Open access peer-reviewed chapter
Abstract
This chapter discusses the future of perovskite solar cells (PSCs) as a new generation of photovoltaic technologies to replace traditional silicon-based solar cells. PSCs have properties such as high efficiency, low processing cost, and flexibility in form, and, therefore, can be implemented in various applications such as building-integrated photovoltaics (BIPV), flexible electronics, and wearable electronics. Nevertheless, some issues still need to be solved in commercialising PSCs, such as stability issues, scaling-up issues, and policy barriers. However, the prospects for market development are vast, and PSCs can revolutionise the solar industry on the planet. In this chapter, the most recent methods for the synthesis of small- and large-scale perovskite-based solar cells are described. This chapter also explores some of the new research areas of interest, including tandem solar cells, perovskite-based multi-junction solar cells, and perovskite quantum dots, all expected to advance the photovoltaic efficiency and versatility further. Further, the evolution of perovskite-silicon heterojunctions, all perovskite tandem cells, and indoor photovoltaics show the growing area of perovskite utilisation. If PSCs are to overcome certain challenges and further the research, it can change the face of solar energy as a clean, efficient, and diverse option for the future.
Keywords
- perovskite solar cells (PSCs)
- photovoltaics
- high efficiency
- tandem solar cells
- building-integrated photovoltaics (BIPV)
- flexible electronics
1. Introduction
With the world’s energy consumption increasing and the onset of climate change, the role of renewable power sources can hardly be overestimated. The oil, gas, and coal that have been the foundation of the global economy for more than a century are now recognised as a significant source of environmental issues, including pollution and climate change. This has shifted the world energy mix where renewable energy is essential, particularly solar energy, in addressing climate change. Solar energy, as energy from the sun, is one of the abundant sources of renewable energy [1, 2]. It is a clean energy source that can meet the world’s energy demands without the social and environmental vices associated with fossil energy resources. Hence, solar energy technologies, particularly solar cells, have been more focused on research and development in the quest for green power technologies [3, 4, 5, 6].
Solar or photovoltaic cells are electrical appliances that generate electric power through the photovoltaic process. These are the basic building blocks of solar panels widely applied in residential, commercial, and industrial applications. Solar cells are particularly attractive for their capacity to generate power without generating any greenhouse gases [7, 8]. This capacity makes solar cells a vital weapon in the war against global warming [9]. Over the past few decades, there has been a huge advancement in the solar cell and, therefore, the efficiency, cost, and overall scalability of solar cells [10, 11]. The first generation of solar cells is the crystalline silicon solar cells that have gained the market due to the efficiency and the durability of the solar cells [12]. But in the need to push these machines further and bring down the cost of production even more, the search for other materials and technologies has been called for. Out of all the new materials for solar cells, perovskite and double perovskite have been researched more because they possess certain features and benefits over silicon solar cells [13, 14]. Perovskites are a class of materials with the general formula ABX3, where ‘A’ and ‘B’ are cations of different sizes, and ‘X’ is an anion that coordinates with both [15]. These materials have displayed good optoelectronic properties such as high absorbance, tunable band gap, and long carrier diffusion that make them suitable for photovoltaic applications [16]. Other structures have also been investigated in photovoltaics, including double perovskites, which are obtained when two different cations occupy the ‘B’ site cation. These materials can offer increased stability, reduced toxicity, and electronic character adjustability by selection of cations and anions. For instance, the double perovskite La2NiMnO6 with the designed bandgap is the great advancement in achieving high efficiency and stability of the solar cell materials [17]. The fact that perovskite and double perovskite materials have some advantages over silicon-based solar cells has led to global interest in the study of their properties and the development of ways to improve their efficiency and address issues related to stability and production. In this chapter, the reader will be acquainted with the fundamentals of perovskite solar cells and the materials used in the formation of perovskite solar cells, the manufacturing process of perovskite solar cells, and the efficiency and stability of the perovskite solar cells, and the prospects of perovskite solar cells for the future market of solar cells.
2. Fundamentals of perovskite solar cells
2.1 Crystal structure and chemical composition
The perovskite materials are defined by their specific crystal structure and are generally represented by the formula ABX3. In this structure:
‘A’ cation: Usually a big organic or inorganic cation (e.g. methylammonium (MA+), formamidinium (FA+), or caesium (Cs+)).
‘B’ cation: A smaller metal ion, usually lead (Pb2+) or tin (Sn2+), situated in the centre of the coordination sphere.
‘X’ anion: A halide ion meaning an ion of chlorine, bromine, or iodine [18].
This results in a three-dimensional structure in which the ‘B’ cation is surrounded by six ‘X’ anions to form an octahedral structure, and the ‘A’ cation is located in the space in between the octahedra. This is because the ‘A’, ‘B’, and ‘X’ positions can be occupied by many elements, and these can be adjusted to provide the most suitable physical or chemical characteristic for a particular use (Figure 1) [20].
