Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H32: Theoretical Modeling of Materials for Solar Energy ConversionFocus
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Sponsoring Units: DCP Chair: Giulia Galli, University of Chicago Room: 332 |
Tuesday, March 15, 2016 2:30PM - 3:06PM |
H32.00001: Predicting materials for solar energy conversion: ab-initio spectroscopy of heterogeneous interfaces Invited Speaker: Giulia Galli We will discuss some progress in predicting materials for solar energy conversion using ab initio calculations, in particular we will focus on heterogeneous interfaces between photo-electrodes and water and between nanocomposites. We will also address the problem of building much needed tighter connections between computational and laboratory experiments. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:18PM |
H32.00002: Ultrafast carrier dynamics in BiVO$_{\mathrm{4}}$ thin film photoanode material: time-resolved THz spectroscopic study. Wesley Belleman, L. Zhou, B. Giri, B.J. Dringoli, P.M. Rao, L.V. Titova Recent demonstrations of 3{\%} solar conversion efficiency in thin film BiVO$_{\mathrm{4}}$ make it a promising photoanode material for photoelectrochemical water oxidation [1]. With a bandgap of 2.4 eV, it strongly absorbs UV and visible light up to 520 nm. However, its efficiency is limited by extremely poor carrier mobility, with values from 0.01 to 1 cm$^{\mathrm{2}}$ /Vs reported in the literature, and often attributed to formation of small polarons [2]. The precise nature of conductivity in BiVO$_{\mathrm{4}}$ is, however, not well-established. We use time-resolved terahertz (THz) spectroscopy as a non-contact probe of microscopic photoconductivity of a 100 nm-thick BiVO$_{\mathrm{4}}$ film. THz spectroscopy allows probing the dynamics of photo-injected carriers over nanometer length scales, and thus provides insight about transport of carriers inside the 100-200 nm grains. We find that intra-grain mobility may be as much as several orders of magnitude higher than macroscopic mobility that is affected by the grain boundaries. References [1] P. M. Rao et al., Nano Lett. 14, 1099 (2014) [2] A.J.E. Rettie et al., Appl. Phys. Lett. 106, 022106 (2015). [Preview Abstract] |
Tuesday, March 15, 2016 3:18PM - 3:30PM |
H32.00003: Emerging materials with novel electronic properties for solar light harvesting and conversion Iffat Nayyar, Tiffany Kaspar, Martin Mcbriarty, Sara Chamberlin, Niranjan Govind, Scott Chambers, Peter Sushko The optical absorption and electronic transport in complex oxides can be tuned by judicious selection of the lattice structure and control of chemical composition and prevalent oxidation state of the transition metal species. Optical spectra for solid solutions of metal oxides are complex; we focus on revealing the electronic structure and orbital nature of the transitions, which is crucial for rational materials design. We applied state-of-the-art \textit{ab initio} methods, including time-dependent DFT, to screen thermodynamically favorable configurations and determine the effects of the local environment on the transition energies and relative intensities. Here we focus on the magnetite-type Fe$_{\mathrm{3-x}}$Cr$_{\mathrm{x}}$O$_{\mathrm{4\thinspace }}$mixed spinel solid-solutions, which can only be doped to x $\le $ 2, since Cr strongly prefers to occupy octahedral sites. In the interval of 0 $\le $ x $\le $ 2, the electronic structure of Fe$_{\mathrm{3-x}}$Cr$_{\mathrm{x}}$O$_{\mathrm{4}}$ undergoes transformations resulting in several qualitatively different types of optical transitions sensitive to the Cr concentration. We find the lowest band gap and high room-temperature conductivity, in$_{\mathrm{\thinspace }}$agreement with experiment, for x$=$1 (Fe$_{\mathrm{2}}$CrO$_{\mathrm{4}})$, and attribute this conductivity to the thermally-driven electron hopping between the octahedral-site Fe$^{\mathrm{2+}}$ and tetrahedral-site Fe$^{\mathrm{3+}}$. We compare Fe$_{\mathrm{3-x}}$Cr$_{\mathrm{x}}$O$_{\mathrm{4}}$ with the corundum $\alpha $-(Fe$_{\mathrm{1-x}}$Cr$_{\mathrm{x}})_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$and $\alpha $-(Fe$_{\mathrm{1-x}}$V$_{\mathrm{x}})_{\mathrm{2}}$O$_{\mathrm{3}}$, where electron transfer from the Cr and V 3$d$ to the unoccupied Fe 3$d^{\mathrm{\ast }}$ orbitals reduces the band gap to 1.6 and 0.6 eV (from 2.1 eV in $\alpha $-Fe$_{\mathrm{2}}$O$_{\mathrm{3}})$. [Preview Abstract] |
