium-Doped Indium Oxide—Pushing the Limits of Optoelectronic Properties in Sputtered Transparent Conductive Oxides
Esteban Rucavado1,Raphaël Monnard1,Sylvain Dunand1,Jérémie Werner1,Federica Landucci1,Quentin Jeangros1,Aicha Hessler-Wyser1,Mathieu Boccard1,Monica Morales-Masis1,Christophe Ballif1
Ecole Polytechnique Federale de Lausanne1Show Abstract
Optimal transparent conductive oxides (TCOs) are essential to reduce parasitic absorption losses in optoelectronic devices. Hydrogenated indium oxides (IO:H, ICO:H, IWO:H) lead the race, as they have electron mobilities > 100 cm2/Vs, low absorption in the visible and near-infrared parts of spectra and they can be easily deposited over a wide range of substrates. During deposition, the introduction of water hampers the material crystallization, hence as deposited films have amorphous microstructure. After annealing at 200°C, these materials experience a phase transition and coalesce in big crystalline domains, which are linked to the high electron mobility. Nonetheless, the presence of water during deposition is a drawback to upscaling humidity sensitive technologies to production.
In this work, we propose an alternative TCO sputtered without the intentional introduction of water during deposition: zirconium-doped indium-oxide (or IO:Zr). Using a combination of optoelectronic characterization, high-end electron microscopy techniques, electron recoil dispersive analysis and Rutherford backscattering, we fully characterize the material and explain the fundamental mechanisms limiting its electron transport. Even without the intentional introduction of water during deposition, these films have an amorphous microstructure and after annealing at 200 °C form crystallites with average size ~ 320 nm. With an electron mobility > 100 cm2/Vs and free carrier density as high as 2.5 × 1020 cm-3, 100 nm-thick films have a wider bandgap (between 3.5 eV and 3.9 eV) than the afore mentioned In-based TCOs. In addition, we found that the main limiting mechanism of electron transport in IO:Zr is ubiquitous phonon scattering.
Motivated by the high conductivity of the material, we reduced the thickness of the films from 100 nm to 15 nm to further reduce the optical absorptance. The thinnest films have an optical absorptance close to the glass substrate in which they were deposited- while still presenting high electron mobility (50 cm2/Vs), and high free carrier density. Interestingly all films, from 100 nm down to 15 nm-thick films, show the presence of large crystalline grains after annealing at 200 °C.
Finally, to demonstrate the applicability of the material, IO:Zr thin films with different thickness were applied as front electrode in silicon- and perovskite-based solar cells; showing in all cases an improvement in current density thanks to high transparency of IO:Zr as compared to ITO.
5:00 PM - EN19.04.32
Structural and Electronic Effects of Sb and Na:Sb Doping in Solution Processed Cu2ZnSnS4 Film Solar Cells
Devendra Tiwari1,Tristan Köhler2,Mattia Cattelan1,Neil Fox1,Reiner Klenk2,David Fermin1
University of Bristol1,Helmholtz-Zentrum Berlin 2Show Abstract
Dopants and additives such as alkali-ions can substantially affect the PV performance of thin-film semiconductor PV 1. For instance, Na has been linked to the passivation of grain boundaries 2, while Sb decreases the crystallization temperature of CIGS 3and. In this work, we shed new light on the role of Na and Sb in the performance of Cu2ZnSnS4 (CZTS) solar cells employing temperature dependent photoluminescence, impedance spectroscopy photoemission microscopy. CZTS films are generated using a single solution precursor from metal salts and thiourea dissolved in DMF-Isopropanol mixture. The precursor is spin-coated onto Mo-coated glass substrates and finally annealed at 550 oC under S atmosphere. The process yields polycrystalline films with a band gap of 1.4 eV consisting of Cu-poor and Zn-rich composition with tetragonal phase. Electron microscopy shows the formation of 1.2 mm films with more crystalline (grain sizes up to 700 nm) and compact morphology upon the inclusion of Sb and Na:Sb. This is due to the lowering of crystallization enthalpy as determined by differential scanning calorimetry. The quantitative analysis of X-ray diffraction reveals decrease in the isotropic thermal parameter, associated with the atomic site disorder, especially for Sn site upon Sb doping.4 Solar cells are completed to have a substrate configuration of: glass/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Ni−Al with individual cells scribed with a total area of 0.5 cm2. Statistical analysis of J-V measurements over 200 devices under AM1.5 G spectrum with 100 mW/cm2 demonstrates an overall improvement in fill-factor (FF) and open-circuit voltage (VOC) with better conformity resulting in a rise of average power conversion efficiency (h) from 3.2 ± 0.6 to 5.2 ± 0.3% on doping. The best performing device with Na:Sb doping yielded an 14.9 mA cm−2 short-circuit current, 610 mV VOC, 63% FF and h of 5.7%, rating amongst top reported for pure sulphide kesterite.5 Recently, a computational study predicted that depending on concentration, Sb doping may either lead to the appearance of states in the gap detrimental to the performance or may boost the performance through lowering the Sn disorder .6 For the first time, we monitor the electronic states due to Sb doping employing low-temperature photoluminescence and surface sensitive energy filtered ultraviolet photoemission spectroscopy which allows the rationalizing the systematic change in device efficiency.