Figure 1.
A typical cubic unit cell (left) and the crystal (right) of perovskite ABX3. Reproduced from Ref. [19] under CC BY 4.0.
The double perovskites are slightly more complex than the traditional perovskite structure; here, two different cations are present in the B′ site and are denoted by B′ and B″. In this manner, one can get the formula A2B′B″X6 (Figure 2).
Figure 2.
Double perovskite crystal structure, with B′ and B" lying in alternate octahedra B′X6 and B"X6. Adapted from Ref. [21], under CC BY 4.0.
The two different cations can change the electronic, stability, and band gap of the material making the double perovskites of particular interest in photovoltaic applications [22]. For example, La2NiMnO6 is a double perovskite in which Ni2+ and Mn4+ occupy the B′ and B″ sites, respectively; they exhibit extraordinary magnetic and electrical properties that can be applied in solar cell [23]. Another important feature of perovskite materials which opens the possibility to tune them for the best performance in solar cells is the possibility to control the chemical composition and structure of the material [24].
2.2 Bandgap engineering
A crucial parameter of semiconductor material used in photovoltaic is its bandgap, which is the energy gap between the valence band and the conduction band. This defines the ability of the material to absorb and convert the solar spectrum to electricity depending on the bandgap. The bandgap for the best efficiency of the solar cell should be around 1.3–1.5 eV, enabling the material to capture a large part of the solar spectrum while providing a high voltage in the open circuit [25]. The bandgap can also be engineered in perovskite materials by altering the composition of the ‘A’, ‘B’, and ‘X’ sites to fit the range required [26]. For example, in the most basic perovskite-type material, methylammonium lead iodide (MAPbI3), the bandgap is about 1.55 eV, which is very close to the maximum achievable for single-junction solar cells. When one or several halides at the ‘X’ site are replaced, for example, iodine at the ‘X’ place can be replaced by bromine to get MAPbBr3, the bandgap increases, and the material can absorb higher-energy lower wavelength light. Likewise, the substitution of lead with tin (in MASnI3) results in a decrease in bandgap to allow the material to absorb lower energy photons [27]. In double perovskites, the cations B′ and B″ also contribute to the bandgap engineering. For instance, in the perovskite La2NiMnO6, the bandgap can be controlled by varying the concentrations of Ni2+ and Mn4+ or by introducing impurities in the structure [26]. That is, the ability to fine-tune the bandgap through careful control of composition can be a powerful asset in the development of perovskite solar cells by enhancing light absorption and thus increasing the device’s efficiency.
2.3 Charge carrier dynamics
Apart from light absorption, the performance of a solar cell is equally measured by the ability to create and collect the photogenerated charge carriers, electrons, and holes. In perovskite solar cells, the processes of generation, separation, and transport of charge carriers depend on the perovskite material’s electronic structure and defect states [28]. In the absorption process, when a photon is incident on the perovskite material, it provides energy to the valence band electron to move to the conduction band, creating a hole in the valence band. This process creates an electron-hole pair, which has to be transported separately and turned into a current at the electrodes. The efficiency of this process depends on several factors, including:
Carrier mobility : High mobility so that the electrons and holes in the material can freely shuttle between them and reduce the chances of recombination.Diffusion length : The distance that, on average, a carrier can travel before recombining is usually the definition of the letter. Due to the long diffusion lengths of more than a 100 nanometres, perovskites are applicable for charge collection.Recombination rates : It is also essential to have a low recombination frequency to facilitate the flow of a high number of carriers to the electrode.
Some perovskites have exhibited dynamic features of charge carriers, low density of defects, and good crystallinity, which is beneficial for efficient charge production and transport. Nevertheless, vacancies, interstitials, and grain boundaries are always available and can pose as recombination sites, reducing the solar cell’s efficiency [29]. Some of these defects originate from material chemistry, while others come from the fabrication processes, and these are areas of active research in perovskite photovoltaics. Thus, the crystal structure, band gap control, and charge carrier behaviour are vital factors of perovskite solar cells. For these reasons and the fact that perovskites are characterised by adjustable band gaps and outstanding charge transport properties, these materials are considered the future for the development of photovoltaic technologies. Still, stability, toxicity, and scalability issues arose, which is why the researchers must address them in the future.
