**3. Effect of combination of natural dyes**

The dye in DSSCs has a vital role in harnessing solar energy from the sun and converts it into useable electrical energy. The primary charges in the dyes separate through photo-excitation, and photo-excited dyes inject electrons into the conduction band of semiconductor material. A dye should fulfill some prerequisites to be considered efficient dye: (1) binding firmly with the semiconductor material; (2) higher molar absorption capabilities for maximum absorption from visible to IR-region; (3) fast electron transfer; (4) LUMO of the dye should be higher than the conduction band of semiconductor for efficient electron injection into the semiconductor material; (5) HOMO of the dye should be lower than the redox couple for efficient regeneration of oxidized dye; and (6) slow degradation (or do not degrade at all) [16, 23–25]. The dyes used in DSSC are divided into three types: metal complexes dye sensitizer, metal-free organic dye sensitizer, and natural dye sensitizer. Metal complexes dye sensitizers, such as polypyridyl complexes of Ruthenium (Ru), Osmium (Os), metal porphyrin, phthalocyanine are the most efficient and durable dye for DSSC application. However, these dyes have a complex synthesis process, release chemicals as a by-product, and require rearearth material for the synthesis process. As a result, the overall fabrication process highly depended on the rear earth material that is neither sustainable nor economical. On the other hand, metal-free organic dye sensitizer has advantages over metal complex dye sensitizer, reducing the use of rear-earth material, higher molar absorption co-efficient, and preprocessing color. However, these advantages are offset by their instability, tedious manufacturing process, tendency to undergo degradation, and toxicity. These significant limitations influenced scientists to work on possible replacements for metal complexes or metal-free organic dye sensitizers [16].

Over the years, significant research has been done to determine the possibility of replacing sensitized dye. Natural dye has several advantages over sensitized dyes. These include low production cost, high availability, easy access, simple fabrication technique, biodegradable, environment friendly, purity grade, non-toxic, and reducing the use of rear-earth material. Natural dye-based DSSCs have attracted considerable attraction as an alternative way to produce low-cost dyes to a large extent by extracting dyes from natural resources. In nature, some vegetables, fruits, flowers, leaves, seeds, roots, stems, bacteria, and algae exhibit various colors due to plant pigmentation [16]. The natural dyes are four major families which are chlorophyll, anthocyanin, carotenoids, and flavonoids [26, 27].

Chlorophyll, which is the most widespread pigment occurring naturally in plants, fungi, bryophytes and algae. The molecular structure of a chlorophyll consists of a Magnesium-containing tetrapyrrolic ring, encircled by other side chains. The chlorophylls are classed mainly as chlorophyll-a, chlorophyll-b, chlorophyll-c1, chlorophyll-c2, chlorophyll-d, and chlorophyll-f. They absorb light from red, blue, and violet in the visible wavelengths with an absorptionmaximumof 670 nm while reflecting green wavelengths. Chlorophyll dye molecule create an electronic coupling with the conduction band of semiconductor material through the carboxylic groups, which helps to anchor the dye molecules and transfer injected electron efficiently from the dye sensitizer to the conduction band of semiconductor material [16, 28]. **Figure 3** shows the basic molecular structure of chlorophyll and the binding chlorophyll and semiconductor material (e.g., TiO2).

dye regeneration takes place due to the acceptance of electrons from I� ion redox

However, some undesirable reactions are simultaneously taking place, such as nonradiation relaxation (Eqs. (5) and (6) no. red arrow in **Figure 2**), recombination of injected electrons with the oxidized dye (Eqs. (6) and (7 no. red arrow in **Figure 2**)

> 1 2 I3

: Excited dye upon illumination; D<sup>+</sup>

Nemours researchers are working on to improve cell performance by different means, such as modifying the TCO/semiconductor material interface by blocking layer; modifying semiconductor material by doping, annealing time, radiation,

TiO2∣<sup>D</sup> <sup>þ</sup> <sup>h</sup><sup>ν</sup> ! TiO2∣D<sup>∗</sup> Excitation of dye upon illumination (1)

þ e� Oxidation of dye due to injection of electrons in TiO2 photoanode

� Restoration of electrolyte at the counter electrode (4)

TiO2∣D<sup>∗</sup> ! TiO2∣D Recombination of dye (5)

TiO2∣D<sup>þ</sup> þ e� ! TiO2∣D Dye recover to ground state (6)

� þ 2e�∣TiO2 ! 3I� Recombination of electrolyte (7)

**Figure 2**). In brief, the sequence of events in a DSSC is as follows [16]:

� ! TiO2∣D þ

� (Eq. (3)). To complete the circle, by electron

� ions at the cathode (Eq. (4)).

