Free base, Cu(II) and Zn(II) complexes of the 2,7,12,17-tetrapropionic acid of 3,8,13,18-tetramethyl-21H,23H porphyrin (CPI) in solution and bounded to transparent monolayer TiO2 nanoparticle films were studied to determine their adsorption on TiO2 surface, to measure the adsorption kinetics and isotherms, and to use the results obtained to optimize the preparation of DSSC photovoltaic cells. Adsorption studies were carried out on monolayer transparent TiO2 films of a known thickness. Langmuir and Frendlich adsorption constants of CPI-dyes on TiO2 monolayer surface have been calculated as a function of the equilibrium concentrations in the solutions. The amount of these adsorbed dyes showed the accordance with Langmuir isotherm. Kinetic data on the adsorption of dyes showed significantly better fits to pseudo-first-order model and the evaluated rate constants linearly increased with the grow of initial dye concentrations. The stoichiometry of the adsorption of CPI-dyes into TiO2 and the influence of presence of coadsorbent (chenodeoxycholic acid) have been established. The DSSC obtained in the similar conditions showed that the best efficiency can be obtained in the absence of coadsorbent with short and established immersion times. 1. Introduction TiO2 is a stable, semiconductor with large band gap well known for its considerable applications in dye-sensitized solar cell (DSSC) systems [1, 2] and, for this, several preparation methods have been performed [3–6]. TiO2 nanoparticles layer, together with the dye-sensitizer loaded on its surface, is one of the most important parts of the DSSC structure and, therefore, the optimization of these parameters can enhance the DSSC efficiency. In the search of new dyes with high extinction coefficient and high DSSC performances, new ruthenium dyes [7], porphyrin dyes [8], and other metal-free dyes [9, 10] are synthesized. Porphyrins are particularly interesting as photosensitizers for DSSC for the absorption in the 400–450？nm region of Soret band and in the 500–700？nm region of Q-bands and also for the appropriate LUMO and HOMO energy levels; this makes them promising candidates as substitutes for ruthenium dyes in DSSC applications [11]. The increase in the use of porphyrin dyes as sensitizers in DSSC has been very important to clarify the role of natural porphyrins as light harvesting in the photosynthesis; the imitation of this process has been obtained using chlorophyll derivatives as dye sensitizers for nano-TiO2 films [12, 13], showing also the importance of free carboxylic groups for the anchoring on the
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