Browsing by Author "Havu, Ville"

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  • Alkali Postdeposition Treatment-Induced Changes of the Chemical and Electronic Structure of Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers
    (2019-01-23) Malitckaya, Maria; Kunze, Thomas; Komsa, Hannu Pekka; Havu, Ville; Handick, Evelyn; Wilks, Regan G.; Bär, Marcus; Puska, Martti J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The effects of alkali postdeposition treatment (PDT) on the valence band structure of Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers are addressed from a first-principles perspective. In detail, experimentally derived hard X-ray photoelectron spectroscopy (HAXPES) data [ Handick, E.; et al. ACS Appl. Mater. Interfaces 2015, 7, 27414-27420 ] of the valence band structure of alkali-free and NaF/KF-PDT CIGSe are directly compared and fit by calculated density of states (DOS) of CuInSe2, its Cu-deficient counterpart CuIn5Se8, and different potentially formed secondary phases, such as KInSe2, InSe, and In2Se3. The DOSs are based on first-principles electronic structure calculations and weighted according to element-, symmetry-, and energy-dependent photoionization cross sections for the comparison to experimental data. The HAXPES spectra were recorded using photon energies ranging from 2 to 8 keV, allowing extraction of information from different sample depths. The analysis of the alkali-free CIGSe valence band structure reveals that it can best be described by a mixture of the DOS of CuInSe2 and CuIn5Se8, resulting in a stoichiometry slightly more Cu-rich than that of CuIn3Se5. The NaF/KF-PDT-induced changes in the HAXPES spectra for different alkali exposures are best reproduced by additional contributions from KInSe2, with some indications that the formation of a pronounced K-In-Se-type surface species might crucially depend on the amount of K available during PDT.
  • All-electron density functional theory and time-dependent density functional theory with high-order finite elements
    (2009) Lehtovaara, Lauri; Havu, Ville; Puska, Martti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We present for static density functional theory and time-dependent density functional theory calculations an all-electron method which employs high-order hierarchical finite-element bases. Our mesh generation scheme, in which structured atomic meshes are merged to an unstructured molecular mesh, allows a highly nonuniform discretization of the space. Thus it is possible to represent the core and valence states using the same discretization scheme, i.e., no pseudopotentials or similar treatments are required. The nonuniform discretization also allows the use of large simulation cells, and therefore avoids any boundary effects.
  • All-electron time-dependent density functional theory with finite elements: Time-propagation approach
    (2011) Lehtovaara, Lauri; Havu, Ville; Puska, Martti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We present an all-electron method for time-dependent density functional theory which employs hierarchical nonuniform finite-element bases and the time-propagation approach. The method is capable of treating linear and nonlinear response of valence and core electrons to an external field. We also introduce (i) a preconditioner for the propagation equation, (ii) a stable way to implement absorbing boundary conditions, and (iii) a new kind of absorbing boundary condition inspired by perfectly matched layers.
  • Analysis of reduced finite element schemes in parameter dependent elliptic problems
    (2001-12-14) Havu, Ville
     Doctoral dissertation (article-based)
    This thesis presents an analysis of modified finite element schemes applied to parameter dependent elliptic problems prone to locking. Two different problems of similar type are considered: the problem of anisotropic heat conduction and the thin shell problem.
