Overview of the Current State of Flexible Solar Panels and Photovoltaic Materials

Citation: Dallaev, R.; Pisarenko, T.; Papež, N.; Holcman, V. Overview of the Current State of Flexible Solar Panels and Photovoltaic Materials. Materials 2023,16, 5839. https:// doi.org/10.3390/ma16175839 Received: 10 August 2023 Revised: 18 August 2023 Accepted: 23 August 2023 Published: 25 August 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Review Overview of the Current State of Flexible Solar Panels and Photovoltaic Materials Rashid Dallaev * , Tatiana Pisarenko, Nikola Papež and Vladimír Holcman Department of Physics, Faculty of Electrical Engineering and Communication, Brno University of Technology, Technická2848/8, 616 00 Brno, Czech Republic; [email protected] (T.P.); [email protected] (N.P.); [email protected] (V.H.) *Correspondence: [email protected] Abstract: The rapid growth and evolution of solar panel technology have been driven by continuous advancements in materials science. This review paper provides a comprehensive overview of the diverse range of materials employed in modern solar panels, elucidating their roles, properties, and contributions to overall performance. The discussion encompasses both traditional crystalline silicon-based panels and emerging thin-film technologies. A detailed examination of photovoltaic materials, including monocrystalline and polycrystalline silicon as well as alternative materials such as cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and emerging perovskite solar cells, is presented. Furthermore, the impact of transparent conductive materials, encapsulation polymers, and antireflective coatings on solar panel efficiency and durability is explored. The review delves into the synergistic interplay between material properties, manufacturing processes, and environmental considerations. Through a comprehensive survey of materials utilized in modern solar panels, this paper provides insights into the current state of the field, highlighting avenues for future advancements and sustainable solar energy solutions. Keywords: solar panels; photovoltaics; perovskite-based; silicon-based; flexible 1. Introduction From the first half of the 19th century to the present day, the topic of photovoltaics has been enriched by theoretical and practical research. French physicist Alexandre Edmond Becquerel discovered the photovoltaic effect in 1839. Fundamental research was continued in 1873 by Willoughby Smith (conductivity of selenium), and Charles Fritz in 1883 created the world’s first module of selenium elements. Then followed the discovery by the German physicist Heinrich Rudolf Hertz in 1887 of a new phenomenon: “external photoelectric effect”. From 1888 to 1891, Alexander G. Stoletov successfully continued further research in this area. The theoretical justification for the photoelectric effect was made by Albert Einstein in 1907. The acute demand for semi-conductor cells has expanded and accelerated research in the field of engineering and photovoltaics. In 1916, Polish scientist Jan Czochralski discovered the processes of crystal formation by growing single crystals from a melt. Experiments were carried out with a thin film of copper oxide (1934) and with silicon-based samples (1940), which were marked by the discovery of the p-n junction in semiconductors; the theoretical model of the p-n junction by William Bradford Shockley laid the foundation for the development of modern solar cells. Daryl Chapin, Calvin Fuller, and Gerald Pearson introduced crystalline silicon solar cells in 1953. Numerous research teams are presently engaged in photovoltaics research across global universities and research establishments. This research can be categorized into three domains: enhancing the cost-effectiveness and efficiency of contemporary solar cells to establish their competitive stance among alternative energy sources, innovating new technologies through novel architectural approaches for solar cells, and advancing novel materials that enhance the conversion of light energy into electric current or act as superior absorbers and conductors of light-induced charges. Materials 2023,16, 5839. https://doi.org/10.3390/ma16175839 https://www.mdpi.com/journal/materials Materials 2023,16, 5839 2 of 32 In this regard, this particular review paper seeks to provide a comprehensive and up-to-date examination of the current state of flexible solar panels and photovoltaic materials. In particular, the focus is on elucidating the intricate relationship between the materials employed in solar panels, their inherent properties, the roles they play within the photovoltaic system, and their profound influence on the overall performance of flexible solar panels. With a growing array of materials being explored for photovoltaic applications, ranging from traditional silicon-based semiconductors to emerging organic, perovskite, and thin-film materials, understanding the nuances of each material’s characteristics has become pivotal. This review aims to bridge the knowledge gap by systematically analyzing and comparing the key attributes of various materials, including their electronic properties, bandgap engineering, charge transport mechanisms, and stability. By delving into the fundamental science behind these materials, we aim to provide readers with a clear understanding of how material selection shapes the efficiency, durability, and adaptability of flexible solar panels. Furthermore, the roles of encapsulation layers, substrates, and transparent conductive materials are equally paramount in determining the performance and reliability of flexible solar panels. This is why the aim is also to explore how these components contribute to enhancing light absorption, electron-hole separation, and overall device stability, while also addressing challenges such as mechanical flexibility, weather resistance, and costeffectiveness. Ultimately, by examining the intricate interplay between photovoltaic materials and panel design, this review aspires to equip researchers, engineers, and policymakers with a comprehensive survey of the current landscape. It aims to foster a deeper appreciation for the materials-driven advancements in flexible solar panels and highlight the pivotal role that material innovation plays in shaping the trajectory of solar energy technology. As the world continues to transition toward renewable energy solutions, a nuanced understanding of these materials and their influence on solar panel performance will undoubtedly accelerate the realization of a sustainable and resilient energy future. This comprehensive review paper meticulously assembles and evaluates an extensive spectrum of data drawn from diverse sources, encompassing scientific literature, cuttingedge research articles, and insightful industry reports. By amalgamating these diverse resources, the review offers an all-encompassing elucidation of the prevailing landscape concerning flexible solar panels and photovoltaic materials. 2. Current State, Market Shares, and Future Outlook The rapid development of solar energy, using innovative world technologies, is the main competitor, and in 2050 it will be predominant in the market for energy-friendly technologies, which will cover all the electric energy needs of the population by the end of the century [ 1 ]. The annual amount of solar energy coming to the Earth is 1018 kWh, while the land surface accounts for about 20% of this energy. The energy characteristics of photocells are mainly determined by the following parameters: the intensity of solar radiation, the magnitude of the load, and the operating temperature [ 2 ]. Since the distance from the Earth to the Sun changes during the year within 150 million km, the amount of solar radiation also varies from 1325 to 1420 W/m2[3]. One of the promising areas for solving the problem of energy supply to consumers is the development and implementation of renewable energy sources (RES). Their share of world consumption in 2020 was around 24% and according to forecasts this number will increase by ~50% by the year 2040 [3]. The dynamically developing solar energy complex puts forward a number of tasks for all economic entities. The main function of the solar energy complex (SEC) is the production and redistribution of energy obtained through the use of solar energy. The production of heat and electricity is based on two main technologies that make it possible to divide the solar energy complex into two subtypes: photovoltaic SEC and solar thermal SEC. Materials 2023,16, 5839 3 of 32 Photovoltaic SEC uses technologies for direct conversion of solar energy into electricity, and solar thermal SEC uses technologies for converting solar energy into heat (using solar collectors) and technologies for concentrating solar energy for its subsequent conversion into electricity [4–8]. Solar power plants have long been an integral part of the energy balance of the largest economies in the world. According to IRENA, over the past 10 years the installed capacity of solar power plants in the world has grown more than 17 times, from 41.6 GW to 714 GW. At the same time, 127 GW of new capacities were installed in 2020 alone. The production of solar panels in the world is also growing steadily. According to the American consulting company Clean Energy Associates (CEA), the global capacity for the production of solar panels themselves by the end of 2021 reached about 400 GW, and the capacity for the production of new elements for panels is 325 GW [9]. From 1999–2008 the main contribution to the development of solar energy was made mainly by four countries: Japan, Germany, the USA, and Spain. Since 2004, the volume of annual installed capacities has increased dramatically in Germany, and it has done so since 2007 in Spain [10]. By the end of 2008, the installed capacity of solar panels worldwide reached 15 GW, of which 65% are located in Europe, 15% in Japan, and 8% in the USA. With record growth in installed capacity in Spain (from 560 MW in 2007 to 2511 MW in 2008) and notable growth in installations in Italy (from 42 MW in 