Electrolytes And Dyes In Dye-sensitized Solar Cell Quantum Design

In recent years, more and more attention has been paid to the utilization of solar energy for the shortage of traditional energy. Dye-sensitized solar cells (DSC) are attracting widespread academic and commercial interest for their high energy conversion efficiency and low cost. A new record conversion efficiency of 11% has already been achieved in the laboratory of Gr?tzel; however, its practicality still remains challengeable for the volatility of organic liquid electrolyte. The leakage of liquid electrolyte limits the long-term performance of DSC. In this part of our work, framework with high thermal stability was employed to fabricate quasi-solid-state DSC with moderate efficiency and long-term stability. The main results and conclusions are summarized as follows:1．Polyether-grafted ZnO nanoparticles are prepared through sol-gel method, and the nanocomposite was applied as solidifier to produce quasi-solid-state solar cells.A stable gel electrolyte was prepared by dispersing PEGMe-ZnO nanoparticles into liquid electrolyte. The ionic conductivity of this gel electrolyte reaches 3．34×10^-4 S/cm at 303K. A quasi-solid-state DSC based on the gel electrolyte yields the energy transfer efficiency of 3．1% at AM1．5 direct irradiation of 75mW/cm light intensity. Addition of 4-tert-butylpyridine into the electrolyte results in dramatically improved Isc and the overall efficiency is also improved to 5．0%. In addition, the quasi-solid-state DSC maintains 93% of its initial efficiency value after storage at 55°C for 34 days.In addition, two kinds of TiO₂ electrode were produced with sol-gel method (A) and ball milling method (B). The size of TiO₂ particle in electrode B is comparatively larger than electrode A. When these two electrodes were employed in DSC with liquid electrolyte, the performance of electrode A preceded electrode B for the larger resistance in electrode B. While they were applied in quasi-solid-state DSC, the performance of electrode B excelled that of electrode A. The larger interspace between particles in electrode B is advantageous for the penetration of gel electrolyte, which means the contact property between the electrode and electrolyte is the crucial factor to determine the overall efficiency of quasi-solid-state DSC. This conclusion is valuable to guide the optimization of quasi-solid-state DSC.2．Water was applied as the plasticizer to fabricate PVDF-HFP porous film, and the polymer film was employed as framework to produce quasi-solid-state DSC. The liquid uptaking ability of porous film as well as the ionic conductivity of gel polymer film increases with the increase of the amount of plasticizer water. The resultant gel electrolyte was applied to produce quasi-solid-state DSC. The contact performance between TiO₂ electrode and this polymer electrolyte was improved for its porous structure. It is indicated that narrower cell gap is advantageous to energy transfer efficiency although there is no distinguished difference between their ionic conductivity values. The energy transfer efficiency of quasi-solid-state solar cells with cell gap of 30μm reaches 6．0% compared with 8．6% of solar cells with liquid electrolyte. The DSC fabricated with this gel polymer electrolyte displayed better thermal stability than those with liquid electrolyte. Water as plasticizer is environmentally friendly and safe, and it is favorable for mass production of DSC.The dye acting as photosensitizer is one of the key components for the efficiency of the cells. Numerous experimental researches on dyes have been conducted widely; however, theoretical investigations on them remain limited. In the second part of our work, we tried to elucidate the process related to their photoelectric property of the ruthenium dyes and organic dyes in order to provide guideline for molecular modification. The main results and conclusions are summarized as follows:1．Several factors related to the dyes determine the efficiency of solar cells, such as light harvesting efficiency, quantum yield of charge injection etc. These steps of N3^4- and its two similar complexes (complex 1 and 2, see figure 6．1) were thoroughly examined with density functional theory (DFT) method in our work. Our computational results indicated that variety of ligand L has little effect on the molecular structures of this series of complexes. The electronic structures of dyes vary greatly by the replacement of ligand L. The orbital analysis shows that the orbital compositions of complex 1 and 2 share more common characters than N3^4-.The absorption spectra of the polypyridyl complexes can be well reproduced with theoretical methods. According to our computational results, low light harvesting efficiency is probably the main cause for the low efficiency of complex 1 and 2．Energy gaps between the conduction band edge of TiO₂ and excited state redox potential of dyes can be employed as the driving force for electron injection. The driving forces of ultrafast and slower component for complex 1 and 2 are much smaller than N3^4-. The comparatively slower electron injection rate brings out the lower efficiency of complex 1 and 2． To summarize, N3^4- has not only intense and broad absorption bands but larger driving force for electron injection. These factors are all advantageous for its excellent photoelectrochemical performance. Thorough theoretical investigation of these polypyridyl complexes provides us with valuable clue to design new efficient photosensitizer conveniently.2．DFT method has also been employed to examine the light harvesting and electron injection process from excited-state coumarin dyes into conduction band of semiconductor electrode. The simulated UV-visible absorption peaks of coumarin dyes fit well with the experimental data. According to our computational results, the expanding of conjugated system always results in red shift of absorption band. The absorption band of NKX-2398 and NKX-2388 are situated in shorter wavelength region compared with that of NKX-2311, which means the light harvesting efficiency of NKX-2311 is higher than the other two dyes. That is the reason why the overall efficiency of these two dyes is lower than NKX-2311．Energy gaps between the conduction band edge of TiO₂ and excited-state redox potential of dyes can be employed as the driving force for electron injection. Our computation indicated that the driving force of NKX-2311 is larger than that of NKX-2586．The predominant performance of NKX2311 roots in its high light harvesting efficiency and favorable excited state energy level.As HOMO energy level is linear with redox potential, simulated absorption spectrum and HOMO orbital energy level can be employed to judge the efficiency of the dyes. These two parameters can be obtained with theoretical method, and then the efficiency of coumarin dyes may be figured out before they are synthesized. By means of this method, it is found out that introduction of electron-donating substituents at site d or two kinds of substituents with steric effect into molecular structure of NKX-2311 is favorable for its efficiency. Our computational method is proved to be a powerful tool for molecular modification of coumarin dyes which will save us much time and money.