  • Computational Screening of Catalysts in Oxygen Evolution Reaction
    (2023-05-16) Nummi, Juha
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Defects for catalysts for the oxygen evolution reaction
    (2023-10-02) Aaltio, Anton
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Dielectric preconditioner in density functional theory self-consistent field calculations
    (2018-02-13) Soikkeli, Ari-Pekka
     Perustieteiden korkeakoulu | Master's thesis
    To gain theoretical knowledge of the behavior of electrons, one must solve the Schrödinger equation. However, as the number of the electrons grows, this task becomes increasingly difficult. Density functional theory provides means to simplify the calculations. The theory relies on the Kohn-Sham equations which must be solved using iterative methods. To ensure the convergence of the iteration, mixing and preconditioning techniques often need to be employed. In this thesis, a new way of preconditioning is implemented as a part of an existing electronic structure simulation code package, the Fritz Haber Institute "ab initio molecular simulations". The preconditioner uses the static dielectric function of the system to speed up convergence. This implementation is tested against preexisting methods in a variety of test systems. In simple systems that contain only a few atoms the dielectric preconditioner managed to slightly decrease the amount of iterations required for the calculation to converge compared to other tested methods. However, the current implementation of the dielectric preconditioner was discovered not to improve convergence in periodic systems. As an additional setback, the increased computational complexity of calculating the dielectric function outweighs the time saved by reducing the number of iterations. Nevertheless, the preconditioner did perform well in a metallic system, which are known to be challenging to calculate.
  • Efficient all-electron hybrid density functionals for atomistic simulations beyond 10 000 atoms
    (2024-07-14) Kokott, Sebastian; Merz, Florian; Yao, Yi; Carbogno, Christian; Rossi, Mariana; Havu, Ville; Rampp, Markus; Scheffler, Matthias; Blum, Volker
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Hybrid density functional approximations (DFAs) offer compelling accuracy for ab initio electronic-structure simulations of molecules, nanosystems, and bulk materials, addressing some deficiencies of computationally cheaper, frequently used semilocal DFAs. However, the computational bottleneck of hybrid DFAs is the evaluation of the non-local exact exchange contribution, which is the limiting factor for the application of the method for large-scale simulations. In this work, we present a drastically optimized resolution-of-identity-based real-space implementation of the exact exchange evaluation for both non-periodic and periodic boundary conditions in the all-electron code FHI-aims, targeting high-performance central processing unit (CPU) compute clusters. The introduction of several new refined message passing interface (MPI) parallelization layers and shared memory arrays according to the MPI-3 standard were the key components of the optimization. We demonstrate significant improvements of memory and performance efficiency, scalability, and workload distribution, extending the reach of hybrid DFAs to simulation sizes beyond ten thousand atoms. In addition, we also compare the runtime performance of the PBE, HSE06, and PBE0 functionals. As a necessary byproduct of this work, other code parts in FHI-aims have been optimized as well, e.g., the computation of the Hartree potential and the evaluation of the force and stress components. We benchmark the performance and scaling of the hybrid DFA-based simulations for a broad range of chemical systems, including hybrid organic-inorganic perovskites, organic crystals, and ice crystals with up to 30 576 atoms (101 920 electrons described by 244 608 basis functions).
  • Ferrofluidic Manipulator : Theoretical Model for Single-Particle Velocity
    (2023-10-01) Cenev, Zoran M.; Havu, Ville; Timonen, Jaakko V.I.; Zhou, Quan
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Theoretical models have crucial importance in the design of algorithms for feedback control of robotic micromanipulation platforms. More importantly, theoretical models provide an understanding of the limits of each manipulation approach along with the identification of the key parameters that influence motion performance. Here, we provide a mathematical framework and numerical implementation of the velocity field for a single diamagnetic particle pinned at the air–ferrofluid interface when it is actuated by one or more inclined electromagnets in a system previously introduced as the ferrofluidic manipulator (Cenev et al., 2021). The theoretical model uses a magnetic dipole approximation for a dipole located between the tip of a solenoid and the end of the coil. The model reveals that the forces due to gravitation, the applied magnetic field, and the capillary action have decreasing contributions to the overall velocity field, respectively. The model assumes overdamped dynamics, and therefore, it is time-independent. The model is valid for an infinite number of electromagnetic solenoids. The theoretical predictions are in good agreement with the estimations from experimental data realized for one and two actuated solenoids.
  • First-principles modeling of alkali metal post deposition treatment effects in CIGS solar cells
    (2017) Fedina, Maria; Komsa, Hannu-Pekka; Havu, Ville; Puska, Martti J.
