| With the high energy conversion efficiency, low cost and environment friendly characteristics, dye-sensitized solar cells (DSSC), have proven to be extremely versatile in commercial applications. Research in enhanced performance and stability of the nano-crystals TiO2 photo-anode of DSSC has also been promising. The challenges at present are the optimization of interface characteristics of the TiO2 photo-anode thin film and its particle size and pattern, the improvement of TiO2 electrode material and structure, suppress of electronic recombination as well as adjusting TiO2 electronic band structure, and improving the synergistic effect of its structural properties so that the efficiency of the photoelectric conversion can be improved.The paper takes use of the first-principle theory of Density Functional Theory to calculate the electronic band structure of TiO2 nanocrystals doped with different metal ions and their influence on photoelectric performance. Furthermore, the bandgap structures of Sr2+、La3+、Cr3+、Sn4+doped TiO2 nanocrystals.were calculated by First principle theory in DFT, and the results show that all these four metal ions have led to the change of the electronic band structures of TiO2 nanocrystals. After the doping of Sr2+、La3+ into TiO2 nanocrystals, the Fermi level has entered the valence band of TiO2 and represents p-type semiconductor property. Fermi level entered the valence bond band of the doping of La3+but the depth of the entrance is shallower than Sr2+which contributes to the light absorption to make light absorption. After the doping of Cr3+, its Fermi level goes through the impurity energy level, showing the property of n-type semiconductor. The doping of TiO2 nanocrystals into the Sn4+belongs to the doping isoelectronic impurity. According to the factors listed above, we can speculate that the photoelectric conversion performance of TiO2 nanocrystalsdoped in Sr2+、La3+、Cr3+ will be improved. The separation of electron hole is more effective.TiO2 and different metal ions doped TiO2 nanocrystals were synthesized by hydrothermal method using tetrabutyl titanate as precursor, and were used to prepare thin film anode of DSSC. The representations show that the size of TiO2 nanocrystals is about 20nm while the size of TiO2 nanocrystals doped with Sr2+is 15 nm. Both of them show a porous mesh structure and the doping of Sr2+promotes the generation of anatase-phase. Sr atom has been doped into the crystal lattice successfully, led to the change of the electronic band structure. The experimental results show the doping of Sr2+can improve the performance of photoelectric. When the doping amount of Sr2+raised upto 1moL%,η raised from 7.61% to 7.92%. These reasons are attributed to several aspects:first of all, the synthesis parameters, doping amount and film structure which be accurately controled by the appropriate doping of Sr2+is suitable for the photoelectric property of DSSC; after this has been applied on DSSC, the test result confirms the optimized anode structure is composed of TiO2/TiO2/Sr-TiO2. The test results of Mott-Schottky indicate that the doping of Sr2+ which contributes to the increase of photo-voltage. The test results of Mott-Schottky indicate that the doping of Sr2+leads to the negative shifts of the flat-band which contributes to the increase of photo-voltage. The analysis result shows that the doping of Sr+increases the resistance, suppresses the electronic interfacial recombination of the electronic interfacial and promotes the electronic transport velocity. The lifetime of electron has been increased and the electric charge collection efficiency has been improved. Thus the photocurrent density Jsc, the impact factor (FF) and photoelectric conversion efficiency η have all been improved. These experimental results are identical with the theoretical calculation results basically.Nanocrystalline TiO2 doped with La3+was synthesized by hydrothermal method and then the photoanode with the structure of TiO2/TiO2/La-TiO2 was prepared. The analysis results show that La3+doping promotes the generating of anatase phase TiO2, and Anatase phase was obtained when the volume of addition is more than 0.8%. The La atom has been successfully doped into the lattice of TiO2. After the doping of La3+, the flat band potential of TiO2 negatively moves for 0.029V and increases the Voc. The doping of La3+ increases the resistance between TiO2 photo-anode and the electrolyte and impedes the electronic interfacial recombination between them. Moderate doping of La3+ can promote the electronic transport velocity and decrease the electronic interfacial recombination which can raise the electric charge collection efficiency up to 93.69% from 68.38%. On the basement of the above factors, the photoelectric conversion efficiency of La-TiO2 Nano-crystals photo-anode DSSC can reach its maximum value of 7.25% when the doping volume reaches to 0.8moL%. It has raised 16.94% compared to the DSSC without being synthesized by the thin film doping with TiO2. The performance of DSSC is totally improved.The colloid of TiO2 Nanocrystals doped with Cr3+and Sn4+has also been synthesized by hydrothermal method. The analysis results show that as the doping volume of Cr3+being improved, TiO2 nanocrystals of the anatase phase increase its proportion and it will reach 100% after the addition volume is more than 0.8%. While the doping of Sn motivates the formation of rutile phase TiO2 nanocrystals. When the doping volume reaches 0.75moL%, it will be totally turned into rutile phase TiO2 nanorods with a length of 100-200 nm. After that, the photo-anode with structure of TiO2/TiO2/Sn-TiO2 was synthesized, and the results show that the photoelectric conversion efficiency of TiO2-0.1moL% Cr has raised 7.40% compared with those without doping, while on the other hand, the properties of DSSC with Sn-TiO2 photoanode obviously drop down. The tests show that the doping of Cr3+can negatively shift the Fermi level which then improves the voltage of open circuit of DSSC and increases the resistance. Thus it finally impedes the electronic interfacial recombination. The analysis results show that when the doping volume of Cr3+ is 0.1moL%, the time of electrical transmission is short, the electron diffusion coefficient is high and the rate of electrical transmission is fast. The electronic life time of it is short and there is a large amount of depletion which makes differences of the short circuit current density and increase the collection efficiency of the charge. Development of the three layers TiO2/TiO2/Mmetal-ions-TiO2 nanocrystals photoanode helps to improve the photoelectric conversion efficiency; relationship established in structure and performance may reveal the mechanism on the photoelectric conversion efficiency. The Rw、τn、ηcc, are the significant keys to the improvement of the photoelectric performance of DSSC. |
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