2007 to 258 MW in 2008) and South Korea (from 43 MW in 2007 to 274 MW in 2008) 2008 was a unique year in the development of solar energy [10]. In 2006, the first solar installations appeared in Belgium (2 MW), France (8 MW), and Greece (1 MW); in 2007, they appeared in the Czech Republic (3 MW) and Portugal (14 MW). In one or two years, these European countries, as well as the leaders, increased the volume of annual installations many times over. Thus, in 2008, 48 MW of solar panels were already installed in Belgium, 46 MW in France, 11 MW in Greece, 51 MW in the Czech Republic, and 50 MW in Portugal. Such a noticeable increase in installation volumes is explained by the introduction of special bonus tariffs for solar electricity [10]. Germany accounted for the lion’s share of the European PV market in 2010 with 7.408 GW of installed capacity, equivalent to 56%. The second country in terms of installed capacity in Europe in 2010 was Italy with 2321 GW or 18% of the market share. The Czech Republic had a fairly significant share: 11% and 1.49 GW of installed capacity by 2010. France closes the top five European market leaders with 719 MW of installed capacity, which is 5% [11]. The growth of solar energy in Europe once again surpassed all analysts’ forecasts. Despite stagnation in many sectors of the economy and resistance to the further implementation of large solar projects in some countries (for example, in the Czech Republic), 21,939 GW of solar installations were connected to the grid on the continent, which is almost 1.7 times more than in 2010 (13 GW) [12]. For the first time in history, Italy became the leader in terms of installed new capacity (9.3 GW), leaving behind Germany with 7.5 GW. It should be noted that the joint share of Italy and Germany amounted to more than 60% in the total installed capacity in 2011 in Europe. They are followed by France (1.7 GW), Belgium (974 MW), and the UK (784 MW) [11]. Analyzing the segmentation of the European photovoltaic market, one should note the predominance of the share of large ground-based solar stations in countries such as Bulgaria, Romania, Slovakia, and Spain (more than 85% of the cumulative capacity). Solar panels in the residential sector are dominated by the Czech Republic (over 60%), the Netherlands (about 70%), and Denmark (about 98%). In Sweden, Austria, Hungary, and Slovenia, the predominance of the commercial sector is noticeable (more than 50%) [13]. Numerous studies in the field of photovoltaics to find the ideal formula for the production of solar cells with higher efficiency and low cost have found a design solution in creating convenient and environmentally friendly flexible (folding) solar panels of various modifications. Materials 2023,16, 5839 4 of 32 The global solar photovoltaic (PV) market has been growing since 2005 by an average of 40% per year. This is far more than any other industry [14]. Solar PV is expected to create more than 2 million jobs in the next 20 years and reduce greenhouse gas emissions into the atmosphere by 350 million tons of CO 2 , which is equivalent to stopping 140 coal-fired power plants. The total solar PV capacity in 2030 will exceed 650 GW [15,16]. If at the early stages of the development of solar energy at the regional level it could be described as monocentric, now the emergence of new growth poles is an active process. In North America, Canada joined the “solar club”, which until recently included only the United States, Europe, Great Britain, and Belgium, who were then joined by Germany, Italy, France, and Spain. In the future, Bulgaria and the Czech Republic may become leaders. Of the traditional European centers for the development of the industry, Germany, Italy, and Spain still retain their importance. Over the past 10 years, the Asian world center has been actively developing, led by China, Japan, and India. Along with the dynamic development of large centers, there is a mass of less significant ones, which are nonetheless contributing to a change in the structure of the location of industry facilities. These new centers are likely to become locomotives for the development of global solar energy. The main global trend is the preservation of leadership for Europe and the gradual spread of the industry in North America and Asia. In the countries of South America and Africa, solar energy has not yet become so widespread, but in the future these countries represent some of the main regions for the development of the industry [17]. At the end of 2017, the installed capacity of global solar PV exceeded 400 GW and covered approximately 2% of global electricity demand. More than 90% of the current global production of modern solar photovoltaic panels use wafer-based crystalline silicon technology [18]. Most flexible solar panels are used at solar stations operating in various climatic zones, regardless of weather conditions. Experts predict by 2040–2050 the transition from rigid modifications to a new generation of “solar films” will occur. Attention is drawn to the excellent mechanical properties of polymers, their ability to be processed and the variety of their forms and derivatives, as well as their high absorption coefficient in the optical range, which allows for their use in the form of ultra-thin (several hundred nanometers) films deposited from solutions at ordinary pressure on flexible substrates with unlimited area. This makes it possible to manufacture polymer solar cells (SC) using cheap mass production methods such as inkjet printing and stamping [19]. To provide mankind with energy in 2050, it is necessary to generate up to 71 TW of electricity annually. The only technology that can solve this problem is roll-to-roll technology: the technology of flexible printed electronics. Two of the leaders in the development of flexible polymer and composite solar cells are Konarka Technologies Inc. and Solarmer Energy Inc. (USA). At Solarmer Energy Inc., in 2011, the highest efficiency of a tandem polymer SC~8.13% was achieved, and at Konarka Technologies Inc., in February 2012, a record-breaking efficiency of single-layer polymer SCs was obtained and remains so to date at 9% (on laboratory samples) [20]. Such studies in the development of solar cells based on optically active conjugated polymers and polymer–inorganic nanoparticle composites, which are multilayer polymer and composite structures in which the layers consist of various polymers and nanoparticles, are conducted at the Organic Electronics Group of the Ioffe Institute (A. F. Ioffe RAS). The ultimate goal of the work is the development of technology and experimental samples of solar cells based on light-sensitive polymers and polymer–inorganic nanoparticle composites. Due to the unique set of properties characteristic of optically active conjugated polymers and their composites with inorganic nanoparticles, such structures will effectively absorb light in various spectral ranges, from ultraviolet to infrared [21–26]. Due to the fact that such polymers are soluble in common organic solvents, it becomes possible to deposit composites on substrates using inkjet and cold stamping technologies. This makes it possible to integrate the technology of composite solar cells into the tech- Materials 2023,16, 5839 5 of 32 nology of flexible printed electronics. The development of solar cells based on multilayer composite structures (polymer–inorganic nanoparticles) will increase the efficiency of such devices and increase the operating time of hybrid solar cells. To date, the efficiency of energy conversion in polymer tandem solar cells obtained in laboratory conditions reaches ~9%, which is already approaching the commercially competitive values of 10–11% [20]. 3. Classification of Solar Panel Types Solar panels are divided into three large families [27]: (1) Thin-film solar panels consist of stretched films that can be easily installed in any convenient place. They are not afraid of dust and can work even in adverse conditions. In cloudy weather, their effectiveness is reduced by 20%. They are inexpensive, but require a large area for installation. (2) Monocrystalline batteries are made from a large number of individual cells, which are filled with silicone. Thanks to their waterproofing, they are effectively used in shipping. Monocrystalline batteries are relatively light in weight and compact in size. They are distinguished by flexibility, light weight, compactness, reliability, and durability. They are easy to install and dependent on direct sunlight. In this case, even light cloud cover can lead to a cessation of energy production. (3) Polycrystalline solar panels contain cells composed of crystals pointed in different directions. This makes it possible to capture diffused light and be less dependent on direct illumination. They are successfully used to illuminate houses, office buildings, and even streets. Below are the physical and technical characteristics that significantly determine the functional component of the solar battery, regardless of the material of manufacture: • Mechanical: geometric parameters; weight; size; number of cells; type and width of connectors. • Temperature: one of the key factors is the change in efficiency when the temperature rises (in extreme conditions) by a certain unit of magnitude (usually 1 degree). • Electrical or current-voltage: (CVC) power; open-circuit voltage; rate of change in current strength at maximum load; efficiency of individual cells and the panel as a whole . •Functional: ease of use; performance; maintenance. • High quality: service life; degree of degradation of cells; efficiency; versatility; environmental friendliness. At the moment, eight main technologies can be noted in the production of highly efficient solar panels [28]: •PERC (Passivated Emitter Rear Cell): dielectric layer on the reverse side of the cell. •Bifacial: bilateral. •Multi busbar: multiline. •Split panels: half. •Dual glass: frameless, with double glass. •Shingled cells: inseparable elements. •IBC (interdigitated back contact cells): interlaced contacts at the back of the cell. •HJT (heterojunction cells): heterostructural cells. Five main types of solar panels using the latest solar photovoltaic technology in 2020 are presented in Figure 1: Materials 2023,16, 5839 6 of 32 Materials 2023, 16, x FOR PEER REVIEW 6 of 32 Figure 1. Different types of solar photovoltaic technology. Single-crystal silicon is a classic photovoltaic material; however, the production of structures based on it is a technologically complex and expensive process. Therefore, in recent years, more and more attention has been paid to materials such as amorphous silicon (a-Si:H), gallium arsenide, and polycrystalline semiconductors [28,29]. Direct conversion of solar radiation into electrical energy is carried out by solar photovoltaic cells (batteries, installations). The most widespread are solar photovoltaic installations (SPVI) based on three types of silicon: single-crystal, polycrystalline, and amorphous. In industrial production, there are SPVIs with the following efficiency [2]: (1) Single-crystal: 15–16% (up to 24% on prototypes). (2) Polycrystalline: 12–13% (up to 16% on prototypes). (3) Amorphous: 8–10% (up to 14% on prototypes). To develop cheap, high-efficiency Si-based solar cells using plasma-chemical technologies, it is necessary to use cheap multicrystalline substrates. It is promising to use thin multicrystalline Si layers deposited on mechanically strong substrates (glass, ceramics), since a silicon thickness of about 10 µm is sufficient for efficient operation of the SC. It is possible to create tandem heterojunction thin-film HIT SCs with a photoconversion efficiency higher than in the best samples of single-junction HIT structures (>25%). High Sibased efficiencies are likely to be obtained with triple or quad semiconductor junction technology [30]. HJT solar cells use a conventional crystalline silicon core with additional thin-film layers of amorphous silicon on either side of the cell, forming a so-called heterojunction. Unlike conventional P-N junction cells, multilayer heterojunction cells can greatly improve efficiency. Up to 26.5% efficiency has been achieved in laboratory tests when combined with IBC technology [28]. An especially promising technology involves the utilization of crystalline silicon thin films on glass substrates. This innovative approach capitalizes on the benefits of crystalline silicon as a solar cell material—its abundance, non-toxic nature, high efficiency, and long-term stability—while also harnessing the cost-effective advantages inherent in employing a thin-film methodology [31,32]. Another intriguing facet of thin-film solar cells is their capability to be deposited onto various materials, including flexible substrates like PET. This characteristic introduces novel opportunities for diverse applications [33]. As of December 2014, the pinnacle achievement in solar cell efficiency stood at 46%, attained through the utilization of multi-junction concentrator solar cells. This remarkable accomplishment was the result of collaborative efforts between Soitec and CEA-Leti in France, in conjunction with Fraunhofer ISE in Germany [34]. High-tech production allows manufacturers to produce products with amazing plasticity, with a light and thin module. To give thinness and lightness to flexible film solar cells, polymer sputtering with aluminum conductors is used. The panels can be rolled up and folded into a compact tube without damaging anything. The three basic components of these panels are flexibility, thinness, and lightness. Figure 1. Different types of solar photovoltaic technology. Single-crystal silicon is a classic photovoltaic material; however, the production of structures based on it is a technologically complex and expensive process. Therefore, in recent years, more and more attention has been paid to materials such as amorphous silicon (a-Si:H), gallium arsenide, and polycrystalline semiconductors [28,29]. Direct conversion of solar radiation into electrical energy is carried out by solar photovoltaic cells (batteries, installations). The most widespread are solar photovoltaic installations (SPVI) based on three types of silicon: single-crystal, polycrystalline, and amorphous. In industrial production, there are SPVIs with the following efficiency [2]: (1) Single-crystal: 15–16% (up to 24% on prototypes). (2) Polycrystalline: 12–13% (up to 16% on prototypes). (3) Amorphous: 8–10% (up to 14% on prototypes). To develop cheap, high-efficiency Si-based solar cells using plasma-chemical technologies, it is necessary to use cheap multicrystalline substrates. It is promising to use thin multicrystalline Si layers deposited on mechanically strong substrates (glass, ceramics), since a silicon thickness of about 10 µ m is sufficient for efficient operation of the SC. It is possible to create tandem heterojunction thin-film HIT SCs with a photoconversion efficiency higher than in the best samples of single-junction HIT structures (>25%). High Si-based efficiencies are likely to be obtained with triple or quad semiconductor junction technology [30]. HJT solar cells use a conventional crystalline silicon core with additional thin-film layers of amorphous silicon on either side of the cell, forming a so-called heterojunction. Unlike conventional P-N junction cells, multilayer heterojunction cells can greatly improve efficiency. Up to 26.5% efficiency has been achieved in laboratory tests when combined with IBC technology [28]. An especially promising technology involves the utilization of crystalline silicon thin films on glass substrates. This innovative approach capitalizes on the benefits of crystalline silicon as a solar cell material—its abundance, non-toxic nature, high efficiency, and longterm stability—while also harnessing the cost-effective advantages inherent in employing a thin-film methodology [31,32]. Another intriguing facet of thin-film solar cells is their capability to be deposited onto various materials, including flexible substrates like PET. This characteristic introduces novel opportunities for diverse applications [33]. As of December 2014, the pinnacle achievement in solar cell efficiency stood at 46%, attained through the utilization of multi-junction concentrator solar cells. This remarkable accomplishment was the result of collaborative efforts between Soitec and CEA-Leti in France, in conjunction with Fraunhofer ISE in Germany [34]. High-tech production allows manufacturers to produce products with amazing plasticity, with a light and thin module. To give thinness and lightness to flexible film solar cells, polymer sputtering with aluminum conductors is used. The panels can be rolled up and folded into a compact tube without damaging anything. The three basic components of these panels are flexibility, thinness, and lightness. Materials 2023,16, 5839 7 of 32 The solar panel, called eArche, created with the participation of Australian company Energus, is a super-flexible, ultra-thin solar array that can be placed on building facades, bus roofs, awnings, or other surfaces. According to project leader Dr Zhengrong Shi, the innovative panel has virtually unlimited potential, with 40 kW systems already in operation at three locations across Australia. According to the manufacturer, eArche has a thickness of 5–6 mm and weighs only about two tons per 100 kW, while conventional solar roof systems weigh about eight tons per 100 kW and cost about the same. The flexible photovoltaic panel can be custom-made to suit the individual sizes of the roofs and walls of buildings. It is also possible to use the technology for land vehicles, yachts, vending machines, and more [ 35 ]. 4. Classification of Photovoltaic Materials and Manufacture Technologies Industrial solar panels can be classified either by design features (standard design of a rigid solar battery, rigid and flexible panels made using various types of semiconductors) or by the type of working photovoltaic layer. In terms of the latter, there are the following classifications: (1) SILICON-BASED Crystalline silicon modules are characterized by two voltage ranges: 40–45 V for open circuit voltage and 32–38 V at the maximum power point (for modules in systems with an operating voltage of 24 V), as well as 20–25 V for open circuit voltage and 15–17 V at the point of maximum power (for modules in systems with an operating voltage of 12 V ). Typical efficiency values for modules based on multicrystalline silicon lie in the range of 13–17% [36], and for monocrystalline in the range of 16–19% [37,38]. •Monocrystalline (semi-flexible modifications) Single-crystal silicon SCs (c-Si-SCs) are made from wafers 300 µ m thick by doping them, creating ohmic contacts (solid rear and grating front), and texturing to impart antireflection properties. There are several types of construction of single-crystal and thinfilm solar cells (SCs), which differ in the method of formation, structure, and arrangement of contacts [39,40]. •Polycrystalline Polycrystalline silicon solar cells generally perform better than some other types of solar cells at high temperatures. This makes them well suited for regions with high levels of sunlight and elevated temperatures. •Amorphous It is predicted that by 2023–2025 amorphous cells will take a leading position in the development of solar energy. The third generation includes solar cells with which it is possible to overcome the known efficiency limit of the Shockley–Kweiser photoelectric conversion [41]. •CZTS—sulfide of copper, zinc, tin, and derivatives of CZTSe and CZTSSe An emerging material for use in photovoltaic solar cells, CZTS silicon-based photovoltaic layers offer the advantages of abundance, non-toxicity, and a direct bandgap, making them an attractive candidate for solar cell applications. However, challenges related to efficiency, manufacturing scalability, and material quality need to be addressed to fully harness their potential in commercial solar panel technology. A transparent conductive layer, often made of materials like zinc oxide (ZnO) or indium tin oxide (ITO), is deposited on top of the CZTS absorber layer. This layer allows light to