     A4 Artikkeli konferenssijulkaisussa
    The efficiencies of Cu(In, Ga)Se-2(CIGS) solar cells have increased very fast, thanks the alkali post deposition treatment (PDT). In the present work, we have considered the role of alkali metal atoms in the efficiency enhancement. First, we have investigated the effect of alkali metal atoms in the bulk CuInSe2(CIS) absorber and at the grain boundaries in terms of formation and migration energies and charge transition levels. We found that the copper sublattice is the most preferable for all the alkali metal atoms. A detailed comparison between different alkali metal atoms, with respect to the behavior at grain boundaries and in grain interiors has been done. Moreover, we have studied how alkali metal atoms contribute in the formation of the secondary phases. The secondary phase formation during the PDT process has been suggested on the basis of calculated reaction enthalpies.
  • First-Principles Modeling of Point Defects and Complexes in Thin-Film Solar-Cell Absorber CuInSe2
    (2017) Malitckaya, Maria; Komsa, Hannu-Pekka; Havu, Ville; Puska, Martti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Point defects and complexes may affect significantly physical, optical, and electrical properties of semiconductors. The Cu(In,Ga)Se2 (CIGSe) alloy is an absorber material for low-cost thin-film solar cells. Several recently published computational investigations show contradicting results for important point defects such as copper antisite substituting indium (CuIn), indium vacancy (VIn), and complexes of point defects in CuInSe2. In the present work we study effects of the most important computational parameters especially on the formation energies of point defects. Moreover, related to defect identification by the help of their calculated properties we discuss possible explanations for the three acceptors, which occur in photoluminescence measurements of Cu-rich samples. [S. Siebentritt et al., Progress in Photovoltaics: Research and Applications 2010, 18, 390, S. Siebentritt et al., Physica Status Solidi (c) 2004, 1, 2304.] Finally, new insight into comparison between theoretical and experimental results is presented in the case of varying chemical potentials and of formation of secondary phases.
  • First-Principles Modeling of Point Defects and Complexes in Thin-Film Solar-Cell Absorber CuInSe2
    (2017-06-01) Malitckaya, Maria; Komsa, Hannu Pekka; Havu, Ville; Puska, Martti J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Point defects and complexes may affect significantly physical, optical, and electrical properties of semiconductors. The Cu(In,Ga)Se2 alloy is an absorber material for low-cost thin-film solar cells. Several recently published computational investigations show contradicting results for important point defects such as copper antisite substituting indium (CuIn), indium vacancy (VIn), and complexes of point defects in CuInSe2. In the present work effects of the most important computational parameters are studied especially on the formation energies of point defects. Moreover, related to defect identification by the help of their calculated properties possible explanations are discussed for the three acceptors, occuring in photoluminescence measurements of Cu-rich samples. Finally, new insight into comparison between theoretical and experimental results is presented in the case of varying chemical potentials and of formation of secondary phases.
  • Heavy Alkali Treatment of Cu(In,Ga)Se2 Solar Cells: Surface versus Bulk Effects
    (2020-02) Siebentritt, Susanne; Avancini, Enrico; Bär, Marcus; Bombsch, Jakob; Bourgeois, Emilie; Buecheler, Stephan; Carron, Romain; Castro, Celia; Duguay, Sebastien; Félix, Roberto; Handick, Evelyn; Hariskos, Dimitrios; Havu, Ville; Jackson, Philip; Komsa, Hannu Pekka; Kunze, Thomas; Malitckaya, Maria; Menozzi, Roberto; Nesladek, Milos; Nicoara, Nicoleta; Puska, Martti; Raghuwanshi, Mohit; Pareige, Philippe; Sadewasser, Sascha; Sozzi, Giovanna; Tiwari, Ayodhya Nath; Ueda, Shigenori; Vilalta-Clemente, Arantxa; Weiss, Thomas Paul; Werner, Florian; Wilks, Regan G.; Witte, Wolfram; Wolter, Max Hilaire
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post-deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open-circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single-phase Cu-alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.
