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This is the fastest growing technology with an average increase of 48% since 2002 (Kropp 2009). Six main types of solar PV which are used to transform solar energy directly into electricity are crystalline silicon, thin film solar cells, concentrated solar PV, organic/polymer cells, hybrid solar cells and dye‐sensitized solar cells (DSSCs) (Pandey et al. 2016). Apart from these main types, there are some other solar cells based on advanced technologies. Tuning of the band gap of solar cells using nanoscale composites revealed enhanced power conversion efficiency. These are often termed as third‐generation PV (tandem cells, impurity‐band and intermediate‐band devices, hot‐electron extraction and carrier multiplication) based on nanostructures. In the field of nanotechnology, carbon nanotubes, quantum dots and ‘hot‐carrier’ flat‐plate device based solar PV cells are produced (Razykov et al. 2011).

Under the crystalline silicon solar cells which are one of the categories of solar PV, there are mono‐crystalline, poly‐crystalline and GaAs‐based solar cells. Mono‐crystalline is still popular among the manufactures due to high efficiency and easy availability; however, its cost is high for both manufactures and end users. So, other cost‐effective options are also evaluated to further decrease the cost, and ploy‐crystalline offers a good deal in terms of production cost. Another alternative under the category of crystalline silicon cell is GaAs‐based solar cells which provides high efficiency, and these are also low‐weight. However, again, its cost is high compared with other types of crystalline solar cells. These are resistant to high heat which makes them suitable for the concentrated PV (used in power generation), hybrid use and space applications (Deb 1998).

Thin‐film solar cells are of three types, namely amorphous Si, CdS/CdTe and CIS/CIGS (copper indium gallium selenide). Amorphous Si‐based thin‐film solar cells are further classified into three types: single junction, double junction and triple junction (El Chaar et al. 2011). Thin‐film solar cells require less manufacturing materials which makes them cheaper compared with crystalline Si‐based cells. Amorphous Si‐based solar cells have higher absorption rate of light (40 times due to non‐crystalline and disordered structure) which makes them more popular than CdS/CdTe and CIS/CIGS among the same category owing to the higher efficiency of the former (Pandey et al. 2016). Let us consider a particular example of CdTe solar cell, where an experimental study (Soliman et al. 1996) to enhance the characteristics of CdTe showed that to produce better cells, chemical heat treatment is required. Another example in the same category is CIGS which has been popular because of its laboratory‐scale efficiency of about 20.3%. In the area of thin films, there is ongoing research to enhance the efficiency and lifetime of these cells (Pandey et al. 2016).

Concentrated solar PV (CPV) system is gaining popularity nowadays due to its high efficiency which is the major requirement to make it cost‐effective technology and also to make it feasible at user end. Different classification of concentrated solar irradiation based on a study (Looser et al. 2014) is shown in Figure 2.2. CPVs are used to generate electricity as well as heating of water to low or medium temperature by extracting heat using active cooling i.e. using heat transfer fluid. For the long‐term applications of CPV in different sectors, various studies are conducted worldwide. In a particular example, at the Institute of Nuclear Research in Taiwan, Kuo et al. 2009 worked on the design and development of the 100 kW high‐concentration photovoltaic (HCPV) with passive cooling system. This institute receives solar radiation of 850 W/m², with this solar radiation system module efficiency reported to be 26.1% with a concentration ratio of 476×.

Organic/polymer solar cells have efficiency between 8 and 10% (Dou et al. 2012). In addition to the low efficiency, these cells are used as an alternative material due to various properties such as low manufacturing cost, low weight and good mechanical flexibility. Globally many laboratories have developed high‐performance solar cells using P3HT (poly [3‐hexylthiophene]) as the donor and PCBM ([6, 6]‐phenyl C60 butyric acid methyl ester) as the acceptor and/or BHJ (bulk hetero‐junction) structures (Bagienski and Gupta 2011; Devi et al. 2011). Further, from an environmental point of view, these types of cells are the most desirable ones.

Figure 2.2 Classification of common technologies and system set‐up for concentrated solar irradiance conversion.

Source: Based on ref. Looser et al. (2014).

Hybrid solar cells offer a right blend of inorganic and organic materials. At present, this type of cells are gaining popularity due to cheap processing techniques of organic materials. Choice of organic and inorganic materials opens various options for the chemical synthesis and molecular design of hybrid solar cells (Pandey et al. 2016). Inorganic part of the cell possesses high charge‐carrier mobility while the organic part has strong optical absorption which makes them one of good options for energy fulfilment.

DSSCs are simple to manufacture, similar to hybrid solar cells with low cost, low toxicity and ease of production. These cells have the potential in the solar industry in near future. At present, these cells cannot be used in commercialized PV systems owing to their poor efficiency (8–12%), a major concern for the solar cells in this category (Pandey et al. 2016). Recently, a new profitable platinum‐free counter electrode for DSSCs has been reported (Ahmad et al. 2014). Graphene nanoplatelets (GNPs) or multi‐wall carbon nanotubes (MWCNTs), or various weight % of hybrid GNPs and MWCNTs mixtures were used to make counter electrodes. A marginal increase in conversion efficiency was reported in the study. Using Ru (II) dyes, the efficiency of current DSSCs was reported to reach 12% (Sharma et al. 2018), which is still less as compared with the efficiency of the first‐ and second‐generation solar cells (20–30%).

Several researches are conducted in the techniques for improving the efficiency of PV panels apart from the advancement of cell material itself. Because of fluctuating solar flux, PV systems are not efficient to capture all available energy. So, to capture all the available solar energy, solar tracking (one axis and two axis) is performed. To increase the solar radiation collection, a tracker keeps PV photo thermal panels in a particular position perpendicular to sun rays during the day (Roth et al. 2005). One of the studies concluded that use of two‐axis tracking surfaces increases the total daily collection of solar radiation by approximately 41.34% compared with fixed one (Mamlook et al. 2006). A CSP system consisting of parabolic, trough‐shaped mirrors focuses the sunlight on the tubes which contain a heat transfer fluid. Temperature is raised to 734 °F by repeated exchange of heat. Heated fluid is used to generate the super‐heated steam which powers turbine generators to produce electricity (Devabhaktuni et al. 2013).

A different technique in which solar energy is not concentrated is becoming popular in a short span of time. In this technique, flat plates and evacuated tubes are used as solar energy collectors for heating and cooling purposes. This technique is cost‐effective with good efficiency and can be used in low‐intensity solar areas. Insulated copper tubes consisting of water or air are used to absorb solar energy. Water or air present in the tubes is heated up before returning to the storage system (Kannan and Vakeesan 2016). In a modification evacuated tube collector is used where heating pipes are shielded by vacuum. This modification is 20–45% more efficient than flat‐plate collectors (Mangal et al. 2010).