Session B7: Focus Session: Computational Design of Materials - Nanostructured and Energy Materials

The Harvard Clean Energy Project: High-throughput screening of organic photovoltaic materials using first-principles electronic structure theory

We present the Harvard Clean Energy Project (CEP) which is concerned with the computational screening and design of new organic photovoltaic materials. CEP has established an automated, high-throughput, in silico framework to study millions of potential candidate structures. This presentation discusses the CEP branch which employs first-principles computational quantum chemistry for the characterization of molecular motifs and the assessment of their quality with respect to applications as electronic materials. In addition to finding specific structures with certain properties, it is the goal of CEP to illuminate and understand the structure-property relations in the domain of organic electronics. Such insights can open the door to a rational, systematic, and accelerated development of future high-performance materials. CEP is a large-scale investigation which utilizes the massive computational resource of IBM's World Community Grid. In this context, it is deployed as a screensaver application harvesting idle computing time on donor machines. This cyberinfrastructure paradigm has already allowed us to characterize 3.5 million molecules of interest in about 50 million DFT calculations.   

The Harvard Clean Energy Project: High-throughput screening of organic photovoltaic materials using cheminformatics, machine learning, and pattern recognition

Organic photovoltaic devices have emerged as competitors to silicon-based solar cells, currently reaching efficiencies of over 9\% and offering desirable properties for manufacturing and installation. We study conjugated donor polymers for high-efficiency bulk-heterojunction photovoltaic devices with a molecular library motivated by experimental feasibility. We use quantum mechanics and a distributed computing approach to explore this vast molecular space. We will detail the screening approach starting from the generation of the molecular library, which can be easily extended to other kinds of molecular systems. We will describe the screening method for these materials which ranges from descriptor models, ubiquitous in the drug discovery community, to eventually reaching first principles quantum chemistry methods. We will present results on the statistical analysis, based principally on machine learning, specifically partial least squares and Gaussian processes. Alongside, clustering methods and the use of the hypergeometric distribution reveal moieties important for the donor materials and allow us to quantify structure-property relationships. These efforts enable us to accelerate materials discovery in organic photovoltaics through our collaboration with experimental groups.   

Thermodynamic Stability of Semiconductors for Photocatalytic Water Splitting

Band structure engineering design of the light-absorbing semiconductors for water splitting has attracted wide attention recently. One of such design is to use the Z-scheme where a photocathode is connected with photoanode to reduce (generate H2) and oxidize (generate O2) water respectively. This requires the conduction band of photocathode and valence band of photoanode to straddle the redox levels of water. However, equally important in this design is the thermodynamic stability of the semiconductors in the aqueous solution upon illumination, i.e., the semiconductors may be oxidized (or reduced) before the water is oxidized (or reduced), causing the corrosion of the photoanode (photocathode). We will present our theoretical study on the thermodynamic stability of a series of photocatalytic semiconductors, including metal oxides, sulfides and nitrides, through the combination of phenomenological models for the semiconductor corrosion and the first-principles total energy and band alignment calculations. We find that almost all sulfides and nitrides are unstable as photoanode, while most of oxides are stable. This limits the choice of the photoanode materials for oxygen evolution. In contrast, for photocathode, most of the considered semiconductors are stable and resistant to reduction, indicating a much wider choice of the photocathode materials for hydrogen evolution.   

Rational design of competitive electrocatalysts for the oxygen reduction reaction in hydrogen fuel cells

The large-scale application of one of the most promising clean and renewable sources of energy, hydrogen fuel cells, still awaits efficient \textit{and} cost-effective~ electrocatalysts for the oxygen reduction reaction (ORR) occurring on the cathode. We demonstrate that truly rational design renders electrocatalysts possessing both qualities. By unifying the knowledge on surface morphology, composition, electronic structure and reactivity, we solve that sandwich-like structures are an excellent choice for optimization. Their constituting species couple synergistically yielding reaction-environment stability, cost-effectiveness and tunable reactivity. This cooperative-action concept enabled us to predict two advantageous ORR electrocatalysts. Density functional theory calculations of the reaction free-energy diagrams confirm that these materials are more active toward ORR than the so far best Pt-based catalysts. Our designing concept advances also a general approach for engineering materials in heterogeneous catalysis.   