  • Improving the Kerker preconditioner for self-consistent eld calculations
    (2013) Saarenpää, Jani
     School of Science | Master's thesis
    In this thesis the convergence of electron density within the density functional theory is studied and possibilities to speed it up are researched. Because the density functional theory leads to non-linear equations the calculations must be done iteratively. To be able to get an electron density for as many atomic configuration as feasible it is important to minimize the number of iterations needed for convergence. One way to reduce the number of iterations is to use the density calculated from earlier iterations. This is called \mixing". Also for metallic systems the density should be "preconditioned" before using it to the next iteration to reduce the number of iterations. In this thesis the Fritz Haber Institute "ab initio molecular simulations" is used as a starting point. It contains an implementation for Pulay mixing and Kerker preconditioning. In most cases these algorithms are enough the get the system to converge in reasonable time. Some systems on the other hand won't converge easily or at all even with Pulay mixing and Kerker preconditioning. One such case is a periodic metallic system where the one periodic dimension is much longer compared with the other two. Also cases that contain a sizable metallic part with a sizeable non-metallic part are hard to converge. To speed up the convergence in the _rst case the isotropic preconditioner is modified into an anisotropic preconditioner. Another way to speed the convergence is to use a better approximation than Thomas-Fermi dielectric function in Kerker preconditioner [1].
  • Large-scale surface reconstruction energetics of Pt(100) and Au(100) by all-electron density functional theory
    (2010) Havu, Paula; Blum, Volker; Havu, Ville; Rinke, Patrick; Scheffler, Matthias
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The low-index surfaces of Au and Pt all tend to reconstruct, a fact that is of key importance in many nanostructure, catalytic, and electrochemical applications. Remarkably, some significant questions regarding their structural energies remain even today, specifically for the large-scale quasihexagonally reconstructed (100) surfaces: rather dissimilar reconstruction energies for Au and Pt in available experiments and experiment and theory do not match for Pt. We here show by all-electron density functional theory that only large enough “(5×N)” approximant supercells capture the qualitative reconstruction energy trend between Au(100) and Pt(100), in contrast to what is often done in the theoretical literature. Their magnitudes are then in fact similar and closer to the measured value for Pt(100); our calculations achieve excellent agreement with known geometric characteristics and provide direct evidence for the electronic reconstruction driving force.
  • Lithium-Ion Battery Technology for Voltage Control of Perpendicular Magnetization
    (2022-07) Ameziane, Maria; Mansell, Rhodri; Havu, Ville; Rinke, Patrick; van Dijken, Sebastiaan
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The voltage control of magnetism is a promising path to the development of low-power spintronic devices. Magneto-ionics—exploiting voltage-driven ion migration to control magnetism—has attracted interest because it can generate large magnetoelectric effects at low voltage. Here, the use of the solid-state lithium-ion battery technology for reversible voltage-controlled switching between perpendicular and in-plane magnetization states in a Co–Pt bilayer is demonstrated. Due to the small size and high mobility of lithium ions, small voltages produce an exceptionally high magnetoelectric coupling efficiency of at least 7700 fJ V–1 m–1 at room temperature. The magnetic switching effect is attributed to the modulation of spin-orbit coupling at the Co–Pt interface when lithium ions migrate between a lithium storage layer (LiCoO2) and the magnetic interface across a lithium phosphorous oxynitride (LiPON) solid-state electrolyte, which is corroborated by density functional theory calculations. Voltage control of magnetism in the battery structure does not show degradation over more than 500 voltage cycles, demonstrating promise for solid-state lithium-based magneto-ionic devices.