� (Eqs. (7) and (8) no. red arrow in

� Oxidation of electrolyte (3)

: Oxidized dye.

(2)

mediator, and I� gets oxidized to I3

*Operation principal of typical DSSC.*

TiO2∣D<sup>þ</sup> þ

I3

2 I

� <sup>þ</sup> 2e�∣CE ! <sup>3</sup>

D: Dye sensitizer; D\*

TiO2∣D<sup>∗</sup> ! TiO2∣D<sup>þ</sup>

1 2 I3

**316**

**Figure 2.**

donation, I� ions regenerated by the reduction of I3

*Solar Cells - Theory, Materials and Recent Advances*

and recombination of injected electrons with *I*<sup>3</sup>

3 2 I

semiconductor material [16]. **Figure 4** shows the basic interaction between antho-

*Effect of Combination of Natural Dyes and the Blocking Layer on the Performance of DSSC*

Carotenoids occur in many plants and algae, as well as several bacteria, and fungi. It contributes to yellow, orange, and red colors and allows them to absorb short-wave visible light [32]. Carotenoids can be divided into two major types: xanthophylls (with oxygen) and carotenes (purely hydrocarbons and without oxygen) [16, 33]. **Figure 5** illustrates the interaction between carotenoids- semicon-

Flavonoids are essential floral pigments. The development of a specific color depends on the accumulation of flavonoid chromophores and other intrinsic and extrinsic factors. Chemically, the flavonoids have a C6- C3- C6 carbon framework with two connected two phenyl rings (A and B) and a heterocyclic ring (C). Depending on the oxidation potential of the C-ring, the particular flavonoids absorb light in the visible wavelength. Till now, over 5000 flavonoids have been identified from different plants. Most of the flavonoid pigment has loosely or unbound electrons. Thus less energy is required for excitation of such electrons is lower compared to the others. As a result, those pigment molecules can be energized by the light within the visible range [16]. The overall cell efficiency of natural dye-based DSSCs is comparably low compared to DSSCs sensitized with sensitized dyes. Due to the inadequate interaction between dyes and semiconductor surface, a significant reduction of the cell's shortcircuit current. The pigment's long structure obstructs the dye molecules to form a bond with the oxide surface of the semiconductor materials effectively. Those are the field of works that are yet to be developed in natural dye DSSCs to achieve highefficiency devices and device stability. To further raise the efficiency of the DSSC combination of dyes has been explored and reported DSSC or to broaden the absorption spectrum [35–39]. A combination of natural dyes with an optimized choice of the extracting solvent enhances the absorption of solar light and allowed utilization of the photon energy more efficiently. As a result, DSSC sensitized with the dye mixture shows higher absorbance, and cumulative absorption properties over the entire visi-

ble region than the DSSC fabricated with single individual dyes [35, 36].

cell performance of natural dye-based DSSC. They also mixed the dyes at five different volume ratios to find the optimized dye mixture. The cell conversion efficiency of DSSC fabricated with individual chlorophyll, and anthocyanin dyes

Kabir et al. studied the effect of chlorophyll and anthocyanin dye mixture on the

cyanin and semiconductor material (e.g., TiO2).

*DOI: http://dx.doi.org/10.5772/intechopen.94760*

ductor material (i.e., TiO2).

**Figure 5.**

**319**

*Carotenoids -semiconductor material (i.e.,TiO2) interaction [34].*

**Figure 3.** *Chlorophyll-semiconductor material (i.e.,TiO2) interaction [29].*

*Basic structure of anthocyanin and anthocyanin-semiconductor material (TiO2) intaraction [31].*

Anthocyanins are also an abundant and widespread group of water-soluble pigments in plants. They absorb light at the longest wavelengths. Depending on the pH value, anthocyanins are responsible for the existence of attractive colors, such as red, orange, magenta, pink, blue, blue-black and purple floral [16, 30]. Generally, the carbonyl and hydroxyl functional groups in the anthocyanin dye sensitizers create an electronic coupling with the semiconductor material's conduction band, which helps transfer the excited electron efficiently to the conduction band of