Tuesday, March 15, 2016 3:30PM - 3:42PM |
H32.00004: New quaternary semiconductor Cu$_{\mathrm{2}}$MgSnS$_{\mathrm{4}}$ and Cu$_{\mathrm{2}}$MgSnSe$_{\mathrm{4}}$ for photovoltaics Kinfai Tse, Guohua Zhong, Yiou Zhang, Xiaoguang Li, Chunlei Yang, Junyi Zhu, Zhi Zeng, Zhenyu Zhang, Xudong Xiao Element substitution of Zn by Mg and Ca is attempted to overcome the problem of potential fluctuation in Cu$_{\mathrm{2}}$ZnSnS$_{\mathrm{4}}$ and Cu$_{\mathrm{2}}$ZnSnSe$_{\mathrm{4}}$ (CZTSSe) due to prevalence of Cu$_{\mathrm{Zn}}+$Zn$_{\mathrm{Cu}}$ defect complex. Through density function theory calculation with hybrid functional, we have shown that Cu$_{\mathrm{2}}$MgSnS$_{\mathrm{4}}$ and Cu$_{\mathrm{2}}$MgSnSe$_{\mathrm{4}}$ (CMTSSe) are stable with respect to secondary phases considered under suitable chemical potential. Stannite CMTSSe is thermodynamically more favorable over the kesterite structure. The alternating Cu and Mg/Sn cation layer of stannite structure may suppress the formation of Mg$_{\mathrm{Cu}}$ antisite due to large stress induced. The electronic and optical properties of CMTSSe are similar to that of CZTSSe with comparable absorption coefficient at the band-edge suggests CMTSSe to be a promising photovoltaic material. [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 3:54PM |
H32.00005: Controlling defects and secondary phases of CZTS by surfactant Potassium Junyi Zhu, Yiou Zhang, Kinfai Tse, Xudong Xiao Cu2ZnSnS4 (CZTS) is a promising photovoltaic absorber material with earth abundant and nontoxic elements. However, the detrimental native defects and secondary phases of CSTS will largely reduce the energy conversion efficiencies. To understand the origin of these problems during the growth of CZTS, we investigated the kinetic processes on CZTS (-1-1-2) surface, using first principles calculations. A surface Zn atom was found to occupy the subsurface Cu site easily due to a low reaction barrier, which may lead to a high ZnCu concentration and a secondary phase of ZnS. These n-type defects may create deep electron traps near the interface and become detrimental to device performance. To reduce the population of ZnCu and the secondary phase, we propose to use K as a surfactant to alter surface kinetic processes. Improvements on crystal quality and device performance based on this surfactant are consistent with early experimental observations. [Preview Abstract] |
Tuesday, March 15, 2016 3:54PM - 4:06PM |
H32.00006: Metal Disorder in Cu$_2$ZnSnS$_4$ (CZTS) Solar Cells from Multi-Scale Simulations Suzanne Wallace, Jarvist Frost, Aron Walsh Kesterite-structured Cu$_{2}$ZnSnS$_{4}$ (CZTS) is a promising earth-abundant and non-toxic material for the active layer of thin-film solar cells due to its high optical absorption coefficient of $>10^4$ cm$^{-1}$ and sunlight matched band gap of 1.5 eV. Device efficiencies are hampered by low open circuit voltage (V$_{OC}$) compared to the optical band gap. One possible origin of this is disorder amongst the Cu and Zn ions. Such disorder could lead to sub band-gap recombination centres due to fluctuations in electrostatic potential from the presence of charged defects. Understanding the origin of these sub-gap states, and the resulting impediment on device performance, is essential to discover design and processing rules for high efficiency kesterite, and other multi-component semiconductor, devices. We investigate this by writing custom Monte-Carlo codes to simulate the on-lattice disorder. A generalised Ising Hamiltonian is parameterised with hybrid density functional theory (DFT) total-energy calculations on defect pairs. The resulting disorder is simulated as a function of temperature, and the order-disorder behaviour and resulting local and long-range electrostatic potential variation due to Cu-Zn disorder is quantified. [Preview Abstract] |
Tuesday, March 15, 2016 4:06PM - 4:18PM |