3. Perovskite materials for solar cells
3.1 Single perovskite materials-methylammonium lead iodide (MAPbI3)
One of the best-investigated perovskite materials for solar cells is methylammonium lead iodide (MAPbI3). MAPbI3 has attracted interest because of its properties, such as a suitable bandgap of about 1.55 eV, high absorption coefficient, and long carrier diffusion lengths, which makes it suitable for high photovoltaic performance [30]. MAPbI3 has the simplest structure of the perovskite family, which is a three-dimensional structure with an ABX3 formula, where A is the methylammonium (MA+), B is lead (Pb2+), and X is iodine (I−). This has made it to record high PCE efficiency over the last decade, with recorded efficiencies standing at over 25% for the most efficient materials in the system [31]. These results give MAPbI3 and other lead-based perovskites the promise of being new-generation solar cells that can replace traditional silicon-based solar cells. However, the utilisation of MAPbI3 in commercial products is limited by several factors as follows. The most critical issue of the material is that it is not as stable as it should be, but that MAPbI3 is quite sensitive to environmental conditions such as moisture, oxygen, and heat. The sensitivity towards these degradation conditions decreases the devices’ effective usage rates over time and, thus, reduces their actual service life [32]. Methods used to enhance the stability of MAPbI3 are encapsulation, compositional engineering, and synthesising of new stable forms of MAPbI3 such as FAPbI3 and mixed cation perovskites. Another critical concern is lead, which is a toxic heavy metal that has adverse effects on the environment and human health. Lead integrated into perovskite solar cells is an issue in mass production and recycling. Current research is being conducted to come up with lead-free perovskites with high efficiency and low toxicity as lead-based perovskites [33].
3.2 Lead-free perovskites
Due to potential lead toxicity issues, there have been significant attempts to synthesise lead-free perovskite materials. Among the most studied alternatives is a group of Tin (Sn)-based perovskites, for example methylammonium tin iodide (MASnI3) [34]. Tin is also a member of group 14 and can also be incorporated into perovskite structures with similar electronic characteristics to lead. Nevertheless, tin-based perovskites have the issue of the fast oxidation of Sn2+ to Sn4+, which causes instability and performance degradation of the devices. To overcome these challenges, researchers have tried out different approaches, one of them being the partial substitution of lead by tin to decrease the toxicity level and increase the efficiency of the perovskite solar cells [35]. The second approach is doping Sn-based perovskites with other elements to improve stability and electronic characteristics. Bismuth (Bi) and antimony (Sb) have also been proposed to replace lead in perovskite solar cells. The organic-inorganic hybrid bismuth perovskites, including (CH3NH3)3Bi2I9, possess stability and non-toxicity issues. However, the efficiency of these perovskites in photovoltaic applications is slightly lower than that of lead perovskites at present [36]. More investigations are directed towards enhancing these materials using compositional design and optimising the processing methods. In general, lead-free perovskites are a good approach to addressing the problem of the environmental impact of lead; however, matching their efficiency to that of lead-based perovskites is a major concern. One of the main current research topics is the improvement of perovskite materials, which should have high efficiency and stability and should not harm the environment.
3.3 Double perovskite materials
Double perovskites, which have two different cations at the B site, are promising materials for developing new photovoltaic materials. Of these, La2NiMnO6 has received considerable attention because of its magnetic and electronic characteristics. In La2NiMnO6, the B sites are occupied by Ni2+ and Mn4+ ions, which make them ordered in such a way that may affect the electronic band structure and magnetic coupling in the material. Since doping and variations in the composition of La2NiMnO6 can be used to control the properties of this material, it is appropriate for photovoltaic uses. For instance, doping of La2NiMnO6 by Sr. may lead to an increase of the density of states, the change of the band structure, and the enhancement of the electronic properties of the material [26]. Sr-doped La2NiMnO6 has been found to improve the efficiency of solar cells since the ratio of light absorption to charge carrier kinetics is improved. In this regard, it has been discovered that the bandgap of La2NiMnO6 can be adjusted to the range of values that is suitable for the conversion of solar photons by the incorporation of dopants and the control of cationic ordering. This flexibility can be employed to fine-tune the material for particular application, which can be a step towards the development of efficient solar cells with longer durability.
However, synthesising high-quality double perovskite thin films has the following challenges. Among the most important factors that determine the device performance and must therefore be controlled during the synthesis process are the cation ordering and the extent of defects that are introduced. More advanced techniques such as PLD and MBE are employed to have a better control over the film forming material and the structure. However, they are costly and difficult to apply on a commercial scale and hence may not be suitable for large-scale production of commercial solar cells. Since it is possible to tune the properties of double perovskites in such a manner, these materials may be crucial in the next generation of photovoltaic uses [37].
