Journal of Materials Science and Engineering B 7 (7-8) (2017) 166-170 doi: 10.17265/2161-6221/2017.7-8.006 D DAVID PUBLISHING Optical Absorption and Adsorption of Natural Dye Extracts on TiO 2 Scaffolds: Comparison between Green Leaf- and Red Fruit-Extracts Hnin Ei Maung, Me Me Theint, Nyein Wint Lwin and Than Zaw Oo Department of Physics, University of Mandalay, Mandalay 100103, Myanmar Abstract: Natural dyes were extracted from green leaves (henna and pomegranate) and red fruits (dragon fruit, rosella and red beet) and used as photosensitizer in DSSCs (dye sensitized solar cells). Two groups of green and red dye extracts exhibit a complementary absorption characteristic. Adsorption capacity of dye on TiO 2 scaffolds was accessed by the red-shift of the UV-Vis and Fourier transform infrared absorption peaks while electrochemical energy levels were estimated by cyclic voltammetry measurement. This optical and electrochemical characterization reveals that henna dye (among green leaf extracts) and rosella dye (among red fruit extracts) present lower band gap energy, better dye adsorption on TiO 2 scaffolds and better energy alignment. It is thus expecting a lower energy photon harvest, increased dye surface area, reduced electron trapping and better electron injection which are favorable for solar cells. Relatively higher efficiency of DSSC using henna and rosella dye extracts is attributed to the speculated optical and electrical characteristics mentioned above. Key words: Natural dye, optical absorption, adsorption, energy level, conversion efficiency. 1. Introduction DSSCs (dye sensitized solar cells) have received an increasing interest due to the simple fabrication process and increasing device efficiency. The device operation is based on light absorption in the dye anchored to the TiO 2 nanoparticles, followed by the electron transfer from the dye excited level into the conduction band of TiO 2, and through the electrodes into the external circuit. The electrolyte facilitates the transport of the electrons and the regeneration of the sensitizer, through a redox mediator [1]. One key component of DSSC is the dye photosensitizer. There are two main types of photosensitizer: metal complex dyes and organic dyes (synthetic and natural) used in the fabrication of DSSCs [2, 3]. Natural dyes have become an alternative to expensive and rare organic sensitizers because of its cost-effectiveness, easy attainability, abundance in Corresponding author: Than Zaw Oo, Ph.D., associate professor, research fields: materials science and device physics. supply of raw materials (flowers, leaves and fruits etc.). At present, the conversion efficiencies of natural dye sensitized DSSCs are quite low [4, 5]. The efficiency can be improved by tailoring the optical absorption of dye as well as electron transport/recombination at each and every interface within the device [6]. In addition to the optical absorption, better dye adsorption to TiO 2 surface and the favorable energy level of dye with neighboring components (TiO 2 electron transporter and redox mediator) is also key requisites for enhancing the device efficiency [7, 8]. In the present work, the optical absorption, adsorption and energy levels of natural dye extracted from green leaves (henna and pomegranate) and red fruits (dragon fruit, rosella and red beet) were studied. They were then correlated to the photovoltaic performance of the DSSCs using these natural dye extracts. 2. Experimental Section Natural dyes were extracted from leaves (henna &
Optical Absorption and Adsorption of Natural Dye Extracts on TiO 2 Scaffolds: 167 pomegranate) and fruits (dragon fruit flesh, rosella and red beet). For the leaf extracts, they were dried under dark condition at 60 C and finely powdered while the drying process was not applied for fruit extracts. The dye solutions were extracted in ethanol at 45 C followed by filtration. For the fabrication of dye loaded TiO 2 samples, TiO 2 paste (Dyesol 18-NRT) was screen-printed on pre-cleaned FTO (fluorine-doped tin oxide) coated glasses followed by sintering at 500 C for 30 min and the mesoporous TiO 2 films were allowed for dye soaking overnight. The dye adsorbed TiO 2 films were rinsed with methanol to remove unwanted solid residues. The UV-Vis absorption and FTIR (Fourier transform infrared) spectra of dye solution as well as dye loaded TiO2 films were recorded using UV-Vis spectrophotometer (Thermo Scientific, Genesys 10S) and FTIR spectrometer (Shimadzu, IRPrestige-21). Cyclic voltammetry measurements were carried out using potentiostat (Digi-Ivy, DY2000) in three-electrode systems consisting of a glassy carbon working electrode, platinum wire counter electrode and Ag/AgCl reference electrode at a scan rate of 50 mv/s. A supporting electrolyte used is 0.1 M KNO 3 in distilled water. DSSCs were assembled by introducing the redox electrolyte (0.05 M I 2 and 0.5 M KI in ethylene glycol) between the dyed TiO 2 working electrode and carbon counter electrode. The current-voltage (J-V) characteristics of the devices were recorded with source meter (Keithley 2420) under 1SUN illumination by a Newport solar simulator. 