Development of dyes and electrolytes for dye sensitized solar cells

Due to exhaustion and environmental hazards of fossil fuels, importance of renewable energy sources has been increased. Fossil fuels have several drawbacks such as environmental pollution and climate change due to the carbon footprint. Also, these type of energy sources are not sustainable. Renewable energies such as solar energy, wind energy, geothermal energy etc. have also been required because of increasing energy consumption of humankind. While some of these renewable energy sources are sustainable some of them are not. Among all renewable energy sources, solar energy comes to the forefront due to being cheap and almost limitless. Also, it is a sustainable energy source and it replenishes at a higher rate than consuming. Solar cells are photovoltaic devices that converts sunlight into the electric energy. There are lots of different type solar cells that use different architecture and utilize different materials. One of these types of solar cells are dye sensitized solar cells that were emerged from nanotechnology. Dye sensitized solar cells are type of cells that have been using dye to capture photons from the sun and convert them into the electrical energy. Dye sensitized solar cells (DSSCs) have been consisted of photoanode, dye, electrolyte and counter electrode. Usually, TiO2 coated FTO substrates have been used as photoanode in dye sensitized solar cells. As counter electrode, Pt coated FTO glass has been utilized. There have been studies to develop new materials for photoanode and counter electrode, but TiO2 and Pt have been considered unchangeable due to their properties such as stability and conductivity. However, dyes that have been utilized for DSSCs have a wide variety. One of the types of dyes is metal complex dyes, especially ruthenium complexes, had been dominated dyes used in dye sensitized solar cells. Ruthenium complex dyes have high thermal stability and very broad absorption range in the UV-Vis area. Although Ru dyes have high efficiency when used in dye sensitized solar cells, ruthenium is a rare earth metal. Thus, using ruthenium dyes makes dye sensitized solar cells are expensive. Another type of dyes is metal-free organic dyes. These dyes can be adjusted to desired band gap or light absorption values by designing their molecules with proper functional groups. In addition, metal free organic dyes can be synthesized with a push-pull structure. With push-pull mechanism e- transfer will be more efficient. Nevertheless, synthesis and purification of metal free organic dyes contains many steps. Final type of dyes in DSSCs area is natural dyes. These dyes can easily be found in nature, especially from flowers, fruits and vegetables. Natural dyes can be extracted with single processes and used as dyes in DSSCs. Even though natural dyes are safer and green approach for dye sensitized solar cells, cells have been used these dyes have low efficiencies. Because of limited light absorption and being unstable against light, natural dyes are not as favored as other dyes. Therefore, developing new dyes for dye sensitized solar cells have been an important topic. Solar spectrum that has been reached to earth surfaces is strong between ~ 250 nm-1200 nm. For this reason, dyes that are selected for dye sensitized solar cells should show high absorbance in this region. Co-sensitization, which using two or more dyes with different absorption at the same time, is one of a method for dye sensitized solar cells. By using co-sensitization, more broad absorption can be obtained, thus, more photons from sun can be captured. In addition, using two dyes can prevent dye aggregation on semiconductor surfaces if dyes were selected with proper molecular structures. Finally, by using more than one dye, new energy levels can be introduced to overall energy levels of dye sensitized solar cells components with co-sensitization. In this way, recombination can be prevented or decreased. One of the other components worth investigations for dye sensitized solar cells are electrolyte. Traditional electrolyte for dye sensitized solar cells is a redox couple dissolved in organic solvents with some additives. Generally, I-/I3- redox couple dissolved in acetonitrile have been used with the addition of 4-tert-butylpyridine. However, using an organic solvent brings some problems to the stability of cells. Organic solvents that have been using for electrolyte have low boiling point. Thus, these liquid electrolytes are tended to leakage and evaporation from solar cells. In long term, these leakage and evaporation will cause to decrease at the efficiency of solar cells. For overcoming these problems, different phases of electrolytes such as quasi-solid or solid have been utilized. Solid electrolytes eliminate leakage and evaporation. However, interface between solid electrolyte with photoanode is limited due to electrolyte being solid. This causes poor charge movement between electrolyte and photoanode. Therefore, efficiency of solar cells that uses solid electrolyte are lower than liquid electrolytes. On the other hand, quasi-solid materials show properties from both liquid and solid phases. While quasi-solid electrolytes are not very fluid when comparing to liquids, charge movement is better than solid electrolytes. In this way, they eliminate the problems of liquid electrolytes and do not yield low efficiency when used in solar cells comparing to solid electrolytes. Also, some properties such as self-healing, anti-freezing and self-adhesiveness make quasi-solid electrolytes are attractive for preparing solar cells with different application areas. Within the scope of thesis, 4-tert-butylsulfonyl and carboxylic acid containing Zn and TiO naphthocyanines were synthesized and used as dye for dye sensitized solar cells. Additionally, different ratio of YD2 porphyrine and synthesized dyes were used for co-sensitization and prepared dye cocktail were investigated for dye sensitized solar cells. From theoretical studies it can be understand that while ZnNSPPc had a planar structure, planarity of TiONSPPc was distorted. From the electrochemical characterization techniques such as CV and SWV, synthesized ZnNSPPc and TiONSPPc dyes HOMO and LUMO values were found to be suitable to use in DSSCs. When only synthesized naphthocyanines were used as dyes in dye sensitized solar cells, Zn-naphthocyanine was yielded 5.59% and TiO-naphthocyanine was yielded 6.23%. When YD2-synthesized naphthocyanine coctails were utilized for dye sensitized solar cells, highest efficiency of 11.64% was obtained with YD2:Zn-naphthocyanines (3:1) mixture. Another study performed in this thesis was theoratically investigation of Fe3+, Co3+ and Zn2+ ions metal complexes with gallic acid and isothiocyanate ligands. In this study, HOMO-LUMO energies were calculated and frontier molecular orbitals were obtained via DFT method. Dye compatibility for dye sensitized solar cells with these parameters were discussed in the thesis and the most suitable dye was determined as Co3+-gallic acid-isothiocyanate complex. Other study on dyes within the scope of thesis is synthesizing metal complex dyes of Co3+ and Zn2+ with diphenylcarbazone and tartarazine. Prepared dyes were used as dye separately for dye sensitized solar cells. While tartrazine did not form complexes with these ions, diphenylcarbazone complexes were synthesized successfully. Highest efficiency was obtained with Zn-diphenylcarbazone complex as 1.91%. In addition, novel quasi-solid gels were developed and used as electrolyte for dye sensitized solar cells within the scope of thesis. One of the studies about this topic is developing gellan gum based gels that contain different amount LiCl and poly(3-hexylthiophene). Obtained gel electrolyte showed anti-freezing and self-adhesive properties due to the hygrosopic nature of LiCl. These properties make this gel electrolytes potential candidates for different usage areas. DSSCs that have been used liquid electrolytes cannot work at below zero temperatures due to the freezing of electrolyte. But using a gel electrolyte that shows resistance to freezing make DSSCs can be used at very cold environmental conditions. Prepared dye sensitized solar cells showed open circuit potential more than 1 V and solar conversion efficiency were obtained as 4.64% with gel electrolyte containing 6 M LiCl. Another study about development of electrolyte is poly(3,4-ethylenedioxythiophene) polystyrene sulfonate doped gellan gum based gels. Solar conversion efficiency of dye sensitized solar cells that used this gel electrolyte found to be 2.01%. In the same study, thickness effect of gel electrolyte was investigated using gellan gum gels and it was observed that thickening gel caused low efficiency for dye sensitized solar cells due to increasing the pathway of charges in electrolyte.