4. Fabrication techniques for perovskite solar cells
4.1 Solution-based methods
4.1.1 Spin coating
Spin coating is one of the most popular perovskite solar cell deposition methods because of its simplicity, low cost, and capability to deposit a uniform thin layer [38]. The technique entails the preparation of a perovskite precursor solution and then placing the substrate on which the solution is to be coated and spinning at high speeds. The centrifugal force then disperses the solution on the substrate in a thin and uniform layer (Figure 3a). This technique is useful for small-scale laboratory applications and has been instrumental in the initial stages of perovskite solar cell development. The spin coating process typically consists of two stages: deposition and drying. The perovskite precursor solution comprising ‘A’, ‘B’, and ‘X’ in the deposition stage is spread on the substrate. The substrate is then spun at high speeds (between 1000 and 5000 rpm) to ensure an even distribution of the solution. The spin speed and concentration of the solution can be used to regulate the thickness of the film. During the drying stage, the solvent is removed while the film is still spinning, resulting in the formation of the perovskite material in the form of a crystalline structure. The drying process should be well controlled to produce high-quality films, uniform grain size, and almost no defects. This can be achieved by solvent engineering, whereby an anti-solvent is added dropwise to the spinning substrate to cause rapid crystallisation or by regulating the temperature and relative humidity during the process. The spin coating method is very efficient for perovskite film deposition in laboratory conditions but unsuitable for large-scale production. The technique is rather complicated to apply over large areas, and the thickness of the film may vary towards the edges. Nonetheless, spin coating continues to be one of the most commonly used techniques for the deposition of perovskite films, especially for materials research and improvement [38].
Figure 3.
Schematics of various solution-based fabrication techniques: (a) spin coating, (b) spray coating, and (c) doctor blade coating. Adapted from Ref. [39], under CC BY-NC 3.0.
4.1.2 Spray coating
Spray coating is another large-area solution-processed deposition technique for perovskite solar cells. Perovskite precursor solution is atomized into fine particles and then coated onto a substrate. The process can be taken outside or inside a controlled setting in order to enhance the quality of the film (Figure 3b). Compared to other deposition techniques, spray coating is used mostly for the deposition of perovskite films on flexible or complex-shaped substrates, which may improve the flexibility of the device. The main advantage of spray coating is its ability to be used for large-area coatings, and thus, it can be used in mass production [40]. The thickness and uniformity of the film that is being formed can be controlled with the help of the characteristics of spraying, for example, size of droplets, distance from the substrate, and the concentration of the solution. Also, the ability to use several spray heads at once allows for the deposition of the layers consecutively—something that is critical when forming multi-layered structures. But spray coating does come with some drawbacks; the thickness of the film is not uniform; the droplets tend to coalesce and there is poor coverage. These problems can be solved by optimising the spray process as well as the post-deposition treatment like thermal anneal or solvent anneal, which will help in getting better quality of the films and higher degree of crystallinity [41].
4.1.3 Doctor blading
Doctor blading or blade coating is another solution-based method even more scalable than spin coating [42]. In this technique, a perovskite precursor solution is applied on a substrate, and then, a blade or knife is used to bar the solution into a layer (Figure 3c). The thickness of the film can be regulated through the distance between the blade and the substrate and the speed of the blade. Doctor blading is advantageous in the sense that it is relatively easy to implement and may be more suited for large-area substrates; therefore, it is more likely to be used in large-scale production. It can be readily incorporated into roll-coating systems in which a thin and continuous material layer is applied to a flexible support medium, such as plastic or metal foil. This makes it possible to produce perovskite solar cells on a large scale, which may lead to a reduction in the cost of production. A major difficulty of the doctor’s blading process is to produce a homogeneous film thickness over a wide area. Some of these factors include the viscosity of the precursor solution, the surface energy of the substrate as well as the drying conditions, and these factors must be well controlled in order to prevent some of the defects that are commonly associated with the process, such as pinholes, non-uniform thickness, or incomplete coverage. Nonetheless, progress in process control and the development of new materials have rendered doctors blading a viable method for the up-scaling of perovskite solar cells [42].
4.2 Vacuum-based methods
4.2.1 Thermal evaporation
Thermal evaporation is one method that works under vacuum, and it is used to deposit perovskite on the substrate. In this process, the perovskite precursor materials are heated at such a level that the material evaporates in a vacuum chamber. The vaporised species then condense to form a thin film on a more relaxed substrate because heat transfers from the substrate to the vaporised species. Thermal evaporation has some merits, such as depositing high-purity films, and the thickness and composition of the films can be well controlled [43]. The other advantage of thermal evaporation is that the deposition of perovskite layers can be done in a clean environment, enabling the control of the film’s composition. This technique is very efficient for depositing multilayer structures where each layer can be deposited rapidly to the next layer. Thermal evaporation is often applied when depositing perovskite onto other materials, such as silicon, to develop tandem solar cells, which are more effective. The high ability of perovskite films to be deposited on different substrates, including flexible and transparent ones, makes thermal evaporation one of the most appropriate techniques for future sun power cell designs [44]. Thermal evaporation, on the other hand, is expensive because it requires vacuum equipment and is unsuitable for large-area deposition, as is the case with solution-based techniques. The process also has energy costs since the precursor materials require heating to high temperatures to evaporate; this may be a limitation to mass production (Figure 4).