1. Prog. Photovoltaics Res. Appl., 2017, 25, 645–667.
2. Adv. Mater., 1998, 10, 31–36.
3. Chem. Mater., 2010, 22, 285–287.
4. Chem. Mater., 2016, 28, 4991–4997.
5. Prog. Photovoltaics Res. Appl., 2016, 24, 879–898.
6. J. Mater. Chem. A, 2017, 5, 6606–6612.
5:00 PM - EN19.04.33
Molecular Ink-Derived Sb2Se3 Nanostructure Photocathodes for Efficient Photoelectrochemical Water Splitting
Wooseok Yang1,Jeiwan Tan1,Hyungsoo Lee1,Jaemin Park1,Yunjung Oh1,Hyunyong Choi1,Jooho Moon1
Yonsei University1Show Abstract
Realization of sustainable hydrogen production via photoelectrochemical (PEC) water splitting is contingent on developing efficient and low-cost photoelectrode. Sb2Se3 recently receives great interest as a promising low-cost light-absorbing material for solar energy conversion. In this talk, we will present a synthetic methodology to produce efficient Sb2Se3 nanostructure photocathodes by a simple spin-coating of Sb-Se molecular ink. The Sb-Se molecular ink is prepared by using the solvent mixture of thioglycolic acid (TGA) and ethanolamine (EA) and the aspect ratio of 1-D Sb2Se3 nanostructures can be controlled by adjusting the relative mixing ratio of TGA and EA. After the deposition of TiO2 and Pt, an appropriately oriented Sb2Se3 nanostructure array exhibits a significantly enhanced PEC performance; the photocurrent reached 12.5 mA cm-2 at 0 V versus reversible hydrogen electrode under AM 1.5 G illumination. The role of carboxylate nucleophile enabling the unique 1-D nanostructures will be elucidated by liquid Raman spectroscopy in conjunction with the observation of the morphological evolution. In addition, the strengths and limitations of Sb2Se3 based photocathode will be discussed with various analyses including incident photon-to-current conversion efficiency, THz spectroscopy, and time-resolved photoluminescence.
5:00 PM - EN19.04.34
Wide-Bandgap CuGa(S,Se)2 as a Top Cell Photocathode for Tandem Water Splitting Devices
Alex DeAngelis1,Kimberly Horsley1,Nicolas Gaillard1
University of Hawaii1Show Abstract
Although several wide-bandgap (1.6-2.0 eV) chalcopyrites (e.g. CuGaSe2 Cu(In,Ga)S2, Cu(In,Al)Se2, (Ag,Cu)GaSe2) have been thus far studied as top cell absorbers for photoelectrochemical (PEC) water splitting tandem devices, CuGa(S,Se)2 is a wide-Eg chalcopyrite that has not yet received any attention in this regard. Thus, we present the performance of wide-bandgap chalcopyrite CuGa(S,Se)2 photocathodes as a top cell for PEC tandem water splitting. To be able to assess the PEC performance of CuGa(S,Se)2 as well as its optical transmittance, transparent conductive fluorinated tin oxide (FTO) was used as a contact. During our study though we learned that synthesizing CuGa(S,Se)2 films on an FTO substrate would degrade the optoelectronic properties of the FTO and so we present a synthesis to circumvent this problem and fabricate functioning CuGa(S,Se)2 PEC photocathodes. We then present the PEC performance of these functioning CuGa(S,Se)2 PEC photocathodes (JSAT≈10 mA/cm2) as well as measurements relevant to its performance as a top cell, such as quantum efficiency of a low-Eg Cu(In,Ga)Se2 shaded by the CuGa(S,Se)2 PEC photocathodes and sub-bandgap transmittance of the CuGa(S,Se)2 PEC photocathodes.