pass through and contributes to electrical conduction. •DSSC, DSC, DISC: dye-sensitized solar cell (“Grätzel cell”) For the manufacture of silicon-free film photovoltaic cells, the main alloys used are: cadmium telluride (CdTe), indium-copper selenide (CIS), and indium-copper-gallium selenide (CIGS). CdTe stands out for its rapidly growing efficiency, stability, and durability. Materials 2023,16, 5839 8 of 32 The use of these alloys has an efficiency of more than 30%. They are more expensive than their silicon counterparts, but thanks to their exceptional characteristics, they have occupied a high-tech niche for new generations of modules. It is important to note that the problem with the disposal of rare earth poisonous elements cadmium (toxic metal), rather expensive ones such as tellurium (comparable to the rarity of platinum), gallium, indium, germanium, and others, increases the technology required for their production. Possible combinations demonstrating the photoelectric effect: Cu, Ga, Ag, Se, Te, Au–Al, and In–S. (2) TELLURIUM-CADMIUM BASED Panels made of CdTe are actively used in the cladding of buildings, where extreme surface heating reaches 70–80 degrees. Cadmium telluride (CdTe) is another promising material for photovoltaics. It has an almost ideal band gap and a very high radiation absorption capacity. CdTe films are quite cheap to manufacture. In addition, it is technologically easy to obtain a variety of CdTe alloys with Zn, Hg, and other elements used to create layers with the desired properties. Like CuInSe2, the best CdTe-based cells include a heterojunction with CdS as the window layer. Tin oxide is used as a transparent contact and antireflection coating. A serious problem with using CdTe is the high resistance of the p-CdTe layer, which leads to large internal losses. However, it is solved by a p-i-n structure with a CdTe/ZnTe heterojunction. CdTe films have a high mobility of charge carriers, and solar cells based on them have high efficiency values (from 10 to 16%) [ 42 ]. Table 1contains the main characteristics of solar cells based on CdTe. Table 1. Main characteristics of solar cells based on CdTe [43–46]. Material Structure S, cm2Uoc, mB Jsc, mA/cm2FF, % Eff-cy: %Company ss/ITO/CdS/CdTc//Cu/Au 0.191 790 20.10 69.4 11.0 IEC ss/SnO2/CdS/CdTe 0.824 840 20.66 74.0 12.8 NREL ss/SnO2/CdS/CdTe 0.313 783 24.98 62.7 12.3 Photon Energy ss/SnO2/CdS/CdTe 0.3 788 26.18 61.4 12.7 Photon Energy ss/SnO2/CdS/HgTeGa 1.022 736 21.9 65.7 10.6 SMU MgF2/ss/SnO2/CdS//CdTe/C/Ag 1.047 843 25.09 74.5 15.8 Univ. South Florida ss/SnO2/CdS/CdTe/Ni 1.068 767 20.93 69.6 11.2 AMETEX ss/SnO2/CdS/CdTe 0.08 745 22.1 66.0 10.9 Georgia Tech. MgF2/ss/SnO2/CdS/CdTe 1.115 828 20.9 74.6 12.9 Solar Cells Inc. Ss/SnO2/CdS/CdTe/Cu/Au 0.114 815 17.61 72.8 10.4 Univ. Toledo CdTe 807 12.7 BP Solar CdTe SCs are quite promising, with ample opportunities for improving and optimizing the production technology and, consequently, for reducing the cost [ 47 – 49 ]. This favorably distinguishes them from solar cells based on a-Si, c-Si, and A IIIB V semiconductors. However, in the production of CdTe SCs, Cd and Te, which are rare earth elements with strong toxic properties, are involved, which to some extent delays the widespread adoption of CdTe solar cells. In the near future, the world community plans to completely abandon the use of Cd in industrial production due to its high toxicity and problems with the disposal of Cd and its compounds. Therefore, when CdTe is chosen as an absorbing material for photovoltaic light conversion, there immediately arises the problem of further utilization of solar cells that have served their time. The latter leads to an increase in the cost of CdTe solar cells and limits their widespread use for light conversion [49]. Tiwari and collaborators [ 50 ] have announced the creation of a flexible CdTe solar cell with a conversion efficiency of 8.6%. In addition, flexible solar cells based on organic semi- Materials 2023,16, 5839 9 of 32 conductors and the so-called dye-sensitized solar cells are currently promising. Batteries of this type can be printed on polymer foil [51]. (3) BASED ON INDIUM-COPPER-GALLIUM SELENIDE The CIS copper-indium selenide film is characterized by high efficiency and the ability to convert up to 20% of solar energy. Elements based on a composite mixture of indium-copper-gallium selenide Cu(InGa)Se2 have the highest absorption coefficient of semiconductors presented in photovoltaics. The film thickness, which does not exceed hundreds of nanometers, provides high conversion efficiency, being the “champion” in terms of durability and efficiency. Flexible copper-indium-gallium diselenide (CIGS) solar cells formed on stainless steel foil achieve 17.4% conversion efficiency [52,53]. At the same time, for CIGS-based elements formed on PI substrates that withstand relatively low deposition temperatures, the conversion coefficient decreases to 11% [ 53 – 55 ]. (4) BASED ON GALLIUM ARSENIDE The best materials to combine with silicon in terms of efficiency are Group III–V semiconductors, especially GaAs, with which laboratory samples of elements have shown an efficiency of more than 32%, or perovskites based on metal halide, for which laboratory samples have shown an efficiency of 29.8%, according to Henry Snaite, Global Energy expert Professor of Physics at the Clarendon Laboratory at the University of Oxford. Group III–V semiconductors are still produced using very expensive and slow molecular beam epitaxy, which makes them prohibitively expensive. In contrast, metal halide perovskites can be produced very quickly at a low temperature using conventional thin film manufacturing processes, making them very attractive from an economic point of view [56]. Gallium arsenide (GaAs) solar cells are a type of high-efficiency photovoltaic technology that utilizes a semiconductor material called gallium arsenide as the absorbing layer to convert sunlight directly into electricity. GaAs solar cells are known for their excellent performance in terms of efficiency, especially in comparison to traditional silicon solar cells. GaAs cells are rapidly gaining popularity nowadays and hundreds of research papers featuring this type of solar cell are published every year [57–62]. Gallium arsenide GaAs and indium phosphide InP cells maintain efficiency at extreme temperatures of several hundred degrees Celsius. Elements (on the order of 2–3 microns) based on GaAs demonstrate conversion efficiency up to 44%, acting as an ideal semiconductor for photovoltaics. Methods for obtaining a multilayer structure based on GaAs with the inclusion of other materials made it possible to significantly increase the efficiency of using the invisible parts of the spectrum. Currently, twoand three-layer photocells are being investigated, which allow one to study a large part of the solar spectrum along the wavelength of solar radiation. For a two-layer photocell on prototypes, an efficiency of 30% was obtained, and for a three-layer photocell, up to 40% was obtained [2]. (5) COMBINED AND MULTILAYER Multilayer (multi-cascade) design increases the efficiency of flexible panels. The hybrid solar battery (PVT), utilizing the excess heat from photovoltaic cells, has opened up prospects for the generation of two energies: thermal and electrical. This symbiosis made it possible to halve the area required for installing thermal collector systems, thereby synchronizing the simultaneous operation of solar collectors and photovoltaic cells. An innovative solution to achieving the efficiency of thin-film solar cells largely depends on the selected semiconductor and growing technology. Representatives of the third generation are thin-film flexible solar panels based on safe natural minerals, organics, and on the properties of physical quantum dots. An alternative to the creation of solar cells is the manufacture of single-junction elements based on multilayer nanoheteroepitaxial structures (NHES) with quantum dots Materials 2023,16, 5839 16 of 32 presence of quasi-2D perovskite components along the grain boundaries of the 3D perovskite structure contributed to the amelioration of defect passivation and the enhancement of internal carrier transportation properties. Moreover, the incorporation of 4M-PEACl induced significant alterations in the surface chemistry of the perovskite films, leading to a more favorable electronic structure for hole extraction. The consequential outcome was a marked enhancement in the efficiency and light stability of perovskite solar cells. Remarkably, the MA-free perovskite solar cells attained a consistent efficiency of 23.7% (24.2% in reverse scan, 23.0% in forward scan), with over 93% of the initial performance retained even after 1000 h of continuous illumination. An innovative proposal was made by a group of scientists from RIKEN–Toray Industries Inc from Tokyo: elastic ultra-thin (3 µ m) solar cells. New batteries recover their shape after being stretched or compressed, are securely attached to fabric or another surface with hot-melt adhesive, and withstand temperatures up to 100 degrees Celsius [105]. The developers of the RIKEN Institute and the University of Tokyo have created an ultra-thin flexible photocell that does not exclude direct contact with water and is not afraid of tensile loads. By integrating flexible photovoltaic systems into textiles, materials with the widest functionality for creating “smart” clothes are formed. Giving products an additional functional load is becoming a sustainable trend in the design and production of clothing [103]. “Wearable” technology is represented by intelligent, technological clothing, footwear, and headwear with built-in electronics with the ability to connect mobile devices. The use of flexible batteries in textile production to create the so-called “photo curtains” is another example. Flexible solar modules while generating energy at the same time protect the room from excessive penetration of sunlight, which ensures a comfortable indoor climate. A technique for designing special-purpose headgear in three-dimensional graphic