Figure 4.
Schematic diagram depicting the fabrication process using thermal evaporation technique. Adapted from Ref. [45] under CC BY licence.
4.2.2 Pulsed laser deposition (PLD)
Another vacuum-based technique employed for the synthesis of perovskite solar cells is called pulsed laser deposition (PLD) which is also used for the deposition of high-quality perovskite thin film with precise control over composition and thickness [44]. In PLD, a high-power laser pulse is incident on a target material and due to this, target material is vaporised and converted into a plasma plume. The material then vaporises and condenses on a substrate in the form of a thin layer of a film. Some of the advantages of PLD are as follows: It is easy to deposit multi-component materials and a stoichiometric transfer of the target material to the film. This is especially helpful in the formation of double perovskite materials because the cation content defines the electronic properties of the compound. The high quality of the films that are produced with smooth surface and clear interfaces is one of the major benefits of the PLD process and it makes it very suitable for creating complex structures of the solar cells like the heterojunction or the tandem cells [46]. It is also versatile in terms of the substrate it can be used on and can be used on flexible materials leading to new device structures. However, PLD is slow, it has low deposition rates, and it involves complicated vacuum systems; therefore, it is not suitable for large-scale production. Another disadvantage of using high energy is that it results to target degradation and non-uniform film in laser ablation. However, as is mentioned above, PLD is still an effective method for R&D of perovskite PVs, especially when the new materials and structures are to be found (Figure 5).
Figure 5.
Depiction of a pulsed laser deposition mechanism for fabrication of perovskite layers. Adapted from Ref. [47] under CC BY licence.
5. Performance and stability metrics of perovskite solar cells
5.1 Power conversion efficiency (PCE)
The most significant characteristic of solar cells is the power conversion efficiency or PCE, which defines the capability of the solar cell to convert light into electricity [48]. Lately, perovskite solar cells have had a very high PCE improvement in the past 10 years; laboratory perovskite-sized devices have PCEs above 25%. This development is because of the properties of the perovskite materials such as high absorption coefficients, tunable band gap, and long diffusion lengths of the carriers. The PCE of a perovskite solar cell consequently depends on the quality of the perovskite layer, the efficiency of the charge transport layers, and the structure of the solar cell. The films should not have any defects, and the grain size should be small and homogenised to enhance the probability of light absorption and reduce the recombination losses. Another aspect that is important for the efficiency is the choice of charge transport materials that assist in the extraction of electrons and holes from the perovskite layer.
5.2 Open-circuit voltage (Voc)
Another parameter that defines the performance of perovskite solar cells is open-circuit voltage (VOC). VOC is the maximum open circuit voltage obtained from the solar cell when no current is flowing through the circuit. It is determined by the difference of the conduction band of the electron transport layer, the valence band of the hole transport layer, and the energy levels of the perovskite material [49]. Perovskite solar cells generally have high VOC values because of the steep absorption edge and low non-radiative recombination. A high VOC is significant in enhancing the PCE since it contributes to the device’s conversion efficiency. Scientists have tried to improve VOC by several approaches, which include the proper alignment of energy in the perovskite and charge transport layers and the minimisation of the defect states in the perovskite film.
5.3 Short-circuit current density (JSC)
JSC stands for short-circuit current density, which is the current density of the solar cell when the terminals of the cell are shorted and is the maximum current that the cell can deliver under standard test conditions. JSC is dependent on the absorption spectrum of the perovskite material, thickness of the perovskite layer, and the quality of the interfaces between the perovskite layer and the charge transport layers [50]. The perovskite material incorporated should be able to capture sunlight especially in the visible light range for the achievement of high JSC values in the solar cells. The thickness of perovskite layer must be tuned to ensure that it can capture the light and at the same time, to reduce the recombination of the carriers. Moreover, the contact between the perovskite layer with the charge transport layers should be designed in a manner that will enhance the extraction of charges while on the same reducing the recombination of charges. Innovations in the structure of perovskite material such as the use of mixed halide perovskites and the deposition of passivation layers have boosted JSC in the recent past. All these advancements have helped to improve the overall performance of PCE in perovskite solar cells, making them potential candidates for commercialisation.
5.4 Fill factor (FF)
The fill factor (FF) measures the quality of the solar cell output characteristics and is given by the ratio of the maximum power output Pm to the product of VOC and JSC [51]. FF depends on the series resistance and the shunt resistance of the solar cell, as well as the recombination activities in the device. A high FF implies that charges are well extracted from the active layer, and there are low resistive losses, which favour high PCE. Measures to enhance FF in perovskite solar cells are the perovskite film surface quality, CTLs’ conductivity, and the interface defect.
