# Structure and Photoelectrical Properties of Natural Photoactive Dyes for Solar Cells
## 2 Methods  
The fresh leaves of the six plants were selected and placed into a vacuum drying oven maintaining 70 \({}^{\circ}\)C to remove moisture. The dried leaves were pulverized into powder in a mortar and weighed 1 g powder, and then mixed 20 mL ethanol with the power. After keeping the admixture away from the light in a brown jar for a week, the efficient natural dyes were obtained through a simple filtering process. The structure of prepared DSSC was mainly made of TiO\({}_{2}\) photo-anode, dye and electrolyte. The specific production process is as our previous works [16]: (a) the TiO\({}_{2}\) electrode was prepared through adding 10 mL isopropyl titanate into water, and keeping the hydrolysis for 3 h, and then adding nitric acid and acetate into the solution, and placing in an environment of 80 \({}^{\circ}\)C; the mixed solution was stirred until it became transparent blue; later, the hydrothermal reaction was executed at 200 \({}^{\circ}\)C for 12 h. After cooling, spin steaming, centrifuging, terpineol ethyl and cellulose were added into the ball grinder; the paste were prepared completely by ball mill, rotary steam and three roll. (b) the screen printing technology was adopted to print the TiO\({}_{2}\) paste to the clean surface of a conductive glass, and the active area of TiO\({}_{2}\) photo-anode was 0.16 cm\({}^{2}\); after ethanol bathing and drying, the TiO\({}_{2}\) photo-anode was sintered, and then treated in TiO\({}_{4}\) solvent. After this, the photo-anode was sintered again. In the later process, the photo-anode was immediately removed after the natural cooling to 80 \({}^{\circ}\)C, and the photo-anode was soaked in the natural dye without light for 24 h. (c) the photo-anode and the platinum plating counter electrode were fitted together into the cell, and in the middle of the two electrodes, the electrolyte solution (including 0.5 M LiI, 0.05 M I2 TBP, GUSCN) was added. The SEM image of thin film of TiO\({}_{2}\) nanoparticles was shown in Figure 2, where the area of TiO\({}_{2}\) film was 16 mm\({}^{2}\) and thickness of the TiO\({}_{2}\) anode layer was about 16-18 \(\mu\)m; here, the characteristic of void and honeycomb of TiO\({}_{2}\) anode layer is benefit for enhancing the adsorption of dyes. Figure 1: Basic working principle of DSSC, which mainly contains four parts: photoanode, natural dyes, electrolyte containing redox couple and counter electrode, where photosensitive layer using the dyes (D1–D6) were extracted from the leaves. Figure 2: SEM micrographs of the TiO\({}_{2}\) nanoparticles on the glass. UV-Vis spectra of dye solutions and dye adsorbed on TiO\({}_{2}\) photo-anodes were measured with TU-1900 spectrometer (Beijing Purkinje General Instrument Co., Ltd., Beijing, China). Cyclic voltammetry properties were performed by using CH Instruments CHI832C Electrochemical Workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). The redox potentials of the dyes were measured in ethanol solvent. The scan range was between \(-1000\) mV and \(+1000\) mV, and the initial scan potential was \(-1000\) mV, at a scan rate of \(50\) mV/s. Here, a glassy carbon working electrode, platinum counter electrode and Ag/AgCl reference electrode were used to form electrode system, where \(0.1\) M KNO\({}_{3}\) was used as the supporting electrolyte [44]. Solar energy conversion efficiency measurements were completed with a solar simulation instrument (PEC-L15, Peccell Technologies, Inc., Yokohama, Japan), and light intensity was adjusted via a reference standard Si-solar solar cell at \(1\) sun light intensity of \(100\) mW cm\({}^{-2}\). Compared with experiment, the chlorophyll was selected as the reference dye in theory, and the ground state structure of chlorophyll a was optimized by using DFT [45], with B3 LYP [46; 47; 48] functional at 6-31G(d) basis set.