CAD (computer-aided design) was proposed, coupled with planar design and modeling modules “Eleandr–konstruktor” [ 106 ] and “Eleandr-KM” [ 106 ]. The result of the experimental study was a developed constructive and technological solution for a special-purpose headgear: a cap with removable batteries [107]. In the case of solar cells integrated into clothing, it is necessary to use substrates thin enough so that the solar cells do not significantly change the “flexibility” of the material onto which they are placed. Note that it is also necessary to carry out work on the creation of a thin and strong material for encapsulating solar cells. The authors note that, taking into account the possible location of a solar battery with an area of 1000 cm 2 on the clothes of an adult, flexible solar cells integrated into clothes can provide the necessary energy consumption [108]. Although it is too early to talk about specific economic indicators of the use of perovskite, since the widespread practical use of this material in solar cells is predicted after 2025, the “Russian mineral” has the prerequisites for a great and successful future. According to experts from the US National Renewable Energy Laboratory (NREL), the production of perovskite panels will be ten times cheaper than that of their silicon counterparts. There are two more advantages of perovskite solar cells: flexibility and transparency. The application of perovskite to a polyurethane substrate is already known about, the efficiency of which in absorbing the sun has reached 5.72% [83]. Scientists from the Moscow Institute of Steel and Alloys (MISiS) and St. Petersburg University of Information Technologies in Mechanics and Optics have developed a perovskite-based solar cell that can simultaneously operate as a battery and as an LED and is based on perovskite halide [83]. Along with traditional monoand polycrystalline solar panels on a rigid frame, flexible thin-film panels are widely used. The technological process of creating thin-film solar cells formed on flexible substrates is relatively simple, and minimal energy consumption significantly reduces the cost of manufacturing “flexible” solar cells. Flexible solar panels are quite widely represented on the market, taking into account their indicative characteristics: Materials 2023,16, 5839 17 of 32 •Current strength; •High efficiency; •Additional features (models with suction cups, with fastening on a backpack); •Reliable functioning; •Efficient work. The production of a-Si solar cells is already at a fairly high technological level. As the main technological process, thin-film technology of plasma-assisted chemical vapor deposition from siliconand germanium-containing mixtures (SiH4, Si2H6, GeH4) is used [42]. Photovoltaic film in micromorphic (polymorphic) panels forms a multilayer a-Si cake with distinct properties in each of the layers. The main characteristics of some solar cells based on silicon (single-crystalline c-Si, microcrystalline mc-Si, thin-film tf-Si, and amorphous a-Si) are presented in Table 3. Table 3. The main characteristics of some solar cells based on silicon. Material Structure S, cm2Uoc, mB Jsc, mA/cm2FF, %Eff-cy Date Company c-Si 4.00 709 40.9 82.7 24.0 9/94 UNSW c-Si 45.7 694 39.4 78.1 21.6 4/94 UNSW c-Si 22.1 702 41.6 80.3 23.4 5/96 UNSW mc-Si 1.00 636 36.5 80.4 18.6 12/91 Georgia Tech. mc-Si 100 610 36.4 77.7 17.2 3/93 Sharp tf-Si 240 582 27.4 76.5 12.2 3/95 Astro Power tf-Si 4.04 699 379 81.1 21.1 8/95 UNSW a-Si:H 1.06 864 16.66 71.7 103 10/90 Chronar a-Si:H 0.99 886 17.46 70.4 10.9 9/89 Glass tech. a-Si:H 1.0 887 19.4 74.1 12.7 4/92 Sanyo a-Si:H 1.08 879 18.8 70.1 1 1.5 4/87 Solarex ITO/c-Si/a-Si 1.0 644 39.4 79.0 20.0 9/94 Sanyo a-Si:H 1.0 891 19.13 70.0 12.0 9/94 Solarex a-Si:H 1.0 923 18.4 72.5 12.3 9/94 Fuji-Elect. a-Si/a-Si/a-SiGe 2320 7.3 73.0 12.4 9/94 Sumitomo a-Si:H 1.0 887 19.4 74.1 12.7 9/94 Sanyo a-C/a-SiML/a-SiC/a-Si 1.0 936 19.6 71.8 13.2 9/94 MutsuiToatsu a-C/a-Si/a-SiC/a-Si 1.0 909 19.8 73.3 13.2 9/94 MutsuiToatsu ITO/a-Si:H/a-SiGe:H 0.28 1621 1 1.72 65.8 12.5 1/92 USSC/Cannon a-Si/k-Si 0.03 1480 16.2 63.0 15.0 9/94 Osaka Univ. a-SiC/a-Si 1.0 1750 8.16 71.2 10.2 9/94 Solarex a-Si/a-Si 1.0 1 800 9.03 74. I 12.0 9/94 Fuji a-SiC/a-SiGe/a-SiGe 1.0 2290 7.9 68.5 12.4 9/94 Sharp a-Si/a-Si/a-siGe 1.0 2550 7.66 70.1 13.7 9/94 ECD/Sovonics p-a-SiO:H/a-Si:H/n-a-Si:H 1.0 899 18.8 74.0 12.5 9/94 Fuji-Elect. ITO/a-Si:H/Si:H/a-siGe 0.27 2541 6.96 70 12.4 2/88 ECD a-Si:H/a-Si:H/a-SiGc:H 1.00 2289 7.9 68.5 12.4 12/92 Sharp a-Si/CuInSe2871 432 16.4 17.4 72.0 68.0 10.3 5.3 9/94 ARCO a-Si/mc-Si 917 575 10.4 30.2 76.0 79.2 7.25 13.75 9/94 Osaka Univ. It should be noted that optimized solar cells based on single-crystal silicon are superior in their parameters to cells based on α -Si in the range of illumination intensities Materials 2023,16, 5839 18 of 32 from medium to low levels due to higher conversion efficiency. At the same time, at a very low illumination level of less than 500 lux, the α -Si:H element outperforms the c-Si element [ 109 , 110 ]. A substantial portion of incident energy in photovoltaic solar energy conversion devices goes to waste, primarily due to the necessity of having a gap within the electronic states’ continuum of the light-absorbing material, which functions as an absorption threshold. When photon energies exceed this bandgap, any excess energy is converted into heat through the thermalization of excited charge carriers. Meanwhile, light with energies below the bandgap remains unutilized [ 41 , 111 , 112 ]. In materials with low bandgaps like germanium or crystalline silicon (c-Si), power conversion losses are mainly attributed to thermalization. However, in materials with bandgaps larger than 1.3 eV—common in thin-film photovoltaic absorbers and oxide-based electrode materials for photoelectrochemical energy storage—transmission of sub-bandgap light becomes the predominant factor [ 113 , 114 ]. This phenomenon even affects solar cells fabricated from c-Si, leading to a significant portion of the solar spectrum being lost through transmission [ 115 ]. The use of nanostructured amorphous silicon in combination with an additional layer made by conventional technology: - Increased the conversion efficiency of the infrared part of the spectrum, while there were no losses in the absorption coefficient in the visible part; - Allowed for the minimization of the overall thickness of the film; - Reduced the rate of degradation by 20–25%. Amorphous silicon solar cells with a p-i-n structure are now used in a wide variety of fields due to the possibility of their fabrication on metal foils, such as stainless steel, and polymer films provided with a metal coating. The use of such substrates is compatible with the technology of mass production of flexible solar cells. Therefore, elements of this type are among the most promising solar energy converters of the near future [116]. Active innovative development of the solar energy complex (development and implementation of advanced technologies for creating varieties of flexible solar panels) is accompanied by its spatial expansion, which is due to these positive aspects: • Reliability (the product is protected from mechanical damage, moisture, and high temperatures, many designs are equipped with covers). • Impact resistance (the panel remains intact when dropped from a height of many meters. The elements are relatively insensitive to overheating and are suitable for creating installations of any power). • Thin and pliable structure (sufficiently large bending radius allows installation of buildings with a non-linear roof shape, dismantling of surfaces of any configuration, and versatility, i.e., the ability to fold and roll into a roll). • Lightness (selenium, silicon, etc., are used to create semiconductors. Polymer sputtering and aluminum conductors lighten the panel by reducing the load on the supporting structures of structures). • Environmental friendliness (materials can be recycled and reused. The source of energy is environmentally friendly, silent, and renewable. The daily influx of sunlight to the Earth is 120 thousand terawatts, which exceeds the need of earthly civilization for energy more than 20 thousand times). • Availability and ease of operation (easy to install, simple equipment maintenance, do not require highly qualified personnel). • Profitability (high overall energy production as power efficiency is maintained in cloudy weather), and a high level of optical absorption of photons increases the efficiency of batteries, simplifies production technology, reduces energy payback time for modules, has a favorable cost, and allows for energy supply to a country (providing autonomous control of heating, ventilation, and lighting, with an energy efficiency index from 14% to 30% and excellent maintainability as it is enough to cut out the damaged area, replace it with a new one and reconnect it to the circuit). • Performance characteristics (wide range of applications, significant working life due to high ability to capture various spectra of solar radiation, high-quality operation in low Materials 2023,16, 5839 19 of 32 light conditions, performance drop with significant temperature fluctuations is not critical, and a snug fit to the surface guarantees resistance to wind loads). Versatility and simplicity, i.e., the ability to increase power by adding new modules. • Aesthetics. Modules can become part of the design idea, be part of the architectural decoration of houses, and play the role of home decoration. Numerous photovoltaic energy conversion systems miss out on harnessing the solar spectrum’s lower-energy photons that lie below the bandgap. This is particularly evident in materials with high bandgaps like amorphous silicon, polythiophenes, and lead halide perovskites. A method of photochemical upconversion, employing triplet-triplet annihilation (TTA-UC) within organic molecules, presents a flexible solution for capturing and converting this untapped light into wavelengths beneficial for the respective device [115]. Research in this area has predominantly