zinc tin sulfide is a forth order semiconducting compounds, increasing interest toward application of solar PV cells. Similar to this material, namely, sulfur selenium alloy, copper zinc tin selenide. These materials have electronic and optical properties same as that of cadmium indium gallium diselenide; hence, it is well suited for solar PV cells applications. It comprises abundant availability and harmless elements. It improves efficiency of solar cells upto 12% compared to cadmium-based and crystalline-based solar cells, and more effort is required to be commercialized in the market [10]. Copper zinc tin sulfide belongs to forth order compound as shown in Figure 2.6. Copper zinc tin sulfide PV cell can attain by replacing the bivalent Zink, trivalent indium, and gallium and tetravalent tin, which forms like kesterite structure. Few authors are reported this PV cell structure in the forms of stannite structure but it is not clear as much of kesterite structure. Crystal energy for stannite is 2.86 meV per atom higher compared to kesterite structure [23]. However, common defect occurred is structure resolution retarded by disarray of the copper and zinc cations.
2.4.2 Dye Sensitized PV Cell
It consists of transparent conducting glass, titanium oxide (TiO2) nanoparticles, dyes, electrodes, and counter electrode as shown in Figure 2.7. Glass substrate layer is in the top, which permits sunlight into the cell simultaneously conducting electrons transfer to external circuit. Titanium oxide nanoparticles are used as anode materials also called negatively doped; it has high photosensitivity and stability. Dye molecules are very important for increasing efficiency through absorbing visible light photons. Natural colorings and synthetic dyes act as donor and acceptor, respectively, which enhance current density and absorbing capacity of infrared region. Three different kinds of electrolytes have been used for the PV cells, namely, inorganic solvents, inorganic ionic liquids produced with salt or mixure salts, and solid electrolyte such as iodine free gel electrolyte. In the backside of dye sensitized cell, another glass substrate is covered by platinum as catalyst and produces cathode materials. It is very low expensive and more appropriate to a thin film solar cell group [19]. Working of dye sensitized PV cells is based on semiconductor that is devised between an electrode and photo sensitized anode in electrochemical system. The advanced dye solar PV cell is also called Gratzel cell, which is invented by B.O’Reganand and M.Gratzel at UC Berkeley in 1988 [4]. Later, it has been developed by scientist at Ecole Polytechnique, Lausanne in 1991 [14]. In 2010, M. Gratzel has honored with Millennium Technology prize for this innovation [20]. This type of cells has more advantages compared to other type of cells, which are cost effective, flexible, and transparent. It has been avoided number of high cost materials such as ruthenium and platinum. Presence of liquid electrolyte makes difficulty in terms of making appropriate in all weather conditions. However, its power conversion efficiency is low compared to other high expensive solar cells. The efficiency of this system should be high than only it can compete with commercially available solar cells.
Figure 2.6 Copper zinc tin sulfide PV cell.
Figure 2.7 Dye sensitized PV cell.
2.4.3 Organic PV Cell
Organic electronics is a branch of study which briefs about tiny organic molecules and polymers; it is incorporated in organic solar cells for power production [7]. Schematic diagram of organic photovoltaic solar cell is shown in Figure 2.8. The photovoltaic effect is used to receive sunlight and transport charge to produce electricity. The polymer solar cell is a type of organic photovoltaic cell, and it is used to produce high volumes at low cost [21]. Due to its flexibility, it is cheap and most appropriate for photovoltaic applications. It has higher absorption coefficient of organic molecules; hence, receiving of large amount of solar light is possible with smaller materials, and it is in the range from few hundreds to few thousand of nanometers. However, this system has few drawbacks such as lesser efficiency, strength, and stability compared to inorganic solar cells.
Figure 2.8 Organic PV cell.
2.4.4 Perovskite PV Solar Cells
The perovskite solar cell is one kind of solar cell which comprises perovskite structured materials, namely, tin-halide–based materials and leadbased materials [29], which is shown in Figure 2.9. Perovskite structured compound called methylammonium lead halides, which is ease to manufacturing of cell and cheap. Efficiency of cell is raised from 3.8% to 21% within 6 years of intervals; it is considered as fastest growing photovoltaic solar cell technology [5, 17, 29]. It reduces production cost without compromising efficiency or increased efficiency, and this type of PV solar cells turned into commercially attractive [1].
2.4.5 Polymer Photovoltaic Cell
Method of fabrication of solar cells is similar to the manufacturing of IC (integrated circuit) and silicon wafer used in computers due to its high refinements. However, high manufacturing cost and difficulties in process are the reason for searching alternative technologies. Few advantages of polymer PV cells compared to silicon PV cells are less weight, less production cost, environmental friendly, and disposable. Polymer PV cells are highly transparent, and it can be used in doors, windows, and electronics; however, it offers 33% of efficiency compared to silicon solar cells; it also undergoes chemical degradation due to solar light and it has lower stability [1, 17]. Owing to tandem structure, polymer solar cells can achieve 10% efficiency as shown in Figure 2.10 [8, 16].
2.4.6 Quantum Dot Photovoltaic Cell
Quantum dot acts as absorbing PV materials in solar cell as shown in Figure 2.11. It can be used for alternative materials, namely, cadmium telluride and silicon. The different ranges of energy level can be obtained by changing size of the dots. The band gap is fixed by using different materials [30]. Different materials are utilized to enhance efficiency of solar cells by storing solar spectrum [6].
Figure 2.9 Perovskite PV cell.
Figure 2.10 Polymer photovoltaic cell.