Authors: M.Pazoki, M. J. Wolf, T. Edvinsson and J.Kullgren
Ion migration has recently been suggested to play critical roles in the operation of lead halide perovskite solar cells. However, so far there has been no systematic investigation of how the monovalent cation affects the vacancy formation, ion migration and the associated hysteresis effect. Here, we present density functional theory calculations on all possible ion migration barriers in the perovskite materials with different cations i.e. CH3NH3PbI3, CH(NH2)2PbI3 and CsPbI3 in the tetragonal phase and investigate vacancy monovalent-cation interactions within the framework of the possible ion migrations. The most relevant ion movement (iodide) is investigated in greater detail and corresponding local structural changes, the relationships with the local ionic dielectric response, Stark effect and current-voltage hysteresis are discussed. We observe a correlation between the energy barrier for iodine migration and the magnitude of the dipole of the monovalent cation. From the data, we suggest a vacancy-dipole interaction mechanism by which the larger dipole of the monovalent cation can respond to and screen the local electric fields more effectively. The stronger response of the high dipolar monovalent cation to the vacancy electrostatic potential in turn leads to a lower local structural changes within the neighbouring octahedra. The presented data reveal a detailed picture of the ion movement, vacancy dipole interactions and the consequent local structural changes, which contain fundamental information about the photo-physics, and dielectric response of the material.
Nano Energy, 38, 2017, pp. 537-543
Authors: Jolla Kullgren, Matthew J. Wolf, Pavlin D. Mitev, Kersti Hermansson and Wim J. Briels
The interplay between energetics and entropy in determining defect distributions at ceria(111) is studied using a combination of DFT+U and lattice Monte Carlo simulations. Our main example is fluorine impurities, although we also present preliminary results for surface hydroxyl groups. A simple classical force-field model was constructed from a training set of DFT+U data for all symmetrically inequivalent (F−)n(Ce3+)n nearest-neighbor clusters with n = 2 or 3. Our fitted model reproduces the DFT energies well. We find that for an impurity concentration of 15% at 600 K, straight and hooked linear fluorine clusters are surprisingly abundant, with similarities to experimental STM images from the literature. We also find that with increasing temperature the fluorine cluster sizes show a transition from being governed by an attractive potential to being governed by a repulsive potential as a consequence of the increasing importance of the entropy of the Ce3+ ions. The distributions of surface hydroxyl groups are noticeably different.
J. Phys. Chem. C, 2017, 121 (28), pp 15127–15134
Authors: M. Pazoki, A. Röckert, M. J. Wolf, R. Imani, T. Edvinsson, and J. Kullgren.
The emergence of highly efficient lead halide perovskite solar cell materials makes the exploration and engineering of new lead free compounds very interesting both from a fundamental perspective as well as for potential use as new materials in solar cell devices. Herein we present the electronic structure of several lanthanide (La) based materials in the metalorganic halide perovskite family not explored before. Our estimated bandgaps for the lanthanide (Eu, Dy, Tm, Yb) perovskite compounds are in the range of 2.0–3.2 eV showing the possibility for implementation as photo-absorbers in tandem solar cell configurations or charge separating materials. We have estimated the typical effective masses of the electrons and holes for MALaI3 (La= Eu, Dy, Tm, Yb) to be in the range of 0.3–0.5 and 0.97–4.0 units of the free electron mass, respectively. We have shown that the localized f-electrons within our DFT+U approach, make the dominant electronic contribution to the states at the top of the valence band and thus have a strong impact on the photo-physical properties of the lanthanide perovskites. Therefore, the main valence to conduction band electronic transition for MAEuI3 is based on inner shell f-electron localized states within a periodic framework of perovskite crystal by which the optical absorption onset would be rather inert with respect to quantum confinement effects. The very similar crystal structure and lattice constant of the lanthanide perovskites to the widely studied CH3NH3PbI3 perovskite, are prominent advantages for implementation of these compounds in tandem or charge selective contacts in PV applications together with lead iodide perovskite devices.
