Dye-Sensitized Solar Cells Using Natural Dyes Extracted from Roselle (Hibiscus Sabdariffa) Flowers and Pawpaw (Carica Papaya) Leaves as Sensitizers

Dye-sensitized solar cells (DSSCs) were fabricated using natural dyes extracted from roselle flowers and carica papaya leaves extract as photosensitizers. The photovoltaic perfomance of the DSSCs were evaluated under 100 mAcm light intensity. The roselle (Hibiscus Sabdariffa) extract sensitized solar cell gave a short circuit current density (Jsc) of 0.180 mAcm, an open circuit voltage (Voc) of 0.470 V, a fill factor (FF) of 0.552, and an overall solar energy conversion efficiency (η) of 0.046%. Also, the pawpaw leaves extract sensitized cell gave a Jsc of 0.094 mAcm, Voc of 0.433 V, FF of 0.544 yielding a conversion efficiency of 0.022%. The cell sensitized by the roselle extract shows better sensitization, which was in agreement with the broadest spectrum of the extract adsorbed on TiO2 film. The sensitization performance related to interaction between the dye and TiO2 surface is discussed.


Introduction
Attempts to create photo electrochemical solar cells by mimicking nature's photosynthesis started in the 1970s with early efforts involving the covering of crystals of semiconductor titanium dioxide with a layer of chlorophyll [1].
Historically the dye-sensitization dates back to the 19th century, when photography was invented. The work of Vogel in Berlin after 1873 can be considered the first significant study of dye-sensitization of semiconductors, where silver halide emulsions were sensitized by dyes to produce black and white photographic films referred in [2].
However, owing to the electrons' reluctance to move through the layer of pigment, the efficiency of the first solar cells sensitized in this way was about 0.01%. In the late 1980s, Scientists discovered that nanotechnology could overcome the problem [1]. The use of dye-sensitization in photovoltaic remained however rather unsuccessful until a breakthrough at the early 1990's in the Laboratory of Photonics and Interfaces in the École Polytechnique Fédérale de Lausanne (EPFL) Switzerland. By the successful combination of nanostructured electrodes and efficient charge injection dyes, professor Grätzel and his co-workers developed a solar cell with energy conversion efficiency exceeding 7% in 1991 [3] and 10% in 1993 [4]. This solar cell is called the dye sensitized nanostructured solar cell or the Grätzel cell named after its inventor.
Dye-sensitized solar cells (DSSCs) are the third generation of photovoltaic devices which is used for the conversion of visible light into electric energy. These new type of solar cells are based on the photosensitization produced by the dyes on wide band-gap mesoporous metal oxide semiconductors; this sensitization is produced by the dye absorption of part of the visible light spectrum [5].
The sensitization approach enables the generation of electricity with irradiation of energy lower than energy of the bandgap of the semiconductor. The progress of such devices occurred with the development of nanostructured semiconductor films onto which light absorbing dye molecules are adsorbed. Many efforts have focused on sensitizer dye, since dye plays a key role in harvesting sunlight and transforming solar energy into electric energy. Several organic dyes and organic metal complexes have been employed to sensitize nanocrystalline TiO 2 semiconductors [6].
Naturally most flowers and leaves show various colours and contain several pigments which are easily extracted and then employed in DSSCs [7]. The leaves of most green plants dye has been investigated in many studies [8,9,10]. Anthocyanins are natural compounds that give colour to fruits and plants [11]. Roselle extract is rich in anthocyanins. It was reported that anthocyanin obtained from roselle are delphinidin and cyanidin complexes [12,13]. The chemical structure of cyanidin and delphinidin in the Hibiscus Sabdariffa dye is shown in figure 4.
In this research work, anthocyanins extracts of roselle flowers, and chrorophyll extract from carica papaya leaves were the natural dyes used as sensitizers in the formed DSSCs. The performances of the formed DSSCs shows that, the roselle extract has higher photosensitized performance as compared to the pawpaw leaves extract.
The Hibiscus sabdariffa dye (commercially known as Roselle) belongs to the family Malvacae and is present in abundance through-out the world and has attained prominence as a jute substitute [14].
Pawpaw, belongs to the family of caricaceae. It is not a tree but a herbaceous succulent plants that posses' selfsupporting stems of spongy and soft wood [15].