3. Results and Discussion 3.1 Optical Absorption of Natural Dye Extracts The natural dyes being studied are categorized into two groups: one from green leaves (henna and pomegranate) and another one from red fruits (dragon fruit, rosella and red beet). Their absorption spectra are shown in Fig. 1. As categorized, the leaf extracts exhibited two peaks (420 nm and 660 nm) corresponding to the green pigment chlorophyll while the fruit extracts produced a single broad peak at 530 nm responsible for red color of anthocyanins with additional bluer peak at 475 nm for red-beet extracts. The absorption characteristics of the leaf and fruit extracts are complementary to each other (the spectral absorption between 500-600 nm is weak in the leaf extracts while it is strong in the fruit extracts). 3.2 Dye-Adsorption on TiO 2 Scaffolds We studied dye-adsorption on TiO 2 by examining the red-shift of the UV-Vis absorption spectra (Fig. 1) and FTIR transmission spectra (Fig. 2) upon loading dye on TiO 2. In Fig. 1, it is observed that upon dye-loading on TiO 2, the absorption maxima (λ max ) are red-shifted for henna and rosella extracts and are almost invariant for the rests. A red-shift of the λ max in absorption spectra suggests a packed molecular conformation of dyes on TiO 2 matrix due to the strong interaction between dye molecules and TiO 2 surface [8, 9]. This study indicates that the adsorption of henna and rosella dyes on TiO 2 is better compared to the respective counterpart dyes. This information is ascertained by FTIR spectroscopic study. Fig. 2 depicts the FTIR spectra of dye and dye loaded on TiO2 films for all dye extracts. At the wavenumber range (3,250-3,650 cm -1 ) and (1,650-1,850 cm -1 ), the vibrational peaks arised are due to the hydroxyl and carbonyl (O-H and C=O) stretching vibration which can attach towards TiO 2. Both anchoring groups were perceived in henna and rosella extracts while the only O-H anchoring group was observed in the rests. The peaks observed at the wavenumber range (2,700-3,200 cm -1 ), (1,550-1,620 cm -1 ) and (1,500-500 cm -1 ) were assigned to C-H stretching, N-H bending and deformation regions respectively [10, 11]. We observed a red-shift of the O-H and C=O stretching vibrations in henna (from 3,382 cm -1 to 3,363 cm -1 and from 1,734 cm -1 to 1,681 cm -1 ) and in rosella (from 3,468 cm -1 to 3,342 cm -1 and from 1,741 cm -1 to 1,661 cm -1 ) upon dye loading
168 Optical Absorption and Adsorption of Natural Dye Extracts on TiO 2 Scaffolds: Fig. 1 UV-Vis absorption spectra of dye vs. dye loaded on TiO 2. Fig. 2 FTIR spectra of dye vs. dye loaded on TiO 2. on TiO2. It suggests that the henna and rosella dyes have shorter side groups which would enhance the dye-adsorption on TiO 2 [12, 13]. For pomegranate, dragon fruit and red beet dyes, the positions of these vibrational peaks kept almost unchanged. The UV-Vis absorption and FTIR spectroscopic studies thus reveal that dye-adsorption capacity is enhanced in henna leave and rosella fruit extracts compared to other extracts being studied. 3.3 Electrochemical Energy Levels of Dye Extracts The energy levels [HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital)] of natural dye extracts were determined by electrochemical CV (cyclic voltammetry) measurement. Fig. 3 presents the cyclic voltammograms of henna and rosella extracts (CV traces of the rests are not shown here). The reduction potential ( E red ) is obtained from the intersection of the two tangents drawn at the rising current and baseline charging current of the CV traces. Using the E red values, the LUMO energy level ) is determined using the equation [14]: (E LUMO E LUMO = (E red + 4.4 ev ). Combining with the energy band gap (E g ) obtained from UV-Vis absorption, the E HOMO of dye extract is calculated by the equation: E HOMO = E g + E LUMO. The electrochemical energy levels and optical band gap energy of the dyes are listed in Table 1. 3.4 Photovoltaic Performance of DSSCs The PCE (power conversion efficiencies) of the DSSCs using leave and fruit extracts as photosensitizers
Optical Absorption and Adsorption of Natural Dye Extracts on TiO 2 Scaffolds: 169 Fig. 3 Cyclic voltammograms of henna androsella dye extracts. Table 1 Calculated energy levels and band-gap of dyes. Dye extracts LUMO (ev) HOMO (ev) E g (ev) Henna -4.05-5.85 1.80 Pomegranate -4.24-6.05 1.81 Dragon fruit flesh -4.14-6.20 2.06 Rosella -3.96-5.93 1.97 Red beet -4.12-6.17 2.05 J (macm -2 ) 0.4 Henna Pomegranate Dragon fruit flesh Rosella Red beet 0.2 0.0 Fig. 4-0.2 J-V curves of DSSCs using natural dye extracts. 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V) Table 2 Photovoltaic device parameters of DSSCs. Sensitizer dye J sc (ma/cm 2 ) Voc (V) FF PCE (%) Henna 0.114 0.289 0.380 0.050 Pomegranate 0.009 0.323 0.350 0.004 Dragon fruit flesh 0.025 0.306 0.430 0.013 Rosella 0.102 0.340 0.380 0.053 Red beet 0.026 0.324 0.410 0.014 were examined under AM1.5 solar irradiation. The J-V characteristics of DSSCs are depicted in Fig. 4 and the device parameters are listed in Table 2. As seen in Table 2, the henna leaf-extracts enabled the PCE of DSSC as high as 0.050% and the rosella fruit-extracts PCE of 0.053% which are relatively higher than those with respective dye-counterparts. It is thus interesting to know why one outperforms the others within each group (leaf or fruit extracts). Examining carefully the device parameters (Table 2) for each group of DSSCs, higher PCE is mainly contributed from increased J sc of the device. Increased photocurrent (J sc ) in these devices is attributed to (i) lower band gap energy of sensitizer dye (Table 2) which can harvest photons of lower energies, (ii) better electron injection towards conduction band of
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