First principles design of the Pd/Co/Pd sandwich-like structure as a promising electrocatalyst for the oxygen reduction reaction on hydrogen fuel cell cathode

In the search of Pt free catalytic materials, experiments have shown an enhancement in the catalytic activity of Pd-Co alloys over Pd surfaces, comparable to the one obtained with Pt, as well as a Pd enrichment of the topmost layers. In this work we present the rational design of a new catalyst material towards the oxygen reduction reaction (ORR) consisting on Pd(111) surface sandwiched with Co as the second layer (Pd/Co/Pd). The calculated reaction free energy diagrams confirm that the proposed sandwich-like structure is highly active towards ORR. The higher catalytic activity of the Pd/Co/Pd system is traced to the change in the electronic local density of states of the Pd surface atoms. Namely the hybridization of the Pd d-states with the Co majority-spin band causes a low-energy shift of the Pd d-band. This results in a reduction of surface reactivity which is favorable for ORR. We have also studied form first principles the stability of the system and evaluated the dissolution potential of Pd in Pd/Co/Pd. The results suggest that the system will be stable in the reaction environment.   

Computational Design of Co-Doping Method for Indium-Reduced Chalcopyrite-Type Photovoltaic Materials

Chalcopyrite-type semiconductor CuInSe$_{2}$ (CIS) is one of the most promising materials for low cost photovoltaic solar-cells. However, from the point of resource security, high concentration of In in CIS is serious disadvantage. In this paper, we propose co-doping method to reduce the concentration of In in CIS-based photovoltaic materials, i.e., 2In are replaced by Zn and Sn. According to the electronic structure calculations by the KKR-CPA-LDA [1] with the self-interaction correction [2], the substitution of Zn and Sn for In does not alter the electronic structure of CIS so much. We extend our co-doping method to enhance the efficiency of solar energy conversion. In addition to Zn+Sn co-doping, we introduce S impurities at Se sites. Due to the phase separation it is found that nano-structures with high concentration of S are self-organized under the layer-by-layer crystal growth condition. Since type-II band alignment is expected between Cu(Zn, Sn)Se$_{2}$ and Cu(Zn, Sn)S$_{2}$, we can expect efficient electron-hole separation in decomposed Cu(Zn, Sn)(Se, S)$_{2}$ [3]. \\[4pt] [1] H. Akai, http://sham.phys.sci.osaka-u.ac.jp/kkr/ \\[0pt] [2] A. Filippetti and N. A. Spaldin, Phys. Rev. B 67 (2003) 125109.\\[0pt] [3] Y. Tani et al., Appl. Phys. Express 3 (2010) 101201.   

Effect of carbon and nitrogen doping on the structure of amorphous GeTe phase change material

Carbon and Nitrogen-doped GeTe are promising materials for use in phase change memories since the addition of C or N increases the stability of the amorphous phase. By combining ab initio molecular dynamics and X-ray scattering experiments, we show that carbon deeply modifies the structure of the amorphous phase through long carbon chains, tetrahedral and triangular units centred on carbon. A clear signature of these units is the appearance of an additional interatomic distance around 3.3 A in the pair correlation function. Besides, the first Ge-Ge and Ge-Te distances are almost not affected by doping. The implications for the vibrational and thermal properties are finally discussed.   

