Fotoeletrodo híbrido formado por TiO2/lignina depositados por spin coating com aplicação em células solares sensibilizadas por corante (CSSC)

Abstract

The growing world energy consumption with a possible depletion of fossil fuels presents an increasing need for the development and use of renewable energy sources. In this way, solar cells have gained space and among the various categories of solar cells, dye-sensitized solar cells (DSSC) have excellent efficiencies in converting light energy into electrical energy. However, for these cells to gain competitiveness in the energy market, it is necessary in addition to efficiency, long life and low manufacturing cost. Thus, in this doctoral thesis, thin hybrid films formed by TiO2 (99% pure titanium, anatase phase) together lignin without modification in its structure were deposited using the “spin coating” technique applied to the DSSC photoelectrodes. TiO2 is not a suitable material for use in CSSC due to its lack of energy absorb in the visible region of the solar spectrum. However, its combination with Lignin allows absorption from the ultraviolet to visible region and the manufacture of a TiO2/Lignin hybrid thin film with excellent efficiency, in addition to low manufacturing cost. Thus, to achieve the objective of this thesis, a “spin coater” equipment was developed and manufactured with alternative materials. For its validation, thin TiO2 films were deposited by spin coating on transparent TiO2 films (commercial). Then, TiO2/lignin hybrid thin films in percentages of 5 (EST5LG), 10 (EST5LG) and 15% (EST5LG) of lignin, were deposited on commercial TiO2 films (opaque) with application on CSSC photoelectrodes. The thin films deposited by “spin coating” were characterized morphologically (UV-vis, SEM, EDS and X-ray) and electrically. The results indicated that the transparent TiO2/TiO2 thin films (commercial) had a thickness of 65.9 μm and an efficiency of ƞ = 12.74%, a value already close to the current 13% of the technology. And for the TiO2/lignin hybrid thin films, the film with 15% lignin (EST15LG) showed the best results. Thickness of 11.59 μm, with band gap reduction from 3.66 to 2.84 eV, current density of 11.06 mA/cm2 and an efficiency of ƞ = 4.65%, superior value of the solar cell mounted only with the opaque thin film (ƞ = 2.00%). The morphological results also indicated that the presence of lignin in the TiO2 matrix behaved both as a good absorber of ultraviolet rays and for the absorption of visible light. And X-ray indicated that the addition of lignin in the structure of TiO2 did not change its structure with the presence of peaks characteristic of TiO2 anatase phase. And the EDS analysis allowed to identify a reduction in the relative proportions of the amounts (Wt%) of the main elements present (Ti/O) in a specific area, from 4.54 (EST5LG) to 3.66 (EST15LG). This reduction allowed to obtain thin films with larger surface area and grain size, so that more dye can be adsorbed on the surface of the film and consequently higher the efficiency of the solar cell. In view of the results, it was possible to conclude that thin films of TiO2 and TiO2/lignin showed good control of thickness and morphology. In order to indicate the possibility of obtaining TiO2/lignin thin films with application in CSSC photoelectrodes in an economical and sustainable way. Furthermore, the developed equipment assembled with alternative materials proved to be efficient for deposition of thin films in dye-sensitized solar cells, by a simple and low cost method.