Physico-chemical interpretations of the adsorption isotherms of D–π–A sensitizers with pyridyl group on TiO2 for dye sensitized solar cells using statistical physics and density functional theory

In this paper, a statistical physics theory was used to analyze the adsorption isotherms of three Donor (D)-π -Acceptor (A) dyes bearing pyridyl group on TiO2 for dye sensitized solar cells. The experimental data of these organic dyes fit with a monolayer model with two binding sites. Our model is established with the statistical physics approach and contain parameters with a clear physical significance rightly connected to the adsorption mechanism of all adsorbates. The main advantage of this development is to give physical meaning to all involved parameters to be able interpret information about physical adsorption at the molecular level which remains unapproachable by means of empirical methods. The modeling by the adequate model provides new microscopic and macroscopic information included in the adsorption process that did not arise in any case of all previous works. The result shows that the values of adsorption energies proved that a chemisorption process occurred. The density functional theory (DFT) determined the different adsorbed modes, the electronic structure, energy level alignment and electronic density difference maps (EDDs) of the three dyes. The energy levels alignment indicates that the dye regeneration in the NI-4/TiO2 system is the most efficient in DSSCs application. The EDDs showed that the electron density decreases and increases at the dye/TiO2 interface, which is beneficial for the intermolecular electron transfer (IET) process. The DFT calculations support the statistical physics modeling and confirm the strong coordinate bonding between the pyridine ring of dyes NI-4, YNI-1 and YNI-2 and the TiO2 surface