H32.00007: First principles investigations on the stability and catalytic reactivity of Cu2O surfaces Liang Li, Yimin Wu, Tijana Rajh, Ian McNulty, Zhonghou Cai, Jeff Guest, Yuzi Liu, Maria Chan Cu$_{\mathrm{2}}$O is an attractive candidate as a next-generation photocatalyst for CO$_{\mathrm{2}}$ reduction because of its high solar spectrum absorption coefficient and small electron affinity. It is observed experimentally, by Electron Paramagnetic Resonance (EPR) and Scanning x-ray fluorescence microscopy (SXFM), that the surface Cu atoms have various oxidation states, and different sites have different affinities for CO$_{\mathrm{2}}$ and intermediate products. In this work, we employ first principles density functional theory (DFT) calculations to calculate the free energies of various low-index Cu$_{\mathrm{2}}$O surfaces and further identify the change of surface Cu oxidation states upon the creation of surface defects and during the photocatalytic process. The reactivity of Cu$_{\mathrm{2}}$O surfaces with various defect types and concentrations are also predicted. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H32.00008: \textit{Ab initio }electron paramagnetic resonance study of 3C-SiC/SiO$_{\mathrm{2}}$ interfaces in SiC-nanofiber based solar cells Taufik Adi Nugraha, Uwe Gerstmann, Wolfgang Gero Schmidt, Stefan Wippermann Semiconducting nanocomposites, e. g. hybrid materials based on inorganic semiconducting 3C-SiC nanofibers and organic surfactants, provide genuinely novel pathways to exceed the Shockley-Queisser limit for solar energy conversion. The synthesis of such functionalized fibers can be performed completely using only inexpensive wet chemical solution processing. During synthesis a thin passivation layer is introduced between the SiC-fiber and surfactants, e. g. the native oxide, whose atomistic details are poorly understood. In this study, we utilize unpaired spins in interfacial defects to probe the local chemical environment with \textit{ab initio} EPR (Electron Paramagnetic Resonance) calculations, which can be directly compared to experiment. Considering a wide variety of possible interfacial structures, a grand canonical approach is used to generate a phase diagram of the 3C-SiC/SiO$_{\mathrm{2}}$ interface as a function of the chemical potentials of Si, O and H, to provide favorable interfacial structures for g-tensor calculations. This study provides directions about specific types of interfacial defects and their impact on the electronic properties of the interface. The authors wish to thank S. Greulich-Weber for helpful discussions. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H32.00009: Porphyrin-based polymeric nanostructures for light harvesting applications: Ab initio calculations Walter Orellana The capture and conversion of solar energy into electricity is one of the most important challenges to the sustainable development of mankind. Among the large variety of materials available for this purpose, porphyrins concentrate great attention due to their well-known absorption properties in the visible range. However, extended materials like polymers with similar absorption properties are highly desirable. In this work, we investigate the stability, electronic and optical properties of polymeric nanostructures based on free-base porphyrins and phthalocyanines (H$_2$P, H$_2$Pc), within the framework of the time-dependent density functional perturbation theory. The aim of this work is the stability, electronic, and optical characterization of polymeric sheets and nanotubes obtained from H$_2$P and H$_2$Pc monomers. Our results show that H$_2$P and H$_2$Pc sheets exhibit absorption bands between 350 and 400 nm, slightly different that the isolated molecules. However, the H$_2$P and H$_2$Pc nanotubes exhibit a wide absorption in the visible and near-UV range, with larger peaks at 600 and 700 nm, respectively, suggesting good characteristic for light harvesting. The stability and absorption properties of similar structures obtained from ZnP and ZnPc molecules is also discussed. [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H32.00010: Fundamental Insights into Aqueous Electrochemical Reduction of CO$_{2}$ on the Ligand-Protected Charged Au$_{25}$ Clusters Dominic Alfonso, Douglas Kauffman, Christopher Matranga Recent breakthroughs in electrochemical studies in our group showed aqueous CO$_{2}$ reduction to CO on atomically precise, inherently charged Au$_{25}$ clusters occurring at low overpotentials. Using first-principles density functional theory and continuum solvation models, the role of the cluster in the reduction process was examined. Free energies of species that were proposed as intermediates in its mechanism were determined. Contrary to previous assumptions, our results show that the fully ligand protected version of the cluster can be ruled out as an active participant. In