Adele Tamboli, NREL
Joel Ager, Lawrence Berkeley National Laboratory / University of California, Berkeley
David Scanlon, University College London
Lydia Wong, Nanyang Technological University
EN19.05: Sb2Se3 and Related Materials
Wednesday AM, April 04, 2018
PCC North, 100 Level, Room 124 B
8:15 AM - EN19.05.01
Sb and Bi Based Semiconductor for Photovoltaic and Lighting Applications
Huazhong University of Science and Technology1Show Abstract
The everlasting demanding for high-efficiency, low-cost solar electricity and lighting motivate the research on new earth-abundant and toxicity-free semiconductors. Sb and Bi based materials are relatively less explored partially because they contain s2 electron lone pairs which often leads to compounds with low crystal symmetry. In this presentation we will discuss two parts: Sb2Se3 for thin film photovoltaics and Bi based halide perovskite for lighting.
For the first part, our efforts and recent progress in Sb2Se3 photovoltaics will be presented. Binary chalcogenide Sb2Se3 has appropriate band gap and excellent optoelectronic properties, nontoxic and earth-abundant composition, and large vapor pressure enabling easy evaporation, making it a possible green alternative to CdTe. Using sprayed ZnO as the buffer layer and rapid thermal evaporation deposited Sb2Se3 as the absorber, ZnO/Sb2Se3 solar cells with certified 5.93% efficiency and outstanding stability was demonstrated1. Through a further Sb2Se3 film optimization, a further 7.6% device efficiency was achieved, but sadly using the CdS buffer layer. The new understanding of Sb2Se3 material properties and device physics will also be briefly presented in this talk.
For the second part, Bi based hybrid and all inorganic halide perovskites as a potential alternative to Pb perovskite for photoluminescence (PL) application will be presented. We investigated these materials for PL because Bi3+ is isoelectronic to Pb2+ but toxicity-free, and more importantly because Bi halide perovskites have low-dimensional crystal structure (0D, 1D or 2D) naturally enjoying a large exciton binding energy which is beneficial for lighting application. We will discuss our recent progress in the synthesis, characterization and lighting application of Bi-based perovskite nanocrystals2, with the emphasis on the Cs3Bi3Br9 nanocrystals with >50% PL yield and excellent stability.
We conclude that Sb and Bi based semiconductors, if carefully engineered to take advantage of their low crystal structure symmetry, are competitive for some optoelectronic applications.
J. Tang et. al. Nat. Energy, 2017, 2, 17046.
J. Tang et. al. Adv. Funct. Mater. 2017, in press.
8:45 AM - EN19.05.02
Photocorrosion-Resistant Sb2Se3 Photocathodes with Earth Abundant MoSx Hydrogen Evolution Catalyst
RajivRamanujam Prabhakar1,David Tilley1
University of Zurich1Show Abstract
The poor stability of high efficiency photoabsorber materials in aqueous media is one factor holding back the realization of photoelectrochemical (PEC) water splitting for large scale, practical solar fuels generation. Here, we demonstrate that highly efficient thin film Sb2Se3–fabricated by a simple, low temperature selenization of electrodeposited Sb–is intrinsically stable towards photocorrosion in strongly acidic media (1 M H2SO4). Coupling with a photoelectrodeposited MoSx hydrogen evolution catalyst gives high photocurrents (5 mA cm-2 at 0 V vs RHE) and high stability without protective layers. A low temperature sulfurization of the Sb2Se3-MoSx stack dramatically improved the onset potential, resulting in high photocurrent densities up to ~16 mA cm-2 at 0 V vs RHE. The simplicity with which these photocathodes are fabricated, combined with the high photocurrents and stability, make Sb2Se3 a strong candidate for scalable PEC cells.
9:00 AM - EN19.05.03
Role of the N-Type Layer and Interface in Sb2Se3 Solar Cells
Univ of Liverpool1Show Abstract
Antimony selenide solar cells are an emerging thin-film technology of growing interest. They benefit from a direct ~1.17eV bandgap, containing no scarce materials, have a simple phase chemistry and an interesting 1D nanoribbon grain structure. Despite the first respectable device efficiency being reported as recently as 2014 and the relative paucity of research, they have already reached efficiencies of 6.5% in excess of other widely studied binary compounds such as SnS. Due to the nascent nature of the technology the optimal cell structure is still to be determined and as a result number of n-type partner layers such as ZnO, CdS and TiO2 have thus far been reported.
In this work we compare devices based on two Sb2Se3 deposition routes, thermal evaporation and close space sublimation, to examine the influence of the n-type partner layer and the interface on device performance. Our results show that while CdS is a suitable partner layer for thermally evaporated Sb2Se3 devices (efficiencies >4%) for CSS deposited layers CdS is unsuitable (<2.5% efficiency). In contrast TiO2 layers are highly effective for CSS material (>5.5% efficiency) but are unsuited to thermally evaporated Sb2Se3 devices (<1% efficiency). We will demonstrate this is due to the degree of S/Se interdiffusion at the interface and thereby linked to the Sb2Se3 deposition process. Device performance will be linked to XPS analysis of band alignments, cross sectional TEM of the interface and deep level transient spectroscopy (DLTS) analysis of defect composition in complete cell structures.
