Yoann Farré

Isoindigo derivatives for application in p-type dye sensitized solar cells

In this study, we have investigated for the first time the use of isoindigo derivatives as sensitizers in NiO-based dye-sensitized solar cells (DSSCs). For this purpose, two indigo sensitizers were prepared and their electronic properties were characterized by UV/visible spectroscopy, cyclic voltammetry and time-dependent density functional theory (TD-DFT). The first dye contains a N,N-di(4-benzoic acid)phenylamine moiety acting as anchoring/donor group, and the isoindigo acting as the acceptor, while the second compound is a dyad which is based on the same structure, but is additionally functionalized with a naphthalene imide unit, acting as a secondary electron acceptor. The electronic properties were also modeled by TD-DFT quantum chemistry calculations and they revealed that a charge transfer band is present between the trisarylamine donor part and the isoindigo moiety. The photovoltaic performances of these new dyes were evaluated in NiO-based DSSCs with both iodide/triiodide and cobalt electrolytes. It turned out that they perform well since the photocurrent was generated up to the wavelength of 700 nm. Altogether, these results underscore the viability of isoindigo dyes for p-DSSCs.

Copper borate as a photocathode in p-type dye-sensitized solar cells

p-Type dye-sensitized solar cells (p-DSSCs) have recently become a major research focus because coupling with n-type DSSCs yields highly efficient tandem DSSCs. Indeed many delafossite-like compounds appear as promising candidates for p-DSSCs due to their deep valence band position and high hole mobility. In this paper, the synthesis of CuBO2 was attempted by a facile sol–gel methodology. Then, the as-obtained particles were used to prepare photocathodes for p-DSSCs with DPP-NDI dye as sensitizer and tris(4,4′-di-tert-butyl-2,2′-bipyridine)cobalt(III/II) as redox mediator. Due to the deeper valence band position compared with classical NiO photocathode, the “CuBO2” based p-DSSC presents an open-circuit photovoltage (Voc) of 453 mV, which is 150 mV higher than that of NiO in the same conditions. The results show that “CuBO2” is a potential alternative for NiO in p-DSSCs.

Molecular-structure control of electron transfer dynamics of push–pull porphyrins as sensitizers for NiO based dye sensitized solar cells

Porphyrin dyes were synthesized for use in p-type (NiO) dye sensitized solar cells based on different design principles. One porphyrin was designed with a significant charge transfer character in the excited state because of push–pull effects of the substituents. Another porphyrin had instead an appended NDI acceptor group (NDI = naphthalene diimide). The dyes were characterized by spectroscopic, electrochemical and DFT methods. Solar cells based on sensitized, meso-porous NiO showed rather poor performance compared to other organic dyes, but with a clear improvement for the dye with the NDI acceptor. Ultrafast transient absorption spectroscopy and nanosecond laser photolysis showed that hole injection into NiO was followed by unusually rapid charge recombination, predominantly on a 50–100 ps time scale, which is likely the main reason for the poor photovoltaic performance. Again the porphyrin with the NDI group showed a more long-lived charge separation that should lead to better dye regeneration in a solar cell, which can explain its better photovoltaic performance.

CuO nanomaterials for p-type dye-sensitized solar cells

In p-type dye-sensitized solar cells (p-DSSCs), NiO is the most commonly used p-type semiconductor. Nevertheless, because of the drawbacks of NiO, much effort has been made to search for suitable substitutes. Herein, three different morphologies of CuO nanomaterials were used to prepare photocathodes for p-DSSCs, which have a deeper valence band and a higher dielectric constant compared to that of NiO. We observe that CuO is unstable in the presence of iodide/triiodide electrolyte, while cobalt complexes with bipyridine ligands are more suitable redox shuttles. We also note that the average transport time in CuO is shorter than that in NiO. Finally, the deep absorbance of CuO in the visible range indicates that suitable sensitizers for the CuO p-DSSC must exhibit high extinction coefficient and absorption bands located in the lower energy part of the solar spectrum (>600 nm) to be exploitable. In this case such CuO based photocathodes represent valuable systems to exploit the near-infrared (NIR) region.

Experimental and theoretical evidences of p-type conductivity in nickel carbodiimide nanoparticles with a delafossite structure type

Nickel carbodiimide (NiCN2) was synthesized using a two-step precipitation-decomposition route leading to a brown powder with gypsum-flower-like morphology and a large specific surface area (75 m2/g). This layered material crystallizes in the 2H structure type of delafossite (space group P63/mmc), which is built upon infinite 2/∞[NiN2] layers connected by linear carbodiimide ([N═C═N]2-) bridges. An X-ray diffraction Rietveld refinement and thermal analyses pointed out some nickel deficiencies in the material, and band structure calculations carried out on the defect compound predicted p-type conductivity in relation to a slight amount of N2-. This p-type conductivity was demonstrated by electrochemical impedance spectroscopy measurements, and a flat band potential of 0.90 V vs SCE at pH 9.4 was measured. This value, which is more positive than those of CuGaO2 and CuCrO2 delafossite oxides and NiO, prompted us to test NiCN2 nanoparticles as a photocathode in p-type dye-sensitized solar cells.