Enhancement in Conversion Efficiency by Surface Modification of Photoanode for Natural Dye-Sensitized Solar Cell

摘要

To avoid the negative effects of current energy systems on environmental pollution and global warming, research have been focused on renewable energy sources for the future. Among the renewable energy sources, solar cells have attracted a great interest as a solution to this problem. As a result, the conversion of solar energy into different forms has been the core of research for the past few years. The conventional solid state silicon-based solar cells, though highly efficient, are yet to become popular for mass applications as they are highly expensive. Therefore, developing low-cost devices for harvesting solar energy is most desirable. Dye-sensitized solar cells (DSSCs) have been considered as one of the most promising photovoltaic technologies because they are generally made from inexpensive components and have a simple designed structure. Numerous metal complexes and organic dyes have been utilized as sensitizers so far; the highest efficiency of DSSCs sensitized by ruthenium complex and osmium complex compounds absorbed on nanocrystalline TiO_2 reached 11-12%. The major drawbacks of ruthenium are its rarity, high cost and the complicated synthesis of ruthenium complexes. The natural dyes anthocyanin/betacyanin obtained from fruits, flowers and leaves can be easily extracted by simple procedures and their cost-effectiveness, non-toxicity and complete biodegradation; therefore, the use of natural dyes in DSSCs has been a major focus of research. Among various wide-bandgap semi-conducting oxides, nanocrystalline TiO_2 (ns-TiO_2) is the most suitable material for the photoanode of DSSC but TiO_2 can utilize only 6% of the total solar irradiation in photocatalysis. Doping with metal oxides has been considered a promising way for improving the photocatalytic efficiency of ns-TiO_2. The increased photoactivity of CuO-TiO_2 may be attributed to the improvement of the light absorption properties and the slowdown of the recombination between the photoexcited electrons and holes during the photoreaction. We have studied the performance of DSSCs sensitized with anthocyanin pigments extracted from black grapes. The solar cell was assembled using CuO-TiO_2 thin film on ITO-coated glass with anthocyanin dye, liquid electrolyte system with LiI: I_2 as a redox couple. The obtained solar conversion efficiency was 4.8% using an irradiation of 100 mW/cm~2.