**2. Basics of DSSC**

However, the sources of these fossil fuel reserves are depleting very fast. The existing sources of energy are inadequate, and if the fuel consumption continued at current usage rates, it would last only about 50 years [3]. On the other hand, burning fossil fuels releases carbon dioxide and other greenhouse gases (e.g., water vapor, methane, nitrous oxide, sulfur dioxide, and other ozone-depleting substance) in the atmosphere, making them the primary contributors to global warming and climate change. Because of the depletion of fossil fuels and global warming, in recent years, researchers are endeavoring severe attempts to find out various ways to meet energy demand around the world. Renewable energy could be an eco-friendly, alternative, sustainable energy resource because they are inexhaustible and will not pollute the environment for us or those of future generations by emitting harmful gases. Many alternative renewable energy sources have already been available, such as solar, hydro, wind, biogas, biomass, geothermal, wave, and tidal energy. Among all renewable resources, solar energy can be the solution to the

Solar energy is the cleanest and most abundant renewable energy source available. The solar cell or photovoltaic (PV) device is used for converting the energy of sunlight into useable electrical energy. The generated energy from solar does not produce any harmful emissions, consumes no fossil fuels, has no moving parts, and requires little maintenance. The development of PV technology is growing, and intensive research works are undertaken worldwide to improve cell performance and reduce the cost of the cell. With a history dating back over 60 years, since the very first silicon bipolar solar cell, the last three decades silicon solar cell has seen immeasurable advancement in both the performance of experimental and commercial cells. First-generation silicon solar cells showed their value in the market with the advantages, including high efficiency (26.6%), high reliability, low cost, ease of fabrication, and environmentally friendly traits [5–8]. Second generation thin-film (e.g., amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium selenide (CIGS)) solar cells are cheaper than the mature Si solar cells; additionally, thin-films are easier to handle and more flexible. However, the shortage of Tellurium and Indium makes it hard to manufacture solar cells commercially. Also, Cadmium is extremely poisonous and medical problems with environmental impact [9]. This significant concern opened the method of exploration of finding other elective materials and further innovation for solar cells. Several new thinnerfilms have surfaced through concentrated research with higher potential, including dye-sensitized solar cell (DSSC), perovskite solar cell (PSC), copper zinc tin sulfide (CZTS) solar cell, organic solar cell (OSC), and quantum dot solar cell (QDSC) [10]. The DSSC belongs to the group of thin-films, functions on a semiconductor generated into an electrolyte and a light-sensitive anode [11]. In 1988, Brian

O'Regan and Michael Grätzel at UC Berkeley, USA initially co-invented the modern version of DSSC and later they further developed this work at the ÉcolePolytechniqueFédérale de Lausanne, Switzerland [12]. Brian O'Regan and Michael Grätzel reported the first modern version of DSSC in 1991 with an efficiency of 7.1–7.9% [13, 14]. DSSC can convert the solar energy to electrical energy by using photosensitive dye. DSSC is fabricated by using conventional roll-printing systems. The semi- transparency and semi-flexibility of DSSC offer a diversity of usages not appropriate for glass-based construction and most of the materials used are lowcost. However, practical elimination of several expensive elements has proven to be difficult, notably Pt and ruthenium (Ru). The energy conversion efficiency of the most recent laboratory-developed module is approximately 14.7% [15]. This chapter is focused on the improvement of efficiency of DSSC by the combination of natural dyes and the blocking layer. In this work, structure and operation principle of the third generation dye-sensitized solar cell (DSSC) has been discussed in the second section. Section 3 explains the combination of natural dyes with an optimized

problem of dwindling fossil-fuel reserves [4].

