Keywords

1 Introduction

Since the last decade, the world is developing at a pace like never before. An important factor behind this evolution is the recent advancements in the field of the energy generation technology. With the growing energy demands, the need for technology which must be economically, environmentally, and socially compatible has also increased. The energy-generating technologies can be classified as conventional and nonconventional [1]. The conventional energy sources have been in use since a long time. These include petroleum, coal, wood, etc. These types of sources have proved to be very beneficial for the human race development but still they have many disadvantages. Most of them cause environmental pollution and also are costly as the energy is needed to be transmitted over long distances after conversion into electricity. On the other hand, there are nonconventional energy sources like wind, solar, and thermal energies. These are inexhaustible and environment friendly but still need to be developed more to be conventionally used. Solar energy as a nonconventional energy source is a very good option for not only bulk electricity production but also for the off-grid purposes. They are also helpful in avoiding the long-distance transmission costs [2, 3].

There are many emerging technologies which have been discussed in this paper. Classification in the PV technology is explained in Sect. 2. Different conventionally used material technologies are described with different structures in Sect. 3. In Sect. 4, modern different emerging PV technologies are clearly explained. Finally, Sect. 5 concludes this paper.

2 Classification

The solar cell technology can be characterized into three eras. Figure 1 shows the different types of the PV technologies based on different types of the materials used. First, generation cells were based on silicon wafers. Silicon is the second most abundant element present on earth and its nontoxic nature makes it suitable for the widespread use in the PV industry [4].

Fig. 1.
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PV technologies classification

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However, due to its high cost an alternative path was required, so second-generation thin-film modules were invented. These modules reduced the material used and hence reduced the cost, but the efficiency was also less as compared to crystalline Si. The third-generation polymer technology is scotch as well as lightweight. It is helpful in meeting concerns regarding the environmental problems. But even this technology has lower efficiency as compared to Si-based ones. Hybrid technology involves the combination of both the crystalline and thin-film modules [5, 6].

3 Conventionally Used Material Technologies

3.1 Crystalline Material

This technology is considered to be the first generation of photovoltaic technologies. Modules are made by combining different silicon cells or GaAs cells. The crystalline silicon-based cells are as yet driving the PV market. The conversion efficiency of single-crystal silicon cell has hit the mark of 26.3% at STC (Standard test conditions, i.e., 25 ºC and 1000 W/m2 sunlight intensity) [7].

3.1.1 Monocrystalline Silicon

It is the most prevalent material in PV modules. It utilizes a p–n junction for its course of action. The front of the cell is encrusted with a blanket of micrometer-sized pyramid structures. Solar cells based on this technology have immensely phosphorous-doped n+ section stacked over p-type boron-doped substrate to engender a p–n junction. Immensely doped p+ field (BSF) sections are framed on the back facade of the silicon substrate; its aim is to curtail the recombination of minority carriers. Its structure is shown in Fig. 2. The cells based on this technology are typically 5 inches squares [8, 9].

Fig. 2.
figure 2

Structure of crystalline solar cells [8]

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3.1.2 Polycrystalline Silicon

This technology was introduced to reduce the production cost of silicon ingots. The wafers of these cells are formed by pouring the molten silicon in a cubical mold. The liquid silicon is then allowed to cool down and solidify. The solidified block is sliced to form perfectly square cells [10]. Polycrystalline silicon has high packing factor. Nowadays, new sorts of back-contact polycrystalline cells are created by different organizations. Among these, metal wrap through (MWT) cells and emitter wrap through (EWT) cells are mostly used for PV cells [10, 11]. The efficiency of the poly-c-Si-based modules by Trina solar corporation at STC is found to be 19.9% [12, 13].

3.1.3 Gallium Arsenide (GaAs)

GaAs cells are having high energy conversion efficiency as compared to mono-c-Si and poly-c-Si cells. But due to high cost, it is not commercially used. It has high-temperature coefficient and hence is suitable for use in the space applications and concentrated PV modules. It possesses lighter weight as compared to c-Si [14]. GaAs can be further alloyed with phosphorous (P), indium (I), aluminum (Al), or antimony (Sb) to improve the efficiency. The efficiency of alloying increases due to the formation of multi-junction structure [15,16,17].

3.2 Thin-Film Material

This technology is considered to be the second generation of photovoltaic technologies. Thin-film technology extensively reduces amount of semiconductor material used and hence reduces the production costs. But due to high radiation capture losses, its efficiency is lower than c-Si cells [18, 19]. Gallium arsenide (GaAs), CdS, and titanium dioxide (TiO2) are the materials that are most commonly used [20, 21].

3.2.1 Amorphous Silicon

This material is having about 40 times superior absorptive rate of light as comparison to mono-c-Si. Due to high efficiency, it is most commonly used material in the thin-film cells. As shown in Fig. 3, a-Si cells due to high band gap of 1.7 eV absorb very broad range of the light spectrum [22].

