3 research outputs found
Optical Absorption Spectra and Excitons of Dye-Substrate Interfaces: Catechol on TiO<sub>2</sub>(110)
Optimizing the photovoltaic
efficiency of dye-sensitized solar
cells (DSSC) based on staggered gap heterojunctions requires a detailed
understanding of sub-band gap transitions in the visible from the
dye directly to the substrateās conduction band (CB) (type-II
DSSCs). Here, we calculate the optical absorption spectra and spatial
distribution of bright excitons in the visible region for a prototypical
DSSC, catechol on rutile TiO<sub>2</sub>(110), as a function of coverage
and deprotonation of the OH anchoring groups. This is accomplished
by solving the BetheāSalpeter equation (BSE) based on hybrid
range-separated exchange and correlation functional (HSE06) density
functional theory (DFT) calculations. Such a treatment is necessary
to accurately describe the interfacial level alignment and the weakly
bound charge transfer transitions that are the dominant absorption
mechanism in type-II DSSCs. Our HSE06 BSE spectra agree semiquantitatively
with spectra measured for catechol on anatase TiO<sub>2</sub> nanoparticles.
Our results suggest deprotonation of catecholās OH anchoring
groups, while being nearly isoenergetic at high coverages, shifts
the onset of the absorption spectra to lower energies, with a concomitant
increase in photovoltaic efficiency. Further, the most relevant bright
excitons in the visible region are rather intense charge transfer
transitions with the electron and hole spatially separated in both
the [110] and [001] directions. Such detailed information on the absorption
spectra and excitons is only accessible via periodic models of the
combined dye-substrate interface
Revealing the Adsorption Mechanisms of Nitroxides on Ultrapure, Metallicity-Sorted Carbon Nanotubes
Carbon nanotubes are a natural choice as gas sensor components given their high surface to volume ratio, electronic properties, and capability to mediate chemical reactions. However, a realistic assessment of the interaction of the tube wall and the adsorption processes during gas phase reactions has always been elusive. Making use of ultraclean single-walled carbon nanotubes, we have followed the adsorption kinetics of NO<sub>2</sub> and found a physisorption mechanism. Additionally, the adsorption reaction directly depends on the metallic character of the samples. FranckāCondon satellites, hitherto undetected in nanotubeāNO<sub><i>x</i></sub> systems, were resolved in the N 1<i>s</i> X-ray absorption signal, revealing a weak chemisorption, which is intrinsically related to NO dimer molecules. This has allowed us to identify that an additional signal observed in the higher binding energy region of the core level C 1<i>s</i> photoemission signal is due to the Cī»O species of ketene groups formed as reaction byproducts . This has been supported by density functional theory calculations. These results pave the way toward the optimization of nanotube-based sensors with tailored sensitivity and selectivity to different species at room temperature
Understanding Energy-Level Alignment in DonorāAcceptor/Metal Interfaces from Core-Level Shifts
The molecule/metal interface is the key element in charge injection devices. It can be generally defined by a monolayer-thick blend of donor and/or acceptor molecules in contact with a metal surface. Energy barriers for electron and hole injection are determined by the offset from HOMO (highest occupied) and LUMO (lowest unoccupied) molecular levels of this contact layer with respect to the Fermi level of the metal electrode. However, the HOMO and LUMO alignment is not easy to elucidate in complex multicomponent, molecule/metal systems. We demonstrate that core-level photoemission from donorāacceptor/metal interfaces can be used to straightforwardly and transparently assess molecular-level alignment. Systematic experiments in a variety of systems show characteristic binding energy shifts in core levels as a function of molecular donor/acceptor ratio, irrespective of the molecule or the metal. Such shifts reveal how the level alignment at the molecule/metal interface varies as a function of the donorāacceptor stoichiometry in the contact blend