Solar Cells

Solar cells or PVs industry is dramatically growing and developing in terms of manufacturing technologies, efficiencies, design, materials, and cost. The research points for solar cells are concerned with cost-

Figure 9.1 Solar cells challenges.

effectiveness, light trapping in the cell, absorption, minimization for leakage currents and thermal stability (Fig. 9.1). Different solar cell types are introduced as three generations. The first one is characterized by monocrystalline silicon wafers, followed secondly with amorphous silicon. Thirdly, the low-cost section of solar cells; organic-based photovoltaics (OPVs) [15]. Solar cells' common materials are mostly light materials. This lightweight compared with the net output power is considered to be an advantage for solar cells. This selectivity to control high power with low weight structures. Additionally, these light structures could be established commercially at any remote location required. This causes a reduction in many financial sectors in production. That’s why it is recommended to use photovoltaic systems in many areas. As discussed before, PVs are built from two sections: P-type and N-type. These layers account for acceptors and donors for charge carriers. The intersection region between those two layers is called the depletion region and this phenomenon is responsible for the junction. The junction is the channel where electrons flow through P and N regions [16].

The main dominant semiconductor is the crystalline silicon. It possesses the interest of all commercial sectors with a percentage that surpasses 90% of the worldwide solar cells market. Since there are different structures for solar cells, the semiconductor singlejunction contributes 25% of these industries. As discussed before, according to Shockley-Queisser analysis, the single junction could not be improved over a certain limit [4]. Modifications in a structure such as tandem structure which is composed of multiple junctions can push overcoming Shockley-Queisser limit. Practically, after very careful and accurate experiments, multi-junction solar cells have overcome the percentage of 40% under specific conditions of focusing light. However, in theory it could achieve more than 50% to contribute to crucial applications [17].

Types of Solar Cells

Solar cell technology is dominated by silicon solar cells. About 80% of the functioning cells are based on semiconducting materials. The other type of cells is either organic based cells or modified structures for semiconducting based cells. In this section, a brief description of the different types of solar cell materials is provided.

Inorganic Solar Cells

Solar cells are classified either organic or inorganic based on the nature of the active material [absorbance layer) of the p-n junction. Inorganic materials used in solar cells have high electron mobility, electrical conductivity, and thermal stability. Their characteristics give advantages over organic solar cells considering optical output power and reliability as shown in Fig. 9.2. A wide range of materials can be used for inorganic materials-based solar cells. Materials like indium gallium nitride (InGaN), gallium indium phosphide (GalnP), and gallium arsenide (GaAs) are commonly used as choices for active materials. Ш-nitride systems enable high-performance solar cells due to their WBG gaps. Several modifications are introduced for inorganic solar cells to enhance the power conversion efficiency. WBG gaps structures and photonic crystals are two main techniques used to allow maintaining higher light trapping performance.

Since WBG materials reduce the leakage current, they are designed for enhancing the behavior of the solar cells. Devices based on substrates like SiC show better performance [18]. The fabrication and preparation processes for such materials operating under very high temperatures can reach up to 1200 C. That’s why the structure

Figure 9.2 Comparison between semiconductors to organic solar cells.

developed using such materials can be stabilized firmly under high temperature compared with semiconductors like crystalline silicon [19].

Organic Solar Cells

The concept of green chemistry is applied in fabricating organic solar cells. It is the design and processing of chemical products that reduce or eliminates the use or production of harmful substances. Such a process produces tiny waste, generates environmentally byproducts, and uses energy efficiently. Green chemistry relates to the life cycle of the solar cell, including its design, manufacture, and usage [20]. Consequently, organic solar cells have been considered as an alternative for its low cost compared with inorganic ones. However, organic materials owe lower electrical conductivity, and hence carriers’ mobility is low. Recent research is focusing on increasing power conversion efficiency for organic solar cells. Therefore, there are different structures and types for the establishment of organic solar cells. Each type exhibits a modification either in the structure or usage of different materials for enhancement of efficiency. Figure 9.3 summarizes the modification introduced to organic solar cells to enhance its efficiency.

Figure 9.3 Modifications to organic solar cells.

Tandem Cells

The simultaneous increase in the number of cells will lead to higher conversion efficiency. The technique of implementing this approach is tandem solar cells. Tandem solar cells are a formulation for a multi-junction structure with a combination of two cells, at least. The two subcells are connected, with one at the top and the other at the bottom. A layer is positioned between the two cells to decrease the recombination of electrons and holes again [21]. The net current of the cell is the minimum short-circuit current between the two cells. The tandem cell's open-circuit voltage is the summation of the two connected cells together of an open circuit voltage as summarized in Fig. 9.4 [22]. This structure has produced more carriers through active material with such analysis.

Perovskite Solar Cells

Perovskite solar cells are a branch of solar cells based on materials with a specific orientation of ciystal structure. Materials have the

Figure 9.4 Tandem solar cells principle.

Figure 9.5 Main principles for perovskite solar cells.

structure of ABX3 with the same crystal structure as calcium titanite, where A and В elements act as cation X acts as an anion. X element must be either oxygen or halide. However, it is more commonly studied as oxygen than halide since oxides have higher electrical properties [23]. Figure 9.5 shows an exhibition for the fundamental points for the ABX3 molecular structure. It has excellent charge carrier transport based on its crystallinity and the length where the carrier can transport. The perovskite cell exhibits a narrower bandgap in 3D structures compared with the 2D structure [24]. The narrow bandgap is beneficial for PV devices [25]. Perovskite solar cells have power conversion efficiency more than conventional organic active materials.

Photonic Crystals

Photonic crystals (PCs) can be implanted in organic and inorganic solar cells. PCs are periodic structures established through the active material. For example, inorganic materials have low electron mobility compared with semiconductors. Also, their light absorption is much lower compared with silicon. Photonic crystals invoke photonic gaps in order to prevent light from spreading by them [26]. This leads to the containment of light in the active material for organic and inorganic solar cells. Control of trapped light in solar cells improves light absorption. The variation of the periodic sequence dimensions of photonic crystals determines the incident

Figure 9.6 Main principles for photonic crystals-based solar cells.

trapped wavelength. Therefore, photonic crystals give a particular property for the required input wavelength [26]. Recent studies show the effect of the shape of the photonic crystals on the absorption magnitude.

The contrast between refractive indices of photonic crystals and active materials excites the manipulation of light. Thus, it increases the optical path of light and consequently causes an increase in absorption. Many plasmonic materials and transparent conducting oxides are used as photonic crystals. Zinc oxide, lithium fluoride, indium tin oxide, and other materials are commonly used to enhance the confinement of light in solar cells. These materials in each proposed structure cause the required refractive index contrast to confine light in the active material. A summary of the three crucial characteristics for the effect of the photonic crystal in the active material is illustrated in Fig. 9.6.