Wissam A. Saidi

Temperature dependent energy levels of methylammonium lead iodide perovskite

Temperature dependent energy levels of methylammonium lead iodide are investigated using a combination of ultraviolet photoemission spectroscopy and optical spectroscopy. Our results show that the valence band maximum and conduction band minimum shift down in energy by 110 meV and 77 meV as temperature increases from 28 °C to 85 °C. Density functional theory calculations using slab structures show that the decreased orbital splitting due to thermal expansion is a major contribution to the experimentally observed shift in energy levels. Our results have implications for solar cell performance under operating conditions with continued sunlight exposure and increased temperature.

Non-additivity of polarizabilities and van der Waals C6 coefficients of fullerenes.

We present frequency-dependent polarizabilities and C6 dipole-dipole dispersion coefficients for a wide range of fullerene molecules including C60, C70, C78, C80, C82, and C84. The static and dynamic polarizabilities at imaginary frequencies are computed using time-dependent Hartree-Fock, B3LYP, and CAM-B3LYP ab initio methods by employing the complex linear polarization propagator and are subsequently utilized to determine the C6 coefficients using the Casimir-Polder relation. Overall, the C60 and C70 average static polarizabilities α(0) agree to better than 2% with linear-response coupled-cluster single double and experimental benchmark results, and the C6 coefficient of C60 agrees to better than 1% with the best accepted value. B3LYP provides the best agreement with benchmark results with deviations less than 0.1% in α(0) and C6. We find that the static polarizabilities and the C6 coefficients are non-additive, and scale, respectively, as N(1.2) and N(2.2) with the number of carbon atoms in the fullerene molecule. The exponent for C6 power-dependence on N is much smaller than the value predicted recently based on a classical-metallic spherical-shell approximation of the fullerenes.

Understanding Structure and Bonding of Multilayered Metal–Organic Nanostructures

For organic and hybrid electronic devices, the physicochemical properties of the contained interfaces play a dominant role. To disentangle the various interactions occurring at such heterointerfaces, we here model a complex, yet prototypical, three-component system consisting of a Cu–phthalocyanine (CuPc) film on a 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) monolayer adsorbed on Ag(111). The two encountered interfaces are similar, as in both cases there would be no bonding without van der Waals interactions. Still, they are also distinctly different, as only at the Ag(111)–PTCDA interface do massive charge-rearrangements occur. Using recently developed theoretical tools, we show that it has become possible to provide atomistic insight into the physical and chemical processes in this comparatively complex nanostructure distinguishing between interactions involving local rearrangements of the charge density and long-range van der Waals attraction.

In situ atomic-scale visualization of oxide islanding during oxidation of Cu surfaces

Oxidation of Cu occurs via Cu2O islanding on an oxide wetting layer at a critical thickness of two atomic layers. The transition from 2D wetting-layer growth to 3D oxide islanding is driven energetically arising from the Cu-Cu2O interfacial interaction.

Controlling nucleation, growth, and orientation of metal halide perovskite thin films with rationally selected additives

Accelerating the progress toward realizing metal halide perovskite solar cells with improved efficiency, stability and reliability requires a deeper understanding of the thin film formation processes. This paper investigates the impact of rationally selected chemical additives in precursor solutions on the nucleation and growth of metal halide perovskite thin films. Computational screening was performed to guide the selection of tetrahydrothiophene oxide as an additive with stronger solvation efficacy than all other commonly used solvents. In situ grazing incidence X-ray diffraction measurements show that the additives suppress the formation of homogeneous nuclei as well as crystalline intermediate structures. Instead, heterogeneous nucleation on the substrate surface and growth of a thin film with a strongly preferential crystallographic orientation occur directly from the precursor solution. Density functional theory calculations show that the crystallographic orientation of the thin films can be tuned by altering the surface energies with the chemical additives. The crystallographic orientation of the thin films is found to have a significant impact on the open circuit voltage of solar cell devices, highlighting the importance of controlling the metal halide perovskite thin film orientation for improved solar cell efficiency.

Experimental and theoretical comparison of gas desorption energies on metallic and semiconducting single-walled carbon nanotubes.

Single-walled carbon nanotubes (SWNTs) exhibit high surface areas and precisely defined pores, making them potentially useful materials for gas adsorption and purification. A thorough understanding of the interactions between adsorbates and SWNTs is therefore critical to predicting adsorption isotherms and selectivities. Metallic (M-) and semiconducting (S-) SWNTs have extremely different polarizabilities that might be expected to significantly affect the adsorption energies of molecules. We experimentally and theoretically show that this expectation is contradicted, for both a long chain molecule (n-heptane) and atoms (Ar, Kr, and Xe). Temperature-programmed desorption experiments are combined with van der Waals corrected density functional theory, examining adsorption on interior and exterior sites of the SWNTs. Our calculations show a clear dependence of the adsorption energy on nanotube diameter but not on whether the tubes are conducting or insulating. We find no significant experimental or theoretical difference in adsorption energies for molecules adsorbed on M- and S-SWNTs having the same diameter. Hence, we conclude that the differences in polarizabilities between M- and S-SWNTs have a negligible influence on gas adsorption for spherical molecules as well as for highly anisotropic molecules such as n-heptane. We expect this conclusion to apply to all types of adsorbed molecules where van der Waals interactions govern the molecular interaction with the SWNT.