9:15 AM - EN19.05.04
ZnO / Sb2S3 Core-Shell Nanowire Heterostructures for ETA Solar Cells
Thomas Cossuet1,Romain Parize1,Atanas Katerski2,Inga Gromyko2,Odette Chaix-Pluchery1,Laetitia Rapenne1,Hervé Roussel1,Erki Karber2,David Munoz-Rojas1,Estelle Appert1,Malle Krunks2,Vincent Consonni1
Université Grenoble Alpes, CNRS, Grenoble INP, LMGP1,Laboratory of Thin Film Chemical Technologies, Tallinn University of Technology2Show Abstract
ZnO nanowire (NW) arrays have emerged as promising building blocks for a wide variety of optoelectronic and photovoltaic devices, including the extremely thin absorber (ETA) solar cells. In this novel architecture, a direct p-type semiconductor absorber is typically deposited on a ZnO NW array to form core-shell p-n heterojunctions following the type-II band alignment. Increasing interest has been dedicated to this radial architecture owing to efficient light trapping and charge carrier management together with the use of a low amount of materials.1
Antimony trisulfide (Sb2S3) is a p-type semiconductor with a 1.7 eV band gap energy and a high absorption coefficient that has been integrated into mesoporous-TiO2-based dye-sensitized solar cells, showing a power conversion efficiency (PCE) as high as 7.5%.2 It is usually grown by low-cost, low-temperature chemical deposition techniques, which still make its combination with ZnO NWs difficult owing to their instability in acidic conditions.
In this work, the crystallization process of Sb2S3 thin films is investigated by in situ x-ray diffraction and in situ Raman spectroscopy, revealing the intermediate formation of a metallic antimony phase and showing the optimal annealing temperature of 270°C.3 Furthermore, an 8 nm-thick TiO2 protective layer is grown by atomic layer deposition onto ZnO NW arrays grown by chemical bath deposition. Sb2S3, as an absorbing shell, is subsequently deposited by chemical spray pyrolysis. The Sb2S3 10 nm-conformal shell with high crystalline quality covers the ZnO/TiO2 NW arrays from the bottom to the top. The photovoltaic performance of the ZnO/TiO2/Sb2S3 core shell NW heterostructures using P3HT as hole transporting material results in a promising PCE of 2.3% with a high Jsc of 7.5 mA/cm2, when considering that the Sb2S3 shell is 10 nm-thick, and a high Voc of 656 mV.4 The use of low-cost, surface scalable chemical deposition techniques for the fabrication of the whole ZnO/TiO2/Sb2S3 structure opens the way for improving the performances of ZnO NW-based ETA solar cells.
1E.C. Garnett et al. Annual Review of Materials Research 41 (2011), 269-295
2Y.C. Choi et al. Adv. Funct. Mater. 24 (2014), 3587
3R. Parize et al. Materials & Design 121 (2017), 1-10
4R. Parize et al. The Journal of Physical Chemistry C 121 (2017), 9672-9680
9:30 AM - EN19.05.05
Sb2Se3 Thin Film—Growth and Characterization
Paulo Fernandes5,6,2,A. Shongalova1,2,M. R. Correia2,S. Ranjbarrizi3,S. Garud3,Bart Vermang3,4,J.M.V. Cunha5,P. M. P. Salomé5
Satpayev University1,I3N, Institute for Nanostructures, Nanomodelling and Nanofabrication2,IMEC3,Hasselt University4,INL5,Instituto Superior de Engenharia do Porto/ IPP6Show Abstract
It is well known that one way of creating an environmentally friendly energy production momentum, which allows mitigating the effects of global warming, is closely linked to the commercial relevance of renewable energy production systems. Photovoltaic (PV) energy can play an important role in this field. Currently dominated by technology based on Si this technology has some drawbacks that prevent a greater market presence. High energy payback time and low industrial production rate, among others, are constrains to a higher PV share in the energy production systems in most countries. Due to monolithic integration, lower energy processes and lower material demand, thin film technology presents good arguments to overcome Si technology. CIGS and CdTe based PV cells are currently the most powerful representatives of thin film technology on the market. However, the solar cells based on these materials present problems related to the scarcity and toxicity of some elements that compose them. Alternative materials are currently been studied, such like Cu2ZnSn(S,Se)4, to be applied as absorber layer in the solar cell structure. But due to its complexity and restricted growth conditions some difficulties are been encountered. These facts have prevented the production of devices with efficiencies compatible with their commercialization.
