# Structure and Photoelectrical Properties of Natural Photoactive Dyes for Solar Cells
## 3 Results and Discussion  
### UV-Vis Absorption Spectra in Experiment and Theory  
The UV-Vis absorption spectra of the dyes in ethanol and dye adsorbed on TiO\({}_{2}\) photo-anodes were measured, and the corresponding absorption spectra are presented in Figure 3a,b. The detected absorption peaks corresponding to the isolated dyes and dyes adsorbed on TiO\({}_{2}\) are listed in Table 1 in detail. As shown in Figure 3a, the absorption spectra of those six kinds of dyes have similar appearance, in which the absorption peaks are mainly distributed in the visible light regions of 400-420 nm and 650-700 nm. It is worth noting that all the absorption spectra of the investigated dyes have the characteristics of double peaks, which are advantageous to the absorption of dyes on the sun light. As listed in Table 1, the two absorption peaks of the six dyes are focused on about 412 nm and 664 nm. The absorption spectra of the dyes investigated in the present work are consistent with the absorption spectrum of chlorophyll [62; 63]. In addition, the absorption spectra of dyes adsorbed on TiO\({}_{2}\) photo-anode are shown in Figure 3b, and the corresponding absorption peaks are listed in Table 1. As listed in Table 1, the absorption peaks in the region of 650-700 nm exhibit different extent of red-shifts compared with that of the isolated dyes, in which the red-shift values are 7.5 nm, 10 nm, 6.5 nm, 29.5 nm, 7.5 nm and 6.5 nm for D1, D2, D3, D4, D5 and D6, respectively. \begin{table}
\begin{tabular}{c c c} \hline
**Dye** & **Absorption Peaks (nm)** & **Dye/TiO\({}_{2}\)** & **Absorption Peaks (nm)** \\ \hline D1 & 416.0, 664.0 & D1/TiO\({}_{2}\) & 416.5, 671.5 \\ D2 & 419.5, 658.5 & D2/TiO\({}_{2}\) & 421.5, 668.5 \\ D3 & 412.5, 663.5 & D3/TiO\({}_{2}\) & 416.5, 670.0 \\ D4 & 411.5, 665.5 & D4/TiO\({}_{2}\) & 69.0 \\ D5 & 413.5, 664.0 & D5/TiO\({}_{2}\) & 413.5, 671.5 \\ D6 & 412.0, 665.0 & D6/TiO\({}_{2}\) & 416.5, 671.5 \\ \hline \end{tabular}
\end{table} 
Table 1: Absorption peaks of isolated dyes and dye adsorbed on TiO\({}_{2}\). To compare with the experimental results, the excited state properties of chlorophyll was investigated via TD-DFT method with different functionals at 6-31G(d) basis set based on the optimized ground state structure of chlorophyll, due to the fact that chlorophyll accounts for the largest proportion in the green plant leaves. The simulated absorption spectra of chlorophyll by using different functionals are presented in Figure 4, and the corresponding absorption peaks are listed in Table 2. Combined with the results of Figure 4 and Table 2, it can be seen that the simulated absorption spectrum of chlorophyll by using PBEPBE functional is more consistent with the experimental results; so we analyze the excited state properties of chlorophyll based on the results calculated by the PBEPBE functional. The calculated excitation energies, absorption peaks, electronic transition configurations and corresponding oscillator strengths of the first ten excited states are listed in Table 3. As listed in Table 3, the excited state S8 corresponds to the first maximum absorption peak 439.66 nm with the oscillator strength of 0.3846, which is mainly formed by the electrons transition from HOMO \(-\) 1 to LUMO + 1. In addition, the excited state S1 corresponds to the second maximum absorption peak at 621.44 nm with the oscillator strength of 0.3042, which is mainly originated from the electrons transition from HOMO to LUMO. Moreover, the excited state S7 corresponds to the third maximum absorption peak at 456.17 nm (the oscillator strength of 0.2823), which mainly comes from the electrons transition from HOMO to LUMO + 1. \begin{table}
\begin{tabular}{c c c c} \hline
**Peak** & **Cam-B3 LYP** & **MPW1 PW91** & **PBEPBE** \\ \hline \(\lambda_{\text{max1}}\)1  & 376.57 (0.9972) & 375.28 (0.8763) & 439.66 (0.3846) \\ \(\lambda_{\text{max2}}\)2  & 602.89 (0.3746) & 586.59 (0.3816) & 621.44 (0.3042) \\ \hline \end{tabular}
\end{table}