revolved around two distinct yet interrelated approaches [ 117 , 118 ]. Firstly, the incorporation of lanthanoid cations (primarily erbium and ytterbium) into a solid-state matrix has been explored [ 119 ]. These materials leverage their well-defined and long-lasting atomic states, enabling the absorption of multiple photons and subsequent upconverted luminescence. As rare-earth materials absorb within the infrared region of the solar spectrum, lanthanoid upconversion (L-UC) holds promise, particularly for enhancing c-Si solar cell performance [120–122]. Experiments with lanthanide boost converters have shown that the advantage of the boost converter layer is limited by the high light intensity required to achieve high conversion quantum efficiency. To address this limitation, boost-converting materials can be combined with quantum dots or plasmonic particles to improve conversion efficiency and improve the applicability of boost converters in commercial solar cells. The defining directions in the development of various types of flexible solar panels are technologies in the study of fundamentally new materials, with characteristic indicators of reliability, durability of the service life, and compliance with the price and quality of the product. The comparative cheapness of solar cells formed on flexible substrates compared to solar cells created on glass is associated primarily with the possibility of using the so-called roll-to-roll method in their production. In addition, the reduction in energy consumption when creating solar cells on flexible substrates compared to batteries formed on glass is associated with the use of plastic substrates as flexible substrates, which do not require significant energy costs during their formation. Therefore, despite the fact that in recent years the production of thin-film silicon solar cells formed on glass substrates with an area of up to ~6 m 2 [ 123 , 124 ] has been significantly increased, there is a need to develop new approaches to the creation of solar cells that allow for increasing the productivity of technological lines by an order of magnitude while significantly reducing costs. To compete with large-area photovoltaics formed on glass plates, the method of manufacturing photovoltaic modules based on “roll coating” must solve a number of problems related to substrate cost reduction, technological simplicity, and low-cost strong encapsulation. Taking into account the importance of all the above tasks associated with the production of “flexible” solar cells, the authors considered the problems associated with low-temperature deposition and the formation of a sequential structure of a solar cell [ 125 ]. The rapidly developing industry of the solar energy complex offers the most durable, productive, and powerful examples of flexible solar panels based on the improvement of traditional and the use of fundamentally new materials: - Manufactured using silicon-free technology; - Based on amorphous silicon; - Based on nanoheteroepitaxial structures with quantum dots. Interest in solar cells on flexible substrates is increasing from developers of autonomous systems of orbital stations and telecommunications satellites, as well as specialists working in the field of rural electrification in developing countries. Small aircraft, automobiles, and other consumers of electrical energy can obtain some of the energy they need from the ambient lighting of their exposed surfaces using flexible photovoltaic cells. Materials 2023,16, 5839 20 of 32 Another application of “flexible” photovoltaics is the integration of small photovoltaic systems into clothing to power portable electronic devices [109,126,127]. In addition to the need to optimize the material and structure of the solar battery in the case of using flexible substrates, it is necessary to solve the problem associated with the series connection of individual solar cells in a photovoltaic module [ 128 ]. Monolithic serial connection of individual elements allows for an increase in the voltage of photovoltaic modules for low-cost photovoltaic systems [129]. In [ 130 ], the authors present the results of research on optimizing the low-temperature deposition of solar cells based on hydrogenated silicon films consisting of amorphous hydrogenated silicon, nanocrystalline hydrogenated silicon, and protocrystalline hydrogenated silicon (pc-Si:H). The authors also investigated solar cells based on an optimized solar cell using pc-Si:H deposited on a flexible low-cost polymer substrate at substrate temperatures of 40 ◦C<Td< 110 ◦C . The experiments performed showed that for a temperature T d = 110 ◦ C, in the case of using non-textured PET foil, the conversion efficiency was 9%. A solar module with an area of 4 × 10 cm 2 , based on a single-element α -Si pin structure formed on polyethylene naphthalate (PEN), demonstrated a conversion efficiency of 3% [131]. It is assumed that the optimal material for creating an absorbing layer of α -Si solar cells is undoped protocrystalline silicon [ 132 ], which has better electronic parameters than α -Si:H and nc-Si:H. When using low substrate temperatures Td < 100 ◦ C, pc-Si:H is the only optimal material for creating a thin-film Si-based solar cell [ 133 ]. At the same time, at T d > 200 ◦ C, the parameters of the solar cell improve when moving from α -Si:H to pc-Si:H when forming a light-absorbing layer [134]. In [ 108 ], the author presents a review of available data on the properties of hydrogenated silicon films, conditions for obtaining protocrystalline silicon, and the parameters of solar cells based on their flexible substrates. The use of flexible substrates (stainless steel foil or polymer foil) somewhat simplifies and reduces the cost of forming a solar cell, which is partly due to the possibility of using the “roll to roll” technique. At the same time, in order to form solar cells based on hydrogenated silicon films on flexible polymer substrates, it is necessary to refine the technology for producing solar cells based on this material at relatively low temperatures (<150 ◦ C). The preferred material for forming the active layer in this case is protocrystalline silicon pc-Si:H, the technological conditions for the formation of which significantly depend on the ratio of gases in the hydrogen–silane mixture and the design of the reactor for depositing films of the structure. Currently, silicon solar cells have been obtained on non-textured PET foil at Td = 110 ◦C with a conversion efficiency of ≈5% [128]. The possibility of depositing films onto stainless steel and heat-resistant polymer substrates allows for the creation of flexible, portable solar modules and the carrying out of deposition over relatively large areas. The main problem of these modules based on amorphous silicon is low resources, due to the removal of hydrogen from the films under the influence of temperature and solar radiation (the Stebler–Vronsky effect) and the low efficiency of the module, which lies in the range of 4–7% [135]. For the first time, cylindrical solar cells were developed by the American company Solyndra, using copper, gallium, indium, and selenium. Cylindrical solar panels capture more light, showing high performance. The presence of a cylindrical shape means resistance to strong winds, with gusts up to 200 km/h. The first large roll-up flexible solar array was used in NASA’s Solar Array Flight Experiment (SAFE) program. The first foldable flexible solar panel was installed on the Communications Technology Satellite (CTS) [136]. Oerlikon Solar (Switzerland) and Applied Materials (USA) have developed similar technological processes for the production of solar cells and tandem-type modules: an amorphous silicon layer is supplemented with a layer of silicon microcrystallites [ 137 ]. The band structure of amorphous silicon is such that it allows the absorption of photons in a slightly different part of the spectrum than for crystalline modules (Figure 2). Therefore, Materials 2023,16, 5839 21 of 32 the combination of layers of amorphous and microcrystalline silicon expands the module’s capabilities for converting reflected and scattered solar radiation. In addition, the dependence of these characteristics on ambient temperature for such modules is usually lower than for crystalline ones (Table 4) [138]. Materials 2023, 16, x FOR PEER REVIEW 22 of 32 undergo stable and robust p-doping through the utilization of MoOx. This results in heightened conductivity and improved hole transportability. The incredibly thin foldable transparent conductor exhibits a sheet resistance of 82 Ω/sq.