Natural dyes from flame tree flower, Pawpaw leaf and their mixtures were used as sensitizers to fabricate dye-sensitized solar cells (DSSC). The photoelectrochemical performance of the Flame tree flower dye extract showed an open-circuit voltage (V OC) of 0.50 V, short-circuit current density (J SC) of 0.668 mA/cm 2, a fill factor (FF) of 0.588 and a conversion efficiency of 0.20%. The conversion efficiency of the DSSCs prepared by pawpaw leaf extract was 0.20%, with V OC of 0.50 V; short-circuit current density, J SC of 0.649 mA/cm 2 and FF of 0.605. The conversion efficiency for the flame tree flower and pawpaw leaf dye mixture was 0.27%, with V OC of 0.518 V, J SC of 0.744 mA/cm 2 and FF of 0.69. Although the conversion efficiencies, Jsc and the Voc of the prepared dye cells were lower than the respective 1.185%, 7.49 mA/cm 2 and 0.64V reported for ruthenium, their fill factors (FF) were higher than that of ruthenium (0.497). It was also observed that both the short-circuit current density and the fill factors of the cells were enhanced using mixed dye.
Ddye-sensitized solar cells (DSSCs) are third generation solar cells developed by O’Regan and Gratzell in 1991 [
Naturally most fruits, flowers and leaves show various colours and contain several pigments which are easily extracted and then employed in DSSCs [
The flame tree (Delonix regia), also known as royal Poinciana or flamboyant, is a member of the bean family (Leguminosae) widely regarded as one of the most beautiful tropical trees in the world [
Pawpaw (Carica papaya Linnaeus), belongs to the family of Caricaceae. It is not a tree but an herbaceous succulent plants that posses self supporting stems of spongy and soft wood [
Fresh flame tree flower and pawpaw leaves each of 10 g were separately weighed on an electronic weighing balance and crushed with a porcelain mortar and pestle, each crushed sample was then mixed with 50 cm3 of ethanol (99% absolute) at room temperature in a dark room. Solid dregs in the solution were filtered by filter paper to acquire a pure and natural dye solution. Then, flame tree flower and pawpaw leaf extracts were blended at volume ratio of 1:1 to serve as a natural dye mixture.
The TiO2 film was prepared by blending 0.2 g of commercial TiO2 powder (Degussa, P25), 0.4 cm3 of nitric acid (0.1 M), 0.08 g of polyethylene glygol (MW 10,000) and one drop of a Triton x-100 (a nonionic surfactant). The mixture was well mixed using an ultrasonic bath for
1 h and the resulting paste was spread over an FTO conductive glass plate (SOLARONIX) having 15 Ω/cm2. The TiO2 nano-particles thus produced had a mean particle size of 20 nm. TiO2 pastes were deposited on the FTO conductive glass by rigid squeegee and screen printing procedure (polyester mesh of 90) in order to obtain a TiO2 film with a thickness of 18 µm. The active area of DSSC was 0.54 cm2 (1.4 cm × 0.39 cm). The TiO2 thin film was sintered at 450˚C for 1h to increase compactness of the thin film. The TiO2 film was consolidated through heat treatment, increasing the internal voids of film organization and thus enhancing its absorption performance. Then the sintered TiO2 thin film was immersed for 24 h in natural dyes prepared, allowing the natural dye molecules to be adsorbed on the surface of TiO2 nanoparticles. Anhydrous alcohol was used to remove any natural dye that had not been adsorbed on the surface of TiO2 nanoparticles. Finally, after cleaning, the DSSCs photoelectrode was complete and ready for testing.
Glass insulation spacers in long strips were used in assembling and these were stuck on the four edges of the base plate of conductive glass at the bottom. These formed a space between photoelectrode and counter electrode enabling the injection of electrolyte.
In the performance test of the prepared DSSC, xenon (Xe) light of 150 W was selected to simulate sunlight (AM 1.5), and an I-V curve analyzer (Model 4200 SC) was employed to measure the photoelectric conversion efficiency of the prepared DSSC. The measured results were plotted in an I-V curve, from which the data of opencircuit voltage Voc (V), short-circuit current density Jsc (mA/cm2), fill factor (FF) and conversion efficiency η% were further acquired.
Absorption spectra provide necessary information on the absorption transition between the dye ground state and excited states and the solar energy range absorbed by the dye. Figures 3 and 4 show the absorption spectra of Flame tree flower and Pawpaw leaf dye extracts respectively as against N719 dye. The absorption range of N719 dye was from 400 - 600 nm with an absorption peak at 475 nm. The flame tree flower dye extract had an absorption in the frequency range 350 - 500 nm with an absorption peak at 415 nm. The chlorophyll dye extract from the pawpaw leaf had an absorption peak at 430 nm characteristic of chlorophyll pigment and an absorption range from 350 - 550 nm. Chlorophyll absorbs most strongly in the blue and red regions of the absorption spectra.