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Novel materials and New design for Dye Sensitized Solar Cells Technology

This thesis work was devoted to the study of possible application of new materials for the development of Dye Sensitized Solar Cells (DSSC) with improved solar conversion efficiency and stability. Particular attention was given to the use of novel nanostructured materials for the preparation of more stable non-liquid electrolytes and novel organic dyes. One of the major problems of DSSC is their low stability, due to the high volatility of the electrolyte and the toxicity of some components. Non-liquid electrolytes were prepared by adding to methoxipropyonitrile-based electrolyte 5wt% of inorganic or hybrid nanoparticles. Synthetic clays with different chemical composition (i.e. saponite and talcs) and TiO2 aiming to prepare stable electrolytes. Saponites are phyllosilicates with TOT structure; the nanoparticles size decreases from ca 200 nm up to 50nm. The nanoparticles size can be tuned by varying the amount of H2 O/Si ratio used for the preparation of the synthesis gel. In this work, saponite materials prepared by usind H2 O/Si ratio of 20, 50, 110, 150 were tested (ranging from 200 to 50 nm) were tested. The electrochemical measurements done on cells prepared by using non-liquid saponite-based electrolyte revealed a slight increase of efficiency for all cells. In addition, for all saponite-based devices (except for saponite prepared with H2O/Si ratio of 150 ratio) an increase of short circuit current (Jsc ) suggesting a light scattering phenomenon, due to saponite nanoparticles, allowing to increase the overall cell efficiency. Beside saponite, synthetic talcs (that are magnesium silicates with neutral layers) were used as additive for non-liquid electrolyte preparation. In particular the surface of tested talcs were functionalized, by one-pot synthesis, with organic pendent groups (CH2 CH2 CH2 N H2 or CH2 CH2 CH2 N HCH2 CH2 N H2 ) into the interlayer space. I-V curves of talc-based devices revealed that the addition of talc has positive effect on the overall cell efficiency especially thanks to a possible positive influence of talc on avoiding recombination of electrons. The best performance obtained was an improvement of the relative efficiency of ca +12.5% for the cells with talc. Moreover quasi-solid electrolytes were prepared by adding 20%wt of saponite (both inorganic or hybrid saponite prepared by introducing COOH and NH2 groups on the surface) to ionic-liquid based electrolytes. Obtained results showed that the DSSC performances strongly depend on the nanoparticle size. An increase on the relative efficiency of ca +15% was obtained by using greater saponite particles. Beside layered solids, gel electrolytes were also prepared by adding to ionic liquids T iO2 nanoparticles. Obtained results showed that the solar cells performance increase occurs only by using gel electrolyte prepared by adding 15wt% or 20wt% obtaining an increase of relative efficiency of ca +25.6% with respect to the ionic liquid electrolyte. These results suggested the occurring of a possible Grotthus-like mechanism which allows an increase in the redox couple diffusion. Two antisymmetric squaraine dyes with different lenght of carbon chain were tested. The obtained results indicated that the VG10C8 (squaraine with longer chain) allowed to increase the performances of DSSC devices. It was pointed out that an increase in the carbon chain length allowed to obtain increased relative efficiency of 37% with respect to the dye with the short chain. An emitting dye, based on diphenylaniline group, the so-called D5 dye, was tested in DSSC prepared with a ionic-liquid-based gel electrolyte prepared by adding saponite nanoparticles. The so-prepared DSSC showed good performances with respect to cells prepared by using Ruthenium dyes and ionic liquid electrolyte overcoming the N719 and the Z907. As conclusion, in order to solve the sealing problems, two new cell designs were tested. The first method, called electrolyte bath, consists in soaking the titania electrode into the electrolyte solution before the sealing process. In this way, using gel electrolyte it should be possible to obtain more stable junction and it should be possible to solve the physical problem due to the sealing. Even if worse perfomances were obtained, the Voc enhances of +11% with respect to the reference acetonitrile-based electrolyte. The second method consists in the use of coloured TiO2 nanoparticles. The coloured TiO2 nanoparticles were obtained by soaking the untreated titania nanoparticles into a N719 dye solution for one night; then the mixture was filtered, by using a Buchner funnel, obtaining a coloured powder with the dye absorbed on the T iO2 surface. The powder was injected into a ionic liquid electrolyte. The method was tested adding 5wt% or 10wt% of nanoparticles, coloured and untreated, demonstrating that the coloured particles into the electrolyte don’t affect the performance of the cells.

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Abstract: Novel Materials and new design for DSSC technology This thesis work was devoted to the study of possible application of new materi- als for the development of Dye Sensitized Solar Cells (DSSC) with improved so- lar conversion eciency and stability. A DSSC is a photoelectrochemical device allowing electric power generation from sunlight. The DSSC working principles are illustrated in gure 1. Sunlight is absorbed by the dye which is absorbed on a TiO 2 -based electrode. Absorbed photon excites an electron from the HOMO to the LUMO of the dye. As a consequence, the excited electron is transferred into the titania electrode up to the external circuit. The dye is regenerated thanks to the presence of a redox couple (tipically I =I 3 ) contained into the electrolyte (normally in a liquid phase). Up to now, the best eciency of light Figure 1: Scheme of Dye Sensitized Solar Cell working principles conversion obtained by using DSSC technology is ca 12 % and is reached using Ruthenium complexes as sensitisers, liquid electrolyte (based on acetonitrile or methoxipropyonitrile solvents), TiO 2 nanoparticles for the electrode prepara- tion and Platinium-based counter electrode. In this thesis work, particular attention was given to the use of novel nanos- tructured materials for the preparation of innovative and more stable non-liquid electrolytes and novel organic dyes as light absorbers. The thesis work was de- veloped in the frame of the European project Innovasol concerning Innovative Materials for future generation excitonic solar cells that is devoted to the opti- mization of DSSC technology; thanks to the collaboration of six academic insti- tutions (Cambridge, EPFL, Unito, Unipmn, TUD, Universidade de Campinas) and two industrial partners (Centro ricerche Fiat and Solaronix). One of the major problems of DSSC is their low stability, due to the high volatility of the liquid electrolyte together with the toxicity of some components (solvent used in the electrolyte and dyes). In this work non-liquid electrolytes were prepared by adding to methoxipropyonitrile-based electrolyte 5wt% of inorganic or hybrid 4

Tesi di Master

Autore: Dario Cottafava Contatta »

Composta da 93 pagine.

 

Questa tesi ha raggiunto 105 click dal 29/02/2012.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.