Dye–Sensitized Solar Cells: Using Over 100 Natural Dyes As Sensitizers

Dye-Sensitized Solar Cells (DSSC's) are one of the most promising new energy generation systems for photovoltaic technology. They have emerged as a renewable energy sources as a result of exploiting several new concepts and materials, such as nanotechnology and molecular devices.

In the early 1990's Gratzel's group developed a DSSC consisting of TiO2 electrode sensitized with Ruthenium (II) Complex dye, organic liquid electrolyte with iodine/iodide redox couple and Platinum deposited counter electrode. In these devices a monolayer of the dye is directly attached to the semiconductor surface via carboxyl group, which could realize an efficient injection of charge carriers from a photo-excited dye to the semiconductor. However, this sensitization of TiO2 for solar applications requires not only efficient but also stable and inexpensive sensitizes. So far, several organic dyes and organic metal complexes have been employed to sensitize nanocrystalline TiO2 semiconductors and one of the most efficient sensitizers is a transition metal coordination compound (Ruthenium polypyridyl complex). The complex has intense charge – transfer (CT) absorption in the whole visible range, long excited lifetime and highly efficient metal-to-ligand charge transfer (MLCT).

Other organic dyes, such as phythalocyanine, cyanine dyes, xanthalene dyes, coumarin dyes etc. usually perform poorly in DSSC's because of their weak binding energy with the TiO2 film and low charge transfer absorption in the whole visible range, but these dyes are very cheap and can be prepared easily, compared to Ruthenium polypyridyl complexes. On the other hand, in nature, the fruit, flower, root and leaf of plants show various colors from red to purple and contain various natural dyes that can be extracted by simple procedures. Therefore, it has been emphasized by many researches to obtain useful dyes as photo-sensitizers for DSSC's from natural products, because of the simple preparation techniques, widely available sources, and low cost. Due to these reasons, the importance of the development of low cost solar energy to electricity conversion units in principle is emphasized in this poster.

Over 100 natural dyes extracted from fruits, leaves, flowers, stems, bark and roots of plants growing in Sri Lanka were used as sensitizers of to fabricate dye-sensitized solar cells (DSSC's).

Preliminary investigations on the identification of natural pigments in the dye-sensitization of nanocrystalline n-type TiO2 were carried out. Fresh extracts of various fruits and vegetables were employed as sensitizers in thin layer sandwich type photo electrochemical dye – sensitized solar cells (DSSC's).

After electrical and electronic analysis, of several natural dyes of local plants, it was observed that many useful dyes which could be extracted from natural products by simple procedure could be used as photo sensitizers for DSSC's. It was also observed that dye extracts of Turmeric and Mangoostein yielded better results. 

The current-voltage curves obtained with solar cells employing the photo anode with TiO2 sensitized by different dyes were observed. The values of short circuit current density (Jsc), open circuit voltage (Voc), fill factor (ff), and efficiency (η) obtained for solar cells employing photo anodes with TiO2 sensitized with different fruit / vegetable extracts were noted . The dye extracts of Turmeric root and Mangoostein fruit were found to be superior to those obtained from other dyes , and were  Jsc = 0.64mAcm-2 and 0.778 mAcm-2, Voc = 599.1 mV and 565.2 mV, ff = 69.03 % and 65.66 % ,  η = 0.264 % and 0.34 % respectively. Also Ekkiriya wood, Egg Plant, Karawilla Cabilla yielded Jsc, Voc and ff of 2.32 mA.cm-2, 414.2mV, 56.86% respectively; 2.096 mA.cm-2, 410.4mV, 56.42%, 0.485%; 1.395 mA.cm-2, 443.5mV, 58.58% and 0.362%. Whilst Fire Fern (not endemic to Sri Lanka) was found to display Jsc, Voc, and ff of 4.128mA.cm-2,405.1mV and 47.97% respectively.