Towards Efficient Solar Cells: Optimizing Non-Equilibrium Hyperdoping Methods

Electrical and optical properties of semiconductors are mainly controlled by the concentration of dopants. While the highest dopant concentrations reachable in most traditional doping methods are limited by the equilibrium solubility of the dopant in the pure semiconductor material, much higher concentrations are observed after femtosecond laser treatment of silicon in a sulfur containing atmosphere. Due to altered optical properties \emph{laser-hyperdoped} silicon is considered a promising material for next generation photovoltaic cells. To control the dopant concentration distribution and thereby tune physical properties of the material, we apply advanced adjoint based optimization techniques to the models describing the laser induced melting and diffusion processes. This allows to determine optimal process protocols generating a desired concentration distribution. Applying advanced PDE-contrained optimization techniques to laser hyperdoping thereby opens new avenues for improving the efficiency of photovoltaic cells.   

First-Principles Materials Design of Chalcopyrite-Type Photovoltaic Materials with Self-Organized Nano-Structures

Cu(In, Ga)Se$_{2}$ (CIGS) is a chalcopyrite-type semiconductor and one of the most promising materials for low cost photovoltaic solar-cells. In this paper, based on first-principles calculations, we propose that spinodal decomposition will enhance the conversion efficiency in CIGS. Our calculations are based on the KKR-CPA-LDA [1] with the self-interaction correction [2]. From the calculated mixing energy of CIGS, it is found that the system favors the spinodal decomposition. We also perform Monte Carlo simulations and find that quasi-one-dimensional nano-structures with high concentration of impurities are formed under the layer-by-layer crystal growth condition in CIGS [3]. It is expected that the photo-generated electron-hole pairs are efficiently separated by the type-II interface and then effectively transferred along the quasi-one-dimensional structures in CIGS. Moreover, we can expect multiplication of generated carriers due to the multi-exciton effects in nano-structures [3]. \\[4pt] [1] H. Akai, http://sham.phys.sci.osaka-u.ac.jp/kkr/ \\[0pt] [2] A. Filippetti and N. A. Spaldin, Phys. Rev. B 67 (2003) 125109.\\[0pt] [3] Y. Tani et al., Appl. Phys. Express 3 (2010) 101201.   

Computational Nano-materials Design of Self-Organized Cd(Te,S) and Cd(Te,Se)Type II Nanowire (Konbu-Phase) by Spinodal Nano-Decomposition for High-Efficiency Photovoltaic Solar-Cells

Based on multi-scale simulations combined ab initio electronic structure calculation (KKR-CPA) and Monte Carlo simulation (MCS) of the two-dimensional layer-by-layer crystal growth, we have designed the self-organized quasi-one-dimensional nano-structures (Konbu-Phase) fabricated by two-dimensional spinodal nano-decomposition for high-efficiency photovoltaic solar cells (PVSCs) in Cd(Te$_{1-x}$S$_{x}$), and Cd(Te$_{1-x}$Se$_{x}$). The Konbu-Phase enhances the nano-scale electron-hole separation in PVSCs due to their Type II band alignment. The Konbu-Phase also increases the efficiency of PVSCs by multi-exciton formation using the inverse Auger effect in the self-organized quasi-one-dimensional nanostructures. We also discuss how to fabricate Konbu-Phase starting from the uniform nano-particles made by the photo-chemical reactions. Reference: M. Oshitani, K. Sato, H. Katayama-Yoshida, Applied Physics Express 4 (2011) 022302.   

Half-Heusler semiconductors as piezoelectrics

We use a first-principles rational-design approach to demonstrate the potential of semiconducting half-Heusler compounds as a previously-unrecognized class of piezoelectric materials. We scan a large number of compounds, testing for insulating character and calculating structural, dielectric, and piezoelectric properties. Of the 792 compounds considered, 234 are found to be nonmetallic, of which 189 are further found to be elastically stable. We compare the computed structural parameters to available experimental values for the half-Heusler compounds considered that have been experimentally studied, as reported in the Inorganic Crystal Structure Database. Calculated piezoelectric coefficients ($d_{14}$) and electromechanical coupling factors ($k_{14}$) are often high enough to compare favorably with those of piezoelectrics currently in use. We analyze how factors such as electronegativity and ionic radius influence the piezoelectricity of the compound. Moreover, we show that even if toxic or expensive elements are excluded, we are still left with many combinations having reasonably high piezoelectric response. Our results provide guidance for the experimental realization and characterization of high-performance materials of this class that may find practical applications.   

