Natural Pigments from Plants Used as Sensitizers for TiO2 Based Dye-Sensitized Solar Cells

Four natural pigments, extracted from the leaves of teak (Tectona grandis), tamarind (Tamarindus indica), eucalyptus (Eucalyptus globulus), and the flower of crimson bottle brush (Callistemon citrinus), were used as sensitizers for TiO2 based dye-sensitized solar cells (DSSCs). The dyes have shown absorption in broad range of the visible region (400–700？nm) of the solar spectrum and appreciable adsorption onto the semiconductor (TiO2) surface. The DSSCs made using the extracted dyes have shown that the open circuit voltages ( ) varied from 0.430 to 0.610？V and the short circuit photocurrent densities ( ) ranged from 0.11 to 0.29？mA？cm？2. The incident photon-to-current conversion efficiencies (IPCE) varied from 12–37%. Among the four dyes studied, the extract obtained from teak has shown the best photosensitization effects in terms of the cell output. 1. Introduction Harvesting energy from sunlight using photovoltaic technology is one of the most important research areas because of an ever increasing global energy need. The conventional solid-state silicon based solar cells, though highly efficient, are yet to become popular for mass applications as they are highly expensive. The necessity for developing low cost devices for harvesting solar energy was, therefore, very much desirable. A new hope was generated in this direction when O’Regan and Gr？etzel reported to have achieved an unprecedented high energy conversion efficiency (η) of 7.1% through a dye-sensitized solar cell (DSSC) developed by using nanocrystalline TiO2 thin film electrode sensitized by a highly efficient Ru(II) polypyridyl complex [1]. This has proven that significantly high light-to-electricity conversion efficiency can be achieved through DSSCs as well. Once this was established, such cells attracted greater attention of the scientists particularly because of two reasons; first, their production cost was expected to be quite low due to ease of their fabrication, and second, they are more environment friendly as compared to conventional solid-state silicon based photovoltaic devices [2]. Being optimistic that DSSCs have the potential to become a commercially viable alternative to expensive silicon solar cells, extensive studies have been conducted on such devices during last two decades. A dye-sensitized solar cell is usually composed of a dye-capped nanocrystalline porous semiconductor electrode, a metal counter electrode, and a redox electrolyte mediating electron transfer processes occurring in the cell. The performance of the cell is primarily dependent on the material and quality