## 1 Introduction  
Dyes from bio-resources or natural dyes were the first ones used by human beings, and nowadays, ecological and health concerns have made natural dyes preferable to synthetic ones in different fields such as cosmetic, pharmaceutical, textile, or food industries [1]. Most natural dyes are found in the roots, banks, leaves, bracts, flowers, skins, and shells of plants. These plant pigments are classified into four major categories [1; 2; 3]: (i) Tetrapyrroles, such as green chlorophylls; (ii) Carotenoids, usually red, orange or yellow; (iii) Flavonoids, of which red, purple or blue anthocyanins are an important subgroup; and (iv) Betalains (yellow betaxanthins and red-purple betacyanins). The most abundant pigments are chlorophylls, followed by carotenoids. Carotenoids can coexist with other families, but betalains are incompatible with anthocyanins. These natural colorants have been of interest in different fields and applications [1]. Particularly interesting is the use of natural dyes as sensitizers in dye-sensitized solar cells (DSSCs) [4; 5; 6], their main advantages being a simple extraction procedure, low cost, wide availability, and their environmentally friendly nature [7; 8; 9]. For almost three decades, DSSCs have been considered a focus line of renewable energies, as a promising simple alternative power source. The increase in the number of publications reflects the interest on the subject [10]. The cell efficiency has been steadily improving due to extensive efforts in numerous experimental investigations and theoretical or computational studies, but there are many areas that deserve further research [11] before considering these cells as a feasible commercial product [12]. The DSSC device is an ensemble of various materials that undergo different interactions and processes, and researchers have focused on the study, modification or innovation of each component. For example, the dye or sensitizer, responsible for solar energy caption, should possess certain features in order to effectively play its role for an efficient performance. The research community has attempted to find the best sensitizer and has developed different types of them: Ru-complex dyes, metal-free organic dyes, quantum dot sensitizers, perovskite-based sensitizers, and natural dyes [6; 13; 14]. Dye molecules are adsorbed onto a nano-structured layer of a wide band gap semiconductor (i.e., TiO\({}_{2}\)) to harvest photons. If these solar cells are to be considered a as a green energy generator, the more \"natural\" the dye, the more \"green\" the device. For NDSSCs (DSSCs using natural dyes), the number of publications, scarce in the decade of 2000, has increased since 2009 (Figure 1). As an example of the still alive controversy, the early work by [PERSON] [15] studied 20 natural dyes and [PERSON] [3], only a few months later, published a review pointing out the requirements of photosensizers, why the NDSSC efficiency is low and further recommendations. Something similar occurs nowadays; reviews have been published stating recent progress in natural dyes [16; 17] but there are still challenges in the mass production of NDSSCs [12; 16]. So this subject is still of interest to researches, i.e., conferences and symposiums are being held, and since last year, almost 200 papers related to \"natural dyes and DSSC\" have been published, both from the experimental and theoretical points of view, aimed to improve devices and understanding of the different materials and phenomena involved. In general, and some examples are cited next, most papers include firstly experimental details about dyes. Pigments are obtained from a great variety of plants, some animals or minerals by different methods, and then are characterized through UV-Vis and FTIR spectrophotometry [18; 19; 20]. Further purification, combinations, analysis, and use constitute the body of the studies, together with new ideas about cell implementation.