6. Potential applications and future prospects of perovskites
6.1 Applications
6.1.1 Building-integrated photovoltaics (BIPV)
The perovskite solar cells (PSCs) can be integrated into building structural components due to their versatility in terms of shape and, form and their aesthetics [52]. Perovskites, unlike silicon-based panels, are flexible in colour and transparency, making it possible to integrate them into windows, façades, and roofing systems. For instance, the semitransparent perovskite films allow architects to develop unique designs of buildings and make them energy-efficient and visually appealing.
6.1.2 Flexible electronics
Perovskite films can be easily integrated into flexible electronics. They are also low-density and in thin film form, so they can be applied to flexible systems such as fabrics and curved surfaces to fabricate solar panels [53]. It also opens up new product opportunities in portable and wearable electronics like Foldable solar panels for charging or energy fabrics.
6.1.3 Wearable devices
The application of perovskite material has been considered in wearables because this material is light and has high energy conversion efficiency. Integrating perovskite solar cells with wearable devices will produce a power supply for gadgets like smartwatches, fitness trackers, and other products [54]. Because they can easily be made in different sizes and shapes, they will be suitable for incorporating into the fashions that are worn daily.
6.1.4 Solar-powered vehicles
Perovskite solar cells will probably revolutionise the automotive industry, whereby the cells are installed in cars to provide extra or even primary power. Due to their low weight, flexibility, and high efficiency of operation, they can be mounted on roofs, windows, and other parts of the body of a car. This can help charge electric vehicles (EVs), avoid reliance on external charging, and even increase the driving range by charging all the time. In solar-powered vehicles, perovskites could also be helpful in energy control systems to power the vehicle electronics and reduce the effect on the environment [55].
6.1.5 Portable solar chargers
Perovskite solar cells are light and highly efficient; therefore, they are suitable for portable solar chargers. These chargers can be used to charge small portable electronics like smartphones, tablets, cameras, and the like in a convenient and environment-friendly way. As perovskite-based chargers are flexible, they can be rolled up, folded, or placed in a backpack and other outdoor equipment, which can be very useful when camping, hiking, and other activities without direct access to the main power supply [56].
6.1.6 Space applications
Due to the low weight and high efficiency of perovskite solar cells, they are suitable for use in space, including satellite and space station operation, as well as in space exploration. In space, weight is a significant concern, and since perovskite solar panels are relatively lighter than traditional silicon panels, the cost of launching and operating them is considerably lower [57]. In addition, they can be easily arranged in a compact, foldable format that can be carried around and then unfolded in space. Efforts are still being made to make perovskite more radiation tolerant, thus making them suitable for long-term space expeditions.
6.2 Unique properties of perovskites
Perovskite materials exhibit several unique properties that contribute to their suitability for these applications. The following are some of the properties of perovskite materials that make them suitable for these applications:
6.2.1 High efficiency
Perovskite solar cells have been researched for the last few years due to their high PCE, which is at par with silicon-based solar cells. Perovskites have other favourable optoelectronic characteristics, including a high absorption coefficient, long carrier diffusion length, and low recombination rate, hence high efficiency. Such characteristics enable perovskite solar cells to absorb and convert more sunlight into electricity. The perovskite cells’ laboratory efficiencies are now over 25%, thus at par with the best silicon technologies. Likewise, the versatility of incorporating perovskites with other materials in tandem structures is beneficial for further efficiency improvement, making the perovskite solar cells a premier contender for the next generation of photovoltaic systems.
6.2.2 Low processing costs
This is one of the greatest strengths of perovskite solar cells, mainly because it has been predicted that they will not be costly to produce. Silicon solar cells, on the other hand, involve high-temperature and energy-consuming processes such as crystal growth and wafer fabrication. At the same time, perovskite can be synthesised and deposited at low temperatures through a solution process. Such deposition methods as spin coating, inkjet depositing, and spray coating allow manufacturing at a large scale and a low cost. Moreover, the roll-to-roll printing technique, which has been used for flexible electronics, can also be used to produce perovskite solar cells on flexible substrates continuously. This low-temperature processing, combined with the possibility of scaling up the production, significantly reduces the cost of production of perovskite solar cells and thus can be suitable for large-scale production.
6.2.3 Tunable bandgap
One of the most outstanding characteristics of perovskite materials is the ability to control the bandgap’s value by changing the material’s chemical composition, for example, replacing one or more of the halogen atoms or the cation. This tunability makes it possible for perovskites to be tailored to fit in the various segments of the solar spectrum and is hence suitable for photovoltaic applications. For instance, by choosing particular compositions, it is possible to enhance absorption in specific spectrum regions, thus enhancing the performance of tandem solar cells or multi-junction devices. This flexibility in adjusting the bandgap will allow for creating new perovskite-based photovoltaic applications that are more specialised and will add to the versatility of perovskites in innovative solar systems.