In this work we present a method to grow Sb2Se3 thin film which can be used as absorber layer in a solar cell structures. These films were grown on the top of different substrates such as soda-lime glass, Mo coated soda-lime glass and Si. The Sb-Se precursor’s films were deposited by RF magnetron sputtering and then annealed with an H2Se gas flow. Different annealing temperatures were tested and analyzed. This study also analyses the effects of the use of different substrates on properties of the film. Compositional and morphological analyzes are performed by Energy Dispersive Spectroscopy and Scanning Electron Microscopy, respectively. Two techniques are used to phase identification and structural characterization, namely, X-ray Diffraction and Raman Dispersion Spectroscopy. Special attention is taken to Raman scattering characterization conditions in order to avoid measurement artefacts. Many authors show results with Sb2O3 Raman modes identified as Sb2Se3. In the work we clearly differentiate these modes from each binary compound and show how to avoid the oxidation of Sb. Spectrophotometry is also performed in order to determine absorption coefficient and the band gap energy of the semiconductor.
EN19.06: 2D Materials I
Wednesday PM, April 04, 2018
PCC North, 100 Level, Room 124 B
10:15 AM - EN19.06.01
Layered Material Heterostructures for Photovoltaics and Photocatalysis
Giulia Tagliabue1,Harry Atwater1
California Institute of Technology1Show Abstract
The strong absorption and visible spectrum energy bandgaps for the transition metal dichalcogenides (TMDCs) of molybdenum and tungsten render them as attractive candidates for photovoltaics (PV) and optoelectronics. Further, the atomically thin nature is favorable for efficient separation and collection of photo-excited charge carriers. Thus if the three major optoelectronic criteria, i.e., i) sunlight absorption, ii carrier collection and iii) operating voltage can be addressed for TMDC materials, they may be candidates for high efficiency photovoltaics and photocatalysis1. We recently demonstrated near-unity broad-band absorption of above band-gap photons for < 15 nm TMDC layers2, and have also achieved high external quantum efficiency in < 10 nm thick active layer photovoltaic devices in a pn junction of WSe2/MoS2 with graphene contacts3. To date, achievement of high open-circuit voltage (Voc) has remained an outstanding challenge for achieving high photovoltaic efficiency. We report here high open-circuit voltages (Voc) in TMDC absorbers based photovoltaic devices, achieved by tailoring the conduction and valence band-alignments between a single TMDC absorber layer and carrier-selective contact layers for electron collection (titanium oxide) and hole collection (nickel oxide), respectively. The band alignments measured using X-ray photoelectron spectroscopy indicate the asymmetric and selective nature of the metal oxide carrier-selective contacts, and we observe open circuit voltages exceeding 700 mV under AM 1.5 illumination at 1 Sun. We will also discuss TMDC passivation, and architectures for photocatalysts and photoelectrochemical devices that employ these materials as absorbers and catalysts.
1. Jariwala, D.; Wong, J.; Davoyan, A. R.; Atwater, H. A. ACS Photonics 2017, ASAP.
2. Jariwala, D.; Davoyan, A. R.; Tagliabue, G.; Sherrott, M. C.; Wong, J.; Atwater, H. A. Nano Lett. 2016, 16, (9), 5482-5487.
3. Wong, J.; Jariwala, D.; Tagliabue, G.; Tat, K.; Davoyan, A. R.; Sherrott, M. C.; Atwater, H. A. ACS Nano 2017, 11, 7230–7240.