*Solar Cells - Theory, Materials and Recent Advances*

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As shown in **Figure 1**, a typical DSSC consists of five different parts, such as, (1) transparent conducting oxide (TCO), glass substrate, (2) anode (wide band-gap semiconductor material layer on TCO), (3) photosensitizer, (4) electrolyte and (5) cathode (platinum/carbon layer on TCO). The components of a DSSC are: two transparent conductive oxide [indium tin oxide (ITO), fluorine-doped tin oxide (FTO), Indium Zinc Oxide (IZO) and Aluminum Zinc Oxide (AZO)] glass electrodes. One of the electrodes is the anode, the working anode, which is printed with semiconductor material [TiO2, ZnO, SnO2, SrTiO3, Zn2SnO4, Nb2O5, etc.] nanoparticles (particle size around 20–50 nm). The semiconductor oxides are sensitized with a photosensitizer (metal complex sensitizer, metal-free organic sensitizer or natural dye sensitizer), which absorbs the photons. The other electrode is the counter electrode [platinum or carbon coated TCO] and in between the two working electrodes is the electrolyte containing the redox couple [I/I3 , Br/Br3 , SCN/ (SCN)3 and SeCN/ (SeCN)3 , etc.].

**Figure 2** illustrates the schematic diagram of the basic working principle of a typical TiO2 based DSSC. All other semiconductor-based DSSC such as ZnO, SnO2, etc. works under the same principle. Under illumination, a photo-excited electron is injected from the excited state of the dye (D\*) from the highest occupied molecule orbital (HOMO) to lowest un-occupied molecular orbital (LUMO) (Eq. (1)). The excited electron is injected to the conduction band of the semiconductor material. The injected electron percolates through the semiconductor material by a driving chemical diffusion gradient and is collected at the TCO glass substrate (Eq. (2)). After passing through an external circuit, and reaches the counter electrode, thus,

**Figure 1.** *Schematic diagram of basic structure of DSSC.*

**Figure 2.** *Operation principal of typical DSSC.*

dye regeneration takes place due to the acceptance of electrons from I� ion redox mediator, and I� gets oxidized to I3 � (Eq. (3)). To complete the circle, by electron donation, I� ions regenerated by the reduction of I3 � ions at the cathode (Eq. (4)). 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**) and recombination of injected electrons with *I*<sup>3</sup> � (Eqs. (7) and (8) no. red arrow in **Figure 2**). In brief, the sequence of events in a DSSC is as follows [16]:

$$\text{TiO}\_2|\text{D} + \text{h}\nu \rightarrow \text{TiO}\_2|\text{D}^\* \text{ Exaction of dye upon illumination} \tag{1}$$

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

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

$$
\mathfrak{O}
$$

carbon nanotubes, etc.; modifying the absorption properties of the dyes by Ru dye,

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

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

requisites 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

conduction band of semiconductor material. A dye should fulfill some pre-

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

rophyll, anthocyanin, carotenoids, and flavonoids [26, 27].

binding chlorophyll and semiconductor material (e.g., TiO2).

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 chlo-

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

organic dye, dye mixture, etc. to enhance cell efficiency [17–22].

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

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

sensitizers [16].

**317**

$$\text{TiO}\_2|\text{D}^+ + \frac{\text{3}}{2}\text{I}^- \rightarrow \text{TiO}\_2|\text{D} + \frac{1}{2}\text{I}\_3^- \text{Oxidation of electrolyte} \tag{3}$$

$$\frac{1}{2}\mathbf{I}\_3^- + 2\mathbf{e}^-|\mathbf{C}\mathbf{E} \rightarrow \frac{3}{2}\mathbf{I}^-\text{ Resteration of electrolyte at the counter electrode} \quad \text{(4)}$$

$$\text{TiO}\_2|\text{D}^\* \rightarrow \text{TiO}\_2|\text{D Recombination of dye}\tag{5}$$

$$\text{TiO}\_2|\text{D}^+ + \text{e}^- \rightarrow \text{TiO}\_2|\text{D} \text{ Dy recover to ground state} \tag{6}$$

$$\text{H}\_3\text{}^- + 2\text{e}^- \text{/TiO}\_2 \rightarrow 3\text{I}^- \text{ Recombination of electrolyte} \tag{7}$$

D: Dye sensitizer; D\* : Excited dye upon illumination; D<sup>+</sup> : Oxidized dye.

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,

carbon nanotubes, etc.; modifying the absorption properties of the dyes by Ru dye, organic dye, dye mixture, etc. to enhance cell efficiency [17–22].