6.2.4 Lightweight structure
Perovskite solar cells are much lighter than conventional silicon solar cells, which is a significant advantage in several ways. Firstly, perovskite solar cells are lightweight and, as such, can be applied in aerospace applications, portable electronics, and wearable technology. The low density of perovskites also makes them portable and easy to transport and install, thus cutting the costs that accompany the transport and installation of large solar plants. This property is precious for such applications where conventional rigid and massive silicon panels are not suitable, thus opening new opportunities for the development of solar technology.
6.2.5 Wide absorption spectrum
Perovskites are also characterised by a high absorption coefficient value that is over the spectrum range of UV and NIR. This broad absorption characteristic makes perovskite solar cells able to capture more energy from the sun—another reason for high PCE. Perovskites also have a feature of trapping low-energy photons that other materials may not trap, and this makes them effective even in polluted or low-light conditions, such as those that prevail in cloudy or indoor environments. This broad-spectrum absorption makes perovskites suitable for any lighting conditions and, therefore, suitable for any application in solar energy conversion.
6.2.6 Defect tolerance
Perovskite materials are very immune to defects in their structures, which is a plus for their use. However, they can have high performance, so the quality of the material can be a little low, making producing these cells less sensitive and cheaper. This defect tolerance also enables perovskite solar cells to have better stability and reliability in real life since there will always be some defects in the solar cells. For this reason, the ability to operate at the highest efficiency with imperfections in the material structure makes perovskites stable and reliable for large-scale applications in various configurations and applications.
All of these properties contribute to the conclusion that perovskite materials can change the field of solar energy and further extend their application to other new and advanced technologies.
6.3 Emerging research directions
Because of the fast development of perovskite materials, much attention has been paid to it in both academic research and the industry. Indeed, it is evident that as the field advances new ideas and potential research areas that may help to enhance the performance, stability, and scalability of the perovskite-based technologies are emerging. These include following areas depicting the versatility of perovskite materials in future generation energy technologies:
6.3.1 Tandem solar cells
Tandem solar cells are the most promising development in the photovoltaic field, where perovskite materials are incorporated into silicon or other photovoltaic materials to increase efficiency. These tandem structures can be created by stacking perovskites on top of the conventional silicon cells, and this is because different materials have different absorption characteristics. The first perovskite layer is designed to capture high-energy photons, and the second silicon layer is responsible for capturing low-energy photons for optimum energy conversion. It has the possibility of attaining efficiencies that are much higher than those of single-junction cells that are restricted by the Shockley-Queisser limit. With research advancing further, the concept behind the tandem cells is expected to gain higher levels of commercial appeal, which will lead to better solar solutions that can revolutionise the energy market [58].
6.3.2 Perovskite-based multi-junction solar cells
Multi-junction solar cells using perovskite materials are another way to develop solar cells. These cells accumulate one layer of various photovoltaic materials, each intended to capture specific parts of the solar spectrum [59]. Thus, by controlling the composition and thickness of these layers, scientists strive to go beyond the efficiency of p-n cells. In theory, the various junctions in multi-junction cells can provide efficiencies of over 40% and are thus some of the most effective solar cells. The problem here is how to achieve stable, high-quality interfaces between the layers and the fact that each layer must function as well as possible. Future developments in material technology and in manufacturing perovskite-based multi-junction photovoltaic cells may create a new benchmark for the entire solar power system.
6.3.3 Next-generation photovoltaic technologies
Combining perovskites with future-oriented PV technologies such as OPVs and QDSCs is a current research field. These so-called hybrid technologies are designed to optimise the properties of each technology, thus creating new generations of photovoltaic devices with higher efficiency, flexibility, and adaptability [60]. For instance, perovskite-organic hybrid cells could combine the advantages of being flexible and cheap to produce organic photovoltaics and the high efficiency of perovskites. Likewise, incorporating perovskites with quantum dots—nanoscale semiconductor particles with adjustable photovoltaic characteristics—can boost light absorption and carrier dynamics in solar cells. These next-generation photovoltaic technologies are seen to offer the potential for delivering more flexible and high-performance solar systems for various uses, including portable electronic devices and large power stations.
6.3.4 Perovskite-silicon heterojunctions
Perovskite-silicon heterojunctions significantly improve solar cell design as they take the best of the perovskite and silicon properties. In this case, researchers can stack a perovskite on top of a silicon solar cell to form a heterojunction that offers both the efficiency of perovskite and the stability of silicon [61]. This combination not only improves the total conversion efficiency of power but also leverages the mature silicon manufacturing system and demand. The invention of high-performance and stable perovskite/silicon heterojunctions can change the solar industry and provide a cost-effective route towards exceeding the efficiency limits of conventional silicon-based photovoltaics. These hybrid cells could become one of the most popular technologies in the solar industry as research continues, thus bringing down costs and increasing efficiency.