−1 and a transmittance of 80% at 700 nm. When incorporated into a foldable solar cell, it achieves a maximum power point tracking output of 15.2%. Remarkably, these foldable solar cells can endure over 10,000 folding cycles at a 0.5 mm folding radius, showcasing an unprecedented level of mechanical resilience. Furthermore, these solar cells exhibit superior performance compared to other flexible solar cells based on carbon nanotube transparent electrodes, marking a significant advancement in the field. Elevated criteria for foldable electronics, necessitating resilience under rigorous mechanical demands such as a bend radius of 0.5 mm, have been successfully addressed through the introduction of an ultra-thin matrix composed of MoOx-doped single-walled carbon nanotubes (SWNT) and polyimide (PI). The ultra-thin transparent conductor based on SWNT-PI displayed outstanding mechanical durability, boasting a fineness of 7 µm, a smooth surface morphology, and a high ratio of direct current (DC) to optical conductivity. Capitalizing on the mechanical robustness of SWNT, the SWNT-PI conductor also showcased exceptional adhesion and ultra-thin properties, making it an ideal candidate for applications in foldable electronics [145]. The authors of the work [146] fabricated ultra-thin flexible SCs based on InGaP/GaAs HES with a reverse layer sequence grown by MOCVD on a germanium substrate 100 mm in diameter. The area of ultra-thin flexible SCs was ~1 cm2. Figure 2. Areas of the solar radiation spectrum which are absorbed in the layers of an amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) doped hydrogen cell [135,138]. Researchers from South Korea have achieved the development of solar cells with such slender dimensions that they can be contoured around diverse objects. To realize these ultra-thin solar cells, the scientists undertook the deposition of gallium arsenide semiconductor cells onto an incredibly thin and flexible substrate. Employing a temporary adhesive layer, the cells were affixed to the substrate’s surface, a process executed within Figure 2. Areas of the solar radiation spectrum which are absorbed in the layers of an amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) doped hydrogen cell [135,138]. Table 4. Difference in temperature dependence of various PVMs [138]. PVM Types MSW-180 Pramac Luce MCPH P7LM BYD Stion SN 130 Technology Mono-Si, bilateral α-Si/µ-Si Multi-Si CIGS Voltage Temperature Coefficient, B/◦C−0.152 −0.004 −0.0034 - Current Temperature Coefficient, AKC 0.00044 0.0006 0.00045 - Temperature Coefficient of Power, BT/◦C−0.005 −0.0025 −0.0047 −0.0045 The main technologies for non-silicon solar cells are thin films based on cadmium telluride (CdTe) and composites based on copper, indium, gallium, and selenium (CIGS). The technology for preparing these films also allows scaling over sufficiently large areas and the use of flexible substrates. Plasma chemical or chemical vapor deposition is usually used [ 139 ], as well as magnetron sputtering [ 140 ] and liquid phase inkjet printing followed by heat treatment [ 141 ]. The efficiency of modules based on CdTe technology lies in the range of 8–11%, and CIGS in the range of 9–14%, which is already approaching the efficiency of multicrystalline PEMs [ 142 ]. The record holders for module efficiency are PEMs based on heterostructures of the A3B5 type (24–27%) [ 143 ], however, the high cost of the materials used makes it necessary to produce small-area solar cells from such modules [138]. The research [ 144 ] outlines techniques for achieving ultra-thin GaAs photovoltaics using interlayer adhesiveless transfer-printing of vertical-type devices onto metallic surfaces. These vertical-type GaAs photovoltaic devices utilize bottom electrodes to recycle Materials 2023,16, 5839 22 of 32 reflected photons. Through systematic examinations involving four distinct solar microcell variations, it has been determined that these vertically oriented solar microcells, which are just a quarter of the thickness of equivalently designed lateral-type cells, can produce a comparable level of electrical power to their thicker counterparts. Both the empirical findings and the theoretical analysis presented in this study demonstrate the robustness of the ultra-thin vertical-type solar microcells even under extreme bending, making them well-suited for integration into the fabrication of flexible wearable electronics. Lately, there has been a growing emphasis on foldable electronics technology within both academic and industrial research communities. This technology differs from flexible technology due to its requirement to endure substantial mechanical strains, particularly those resulting from an extremely tight bending radius of 0.5 mm. Achieving foldable devices demands transparent conductors with remarkable mechanical durability, necessitating their ultra-thin dimensions in the micrometer range and integration of the conductive material into a substrate. In the study [ 145 ], a composite film of single-walled carbon nanotubes (CNTs) and polyimide (PI) is synthesized, boasting a mere 7 µ m thickness. This composite film serves as a foldable transparent conductor for application in perovskite solar cells (PSCs). During the high-temperature curing process of the CNT-embedded PI conductor, the CNTs undergo stable and robust p-doping through the utilization of MoOx. This results in heightened conductivity and improved hole transportability. The incredibly thin foldable transparent conductor exhibits a sheet resistance of 82 Ω /sq. −1 and a transmittance of 80% at 700 nm. When incorporated into a foldable solar cell, it achieves a maximum power point tracking output of 15.2%. Remarkably, these foldable solar cells can endure over 10,000 folding cycles at a 0.5 mm folding radius, showcasing an unprecedented level of mechanical resilience. Furthermore, these solar cells exhibit superior performance compared to other flexible solar cells based on carbon nanotube transparent electrodes, marking a significant advancement in the field. Elevated criteria for foldable electronics, necessitating resilience under rigorous mechanical demands such as a bend radius of 0.5 mm, have been successfully addressed through the introduction of an ultra-thin matrix composed of MoOx-doped single-walled carbon nanotubes (SWNT) and polyimide (PI). The ultra-thin transparent conductor based on SWNT-PI displayed outstanding mechanical durability, boasting a fineness of 7 µ m, a smooth surface morphology, and a high ratio of direct current (DC) to optical conductivity. Capitalizing on the mechanical robustness of SWNT, the SWNT-PI conductor also showcased exceptional adhesion and ultra-thin properties, making it an ideal candidate for applications in foldable electronics [145]. The authors of the work [ 146 ] fabricated ultra-thin flexible SCs based on InGaP/GaAs HES with a reverse layer sequence grown by MOCVD on a germanium substrate 100 mm in diameter. The area of ultra-thin flexible SCs was ~1 cm2. Researchers from South Korea have achieved the development of solar cells with such slender dimensions that they can be contoured around diverse objects. To realize these ultrathin solar cells, the scientists undertook the deposition of gallium arsenide semiconductor cells onto an incredibly thin and flexible substrate. Employing a temporary adhesive layer, the cells were affixed to the substrate’s surface, a process executed within a specialized chamber set at a temperature of 170 degrees Celsius. Through a procedure termed “cold welding,” the cells were effectively fused with the substrate. Subsequently, the adhesive layer was meticulously removed, resulting in a significant reduction in the thickness of the photovoltaic cell. However, the technology pioneered by the Korean researchers must still undergo an array of comprehensive assessments and evaluations before it becomes viable for widespread incorporation into the production of various items [144]. A team of researchers from Rice University, Houston Community College, and Brookhaven National Laboratory have developed flexible organic photovoltaic wafers that can be used to generate constant power. The study is published in the journal Chem- Materials 2023,16, 5839 23 of 32 istry of Materials. Organic solar cells are made up of carbon-based materials, including polymers, which store sunlight and convert it into electricity [147]. Scientists from Stanford University have created flexible solar panels that can be made using existing technologies, and any material, for example, an ordinary sticker, can be used as a substrate for them. The authors of the work tested the application of a flexible solar battery on various surfaces (glass, plastic, paper) and the device worked on all of them without loss of efficiency. The researchers point out that the production of such solar cells is not only easier and cheaper than traditional photovoltaic panels of the appropriate size, but also devoid of any waste. Silicon wafers can be reused [148]. Another team of scientists have developed solar panels as thick as spiderwebs. Specialists have created ultra-thin, flexible, and very efficient solar cells. The third part of the cell is the active elements that are engaged in the production of energy, and 2/3 of the cell is the polymer base [149]. The novel technique involves an inverted growth process for the solar cell. These layers incorporate high-energy materials featuring exceptionally high-quality crystals, particularly in the upper layers where the majority of power generation occurs. Not all layers conform strictly to a lattice pattern with uniform atomic spacing. Instead, the cell encompasses a comprehensive array of atomic spacing, facilitating enhanced sunlight absorption and utilization. The bulky and rigid germanium layer is eliminated, leading to a reduction in both the cost and weight of the cell by 94%. By overturning the conventional approach to cell fabrication, the outcome is an exceptionally lightweight and flexible cell that also achieves a groundbreaking level of solar energy conversion efficiency (40.8% under 326 suns’ concentration). Experimental photovoltaic panels that diverge from traditional silicon can be fashioned using quantum heterostructures, such as carbon nanotubes or quantum dots, which are integrated into conductive polymers or mesoporous metal oxides. Furthermore, the application of thin films of these materials onto conventional silicon solar cells can heighten the efficiency of optical coupling into the silicon cell, leading to an overall enhancement in efficiency. By manipulating the dimensions of the quantum dots, the cells can be finely adjusted to absorb distinct wavelengths of light. Although the investigation is still in its preliminary stages, photovoltaics enhanced with quantum dots hold the potential to attain an energy conversion efficiency of up to 42%, attributed to the phenomenon of multiple exciton generation (MEG) [150]. An emerging field has arisen that employs networks of carbon nanotubes as transparent conductors in organic solar cells. These nanotube networks possess flexibility and can be applied to surfaces through various methods. Through specific treatments, nanotube films can achieve significant infrared transparency, potentially facilitating the development of effective low-bandgap solar cells. It is noteworthy that nanotube networks function as p-type conductors, which sets them apart from the conventional n-type transparent conductors. The introduction of a p-type transparent conductor could pave the way for novel cell designs that