10:45 AM - EN19.06.02
Vertical MoS2 Optoelectronic Devices Grown On Flexible Molybdenum Foil Bottom-Contacts
John Robertson1,Kazi Islam1,Maxwell Woody1,Jacqueline Failla1,Jiang Wei1,Matthew Escarra1
Tulane University1Show Abstract
Optoelectronic devices featuring 2D transition metal dichalcogenide (TMDC) semiconductors, such as MoS2, WS2, and MoSe2, hold great promise for the miniaturization of future light emitting and collecting devices. In order for 2D optoelectronic devices to be developed into viable technologies, it is necessary to produce large-area films and vertical device architectures. Vertical devices are expected to increase device performance by reducing the path length of charge carriers by ~100x and by enabling lateral scaling on the order of centimeters. In response, we propose and demonstrate a device architecture that uses a molybdenum film as both the growth substrate for 2D MoX2 (X = S, Se, Te) and also as the bottom contact for vertical MoX2-based optoelectronic devices. This architecture allows for TMDC growth directly on top of its bottom contact, simplifying and improving the device fabrication process. Using a Thermal Vapor Sulfurization (TVS) technique developed in previous works, we show that a high-quality 2D film of MoS2 can be grown on top of molybdenum by reacting the top of the molybdenum layer with sulfur vapor; the remaining molybdenum underneath the MoS2 is used as a conducting bottom contact. This contact scheme requires no external transfers of the MoS2 layer and results in an intimate bottom contact interface relatively free of contaminants. Preliminary multilayer MoS2 photodetector devices on a 150nm molybdenum film were fabricated and characterized. Strong A1g and E2g raman peaks, spaced 25cm-1 apart, indicate a 10-15 layer MoS2 growth. In addition, diffuse and specular reflection measurements indicate an absorption of up to 85% of visible light, as aided by internal reflection off of the MoS2/Mo interface. A 30µm × 20µm Ti/Au grid finger array is used as the top contact. When illuminated from 400nm to 700nm using a supercontinuum laser and laser line tunable filter under 6V source-drain bias, the device shows a 3 order of magnitude increase in spectral photocurrent relative to comparable lateral photodetectors. Dark and illuminated IV curves reveal diode-like behavior, with an induced photocurrent of up to 300nA under monochromatic illumination of ~0.05mW. Palladium and other hole-selective contacts will be used to fabricate large-area 2D MoX2-based Schottky-type photovoltaics with collection areas on the order of cm2. Device simulation using the AFORS-HET software package is used to computationally interpret the performance of the devices and to explore the parameter space of device properties. We also emphasize the potential for these devices to be fabricated on micron-thick molybdenum foils, enabling roll-to-roll fabrication on ultra-lightweight and flexible substrates. This presented technique reveals a viable route for industrial-scale synthesis of nm-thick 2D TMDC photovoltaics, with great potential for applications requiring ultra-lightweight photovoltaics, such as spacecraft and vehicle energy collection.
11:00 AM - EN19.06.03
Effects of Synthesis and Processing on Optoelectronic Properties of Titanium Carbonitride MXene
Kanit Hantanasirisakul1,2,Mohamed Alhabeb1,2,Alexey Lipatov3,Kathleen Maleski1,2,Babak Anasori1,2,Alexander Sinitskii3,Steven May2,Yury Gogotsi1,2
A.J. Drexel Nanomaterials Institute, Drexel University1,Department of Materials Science and Engineering, Drexel University2,University of Nebraska-Lincoln3Show Abstract
MXenes are a rapidly growing class of 2D transition metal carbides, carbonitrides, and nitrides. Their high electronic conductivities make them promising in many applications including transparent conductive coatings, electromagnetic interference shielding, and energy storage. To date, more than 20 MXenes have been synthesized mostly by selective etching using fluorine-containing etchants, and exfoliated into 2D flakes by employing organic- or inorganic-based intercalants. Using different etchants and intercalants results in variation in surface functionality, flake size, number of defects, and interlayer spacing, all of which may affect the optoelectronic properties of MXenes. In this study, we report the effects of synthesis methods and post-synthesis heat treatment on the optoelectronic properties of titanium carbonitride MXene (Ti3CN). Transport properties of free-standing MXene ‘paper’, transparent thin films, and monolayer flakes were studied by temperature-dependent resistivity measurements from 10-300 K. The results show that the films prepared using a large organic intercalant are about three orders of magnitude less conductive than films prepared with a smaller inorganic-based intercalant. Furthermore, upon heat treatment, the conductivity can be greatly improved by removing the intercalated molecules. Our results show how the electronic properties of Ti3CN MXene can be tuned by optimization of synthesis method and post-synthesis heat treatment. A similar approach can be applied to other MXenes to control their optoelectronic properties.
11:15 AM - EN19.06.04
Termination of Ge Surfaces with Ultrathin Germanium Sulfide Layers via Solid-State Sulfurization
Courtney Keiser1,Hui Chen1,2,Shixuan Du2,Hongjun Gao2,Peter Sutter1,Eli Sutter1
University of Nebraska-Lincoln1,Chinese Academy of Sciences2Show Abstract
Germanium is an attractive alternative to silicon due to its substantially higher carrier mobility and good lattice match with GaAs that can facilitate materials integration for high-performance electronics and optoelectronics. But its oxides are unstable and have poor electronic properties. Surface passivation by chalcogens could potentially endow Ge surfaces with properties similar to those of 2D metal chalcogenides, in particular a very low chemical reactivity and complete elimination of dangling bonds. For transition metals, direct sulfurization has been reported as a way of producing high quality few- and single-layer MoS2 and WS2. In the case of Ge, a similar sulfurization might protect the surface against oxidation by formation of germanium sulfide (GeS), a layered chalcogenide semiconductor with a direct bandgap in the visible range and promising optoelectronic and electronic properties.
