# Structure and Photoelectrical Properties of Natural Photoactive Dyes for Solar Cells
## 1 Introduction  
With the development and utilization of renewable energy, much works have been paid attention to the devices converting solar energy into electrical energy, among which the dye-sensitized solar cells (DSSCs) with its characteristics of low fabrication costs, environmentally friendly components and relatively high photoelectrical conversion efficiency (PCE) have received wide attention [1; 2; 3; 4; 5]. In 1991, [PERSON] and [PERSON] [6] created the DSSCs with overall PCE yield of 7.1-7.9% in simulated solar light from low- to medium-purity materials by low-cost process. DSSCs are mainly composed of a light absorbing photoanode, dyes, an electrolyte containing redox couple and a counter electrode [7]. It is remarkable that the dyes play a crucial role in DSSCs because firstly they absorb sunlight [8], followed by injection of the excited electron into semiconductor. In addition to the use of dyes containing metal elements [9; 10; 11], the metal-free organic dyes have been widely applied to the field of DSSCs [12; 13; 14; 15; 16; 17; 18; 19]. However, the shortcomings of high cost, environmental pollution and also expensive chemical synthesis process are worth the attention of the researchers [20]. Fortunately, researchers have found that the application of natural dyes extracted from the plant flowers petals, leaves, roots in the DSSCs, also producing a considerable PCE [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33], which has opened up a friend way for the conversion of solar energy into electricity. For example, [PERSON] et al., used some natural dyes as photosensizers (extracted from Celosia Cristata, Saffron, Cynoglossum, and eggplant peel) [21], in which PCE is found to be 1.38% (Celosia Cristata). Five different natural dyes with various anchoringgroups have been studied from various plants and used them as photo-sensitizers in DSSC [22], showing possess better absorption at 300-550 nm. [PERSON] et al. [23] used a series of natural dye extracted from plants for the DSSCs. Because Delphinidin was the main pigment of _C. ajacis_, interaction between the hydroxyl groups of the Delphinidin and the TiO\({}_{2}\) surface was very efficient, and this sensitizer owned the best photovoltaic performance among all the natural dyes. It was interesting that [PERSON] and co-workers [24] used carbonized mangosteen peel and mangosteen peel dye as a natural counter electrode and a natural photo-sensitizer for DSSC application, respectively; they found a distinctive mesoporous honeycomb-like carbon structure with a rough nanoscale surface in carbonized mangosteen peels, resulting in the highest PCE (2.63%) when using carbonized mangosteen peel and an organic disulfide/thiolate (T\({}_{2}\)/T\({}^{-}\)) electrolyte. Since the first report on Chlorophyll (Chl) and its derivatives as sensitizers in DSSCs [32], the greater sensitizer ability of Chl derivatives with respect to the pure untreated chlorophyll a was evidenced, which result can be explained considering the fact that the low attitude of this natural pigment should be efficiently absorbed on TiO\({}_{2}\) layer when dissolved in some solvent (ethanol, acetone, pyridine, etc.). Generally, the dye extracted from natural species are not pure chlorophyll but natural compounds derived by chlorophyll a and b. Recent study found the copper-Chle6 achieving the following parameters: _FF_ of 0.65, \(V_{OC}\) of 438 mV and PCE of 1.81% [33]. There are also several ways to improvement of solar cell performances by photoanodes modification, such as the photoanodes modification based on H-TiO\({}_{2}\) NPs scattering layer [34] and indium doped ZnO photoanode [35]. Recently, the theoretical method based on quantum chemistry calculation has provided the valuable reference for predicting the relationship between the structure and photoelectrical properties of molecules [36; 37; 38; 39; 40]. [PERSON] et al.