Dopamine Adsorption on Anatase TiO2(101): A Photoemission and NEXAFS Spectroscopy Study

The adsorption of dopamine onto an anatase TiO(2)(101) single crystal has been studied using photoemission and NEXAFS techniques. Photoemission results suggest that the dopamine molecule adsorbs on the surface in a bidentate geometry, resulting in the removal of band gap states in the TiO(2) valence band. Using the searchlight effect, carbon K-edge NEXAFS spectra indicate that the phenyl rings in the dopamine molecules are orientated normal to the surface. A combination of experimental and computational results indicates the appearance of new unoccupied states arising following adsorption. The possible role of these states in the charge-transfer mechanism of the dopamine-TiO(2) system is discussed

Optically Transparent FTO-Free Cathode for Dye-Sensitized Solar Cells

The woven fabric containing electrochemically platinized tungsten wire is an affordable flexible cathode for liquid-junction dye-sensitized solar cells with the I₃^–/I^– redox mediator and electrolyte solution consisting of ionic liquids and propionitrile. The fabric-based electrode outperforms the thermally platinized FTO in serial ohmic resistance and charge-transfer resistance for triiodide reduction, and it offers comparable or better optical transparency in the visible and particularly in the near-IR spectral region. The electrode exhibits good stability during electrochemical loading and storage at open circuit. The dye-sensitized solar cells with a C101-sensitized titania photoanode and either Pt–W/PEN or Pt–FTO cathodes show a comparable performance

Highly Efficient Perovskite Solar Cells with Gradient Bilayer Electron Transport Materials

Electron transport layers (ETLs) with suitable energy level alignment for facilitating charge carrier transport as well as electron extraction are essential for planar heterojunction perovskite solar cells (PSCs) to achieve high open-circuit voltage (<i>V</i>_OC) and short-circuit current. Herein we systematically investigate band offset between ETL and perovskite absorber by tuning F doping level in SnO₂ nanocrystal. We demonstrate that gradual substitution of F^– into the SnO₂ ETL can effectively reduce the band offset and result in a substantial increase in device <i>V</i>_OC. Consequently, a power conversion efficiency of 20.2% with <i>V</i>_OC of 1.13 V can be achieved under AM 1.5 G illumination for planar heterojunction PSCs using F-doped SnO₂ bilayer ETL. Our finding provides a simple pathway to tailor ETL/perovskite band offset to increase built-in electric field of planar heterojunction PSCs for maximizing <i>V</i>_OC and charge collection simultaneously

Contribution de la RMN haute résolution Proton à l’étude de l’ordre local dans des pérovskites hybrides

National audienceLes cellules solaires à base de pérovskites hybrides apparaissent depuis quelques années comme des candidats de choix pour concurrencer les cellules conventionnelles à base de silicium cristallin1. En effet, les rendements photovoltaïques obtenus avec les pérovskites halogénées dépassent aujourd’hui 22 % et leur stabilité temporelle sous irradiation lumineuse ne cesse de s’améliorer2. Ces performances s’obtiennent grâce à l’emploi de solutions solides de plus en plus complexes. Dans le cas particulier des pérovskites à réseau tri-dimensionnel formule générique APbX3, A est un petit cation organique (Methylammonium=MA, Formamidinium=FA,...) ou inorganique (Cs, Rb,...) et X un halogène (Cl, Br ou I).2 Dans ces solutions solides, l’influence des substitutions de A et de X sur la structure et la dynamique est peu appréhendée et la RMN peut s’avérer être un outil de choix.Récemment nous avons montré le fort potentiel de la RMN pour ces matériaux et notamment celle du plomb-207 pour sonder l’effet de la substitution de l’halogène dans MAPbX3 (X=Cl, Br, I)3. Cet exposé vise à discuter des performances de la RMN pour analyser les implications de la substitution du cation organique MA. Nous montrerons quà température ambiante, la RMN du plomb-207 présente peu d’intérêt. Elle est peu sensible à la substitution du cation A et présente des temps de relaxations transversales courts qui empêchent d’envisager des expériences de corrélations. En revanche, le proton apparait comme un candidat de choix pour l’étude de l’ordre local. Les temps de relaxation sont favorables et permettent l’utilisation du couplage dipolaire pour sonder les proximités spatiales (RFDR, BABA)

Heterogeneous Electron Transfer from Dye-Sensitized Nanocrystalline TiO₂ to [Co(bpy)₃]³⁺: Insights Gained from Impedance Spectroscopy

Dye-sensitized solar cells (DSCs) employing the [Co­(bpy)₃]^3+/2+ redox mediator have recently attained efficiencies in excess of 12%, increasing the attractiveness of DSCs as an alternative to conventional photovoltaics. Heterogeneous electron transfer from dye-sensitized nanocrystalline TiO₂ to [Co­(bpy)₃]³⁺ ions in solution, a process known as recombination in the context of DSC operation, is an important loss mechanism in these solar cells. Here, we employ impedance spectroscopy over a range of temperatures to characterize electron storage, transport, and recombination in efficient DSCs based on the [Co­(bpy)₃]^3+/2+ redox mediator, with either the amphiphillic ruthenium sensitizer Z907 or the state-of-the-art organic sensitizer Y123. The temperature dependence of the electron-transport resistance indicates that transport occurs via states at energies lower than commonly assumed for the TiO₂ conduction band edge. We show that a non-exponential dependence of capacitance, transport resistance, and recombination resistance on photovoltage can be interpreted as evidence for partial unpinning of the TiO₂ energy levels. We also find that the nature of the sensitizing dye determines the predominant recombination route: via the conduction band for Y123 and via band gap states for Z907, which is the main reason for the superior performance of Y123. The different mechanisms appear to arise from changes in electronic coupling between TiO₂ donor states and [Co­(bpy)₃]³⁺ acceptor states, as opposed to changes in the density of TiO₂ states or their energetic matching with the acceptor-state distribution. These findings have implications for modeling heterogeneous electron transfer at dye-sensitized semiconductor–solution interfaces in general and for the optimization of DSCs