J. Mater. Chem. A, 5, 2017, pp. 23131-23138
Photon Energy-Dependent Hysteresis Effects in Lead Halide Perovskite Materials
Authors: Meysam Pazoki, T. Jacobsson, Silver H. Jesper and Cruz, Malin Johansson, Roghayeh Imani, Jolla Kullgren, Anders Hagfeldt, Erik M. J. Johansson, Tomas Edvinsson and Gerrit Boschloo.
Lead halide perovskites have a range of spectacular properties and interesting phenomena and are a serious candidate for the next generation of photovoltaics with high efficiencies and low fabrication costs. An interesting phenomenon is the anomalous hysteresis often seen in current–voltage scans, which complicates accurate performance measurements but has also been explored to obtain a more comprehensive understanding of the device physics. Herein, we demonstrate a wavelength and illumination intensity dependency of the hysteresis in state-of-the-art perovskite solar cells with 18\% power conversion efficiency (PCE), which gives new insights into ion migration. The perovskite devices show lower hysteresis under illumination with near band edge (red) wavelengths compared to more energetic (blue) excitation. This can be rationalized with thermalization-assisted ion movement or thermalization-assisted vacancy generation. These explanations are supported by the dependency of the photovoltage decay with illumination time and excitation wavelength, as well as by impedance spectroscopy. The suggested mechanism is that high-energy photons create hot charge carriers that either through thermalization can create additional vacancies or by release of more energetic phonons play a role in overcoming the activation energy for ion movement. The excitation wavelength dependency of the hysteresis presented here gives valuable insights into the photophysics of the lead halide perovskite solar cells.
J. Phys. Chem. C, 121, 2017, pp. 26180-26187
Authors: Imre Bakó, Anikó Lábas, Kersti Hermansson, Ákos Bencsura and Julianna Oláh
The significant cooperative effect between water molecules substantially affects the properties of liquid water. The cooperativity of hydrogen bonds means that the hydrogen bond strength is influenced by the neighboring water molecules. Another descriptor related to cooperativity is degree correlation (or static correlation) describing the probability of hydrogen-bonded molecule pairs participating in additional hydrogen-bonds. Herein we analyze the latter one in liquid water at various temperatures and densities in a series of molecular dynamics simulations with the help of knowledge from network science. We investigated how the applied hydrogen bond criteria (energetic or geometric) influence the obtained results, and showed that the energetic criterion is much more rigorous and reliable, therefore should be used for similar studies. We found that the structure of the subsystems of water molecules with 3 and 4 hydrogen-bonds is distinctly different at low temperature, 3‑hydrogen-bonded water molecules form branched chain structures at all temperature. Deconvolution of the descriptors of the mixing pattern of water molecules according to their donor and acceptor numbers showed that species with complementary hydrogen bonding properties are likely to correlate and form H-bonds with each other, while species with similar H-bond pattern tend to avoid each other. Pearson’s coefficient (global descriptor of the local cooperativity) of the studied networks suggests that at normal density the H-bonded network in liquid water can be described by an uncorrelated network.
Journal of Molecular Liquids, 245, 2017, pp 140-146
Author: Kersti Hermansson
This report discusses some of the most pressing challenges that need to be overcome for computational condensed matter chemistry to become fully accepted, at par with experiments. The prospects are rather bright. By means of a few examples, all connected to the bound water molecule and the hydroxide ion, and their mysteries, the unique capabilities of theoretical calculations to provide new insights and sometimes even surpass experiments in accuracy, will be demonstrated.
Contributions, Sec. Nat. Math. Biotech. Sci., MASA, 38 (1), 2017, pp 17–26
Authors: Samual R. Zukowski, Pavlin D. Mitev, Kersti Hermansson, and Dor Ben-Amotz
The hydration-shell of CO2 is characterized using Raman multivariate curve resolution (Raman-MCR) spectroscopy combined with ab initio molecular dynamics (AIMD) vibrational density of states simulations, to validate our assignment of the experimentally observed high-frequency OH band to a weak hydrogen bond between water and CO2. Our results reveal that while the hydration-shell of CO2 is highly tetrahedral, it is also occasionally disrupted by the presence of entropically stabilized defects associated with the CO2-water hydrogen bond. Moreover, we find that the hydration-shell of CO2 undergoes a temperature-dependent structural transformation to a highly disordered (less tetrahedral) structure, reminiscent of the transformation that takes place at higher temperatures around much larger oily molecules. The biological significance of the CO2 hydration shell structural transformation is suggested by the fact that it takes place near physiological temperatures.