streamline the manufacturing process and enhance overall efficiency. Efficiency and reliability determine the advantage of half-cut solar panels over their solid-cell counterparts, leading to savings in installation time and costs. The ninth edition of ITRPV (International Technology Roadmap for Photovoltaic) predicts that the market share of half-cells will grow from 5% in 2018 to almost 40% in 2028. Half-cell modules have solar cells that are cut in half, improving the efficiency and durability of the module. Traditional panels with 60 and 72 cells will have 120 and 144 cells, respectively. When solar cells are halved, their current is also halved, so resistive losses are reduced, and the cells can produce slightly more power. Smaller cells experience less mechanical stress, so cracking is less likely. Half-cell modules have higher output performance and are more reliable than traditional panels [151]. The beginning of the 21st century has seen a great abundance when it comes to new versions of seemingly long-known materials. Many of them have shown themselves to be extremely promising: graphene, carbon nanotubes, and, since 2012, perovskites [152]. Materials 2023,16, 5839 24 of 32 Evolutionary developments in renewable energy are aimed at increasing the efficiency of solar panels, creating devices that are lighter, thinner, and closer to perfection in general. At the present stage of the development of microelectronics to solve the problem of heat removal and the production of highly efficient solar cells, the main hopes for a technological solution are pinned on graphene. Researchers at the Massachusetts Institute of Technology have found an alternative replacement for expensive materials in solar cells using organic materials with graphene electrodes. Batteries made using graphene show excellent efficiency results. The two-dimensional structure of graphene endows it with amazing properties of flexibility and transparency. Future developments aim to create thinner, more flexible, and more stable solar panels in the future. Solar energy for a century and a half has made a qualitative leap from the household level (energy supply of buildings and outdoor lighting, equipment for surface buoys and signs, children’s toys, parking lots with electric charging stations, cars, and other vehicles) to space development. The pace of development of the alternative energy industry served as an impetus for the intensification of innovation activities, as well as the creation of new high-tech industrial sectors. The activity of each sector of the economy solves the problems of a technological, economic, and environmental nature in the field of alternative energy: • Primary sector: mining and collection of minerals (in particular: copper, silicon, tellurium, cadmium, etc.) •Secondary sector: manufacturing industry (aerospace, construction, etc.) •Tertiary sector: the service sector in all sectors of the economy. •Quaternary sector: intellectual services (technological progress). Flexible solar modules have proven themselves in the field of medicine, communication systems, fire service, navigation and signal signs, police, army, maritime, search and rescue services, traffic surveillance and control systems on freeways, cinema, etc. Manufacturers of flexible panels, depending on the formulation of technical problems, make various modifications on the basis of solar modules of different types. The design and physical features expand the possibilities of their application: fixing onto objects with a complex configuration or non-smooth surface. The relentless growth in demand for cheaper resources is driving the market towards the production of more efficient and cost-effective renewable energy sources. Factors limiting the absorption of sunlight by solar cells and reducing the efficiency of flexible solar cells can be dust, dirt, or bird droppings. A promising solution in the field of solar energy is the use of flexible solar panels, which is due to their geometric and physical parameters. 7. Key Findings and Future Implications Thus, the following key findings of this review can be highlighted: • Solar panel diversity: the review paper revealed a diverse landscape of solar panel technologies, including monocrystalline, polycrystalline, thin-film, and emerging thirdgeneration solar cells. Each technology exhibited distinct advantages and limitations, impacting factors such as efficiency, cost, and manufacturing complexity. • Material influence on efficiency: the study underscored how the choice of photovoltaic material profoundly influences solar panel efficiency. Monocrystalline silicon exhibited high conversion efficiencies due to its uniform crystal structure, while thin-film materials like cadmium telluride and copper indium gallium selenide demonstrated cost advantages but comparatively lower efficiencies. • Bandgap engineering: photovoltaic materials with tunable bandgaps, such as perovskites and tandem solar cells, showcased the potential to optimize energy absorption across the solar spectrum. The manipulation of bandgaps allowed for enhanced performance in various lighting conditions. • Material stability and durability: the review highlighted the importance of material stability in solar panel longevity. Emerging materials like perovskites demonstrated impressive efficiency gains but often faced challenges related to degradation under Materials 2023,16, 5839 25 of 32 environmental stressors. Extensive research focused on enhancing stability to ensure long-term performance. • Defect management and passivation: effective passivation techniques were identified as essential for minimizing defects at material surfaces and interfaces. Silicon-based solar cells employed techniques like hydrogenation, while emerging materials leveraged tunnel oxide passivated contacts (TOPCon) to reduce recombination losses and improve charge carrier extraction. • Multijunction and tandem solar cells: multijunction and tandem solar cells were identified as effective strategies to improve overall efficiency by stacking different materials with complementary absorption spectra. These configurations demonstrated the potential to exceed the efficiency limits of single-junction solar cells. • Cost reduction and scalability: the study emphasized the significance of advancing photovoltaic materials with abundant and non-toxic constituents, fostering costeffective production and widespread scalability. The integration of advanced manufacturing techniques like roll-to-roll processing showcased potential for reducing material and production costs. • Environmental impact: the environmental impact of photovoltaic materials was a central concern. The review paper discussed the importance of sustainable sourcing, recycling methods, and reducing toxic components to align with broader environmental goals. When it comes to future implications, the following points can be considered: • Advancements in material stability and durability: As the adoption of flexible solar panels continues to expand, the need for enhanced durability and long-term stability becomes paramount. Future research efforts should concentrate on developing materials that can withstand a variety of environmental stressors, such as UV radiation, temperature fluctuations, and moisture exposure. By engineering materials with improved stability, solar panels can maintain their performance over extended lifetimes, leading to greater economic viability and reduced maintenance requirements. • Integration into urban infrastructure: The concept of solar-integrated urban landscapes holds immense promise for decentralized energy generation. Future research should focus on optimizing flexible solar panels for integration into existing and new architectural structures, facades, and even wearable devices. This integration not only presents opportunities to generate energy from underutilized surfaces but also contributes to sustainable urban planning, making clean energy generation a ubiquitous part of daily life. • Innovations in manufacturing techniques: achieving cost-effective production methods without compromising efficiency is a persistent challenge in the solar industry. Researchers and engineers should explore novel manufacturing techniques, such as roll-to-roll printing, additive manufacturing, and continuous deposition processes, to enable large-scale production of flexible solar panels. By streamlining production processes, the cost barriers associated with these advanced materials can be reduced, making them more accessible for widespread deployment. • Environmental impact and circular economy: as the world embraces renewable energy, considerations of the environmental impact of solar panel materials, production, and end-of-life management become increasingly relevant. Future research should delve into the development of sustainable material alternatives and explore recycling methods to ensure a circular economy approach. Addressing the environmental aspects of flexible solar panel technology will be instrumental in minimizing its ecological footprint and aligning with broader sustainability goals. • Multi-functionality and energy storage integration: The integration of energy storage capabilities within flexible solar panels holds promise for a more seamless energy supply, enabling power generation even when sunlight is unavailable. Future research should explore ways to incorporate energy storage technologies, such as flexible batteries or supercapacitors, directly into the panel structure. This integration would enhance the versatility and reliability of solar panels, making them even more attractive for diverse applications. Materials 2023,16, 5839 32 of 32 151. Shukir, S.S. Half-Cut Cell Solar Panels to Reduce the Effect of High Temperature and Shadow on the Productivity of Solar Panels. J. Altern. Renew. Energy Sources 2022,3, 1–8. 152. Wali, Q.; Iftikhar, F.J. Effect of Crystallization on the Photovoltaic Parameters and Stability of Perovskite Solar Cells. Polyhedron 2021,199, 115089. [CrossRef] Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.