Fig. 3.
figure 3

Structure of a-Si solar cell

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The Tel solar corporation a-Si solar cell at STC is found to have 12.3% efficient. But when exposed to sunlight, its efficiency decreases by about 30 to 40%. This reduction is caused due to Staebler–Wronski effect (SWE) which can be minimized by thermal annealing at or above 150 ºC [22,23,24].

3.2.2 Cadmium telluride (CdTe or CdS/CdTe)

Photovoltaic solar cells based on CdTe contribute to the major part (about 5.1%) of commercial thin-film module production worldwide. CdTe-based solar cells are the second most normal PV innovation on the planet. The United States is the leading manufacturer of CdTe PV. In addition to high efficiency, these cells can be quickly manufactured and also costs low [25]. Typical CdTe thin-film deposition techniques are shown in Fig. 4.

Fig. 4.
figure 4

CIGS and CdTe solar cell diagrams [45]

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3.2.3 Copper Indium Gallium Diselenide (CIS/CGS/CIGS)

It is fabricated by storing a thin layer of copper, indium, gallium, and selenide on plastic or glass backing. CIGS is a strong arrangement of copper indium selenide (CIS) and copper gallium selenide (CGS), having chemical composition as CuInxGa(1−x)Se2 [26]. As discussed earlier, the more suitable the band gap, the more is the range of the wavelength to be absorbed from the solar radiation. CIGS-based solar panels are the most elevated performing thin-film solar panels till date. These cells contain less of the toxic material cadmium as compared to CdTe cells [27].

Photo-degeneration takes place in CIGS modules when subjected to sunlight just like CdTe modules. Additional barrier coating is required to mitigate this problem [28]. CdS layer protects the CIGS layer from further processes. Further, the ZnO layer as the transparent front contact is deposited by radio frequency sputtering or atomic layer deposition (ALD) [29]. The heterojunction is formed between ZnO and CIGS layer [30]. In June 2016, the Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) held world record for highest efficient CIGS laboratory-sized (0.5 square centimeter) solar cell with 22.6% efficiency [31].

4 Emerging PV Technologies

4.1 Organic/Polymer Material

This underdeveloped technology is the part of the third generation of photovoltaic technologies. The organic polymers have high light absorption coefficient, and hence more light can be absorbed using less material [32]. The demerits of this technology include very low conversion efficiency and less stability due to photochemical degradation [32, 33]. Based on the junction types, these cells can be divided into three basic categories: (1) Single layer [34,35,36], (2) Discrete heterojunction [35,36,37], and (3) Bulk heterojunction (BHJ) [38].

4.2 Hybrid Technology

A hybrid solar cell can be a combination of organic and inorganic materials. Due to combination of high charge carrier mobility of the inorganic material and high light absorption capability of organic materials, this technology got much attention in the recent year. Following are some common hybrid technologies:

  • Perovskite-based cells: This solar cell uses a perovskite-structured compound as active layer for light absorption. The problem with this technology is low sunlight stability of the perovskite material [39]. Shin et al. [40] using methylammonium lead iodide (MAPbI3) perovskite as active layer and lanthanum (La)-doped BaSnO3 (LBSO) as photoelectrode materials achieved photo-conversion efficiency of 22.1% and high photostability of about 1000 h.

  • Multi-junction solar cells: These cells contain three or more p–n junctions using materials having different band gaps. The use of different semiconductors with different band gaps increases the range of the light spectrum absorbed by the cell [41]. Frank et al. [42] developed a 44.7% efficient four-junction GaInP/GaAs//GaInAsP/GaInAs tandem cell.

4.3 Quantum Dots

A quantum dot is a semiconductor crystal having size in nanometers. The band gap of these dots can be changed by changing their size. The change of the band gap changes the range of the solar spectrum radiation absorbed by the material. Hence, it is an attractive technology to be used in multilayer PV cells [43]. These cells are easy to synthesize and less costly. The highest conversion efficiency shown by quantum dots based solar cells till date is 11.3% only [44].

5 Conclusion

Various state-of-the-art solar photovoltaic materials have been discussed in this paper. The first half of the paper is mainly focused on the structure, efficiencies, and manufacturing processes of the conventionally used solar cells. Si-based solar cells still rule the PV industry. There are many emerging technologies which have been discussed in the later sections of the paper. These emerging technologies may prove to be competing with the conventionally used technologies in the near future. But currently the challenges for these emerging technologies are increasing conversion efficiencies and stability under direct solar radiation exposure. The race to develop highly efficient solar cells with low manufacturing cost is never ending. Therefore, further improvements are expected in the near future in the PV technologies.