Enhancing the open-circuit voltage of dye-sensitized solar cells: coadsorbents and alternative redox couples

In Feburary 2008, the oil price easily exceeded $ 100 per barrel due to the weak US dollar and the imbalance between the increasing demands and deficient supplies. People are paying more and more attention to seek for alternative energy sources that would suffice the modern society in the following high-oil-price era. The work in this thesis is associated with some fundamental research in one of the solutions to the energy shortage, photovoltaics. Particularly, the dye-sensitized solar cell was taken as the system where the effects of coadsorbents and alternative couples to the classic iodide/iodine redox were studied and rationalized. The first chapter was a general introduction to the photovoltaics and dye-sensitized solar cells, such as the operating principles and the characteristics of the dye cell. In Chapter 2, we specified all the experimental issues, including the chemicals, materials, film preparation, characterization techniques and data analysis. A short part was also dedicated to the basics of the photovoltaics. We studied the electronic effect of the scattering particles in our devices in Chapter 3. These particles were of 400 nm in diameter and always put on top of the nanotransparent layer to increase the light harvesting of the devices. It was found that the particles gave a small dark current but under illumination, they made a significant contribution to the total photocurrent. Photovoltage and photocurrent transient decay measurements performed under bias illumination showed that the density of electronic states of the light scattering layer was two times smaller than that of a transparent nanoparticle layer. From Chapter 4 to Chapter 7, we systematically studied the function of the coadsorbents in our Application of an ω-guannidino carboxylic acid was found to increase the open-circuit voltage of the device by 50 mV. Coadsorbents with similar structures were then employed with an amphiphilic ruthenium sensitizer, Z-907, to scrutinize this effect. The voltage shift was interpreted as the effect of a negative shift in conduction band and a decrease in the back electron transfer of photoinjected electrons in TiO2 to the triiodide species in the electrolyte, the former being the main reason for the large photovoltage increase. The specificity in the structure of the coadsorbents and its implication were also discussed. In Chapters 8 and 9, we studied the possibility to use some one-electron transfer couples as the replacement of the iodide-based electrolyte. Mainly, 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO), phenoxazine derivatives and tetrathiafulvalene were selected as the candidates and their device performances were evaluated by electrochemical, photochemical and phototransient measurements. The redox potentials of these couples were 100 to 400 mV more positive than that of I- / I3- couple, mitigating the potential mismatch between the Nernst potential of the dye cation and that of the redox mediator. TEMPO/TEMPO+ based devices showed the best conversion efficiency of more than 5 % under AM 1.5 illumination at 100 mWcm-2, a new record at this light intensity for non-iodine and non-pseudohologen electrolytes. On the other hand, the fast recapture of electrons in TiO2 by the oxidized species of the couple was accounted for the lower observed increase in Voc of the devices with these electrolytes than the simple shift in the redox potentials. We drew some final conclusions in Chapter 10 together with our outlooks in the future research and development of coadsorbents and electrolytes for dye-sensitized solar cells.

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