Our group has made original and innovative contributions to the synthesis, discovery, characterization and understanding of fundamental physical and chemical properties of oxides, organic-inorganic hybrid films and nanostructures. It is also interested in the integration of these films and structures in efficient devices and in their applications. Our projects undertaken involve the following three key research axes:
Films preparations by solution-based techniques: electrochemical, chemical deposition, hydrothermal, chemical coating, spin-coating and sol-gel techniques. Oxide film and structure syntheses and characterizations. Hybrid perovskite preparation. Lead-Free perovskite materials.
Strategies for semiconductor surface and nanostructure functionalizing by organic and inorganic components (dyes, quantum dots, perovskite, electroactive molecules…). This includes modelling by DFT approaches in collaboration with the i-CLeHS laboratory at Chimie-Paristech.
Application and integration in devices of advanced materials. Photovoltaic is a strong expertise of the group. In the past we have worked on dye-sensitized solar cells (DSSC), Quantum-Dots Solar cells (QD-SC) and p/n junctions. We have also published strong impact works on light-emitting diodes, sensors and photodetectors.
Our current main researches topics are described in what follows
Perovskite solar cells
Solid state perovskite solar cells (PSCs) were first published in 2012. Since their discovery, their record efficiency for solar irradiation power conversion has raised rapidly and continuously. PSCs are presently the most performant thin film solar cell technology and they are close to the efficiency of the best silicon technology. Pr. Th. Pauporté has been a pioneer in the field since he started researches on PSCs as early as 2013.
The main present bottlenecks for a wide diffusion of this technology are the stability, the cost and the presence of lead, a toxic element, in the most efficient materials. Our group is developing researches to address these three key issues.
More efficient and Stable Perovskite solar cells
We develop structures and material assemblies to get high efficiency PSCs. In our recent researches, additives in the perovskite precursor solution have allowed us to prepare perovskite layers which were highly covering and made of big grains. The unencapsulated devices produced are highly efficient and they exhibit a high stability especially under solar illumination. They also have a good stability when stored in a high humidity atmosphere.
An improvement of the PSCs has been been achieved by engineering the TiO2/perovskite interface using self-assembled layers. We have tested various benzoic acid derivatives and amino-acids. We have shown a significant improvement of the charge transfer and of the interface quality by using chloride functionalized modifiers. It resulted in devices with a higher power conversion efficiency. The interface has been modeled by i-CLeHS and the electron transfer has been quantified. Using this modifier, the transfer is quantitative (99.94%) in agreement with our experimental data.
Another axis of our research to improve the stability of the PSCs is to use 2D/3D compounds. The precursor formulation is adjusted and also additives are employed to reach this objective.
Less expensive Perovskite solar cells
Among the components of PSCs, the most popular hole transporting material (HTM) employed is the Spiro-OMeTAD. This compound is complex to prepare and it entails extra costs. In Collaboration with the Cergy-Pontoise University, we develop new HTMs. They are based on carbazoles, their synthesis is simple and they are much cheaper to produce than Spiro-OMeTAD. Some of them have been shown to be as efficiency as their benchmark counterpart.
Less Toxic Perovskite solar cells
One of the components of hybrid halide perovskites is the lead element. This element present toxic properties and there is an effort of the scientific community to employ less lead or the replace this element in the absorber of the solar cells. In collaboration with Moltech Angers, the lead-deficient family of hybrid perovskites is developed. This family of compound has been patented. They are tested in solar cells.
Also compounds of the perovskite family which do not contain lead are developed and we investigate them for PV application.
Halide Perovskite single crystals for advanced applications
We are developing techniques for the growth of large halide perovskite single crystals. These crystals can be grown at low cost from solutions. The growth is based on varying the solubility of the precursors in a solvent by slowly diffusing an anti-solvent of by playing on the temperature. Nice single-crystals have been obtained. They have been tested for their luminescence properties at various temperatures. They are also investigated for their scintillation properties under X-ray and g-ray excitation.
Electrochemical growth of oxide nanostructures
Electrochemistry is a versatile technique for obtaining nanostructure of wide bandgap oxides. We have especially developed the doping of semiconductors by using various additive elements in electrodeposition baths. We have been mostly interested in the preparation of pseudo-1D nanostructures of ZnO such as nanowires and nanorods. By playing on the electrochemical growth parameters, we have also been able to prepare, in one step, nanowires decorated with platinum, gold or silver nanoparticles.
Doped or decorated ZnO nanowires have been integrated in sensor devices. Palladium has been shown an especially interesting additive for sensor application. Hydrogen nanosensors made of single nanowires contacted on both ends have been prepared. They have been found highly selective to hydrogen and they presented a high sensitivity at room temperature.
The nanostructures are also developed for the photodetection of UV and visible light. For instance miniaturized photodetectors have been prepared and have been shown efficient.
Electrochemical growth of functional coatings
In this part of our research, dense, conformal and rather thick layers of oxide or metal layers are prepared by electrolysis. The precursors are dissolved in solution and by reacting them at the surface of an electrode, a coating is formed. This approach can be used with conducting substrates. By the technique, we have been able to produce thick magnetite layers that can model the deposits formed by corrosion in vapor generators.
For some elements, using aqueous solutions to perform electrodeposition of a desired compound is not possible. It can be due to either a very negative reduction potential or the instability of the ions in water which react and form for instance an oxide over the whole pH range. To circumvent these problems, we develop the electrodeposition in ionic liquid (IL) solution. ILs exhibit large electrochemical windows and can dissociate ionic precursors. ILs act both as the solvent and supporting electrolyte. The experiments are performed in an inert atmosphere. By following this approach, we have done the electrodeposition of silicon, tantalum and zirconium layers.