Here we report solid-state reactions of Ge with sulfur involving exposure to S vapor at elevated substrate temperatures and near atmospheric pressure to produce Ge-sulfides on extended Ge(100) and (111) surfaces. We analyze the reaction kinetics and derive the activation energy of the rate-limiting step of the sulfurization reaction through X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy on samples exposed to the same S dose at different temperatures. Our results show that the sulfurization process gives rise to a sequence of GeSx phases, terminating in few nanometer layers of GeS2. The thickness evolution is thermally activated with a barrier EA = 0.68 eV, attributed to a reaction-limiting diffusion process through the sulfide layer. GeSx terminated Ge exhibits excellent long-term stability against oxidation in air, demonstrating the ability of controllably producing Ge-chalcogenide passivation layers via solid-state reactions in S vapor .
In addition to these results for flat Ge surfaces, we discuss the sulfurization of Ge nanowires. Semiconductor/chalcogenide core-shell nanowires could become heterostructure materials for photovoltaics, photo-electrochemistry, and optoelectronics. Ge nanowires synthesized by the vapor-liquid-solid (VLS)  method and exposed to sulfur vapor at different temperatures and times show the controlled formation of GeS shells. Single nanowire cathodoluminescence measurements establish the optoelectronic properties resulting in these novel one-dimensional core-shell heterostructures .
 H. Chen, C. Keiser, S. Du, H-J. Gao, E. Sutter, and P. Sutter, “Termination of Ge Surfaces with Ultrathin GeS and GeS2 Layers via Solid-State Sulfurization”, under review (2017).
 E. Sutter, B. Ozturk, and P. Sutter,”Selective Growth of Ge Nanowires by Low-Temperature Thermal Evaporation”, Nanotechnol. 19, 435607 (2008).
 C. Keiser, P. Sutter, and E. Sutter, “Controlled Formation of Hybrid Germanium/Germanium Sulfide Core-Shell Nanowire Heterostructure”, in preparation (2017).
11:30 AM - EN19.06.05
Epitaxy of Layered Dichalcogenides for Electronics and Energy Applications
Joan Redwing1,Xiaotian Zhang1,Tanushree Choudhury1,Mikhail Chubarov1,Joshua Fox1,Joshua Robinson1
The Pennsylvania State University1Show Abstract
There is growing interest in the properties and applications of the family of layered metal dichalcogenides, MX2 (X=Se, S), which includes transition metal dichalcogenides (TMDs) such as MoS2 and WSe2 and group IV dichalcogenides particularly SnSe2 and SnS2. While significant scientific advances have been made using monolayer and few-layer flakes exfoliated from bulk chalcogenide crystals, future device development requires the ability to synthesize large area, single crystal films and heterostructures. Our research is aimed at the development of an epitaxial growth technology for layered dichalcogenides, similar to that which exists for III-V and II-VI compound semiconductors, based on gas source chemical vapor deposition (CVD) and metalorganic CVD (MOCVD) in cold-wall reactor geometries. This approach provides excellent control of the precursor partial pressure and reduced pre-deposition upstream of the substrate thereby enabling control over nucleation density, lateral growth rate and film composition for the layer-by-layer growth of 2D films and heterostructures.
Our recent studies have focused on the epitaxial growth of WSe2 and WS2 monolayer films and vertical heterostructures using metal hexacarbonyl and hydride chalcogen precursors on substrates including sapphire, SiC, epitaxial graphene and hexagonal boron nitride. A multi-step precursor modulation growth method was developed to control the nucleation density, size, orientation and the lateral growth rate of monolayer domains on the substrate. Using this approach, coalesced monolayer and few-layer TMD films were obtained on sapphire substrates up to 2” in diameter at growth rates on the order of ~ 1 monolayer/hour. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to the sapphire resulting from a merging of predominantly 0o and 60o oriented domains. Epitaxial growth of SnS2, SnSe2 and Sn(S,Se)2 alloy films on epitaxial graphene have also been demonstrated using evaporated sources. The gas source CVD method also provides a means to study and quantify surface diffusivities and lateral growth rates of domains as a function of growth conditions providing insight into the fundamental mechanisms of monolayer growth. Applications and challenges of this approach in the growth of 2D heterostructures will also be discussed.