General Composition, Function and Parameters of the DSSCs
The cell is composed of four elements, namely, the transparent conducting and counter conducting electrodes, the nanostructured wide bandgap semiconducting layer, the dye molecules (sensitizer), and the electrolyte. The transparent conducting electrode and counter-electrode are coated with a thin conductive and transparent film (such as fluorine-doped tin dioxide (SnO 2 )). The TiO 2 surface is stained with a dye. TiO 2 nanocrystals are used rather than a continuous layer to maximize surface area for light absorption. Between the electrodes is an electrolyte. Upon absorption of photons, dye molecules are excited as shown schematically in Figure 1. Once an electron is injected into the conduction band of the wide bandgap semiconductor nanostructured TiO 2 film, the dye molecule (photosensitizer) becomes oxidized. The injected electron is transported between the TiO 2 nanoparticles and then extracted to a load where the work done is delivered as an electrical energy. The electrons flow through the TiO 2 onto the electrode, through an electric circuit, and then to the counter electrode. The electrolyte carries electrons back to the dye from the counter electrode. Electrolytes containing redox ions is used as an electron mediator between the TiO 2 photoelectrode and the coated counter electrode. Therefore, the oxidized dye molecules (photosensitizer) are regenerated by receiving electrons from the ion redox mediator that get oxidized.

Preparation of the Natural Dye
The leaves of carica papaya were grinded to small particles using blender with water as extracting solvent, and flowers of Hibiscus Sabdariffa were air dried till they became invariant in weight.
The dried flowers of Hibiscus Sabdariffa were left uncrushed because previous attempts proved failure to extract the dye from crushed samples due to jellification [16]. The method of heating in water (Aq.) was used to extract the dye. Distilled water was the solvent for aqueous extraction. 5 g of the sample (Hibiscus Sabdariffa) Flower was measured using analytical scale balance and dipped in 50 ml of the solvent heated to 100°C for 30 minutes after which solid residues were filtered out to obtain clear dye solutions.

DSSCs Assembling
The photoanode was prepared by first depositing a blocking layer on the fluorine doped tin oxide (FTO) glass (solaronix), followed by the nanocrystalline TiO 2 (solaronix). The blocking layer was deposited from a 2.5 wt% TiO 2 precursor and was applied to the FTO glass substrate by spin coating and subsequently sintered at 400°C for 30 minutes.
The 9 µ m thick nanocrystalline TiO 2 layer was deposited by screen printing. It was then sintered in air for 30 minutes at 500°C. The counter electrode was prepared by screen printing a platinum catalyst gel coating onto the FTO glass. It was then dried at 100°C and fired at 400°C for 30 minutes.
The sintered photoanode was sensitized by immersion in the sensitizer solution at room temperature overnight. The cells were assembled by pressing the photoanode against the platinum-coated counter electrodes slightly offset to each other to enable electrical connection to the conductive side of the electrodes. Between the electrodes, a 60 µ m space was retained using two layers of a thermostat hot melt sealing foil. Sealing was done by keeping the structure in a hotpressed at 100°C for 1 minute. the liquid electrolyte constituted by 50 mmols of tri-iodide/iodide in acetonitrile was introduced by injection into the cell gap through a channel previously fabricated at opposite sides of the hot melt adhesive, the channel was then sealed.