  • Modeling Bi-induced changes in the electronic structure of GaAs 1-xBix alloys
    (2013-12-05) Virkkala, Ville; Havu, Ville; Tuomisto, Filip; Puska, Martti J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We suggested recently that the band-gap narrowing in dilute GaAs 1-xNx alloys can be explained to result from the broadening of the localized N states due to the N-N interaction along the zigzag chains in the âŒ
  • Modelling and Numerical Solution of a Fluid Flow and Heat Transfer Problem
    (1998) Havu, Ville
     Helsinki University of Technology | Master's thesis
    Työn tarkoituksena oli mallintaa ja ratkaista numeerisesti virtauslämmönsiirto-ongelma erityisesti pyörivässä koordinaatistossa. Mallinnus toteutettiin käyttäen Navier-Stokesin yhtälöitä virtausongelmalle ja energiayhtälöä lämmönsiirto-ongelmalle. Virtaavat nesteet oletettiin newtonilaisiksi ja kokoonpuristumattomiksi. Itse laskentatehtävät ratkaistiin numeerisesti stabiloidun elementtimenetelmän avulla ELMER-ohjelmistolla. Työssä esitellään kirjallisten lähteiden pohjalta käytetyt menetelmät yleisellä tasolla sekä kuvataan tehtävien ratkaisualgoritmi niin virtaus- kuin energiayhtälönkin osalta. Työn lakennalliset tulokset koskevat putki- ja rakovirtauksia sekä virtausnesteiden lämmönvaihtoa ulkoisen materiaalin kanssa. Ensinnäkin on havaittavissa, että pyörimisliike ei vaikuta laminaarivirtauksen käyttäytymiseen. Samaten virtausratkaisun muoto on riippumaton virtausnesteen nopeudesta. Toiseksi voidaan todeta, että putkivirtauksen tapauksessa vakiomuotoinen virtauksen tulppaprofiili on hyvin käytännöllinen approksimaatio turbulentille virtausprofiilille. Energiayhtälöä ratkaistaessa ongelmia aiheuttavat epäsäännöllisten reunaehtojen synnyttämät singulariteetit. Näitä lukuunottamat yhtälön numeeriset ratkaisut ovat totunnaisia.
  • Nonadiabatic Ehrenfest molecular dynamics within the projector augmented-wave method
    (2012) Ojanperä, Ari; Havu, Ville; Lehtovaara, Lauri; Puska, Martti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We derive equations for nonadiabatic Ehrenfest molecular dynamics within the projector augmented-wave (PAW) formalism. The discretization of the electrons is time-dependent as the augmentation functions depend on the positions of the nuclei. We describe the implementation of the Ehrenfest molecular dynamicsequations within the real-space PAW method. We demonstrate the applicability of our method by studying the vibration of NaCl, the torsional rotation of H2C=NH+2 in both the adiabatic and the nonadiabatic regimes, and the hydrogen bombardment of C40H16.
  • Protective Coating Interfaces for Perovskite Solar Cell Materials: A First-Principles Study
    (2022-03-16) Fangnon, Azimatu; Dvorak, Marc; Havu, Ville; Todorović, Milica; Li, Jingrui; Rinke, Patrick
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The protection of halide perovskites is important for the performance and stability of emergent perovskite-based optoelectronic technologies. In this work, we investigate the potential inorganic protective coating materials ZnO, SrZrO3, and ZrO2 for the CsPbI3 perovskite. The optimal interface registries are identified with Bayesian optimization. We then use semilocal density functional theory (DFT) to determine the atomic structure at the interfaces of each coating material with the clean CsI-terminated surface and three reconstructed surface models with added PbI2 and CsI complexes. For the final structures, we explore the level alignment at the interface with hybrid DFT calculations. Our analysis of the level alignment at the coating-substrate interfaces reveals no detrimental mid-gap states but rather substrate-dependent valence and conduction band offsets. While ZnO and SrZrO3 act as insulators on CsPbI3, ZrO2 might be suitable as an electron transport layer with the right interface engineering.