We investigate a set of donor-π-acceptor (D-π-A) dyes with new acceptor groups for dye-sensitized solar cells, using time-dependent density-functional-theory calculations of the electronic structure and optical absorption. We considered three types of modifications on existing dye structures: (i) replacement of the side cyano group (CN) on the molecular anchor, (ii) insertion and alteration of the intermediate spacer groups, and (iii) modification of the number and positions of cyano CN groups on a phenyl-ring spacer. We find that with these modifications, the dye electronic levels and corresponding optical absorption properties can be gradually tuned, rendering possible the identification of dyes with desirable structural, electronic, and optical properties. For example, dyes with phenyl and CN-substituted phenyl groups are promising candidates for red light absorption and high molar extinction coefficients.
A method for the direct correlation at the nanoscale of structural and optical properties of single GaAs nanowires is reported. Nanowires consisting of 100% wurtzite and nanowires presenting zinc-blende/wurtzite polytypism are investigated by photoluminescence spectroscopy and transmission electron microscopy. The photoluminescence of wurtzite GaAs is consistent with a band gap of 1.5 eV. In the polytypic nanowires, it is shown that the regions that are predominantly composed of either zinc-blende or wurtzite phase show photoluminescence emission close to the bulk GaAs band gap, while regions composed of a nonperiodic superlattice of wurtzite and zinc-blende phases exhibit a redshift of the photoluminescence spectra as low as 1.455 eV. The dimensions of the quantum heterostructures are correlated with the light emission, allowing us to determine the band alignment between these two crystalline phases. Our first-principles electronic structure calculations within density functional theory, employing a hybrid-exchange functional, predict band offsets and effective masses in good agreement with experimental results.
A D-π-A dye (KM-1) incorporating cyanobenzoic acid as a new acceptor/anchoring group has been synthesized for dye-sensitized solar cells (DSCs) with a high molar extinction coefficient of 66 700 M-1 cm-1 at 437 nm. Theoretical calculations show that the hydrogen bond between CN and surface hydroxyl leads to the most stable configuration on the surface of TiO2. In addition, the adsorption of the dye on TiO2 follows a BrunauerEmmettTeller (BET) isotherm. Multilayer adsorption of KM-1 on TiO2 seems to take place particularly at higher dye concentrations. DSC device using KM-1 reached a maximum incident photon-to-current conversion efficiency (IPCE) of 84%, with a solar to electric power conversion efficiency (PCE) of 3.3% at AM1.5 G illumination (100 mW cm-2 ). This new type of anchoring group paves a way to design new dyes that combine good visible light harvesting with strong binding to the metal oxide surface
We discuss the optimal evaluation of endothelial shear stress for real-life case studies based on anatomic data acquisition. The fluid dynamic simulations require smoothing of the geometric dataset to avoid major artefacts in the flow patterns, especially in the proximity of bifurcations. A systematic series of simulations at different corrugation levels shows that, below a smoothing length of about 0.5 mm, the numerical data are insensitive to further smoothing.