J. Phys. Chem. Lett., 8 (13), 2017, pp 2971–2975
Authors: Getachew G. Kebede, Daniel Spångberg, Pavlin D. Mitev, Peter Broqvist, and Kersti Hermansson
In this work, a range of van der Waals type density functionals are applied to the H2O/NaCl(001) and H2O/MgO(001) interface systems to explore the effect of an explicit dispersion treatment. The functionals we use are the self-consistent vdW functionals vdW-DF, vdW-DF2, optPBE-vdW, optB88-vdW, optB86b-vdW, and vdW-DF-cx, as well as the dispersion-corrected PBE-TS and PBE-D2 methods; they are all compared with the standard PBE functional. For both NaCl(001) and MgO(001), we find that the dispersion-flavoured functionals stabilize the water-surface interface by approximately 20%-40% compared to the PBE results. For NaCl(001), where the water molecules remain intact for all overlayers, the dominant contribution to the adsorption energy from “density functional theory dispersion” stems from the water-surface interactions rather than the water-water interactions. The optPBE-vdW and vdW-DF-cx functionals yield adsorption energies in good agreement with available experimental values for both NaCl and MgO. To probe the strengths of the perturbations of the adsorbed water molecules, we also calculated water dipole moments and found an increase up to 85% for water at the MgO(001) surface and 70% at the NaCl(001) surface, compared to the gas-phase dipole moment.
The Journal of Chemical Physics 146, 064703 (2017);
Authors: Jolla Per Kullgren, Matthew Jason Wolf, Kersti Hermansson, Christof Köhler, Bálint Aradi, Thomas Frauenheim, and Peter Broqvist
Reducible oxides such as CeO2 are challanging to describe
with standard density functional theory (DFT) due to the mixed valence states of the cations, and often require the use of additional correction schemes, an
d/or more computationally expen- sive methods. This adds a new layer of complexity when it comes to the generation of Slater-Koster tables and the corresponding repulsive potentials for self-consistent density functional based tight binding (SCC-DFTB) calculations of such materials. In this work, we provide guidelines for how to set up a parameterisation scheme for mixed valence oxides within the SCC-DFTB framework, with a focus on reproducing structural and electronic properties as well as redox reaction energies calculated using a reference DFT method. This parameterisation procedure has been used to generate parameters for Ce–O interactions, with Ce in its +III or +IV formal oxidation states. The generated parameter set is validated through comparison to DFT calculations for various ceria (CeO2) and reduced ceria (CeO2−x ) systems of different dimensionalities ranging from 0D (nano-particles) to 3D (bulk). As oxygen vacancy defects in ceria are of crucial importance to many technological applications, special focus is directed towards the capability of describing such defects accurately.
J. Phys. Chem. C, 2017, 121 (8), pp 4593–4607
Authors: Matthew J. Wolf, Jolla Kullgren, Peter Broqvist, and Kersti Hermansson
We investigate the effects of anion doping with fluorine impurities on the chemistry of the CeO2 (111) facet, using the results of DFT + U
calculations. We consider three prototypical processes: the formation of oxygen vacancies, the adsorption of O2 and H2O molecules, and the re-oxidation of the surface with fragments of the two molecules. We find that the first two of these processes are not strongly affected, but that the presence of F lowers the energy gained in the re-oxidation of the surface in comparison to the healing of an oxygen vacancy, by 1.47 eV in the case of O2 (provided that the F is part of a cluster) and by 0.92 eV in the case of H2O. Based on these results, we suggest that F could enhance the redox chemistry of ceria by toggling between being in the surfaceand on the surface, effectively facilitating the release of lattice O by acting as a “place holder” for it. Finally, we find that the desorption of F as either 1212
F2 or HF is energetically unfavourable, suggesting that F doped ceria should be stable in the presence of O2 and H2O.
J. Chem. Phys. 2017, 146, 044703