6.3.5 Perovskite quantum dots for photovoltaics
Perovskite QDs are a relatively new research area with many advantages over bulk perovskite materials. Quantum dots are semiconductor nanocrystals that have their dimensions in the nanometre size and are capable of quantum confinement effects for their band gap and optical characteristics. In the context of photovoltaics, perovskite QDs can be employed to form high efficiency and stability of the solar cells and enhance light trap and charge carrier dynamics. The size of QDs is small, allowing for the fine-tuning of the electronic characteristics of the material, and the ability to decide what these characteristics are is a significant advantage that the technology has over other approaches. Also, perovskite QDs can be solution-processed, ensuring they can be used in large-scale, low-cost production. Perovskite QDs are still under investigation, and future research may lead to higher efficiency and new applications such as solar windows and portable, flexible solar cells [61].
6.3.6 All-perovskite tandem solar cells
Tandem solar cells made only of perovskite materials incorporate two or more layers of perovskite materials with varying bandgaps arranged in a tandem manner. While conventional tandem cells use perovskites with other materials, such as silicon, all-perovskite tandem cells utilise various perovskite layers designed for different solar spectrum parts [62]. It is possible to achieve very high utilisation of sunlight, and thus, the power conversion efficiencies are higher than those of single-junction perovskite cells. Current work in this field is dedicated to modifying perovskite to achieve high stability and efficiency, achieving the desired bandgap, and optimising the fabrication of the structure to form perfect interfaces between the layers. All-perovskite tandems are considered one of the most promising approaches to creating the next-generation ultra-efficient, lightweight, flexible, and low-cost solar cells.
6.3.7 Perovskite solar cells for indoor photovoltaics
Perovskite solar cells for indoor photovoltaics (IPV) are a fast-emerging area that aims at harvesting energy from artificial lighting [63]. Indoor conditions are characterised by lower light intensity and different spectrums compared to sunlight, and perovskites are well suited to indoor light conditions because of their ability to tune the absorption and their efficiency at low light. Indoor perovskite materials may pave the way to highly efficient IPV systems that can power small devices in smart homes, as well as IoT devices such as sensors, small electronics, wearables, and more, without the need for batteries. This research direction is most suitable for developing indoor energy self-sufficient spaces, minimum battery replacement or recharging requirements, and the emergence of a new generation of energy-efficient indoor technologies.
These new directions demonstrate the richness of perovskite materials and promise to go beyond the state of the art in photovoltaic applications. Therefore, the potential application of perovskite solar cells is limited to conventional solar cells and extended to many other fields. Nevertheless, commercialisation has problems, but constant research and development offer new possibilities for advancing photovoltaic technologies.
7. Conclusion
This chapter seeks to present the subject of this study, that is, perovskite solar cells (PSCs), as the object that holds the potential to transform the solar energy sector. PSCs are morphologically distinct from conventional photovoltaics based on silicon due to their high efficiency, low processing costs, and flexibility, which can be adapted to BIPV, flexible electronics, and wearable devices. Thus, it is creative in addressing energy issues and ensures that solar technology is integrated into people’s lifestyles. However, there are some issues related to the development of PSCs to reach full-scale commercialisation. There are stability issues as PSCs are equally affected by environmental factors that could affect their performance in the long run. Moreover, the transition of the process from a pilot scale to an industrial scale has its own problems in terms of technology and the economy. However, the future of PSCs is not so gloomy, as the development in this field is still ongoing. It is for this reason that tandem solar cells that are combined with other materials, such as PSCs, to offer higher efficiency and perovskite-based multi-junction cells that are developed to capture more of the solar spectrum are believed to be among the most promising novelties of the photovoltaic industry. Perovskite QDs have great potential for fabricating efficient and stable solar cells with excellent optical properties. Some improvements have been reported in perovskite-silicon heterojunction and all perovskite tandem solar cells, which combine the best of both worlds and expand the applicability of the technology. Apart from improving PSC’s performance and flexibility, these advances generate new applications such as high-efficiency solar panels, energy-harvesting devices for smart homes and wearable technologies. If the current challenges are met and the research continues, PSCs can significantly revolutionise the solar industry with efficient, clean, and flexible energy solutions. Hence, as renewable energy is adopted more often, PSCs can assist in providing the world with energy without being detrimental to the environment.
Acknowledgments
The authors are thankful to PV Spoke National Science and Innovation Energy Research Programme and National Research Foundation (GUN:137944 and 118947) and Govan Mbeki Research and Development Centre (GMRDC), South Africa, University of Fort Hare.
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