Oxygen transport in ceria: a first-principles study

Ceria (CeO2) is an important material for environmentally benign applications, ranging from solid-oxide fuel cells (SOFC) to oxygen storage [1-2]. The key characteristic needed to be improved is the mobility of oxygen ions. Optimization of ionic transport in ceria has been the topic of many studies. In particular, it has been discovered how the ionic conductivity in ceria might be improved by choosing the proper kind and concentration of dopants [3]. In this presentation we will approach the problem from a different direction by adjusting structural parameters of ceria via the change of external conditions. A systematic first-principles study of the energy landscape and kinetics of reduced ceria as a function of external parameters reveals a physically transparent way to improve oxygen transport in ceria. \\[4pt] [1] N. Skorodumova, S. Simak, B. Lundqvist, I. Abrikosov, and B. Johansson, Physical Review Letters 89, 14 (2002). \\[0pt] [2] A. Trovarelli, in Catalysis by Ceria and related materials (Imperial College Press, London, 2002). \\[0pt] [3] D. A. Andersson, S. I. Simak, N. V. Skorodumova, I. A.Abrikosov, and B. Johansson, Proceedings of the National Academy of Sciences of the United States of America 103, 3518 (2006).   

First Principles Optical Absorption Spectra of Organic Molecules Adsorbed on Titania Nanoparticles

We present results from first principles computations on passivated rutile TiO$_2$ nanoparticles in both free-standing and dye-sensitized configurations to investigate the size dependence of their optical absorption spectra. The computations are performed using time-dependent density functional theory (TDDFT) as well as GW-Bethe-Salpeter-Equation (GWBSE) methods and compared with each other. We interpret the first principles spectra for free-standing TiO$_2$ nanoparticles within the framework of the classical Mie-Gans theory using the bulk dielectric function of TiO$_2$. We investigate the effects of the titania support on the absorption spectra of a particular set of perylene-diimide (PDI) derived dye molecules, namely brominated PDI (Br$_2$C$_{24}$H$_8$N$_2$O$_4$) and its glycine and aspartine derivatives.   

Stoichiometric anti-phase boundaries in monolayer boron-nitride

We propose a stoichiometric structure for domain boundaries in monolayer boron nitride, on the basis of the seeds of extended topological defect lines that have been observed in graphene under electronic irradiation [1]. The structure consists of a periodic extended line defect with an atomic structure consisting of alternating fourfold and eightfold BN rings. The structure is shown to be lower in energy than non-stoichiometric counterparts, and to lead to the formation of shallow electron and hole bands inside the band gap of the BN bulk matrix. We suggest that the existence of such defect bands may be experimentally observed in optical experiments. \\[4pt] [1] J. Kotakoski, A. V. Krasheninnikov, and J. C. Mayer, Physical Review Letters, 106, 105505 (2011).   

Understanding Heat Dissipation in Carbon Nanotube Resonators: New Insights from Molecular Dynamics Simulation

Dissipation in carbon nanotube (CNT) resonators under conditions of steady state continuous driving has been simulated within classical molecular dynamics (MD) making use of a newly developed ``Phonostat'' algorithm. A previous study on heat flow in CNT resonators using MD simulation showed an anomalous heat dissipation and gateway kind of behavior was proposed. Here, we focus on characterizing the ``gateway'' modes and identify the pathways of heat dissipation by clamping such modes using our phonostat algorithm. In this present study we see three or four different sets of heat flow behavior when different sets of gateway modes are clamped as against a single heat flow behavior presented in the previous study. Our new results are explained in terms of filling of different subsets of phonon modes and the magnitude of friction between them. Our simulation results show that controlling these gateway modes is the key to improve the quality factor (Q) of CNT resonators for sensing applications, which has been a major challenge till now.