(375a) 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.