EN19.07: Bulk Photovoltaic Effect and Novel PV Mechanisms
Wednesday PM, April 04, 2018
PCC North, 100 Level, Room 124 B
1:30 PM - EN19.07.01
Bulk Photovoltaic Effect—Theoretical Limits and Novel Materials
Liang Tan1,Andrew Rappe1
University of Pennsylvania1Show Abstract
The bulk photovoltaic effect (BPVE) is the generation of photocurrents in the bulk of a single-phase material. It holds advantages over traditional photovoltaics based on p-n junctions, such as above-band gap photovoltages, and current generation in the bulk without the need for interface engineering. Despite numerous theoretical and experimental research efforts into the BPVE, there has been no systematic investigation into its maximum magnitude attainable in solid-state materials. In this talk, we present an upper bound on the dominant microscopic mechanism of BPVE: the shift current response. We show that this bound depends on the band gap, band width, and geometrical properties of the material in question. As a proof of principle, we perform first-principles calculations of the response tensors of a wide variety of materials, finding that the materials in our database do not yet saturate the upper bound. This suggests that new large BPVE materials will likely be discovered by future materials research guided by the factors mentioned in this work.
These results imply that small band gap materials can potentially host large BPVE. As examples, we propose materials which are tuned across a band-gap-closing phase transition from a normal semiconductor into a topological insulating phase. This class includes some inorganic layered semiconductors, such as BiTeI, and inorganic halide perovskites, such as CsPbI3. We show that this results in a dramatic enhancement of photocurrent as well as an abrupt reversal in its direction. Using first-principles calculations, we show that that this effect is robust across different materials systems as long as such a transition into a topologically insulating phase is present.
2:00 PM - EN19.07.02
Identifying New Photoferroics—The Modeling and Synthesis of BiCoO3
Kaitlin Hellier1,2,Lauren Garten2,Sue Carter1,Stephan Lany2,David Ginley2
University of California Santa Cruz1,National Renewable Energy Laboratory2Show Abstract
Recent developments in photoferroics have led to increased power efficiencies greater than 8%. However, many of the polar materials that have been used in photoferroic devices have bandgaps of 3 eV or more, making the effectiveness of these materials as solar absorbers limited. This has spurred interest in theoretical modeling and experimental realization of new polar materials, with a focus in band gap tuning. In efforts to explore Bi-based perovskite oxides, electronic structure calculations were performed on BiMO3 perovskite structures to determine band gaps and the electronic dielectric functions (GW approximation), as well as the ionic dielectric constant and the piezoelectric tensors (density functional perturbation theory). BiCoO3 showed promise as a low band gap semiconductor with a predicted gap of 2.08 eV, a polar P4mm structure, and a predicted relative permittivity of 44.6. To further investigate this material’s potential as the solar absorber in a photoferroic device, BiCoO3 films were grown via combinatorial pulsed laser deposition. Utilizing a multi-target deposition with Bi2O3 and CoO, the substrate, temperature, chemistry and partial pressure of oxygen were varied to achieve a tetragonal P4mm structure. X-ray diffraction was used to track the phase formation was tracked as function of composition and temperature. Additionally, x-ray fluorescence and UV-vis absorption were used to characterize the compostion and band gap. Upon achievement of proper composition, epitaxial strain was used to create high quality films with an aligned internal polarization. From these films metal-semiconductor-metal structure was used for electronic and photoferroic testing.
2:15 PM - EN19.07.03
Wurzite CuGaO2 as Strong Candidate Materials for Efficient Ferroelectric Photovoltaics
Donghun Kim1,Seungwoo Song2,Hyun Myung Jang3
KIST1,Korea Research Institute of Standard and Science2,POSTECH3Show Abstract
Despite the potential to exceed the Schockley-Queisser theoretical limit, ferroelectric photovoltaics (FPVs) have performed inefficiently due to their extremely low photocurrents. The reported low photocurrent values are mainly attributed to poor light absorptions. Most ferroelectric materials with sufficiently large polarizations, such as LiNbO3, BaTiO3 (BTO), and PZT, have a very wide band-gap energy of >3.0 eV, which places their absorption onset near the ultraviolet (UV). BiFeO3 is known to have a relatively smaller band gap of 2.7 eV; however, this is still far apart from the optimal band-gap range (1.1-1.5 eV) for PV applications. In this regard, in order to realize the potential of FPVs, it is highly desirable to search for a novel ferroelectric material with both strong polarization and optimal band-gap energy.
We propose a recently discovered material, namely β-CuGaO2 [T. Omata et al., J. Am. Chem. Soc. 2014, 136, 3378] as a strong candidate material for efficient ferroelectric photovoltaics (FPVs). According to first-principles predictions exploiting hybrid density functional, β-CuGaO2 is ferroelectric with a remarkably large remanent polarization of 83.80 μC/cm2, even exceeding that of the prototypic FPV material, BiFeO3. Quantitative theoretical analysis further indicates the asymmetric Ga 3-O 2 hybridization as the origin of the Pna21 ferroelectricity. In addition to the large displacive polarization, unusually small band gap (1.47 eV) and resultantly strong optical absorptions additionally differentiate β