Malgorzata Wierzbowska

Multipeak negative differential resistance from interplay between nonlinear stark effect and double-branch current flow

Multipeak negative differential resistance (NDR) molecular devices are designed from first principles. The effect of NDR is associated with the non-linear Stark shifts and the electron localization within the conductive region and contacts. A deep I(V)-curve well is formed when the aromatic molecule, containing intramolecular hydrogen bond, is connected to each lead by the double-branch contacts. This effect occurs at the same voltage where a single-junction case exhibits only a flat step in the current characteristics. The multipeak oscillations arise from the mutual effect of the Stark shifts located at the electron-rich contacts and parts of the molecule – this opens the route for further tailoring the desired properties.

Separate-Path Electron and Hole Transport Across π-Stacked Ferroelectrics for Photovoltaic Applications

Electron and hole separate-path transport is theoretically found in the π-stacked organic layers and columns. This effect might be a solution for the charge recombination problem. The building molecules, named 1,3,5-tricyano-2,4,6-tricarboxy-benzene, contain the mesogenic flat aromatic part and the terminal dipole groups which make the system ferroelectric. The diffusion path of the electrons cuts through the aromatic rings, while holes hop between the dipole groups. The transmission function and the charge mobilities, especially for the holes, are very sensitive to the distance between the molecular rings, due to the overlap of the π-type orbitals. We verified that the separation of the diffusion paths is not destroyed by the application of the graphene leads. These features make the system suitable for the efficient solar cells, with the carrier mobilities higher than those in the organometal halide perovskites.

High efficiency quasi 2D lead bromide perovskite solar cells using various barrier molecules

This work reports on the high power conversion efficiency (PCE) and high open circuit voltage (Voc) of bromide-based quasi 2D perovskite solar cells. A Voc of more than 1.4 V and, at the same time, a PCE of 9.5% for cells with hole transport material (HTM) were displayed, whereas a Voc value of 1.37 V and a PCE of 7.9% were achieved for HTM-free cells. Bromide quasi 2D perovskites were synthesized using various long organic barriers (e.g., benzyl ammonium, BA; phenylethyl ammonium, PEA; and propyl phenyl ammonium, PPA). The influence of different barrier molecules on the quasi 2D perovskites properties was studied using absorbance, X-ray diffraction, and scanning electron microscopy. No change was observed in the exciton binding energy as a result of changing the barrier molecule. Density functional theory (DFT) with spin–orbit coupling calculations showed that in the case of BA, holes are delocalized over the whole molecule, whereas for PEA and PPA, they are delocalized more at the phenyl ring. This factor influences the electrical conductivity of holes, which is highest for BA in comparison with the other barriers. In the case of electrons, the energy onset for the nonzero conductivity is lowest for BA. Both calculations support the better PV performance observed for the quasi 2D perovskite based on BA as the barrier. Finally, using contact angle measurements, it was shown that the quasi 2D perovskite is more hydrophobic than the 3D perovskite. Stability measurements showed that cells based on the quasi 2D perovskite are more stable than cells based on the 3D perovskite.

Contacts for organic switches with carbon-nanotube leads.

We focus on two classes of organic switches operating due to the photo- or field-induced proton transfer (PT) process. By means of first-principles simulations, we search for the atomic contacts that strengthen diversity of the two swapped current-voltage (I-V) characteristics between two tautomers. We emphasize that the low-resistive contacts do not necessarily possess good switching properties. Very often, the higher-current flow makes it more difficult to distinguish between the logic states. Instead, the more resistive contacts multiply a current gear to a larger extent. The low- and high-bias work regimes set additional conditions, which are fulfilled by different contacts: (i) in the very low-voltage regime, the direct connections to the nanotubes perform better than the popular sulfur contacts, and (ii) in the higher-voltage regime, the best are the peroxide (-O-O-) contacts. Additionally, we find that the switching-bias value is not an inherent property of the conducting molecule, but it strongly depends on the chosen contacts.

Breathing bands due to molecular order in CH3NH3PbI3

Abstract CH3NH3PbI3 perovskite is nowadays amongst the most promising photovoltaic materials for energy conversion. We have studied by ab initio calculations, using several levels of approximation – namely density functional theory including spin-orbit coupling and quasi-particle corrections by means of the GW method, as well as pseudopotential self-interaction corrections, the role of the methylammonium orientation on the electronic structure of this perovskite. We have considered many molecular arrangements within 2 × 2 × 2 supercells, showing that the relative orientation of the organic molecules is responsible for a huge band gap variation up to 2 eV. The band gap sizes are related to distortions of the PbI3 cage, which are in turn due to electrostatic interactions between this inorganic frame and the molecules. The strong dependence of the band gap on the mutual molecular orientation is confirmed at all levels of approximations. Our results suggest then that the coupling between the molecular motion and the interactions of the molecules with the inorganic cage could help to explain the widening of the absorption spectrum of CH3NH3PbI3 perovskite, consistent with the observed white spectrum.

Ferrimagnetism in 2D networks of porphyrin-X and -XO (X=Sc,...,Zn) with acetylene bridges

Abstract Magnetism in 2D networks of the acetylene-bridged transition metal porphyrins M(P)-2(C–C)-2 (denoted P-TM), and oxo-TM-porphyrins OM(P)-2(C–C)-2 (denoted P-TMO), is studied with the density functional theory (DFT) and the self-interaction corrected pseudopotential scheme (pSIC). Addition of oxygen lowers magnetism of P-TMO with respect to the corresponding P-TM for most of the first-half 3d-row TMs. In contrast, binding O with the second-half 3d-row TMs or Sc increases the magnetic moments. Ferrimagnetism is found for the porphyrin networks with the TMs from V to Co and also for these cases with oxygen. This is a long-range effect of the delocalized spin-polarization, extended even to the acetylene bridges.

Gapless edge states in (C,O,H)-built molecular system with p-stacking and hydrogen bonds

The gapless edge states have been found in a 2D molecular system built with light atoms: C,O,H. This prediction is done on the basis of combined density functional theory (DFT) and tight-binding calculations. The system does not exhibit any effect of the spin-orbit coupling (SOC), neither intrinsic nor Rashba type. The band structure and the edge states are tuned with a strength of the p-stacking and O...H interactions. The elementary cell of this noncovalent structure, does not have the 3D inversion or rotational symmetry. Instead, the system transforms via a superposition of two reflections: with respect to the xz and xy mirror planes, both containing the non-periodic direction. This superposition is equivalent to the inversion in the 2D subspace, in which the system is periodic. The energy gap obtained with the DFT method is 0.11 eV, and largely opens (above 1 eV) with the GW and hybrid-DFT approaches. The bands inversion is partial, i.e. the Bloch states are mixed, with the ”swapping” and ”non-swapping” atomic contributions.

Ferroelectric pi-stacks of molecules with the energy gaps in the sunlight range

Ferroelectric

In2O3 Doped with Hydrogen: Electronic Structure and Optical Properties from the Pseudopotential Self-Interaction Corrected Density Functional Theory and the Random Phase Approximation

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