Characterization and Measurement
The current-voltage (J-V) data was obtained using a keithley 2400 source meter under AM1.5 (100 mw/cm 2 ) illumination from a Newport A solar simulator. Scanning electron micrographs of the nanocrystalline TiO 2 films are taken with Carl Zeis SEM. The absorption spectra of the dyes was recorded on Ava-spec-2048 spectrophotometer. The cell active area was 0.5 cm 2 . Thickness measurement was obtained with a Dektac 150 surface profiler. X-ray microanalysis was carriedout with INCA EDX analyzer. Fig. 2a. shows the SEM image of TiO 2 nanoparticles fabricated using screen printing method. The SEM micrograph shows that the TiO 2 nanoparticles produced have a mean particle size of about 20 nm. It also reveals that the surface is porous and has agglomeration.   Nitrogen is present due to the blower that was used to dry the TiO 2 semiconductor. Fig. 3a shows the UV-vis absorption spectra of roselle extract, and pawpaw leaves extract. It was found that the absorption peak of roselle extract is about 540 nm (Fig 3b) while it was deduced that the carica papaya dye absorbs photons best at a wave length peak of 370 nm (Fig 3a). The difference in the absorption characteristics is due to the different type of pigments (anthocyanin for roselle and chlorophyll for pawpaw) and colors of the extracts. After immersion of the TiO 2 -coated electrode (photoanode) in the extracts, observable colors of TiO 2 films turned deep purple for the roselle extract but the film turned light green for the pawpaw extract. In the case of roselle extract, an absorption peak of the photoanode is broader than that of the dye solution ( Fig. 3(c)), with a shift to a higher wavelength (from 540 to 560 nm). The difference in the absorption peak is due to the binding of anthocyanin in the extract to the TiO 2 surface [17].      Based on the J-V curve, the fill factor (FF) and solar cell efficiency ( η ) were determined using equations (1) and (2) respectively.

Results and Discussion
Where V max = maximum voltage (V); J max = maximum current density (mA/cm 2 ); J sc = short circuit current density (mA/cm 2 ); V oc = open circuit voltage (V) and P IRRADIANCE = light intensity (mW/cm 2 ) From the effective absorption area of 0.5 cm 2 of the DSSCs, the averaged values of the light-to-current conversion efficiencies (η) of the DSSCs, shortcircuit photocurrent (J SC ), open-circuit voltage (V OC ), and fill factor (FF) were recorded as presented in Table 1. Obviously, the efficiency of the cell sensitized by the roselle extract was significantly higher than that sensitized by the pawpaw leaf extract. This is due to a higher intensity and broader range of the light absorption of the extract on TiO 2 (Fig. 3c), and the higher interaction between TiO 2 and anthocyanin in the roselle extract which leads to a better charge transfer. Moreover, anthocyanin in the roselle extract (cyanidin and delphinidin) has a shorter distance between the dye skeleton and the point connected to TiO 2 surface compared to the chrorophyll extract from carica papaya leaves that shows lower charge transfer effect due to the limited absorbance of the dye in the visible spectrum [18]. This could facilitate an electron transfer from anthocyanin in the roselle extract to the TiO 2 surface and could be accounted for a better performance of roselle extract sensitization [19]. When the results in this study is compared to a studiy performed under similar conditions by Ahmed et., al [20] who found Jsc of 0.17 mAcm -2 , Voc of 0.46 V, FF of 0.41 and η of 0.033% for Hibiscus Sabdariffa respectively, our result is alitle higher. Also when compared to Kimpa et.al [21], where the conversion efficiency of the DSSC prepared by pawpaw leaf extract was 0.20%, with V OC of 0.50 V, J SC of 0.649 mA/cm 2 and FF of 0.605, their result has better perfrmance. These differences may be attributed to differences in concentrations of phytoconstituents in different parts of the plants for the Hibiscus Sabdariffa dye [16], and due to the extracting solvent used for the case of pawpaw (distilled water for our studies and ethanol for Kimpa et., al).

Conclusions
Sensitization of a dye sensitized solar cell with extracts from roselle flowers was demonstrated. A similar cell sensitized with carica papaya leaves (containing a mixture of carothenoids and appreciable concentration of phenolic acid) extract shows a lower Jsc, Voc, FF and η. Using water as extracting solvent, the energy conversion efficiency (η) of the cells consisting of roselle extract and pawpaw leaves extract was 0.046%, and 0.022%, respectively. The roselle extract has higher photosensitized performance as compared to the pawpaw leaves extract. This is due to the better charge transfer between the roselle dye molecule and the TiO 2 surface which is related to a dye structure, [18].