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. Reviews of Dyes Used in DSSCs
The dyes applied in DSSCs are categorized into two types which are organic and inorganic dyes. Inorganic dyes comprises of metal complex, e.g polypyridyl complexes of Ruthenium and Osmium, metal porphyrin, phthalocyanine and inorganic quantum dots, while organic dye comprises of natural and synthetic dyes.
4.1. Natural Dye Sensitizers
Another type of dye sensitizers used is the organic or natural dye. Natural dyes offers a suitable alternative to high cost inorganic based DSSCs. Naturally, the fruits, flowers and leafs of plant shows different colors from red to purple and include different natural dyes which can be extracted using simple procedure and used for DSSC fabrication [30]. Consideration have been made on natural pigments as a promising alternative sensitizer dyes for DSSC because of their simple production technique, affordable cost, complete biodegradation, easy availability, purity grade, environmental friendly, high reduction of noble metal, and chemical synthesis cost [31–33]. Plant pigmentation results from the electronic structure of pigments reacting with sunlight to change the wavelengths as may be perceived by the viewer. The pigment can be described by the maximum absorption wavelength (λmax) [34]. The performance of natural dye sensitizer in DSSC has been estimated using fill factor (FF), energy conversion efficiency (η) (Jsc), open circuit voltage (Voc), and short circuit current. Many parts of a plant have been tested by Researchers (see Table 1) and various useful dyes have been highlighted as photo-sensitizers for DSSC from natural products [35-42].
Common pigments are (a) Betalains (b) Carotenoids (c) Chlorophyll and (d) Flavonoids as Anthocyanins etc. Structures of some natural dyes employed in DSSCs are shown in Figure 2, 3 and 4.
4.1.1. Flavonoids
 Flavonoids are widely distributed plant pigments. The word “flavonoid” is commonly employed to define a large group of natural products including C6 - C3 - C6 carbon structure or more specifically phenylbenzopyran functionality.
Over 5000 naturally occurring flavonoids have been extracted from various plants, and divided according to their chemical structure as follows: flavonols, flavones, flavanones, isoflavones, catechins, anthocyanin, and chalcones. There are three classes of flavonols which are: flavonoids (2-phenylbenzopyrans), isoflavonoids (3- benzopyrans), and neoflavanoids (4-bezopyrans). Flavonoids contain 15-carbon (C15) based structure with two phenyl- rings joined by three carbon bridges, forming a third ring. The phenyl ring oxidation degree (C-ring) identifies the different colors of flavonoids. However, not all flavonoids have the capability of absorbing visible light, although they have similar structures. Flavonoid molecules are characterized by loose electrons; thus, the energy required for electron excitation to LUMO is lowered, allowing visible light to energize the pigment molecules. Flavonoids regularly occur in fruits, where animals that feed and diffuse the seeds of fruits are attracted, as well as in flowers where insect pollinators are attracted. Many flavones and flavonols absorb radiations most concentrated in ultraviolet (UV) region forming special UV patterns on flowers which are visible to bees. They are also present in the leaves of many species, where they protect plants by screening out harmful ultraviolet radiation from the Sun. Flavonols, Anthocyanins, and proanthocyanidins are three major subcategories of flavonoid compounds.
4.1.2. Carotenoids
Carotenoids are organic pigments found in both chloroplasts and chromoplasts of plants and some other photosynthetic organisms, including some fungi and bacteria. Carotenoids play two important roles in plants and algae: absorption of light energy for use in photosynthesis, and protection of chlorophyll from photo-damage [43]. Carotenoid pigments do make provisions for many flowers and fruits with typically red, yellow and orange colors, and numbers of carotenoid derived aromas. There are over 600 carotenoids known and are divided into two categories; carotenes (pure hydrocarbons) and xanthophylls (which contain oxygen). All carotenoids are tetraterpenoids, meaning that they are produced from 8 isoprene molecules and contain 40 carbon atoms. Generally, carotenoids absorb wavelengths ranging from 400-550 nanometers (violet to green light).
4.1.3. Chlorophyll
Chlorophyll (Chl) is a green pigment found in the leaves of most green plants, cyanobacteria, and algae. There are six types of chlorophyll pigment, and the most occurring type is Chl a. Chlorophyll is a compound known as a chelate which is composed of hydrogen, carbon, a central metal ion joined to a large organic molecule, and some other elements like oxygen and nitrogen. In photosynthesis, absorption of energy is done by chlorophyll for the transformation of carbon dioxide to carbohydrates and water to oxygen. This process converts solar energy to a form that can be utilized by plants. The molecular structure contains chlorine ring with Mg center, together with various side chains and a hydrocarbon trail, depending on the Chl type (Fig. 3). Chls are the most important pigments in natural photosynthetic systems [44, 45]. Their functions consist harvesting sunlight, converting solar to chemical energy, and electrons transfer. Chls include a group of more than 50 tetrapyrrolic pigments [46]. Chls and their derivatives are inserted into DSSC as dye sensitizers because of their beneficial light absorption tendency modes; the most efficient of which is Chl α (chlorine 2) derivative-methyl trans-32-carboxy- pyropheophorbide α. Xiao et al., reported that chlorine 2 has an ability to lead 4semiconductors TiO2 and ZnO surfaces through different modes [47]. Maximum absorption is achieved by Chlorophyll at 670 nm because of an interesting compound that acts as a photosensitizer in the visible light range.
Chlorophyll-a is the primary pigment for photosynthesis in plants with the composition C55H72O5N4Mg (Fig. 3).
It exhibits a grass-green visual color and absorption peaks at 430nm and 662nm.
Chlorophyll-b has the composition C55H70O6N4Mg, the difference from chlorophyll-a being the replacement of a methyl group with a CHO (Fig. 3). It exhibits a blue-green visual color and absorption peaks at 453nm and 642nm.