In the search for sustainable energy sources, dye sensitized solar cells (DSSCs) represent an attractive solution due to their low cost, relatively high efficiencies, and flexible design. Porphyrin-based dyes are characterized by strong absorption in the visible part of the spectrum and easy customization allowing their electronic properties to be controlled by structural variations. Here we present a computational screening study of more than 5000 porphyrin-based dyes obtained by modifying the porphyrin backbone (metal center and axial ligands), substituting hydrogen by fluorine, and adding different side and anchoring groups. Based on the calculated frontier orbital energies and optical gaps we quantify the energy level alignment with the TiO2 conduction band and different redox mediators. An analysis of the energy level-structure relationship reveals a significant structural diversity among the dyes with the highest level alignment quality, demonstrating the large degree of flexibility in porphyrin dye design. As a specific example of dye optimization, we show that the level alignment of the high efficiency record dye YD2-o-C8 [Yella et al., Science, 2011, 334, 629-634] can be significantly improved by modest structural variations. All the presented data have been stored in a publicly available database.

Motivated by the need to control the resistance of metal-graphene interfaces, we have simulated the structural and transport properties of edge contacts upon their formation. Our first-principles calculations reveal that the contacts evolve in a nontrivial way depending on the type of metal and the chemical contamination of the graphene edge. In particular, our results indicate that the origin of the low experimental resistance of chromium-graphene edge contacts is related to their weaker variation upon contamination and defect formation. In summary, by analyzing the distance dependence of the graphene-metal interaction and the relation between the reactivity and forces at the graphene edge, we shed new light on the mechanisms responsible for the diverse performance of experimentally fabricated graphene edge contacts.

Tantalum oxide has a wide range of applications and has drawn much attention especially for its useful properties in resistive random-access memories, in which the Ta oxide composition plays an important role to control the electrical properties of the TaO x thin films. In this paper, we present a way to tune the composition of TaO x thin films by varying the oxygen partial pressure during growth using pulsed laser deposition. TaO x thin films were deposited at room temperature, under oxygen partial pressures ranging from 10-6 mbar to 2×10-2 mbar. Using angle resolved X-ray photoelectron spectroscopy, we show that the composition of the film varies systematically with the oxygen partial pressure during the film growth. We then correlate the oxygen content with the electrical properties of the film and the results show that the composition has a great influence on the resistivity of the TaO x thin films. As the oxygen partial pressure during deposition increases, the percentage of tantalum pentoxide (Ta 2 O 5) as well as the resistivity of the films increases. This experimental approach provides a pathway to control the TaO x thin film stoichiometry and its electrical properties during growth.

The transition between Ta2O5 and TaO2 governs resistive switching in tantalum oxide-based resistive random access memory. Despite its importance, the Ta2O5–TaO2 transition is scarcely described in the literature, in part because the tantalum oxide layer in devices is amorphous, which makes it difficult to characterize. In this paper, we use first-principles calculations to construct the convex hull of the amorphous Ta2O5−x system for 0 ≤ x ≤ 1 and show that oxygen deficiency in tantalum oxide leads to phase-separation into Ta2O5 and TaO2. In addition, our work challenges the conventional interpretation of X-ray Photoelectron Spectroscopy (XPS) spectra of the Ta 4f orbitals. Specifically, we find that TaO2 exhibits both the Ta4+ peak associated with TaO2 and the Ta5+ peak normally associated with Ta2O5. While our simulated Ta2O5 peak originates from a narrow range of oxidation states, the TaO2 peak comes from disproportionated Ta atoms with Bader charges ranging from +3 to +1, the lowest of which are well below Ta atoms in crystalline TaO. Finally, we demonstrate that the XPS blueshift of around 1 eV observed experimentally in amorphous Ta2O5 with respect to crystalline Ta2O5 comes from both the presence of under-coordinated Ta atoms and longer Ta–O bond distances in the amorphous system. Our simulated XPS analysis shows that amorphous XPS spectra may be more complex than previously thought, and hence, caution should be applied when assigning XPS peaks to oxidation states.

A key step in engineering resistive switching is the ability to control the device switching behavior. Here, we investigate the possibility to tune the resistive switching of tantalum oxide (TaOx)-based memristors from a non-switchable state to a switchable state by applying post-fabrication annealing of the devices. The switching of the devices was found to be related to: (1) the oxidation state changes in the TaOx thin film after annealing and (2) the local variations in oxygen stoichiometry in the vicinity of the interface between the TiN electrode and the TaOx active resistive layer. We further discuss the possible mechanism behind the resistive switching after annealing. This experimental approach provides a simple but powerful pathway to trigger the resistive switching in devices that do not show any resistive switching initially.