particular, COOH species on the intact cluster should not be expected to form unless very high potentials are applied. Instead, the calculations suggest that the reduction process would likely occur on a dethiolated gold site. These findings point to the crucial role of such reaction center on the Au$_{25}$ clusters in facilitating the CO$_{2}$ conversion via the formation of low energy COOH intermediates. [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H32.00011: First principle simulations of a bias-dependent electrochemical cell Luana Pedroza, Pedro Brandimarte, Marivi Fernandez-Serra, Alexandre R. Rocha Understanding the local structure of water molecules at the interfaces of metallic electrodes is a key problem in many electrochemical problems. Notably the system is under an external potential bias, which makes the task of simulating this setup difficult. To correctly compute the effect of an external bias potential applied to electrodes, we combine density functional theory and non-equilibrium Green's functions methods, with and without van der Waals interactions. In this work, we apply this methodology to study the electronic properties and forces of water molecules at the interface of different metallic electrodes. We find that the water molecule is sensitive to the sign and magnitude of the applied bias. We also show that it changes the position and orientation of the most stable configuration indicating that the external bias plays an important role in the structural properties of the interface. [Preview Abstract] |
Tuesday, March 15, 2016 5:06PM - 5:18PM |
H32.00012: Photoemission spectra of aqueous solutions of salts from many-body perturbation theory Alex P. Gaiduk, Jonathan H. Skone, Marco Govoni, Giulia Galli The computational design of electrode materials for energy conversion and storage processes requires an accurate description of the energy levels of the electrolyte and of electrolyte/electrode interfaces. Conventional density-functional approximations are in general not well suited for this task as they yield inaccurate orbital energies. Many-body perturbation theory (MBPT) predicts vertical ionization potentials and energy gaps in better agreement with experiments, providing the possibility for an accurate description of the electronic properties of electrolytes. We coupled \emph{ab initio} molecular dynamics [1] with MBPT calculations [2] to investigate the photoemission spectra of a 1 M aqueous solution of NaCl. For the first time we were able to determine the absolute positions of the spectra peaks, with excellent agreement with experiments for both the solute and solvent peak positions. The best results were obtained using wavefunctions obtained from dielectric-dependent [3] hybrid calculations as a starting point for MBPT. \\[8pt] [1]~A.~P. Gaiduk, C. Zhang, F. Gygi, G. Galli, \emph{Chem. Phys. Lett.} 604, 89 (2014); [2]~M. Govoni and G. Galli, \emph{J.~Chem.\ Theory Comput.} 11, 2680 (2015); [3]~J.~H. Skone, M. Govoni, G. Galli, \emph{Phys.\ Rev.~B} 89, 195112 (2014). [Preview Abstract] |
Tuesday, March 15, 2016 5:18PM - 5:30PM |
H32.00013: Coupled experimetal and theoretical study of photon absorption and charge transport in BiVO4 photoanodes for solar water splitting Yuan Ping, Tae Woo Kim, Giulia Galli, Kyoung-Shin Choi Bismuth vanadate (BiVO4) has been identified as one of the most promising photoanode materials for water-splitting photoelectrochemical cells. The major limitations of BiVO4 are its relatively wide bandgap (~2.5 eV) and low electron mobility (~0.2 cm-2V-2S-1), which limit its solar-to-hydrogen conversion efficiency. In this talk we will present the results of a coupled experimental and ab initio theoretical study showing that nitrogen doping together with extra oxygen vacancies lead to both a reduction of BiVO4 band gap and to an increase of the majority carrier density and mobility. In turn these improvements lead to the applied bias photon-to-current efficiency over 2\%, a record for a single oxide photon absorber, to the best of our knowledge[1]. The “codoping” method adopted in our work could also be applied to simultaneously enhance photon absorption and charge transport in other oxides, providing new possibilities for photocatalytic materials. [1] T. Kim, Y. Ping, G. Galli and K. Choi, Nature Communications, 6,8769, (2015). [Preview Abstract] |
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