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

DYE-SENSITIZED SOLAR CELLS:
USING OVER 100 NATURAL DYES AS SENSITIZERS

C I.F. Attanayake ^1*, C. de Silva², B.A.J.K. Premachandra¹,
A.A.P. de Alwis¹,  G.K.R. Senadheera³.

^1.        Department of Chemical & Process Engineering, University of Moratuwa, Moratuwa, Sri Lanka

^2.        Department of Chemical Engineering, City College of New York, New York, U.S.A.

^3.        Institute of Fundamental Studies, Hantana, Sri Lanka and Open University of Sri Lanka, Polgolla, Sri Lanka

^* Corresponding Author, E-mail: ivancamilus2011@gmail.com

 

ABSTRACT

Dye-sensitized Solar Cells (DSSC's) are one of the most promising new energy generation systems of photovoltaic technology to address the impending fossil fuel – based power crisis utilizing renewable energy. These have emerged as a renewable energy source as a result of exploiting several new concepts and materials such as nanotechnology and molecular devices.

In the early 1990's Professor Michael Gratzel and his 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 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 sensitizers. So far, several organic dyes and inorganic metal complexes have been employed to sensitize nanocrystalline TiO₂ semiconductors and one of the most efficient sensitizers is a transition metal coordination compound (Ruthenium polypiridyl complex). This complex has intense charge – transfer (CT) absorption in the whole visible range, long excited life time 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 polypiridyl complexes. On the other hand, in nature, the fruit, flower, stem, bark, root and leaf of plants show various colours form red to purple and contain various natural dyes that can be extracted by simple procedures. Therefore, it has been emphasized by many researchers to obtain useful dyes as photosensitizers 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 in emphasized in this paper. Over 100 natural dyes extracted from fruits, leaves, flowers, stems, bark and roots of plants growing in Sri Lanka were used as sensitizers to fabricate dye-sensitized solar cells.

Preliminary investigation on the identification 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. After electrical and electronic analysis of several natural dyes of local plants, it was observed that dye extracts of Mangoostein fruit rind was found to be superior to those obtained from other dyes, and were Jsc=2.56 mA.cm^-2,   Voc  =  685.3 mV,    ff  = 60.02 %  and  h  =  1.053%. Also Ekkiriya wood, Egg plant, Karawala kabilla and Banana flower yeilded Jsc, Voc, ff and h of 2.32 mA.cm^-2, 414.2 mV, 56.86%, 0.547% respectively ; 2.096 mA.cm^-2, 410.4mV, 56.42% and 0.485% respectively; 1.395mA.cm^-2, 443.5mV, 58.58% and 0.362% respectively; 0.763 mA.cm^-2, 414.0mV, 45.2% and 0.357% respectively.  Whilst Fire fern leaf (not endemic to Sri Lanka) was found to display Jsc, Voc, ff and h of 4.128 mA.cm^-2, 405.1mV, 47.97% and 0.802%  respectively.