Temperature Dependence of the Energy Levels of Methylammonium Lead Iodide Perovskite from First-Principles

Environmental effects and intrinsic energy-loss processes lead to fluctuations in the operational temperature of solar cells, which can profoundly influence their power conversion efficiency. Here we determine from first-principles the effects of temperature on the band gap and band edges of the hybrid pervoskite CH3NH3PbI3 by accounting for electron-phonon coupling and thermal expansion. From 290 to 380 K, the computed band gap change of 40 meV coincides with the experimental change of 30-40 meV. The calculation of electron-phonon coupling in CH3NH3PbI3 is particularly intricate as the commonly used Allen-Heine-Cardona theory overestimates the band gap change with temperature, and excellent agreement with experiment is only obtained when including high-order terms in the electron-phonon interaction. We also find that spin-orbit coupling enhances the electron-phonon coupling strength but that the inclusion of nonlocal correlations using hybrid functionals has little effect. We reach similar conclusions in the metal-halide perovskite CsPbI3. Our results unambiguously confirm for the first time the importance of high-order terms in the electron-phonon coupling by direct comparison with experiment.

Strong reciprocal interaction between polarization and surface stoichiometry in oxide ferroelectrics.

We present a systematic evaluation of the effects of polarization switchability on surface structure and stoichiometry in BaTiO3 and PbTiO3 ferroelectric oxides. We show that charge passivation, mostly by ionic surface reconstructions, is the driving force for the stability of the surfaces, which suggests that varying the substrate polarization offers a new mechanism for controlling surface reconstructions in polar systems and inducing highly nonstoichiometric structures. Conversely, for thin-films the chemical environment can drive polarization switching via induced compositional changes on the surface. We find that the value of the oxygen partial pressure for the positive-to-negative polar transition is in good agreement with the recent experimental value for thin-film PbTiO3. For BaTiO3, we show that it is harder for oxygen control to drive polar transition because it is more difficult to reduce. This study opens up the possibility of real-time control of structure and composition of oxide surfaces.

Ultrafast Dynamics of Photongenerated Holes at a CH3OH/TiO2 Rutile Interface

Photogenerated charge carrier dynamics near molecule/TiO2 interfaces are important for the photocatalytic and photovoltaic processes. To understand this fundamental aspect, we performed a time-domain ab initio nonadiabatic molecular dynamics study of the photogenerated hole dynamics at the CH3OH/rutile TiO2(110) interface. We studied the forward and reverse hole transfer between TiO2 and CH3OH as well as the hole energy relaxation to the valence band maximum. First, we show that the hole-trapping ability of CH3OH depends strongly on the adsorption structure. Only when the CH3OH is deprotonated to form chemisorbed CH3O will ∼15% of the hole be trapped by the molecule. Second, we find that strong fluctuations of the HOMO energies of the adsorbed molecules induced by electron-phonon coupling provide additional channels, which accelerate the hole energy relaxation. Third, we demonstrate that the charge transfer and energy relaxation processes depend significantly on temperature. When the temperature decreases from 100 to 30 K, the forward hole transfer and energy relaxation processes are strongly suppressed because of the reduction of phonon occupation. These results indicate that the molecule/TiO2 energy level alignment, thermal excitation of a phonon, and electron-phonon coupling are the key factors that determine the photogenerated hole dynamics. Our studies provide valuable insights into the photogenerated charge and energy transfer dynamics at molecule/semiconductor interfaces.

Enzyme-Catalyzed Oxidation Facilitates the Return of Fluorescence for Single-Walled Carbon Nanotubes

In this work, we studied enzyme-catalyzed oxidation of single-walled carbon nanotubes (SWCNTs) produced by the high-pressure carbon monoxide (HiPco) method. While oxidation via strong acids introduced defect sites on SWCNTs and suppressed their near-infrared (NIR) fluorescence, our results indicated that the fluorescence of SWCNTs was restored upon enzymatic oxidation, providing new evidence that the reaction catalyzed by horseradish peroxidase (HRP) in the presence of H2O2 is mainly a defect-consuming step. These results were further supported by both UV-vis-NIR and Raman spectroscopy. Therefore, when acid oxidation followed by HRP-catalyzed enzyme oxidation was employed, shortened (<300 nm in length) and NIR-fluorescent SWCNTs were produced. In contrast, upon treatment with myeloperoxidase, H2O2, and NaCl, the oxidized HiPco SWCNTs underwent complete oxidation (i.e., degradation). The shortened, NIR-fluorescent SWCNTs resulting from HRP-catalyzed oxidation of acid-cut HiPco SWCNTs may find applications in cellular NIR imaging and drug delivery systems.