Overview of the Current State of Gallium Arsenide-Based Solar Cells
Department of Physics, Faculty of Electrical Engineering and Communication, Brno University of Technology, Technická 2848/8, 61600 Brno, Czech Republic
Directorate of Research, Development and Innovation Management (DMCDI), Technical University of Cluj-Napoca, Constantin Daicoviciu Street, No. 15, 400020 Cluj-Napoca, Romania
Abstract
As widely-available silicon solar cells, the development of GaAs-based solar cells has been ongoing for many years. Although cells on the gallium arsenide basis today achieve the highest efficiency of all, they are not very widespread. They have particular specifications that make them attractive, especially for certain areas. Thanks to their durability under challenging conditions, it is possible to operate them in places where other solar cells have already undergone significant degradation. This review summarizes past, present, and future uses of GaAs photovoltaic cells. It examines advances in their development, performance, and various current implementations and modifications.
Introduction
Gallium arsenide is a material widely used mainly in semiconductor technologies due to its attractive properties, where it has found many uses. In contrast to silicon, it has become very popular in high electron mobility transistor (HEMT) structures since it does not require any momentum change in the transition between the maximum of the valence Band and the minimum of the conductivity Band, and does not require a collaborative particle interaction. However, hole mobility, in contrast to much higher electron mobility, is similar to silicon—the response times are the same for devices that require cooperation between the motion of holes and electrons. The direct bandgap of GaAs of 1.42 eV is also suitable for diode and photovoltaic (PV) cell applications. It is often extended by so-called alloying, i.e., precise melting of two elements together, in this case, with aluminum, to give Al x Ga 1 − x As. The advantage of a wide bandgap is also the fact that the material remains more semiconductive at higher temperatures, such as in silicon, which has a bandgap of 1.12 eV. With higher temperatures, the thermal generation of carriers becomes more dominant over the intentionally doped level of carriers [1,2]. Therefore, GaAs solar cells have also become the standard for use in demanding temperature conditions. The production of wafers is generally more difficult and expensive. Due to the temperature gradient acting as mechanical stress, more crystalline defects are created: a standard diameter of 6″ wafers is used compared to 12″ for silicon [3]. Single crystals of GaAs are very brittle. Germanium is often used as a substrate, which is suitable for its high mechanical strength and atomic lattice spacing very similar to GaAs [4].
GaAs PV cells belong to III–V group compounds, according to the newer IUPAC notation, already referred to as groups 13–15. Nonetheless, Roman numerals are still familiar, which means this is a semiconductor compound of at least two chemical elements. In 2000, a significant contribution to GaAs was credited to the Nobel prize-winning Russian physicist Zhores Alferov in the field of heterostructures [5].
For GaAs-based solar cells, performance can also be tuned by layering, where one solar cell can contain up to eight thin layers, each absorbing light at a specific wavelength. Such photovoltaic cells are called multi-junction or cascade solar cells. They use tandem fabrication, so they can also be found under the name tandem cells. Each layer contains a different composition and material with a specific bandgap that absorbs light in a particular spectral region. Usually, the top layer has a large bandgap and absorbs most of the visible spectrum up to the bottom layer with a low bandgap, which absorbs light in the infrared region [6]. By covering a wide spectral electromagnetic range, maximum efficiency can be achieved. Other layers are commonly used, such as GaAs, AlGaAs, InP, InGaP, and GaInAs. Due to the mentioned mechanical strength and oriented growth of the Ge crystal lattice, it is possible to make very thin layers, reducing the overall weight of the PV cell.
Multi-junction solar cells, or thin-layer solar cells are referred to as the second generation of solar cells, which has also already been successfully commercialized. It is, therefore, not an experimental technology but a very mature and mastered technology that is already used in many areas. Thanks to such a multi-layered construction, they achieve higher efficiency than conventional single-layer solar cells. In March 2016, Yamaguchi et al. developed the triple-junction PV cell with 37.9% efficiency under 1 Sun, and 44.4% efficiency together with concentrator under 246–302 Suns [7]. In April 2020, a study was published in Nature Energy [8], where the authors of the six-junction PV cell achieved an efficiency of 39.2% and a value of 47.1% at 143 Suns, using the concentrator which was also certified by NREL. They also claimed that further reduction in the limiting series resistance should result in efficiencies over 50%.
Another interesting use of cells was the design of the first holographic diffraction system to incorporate eight solar subcells, more precisely, four different dual-junction PV cells, as can be seen in Figure 1. Darbe et al. declared by simulations 33.2% module conversion efficiency, including external losses, and 63.0% with ideal cells and optics [9].
The most common field using GaAs-based solar cells is the aerospace industry [10,11]. The main reason is their wide spectral coverage, which is much larger in space than on Earth. They are also used in the aviation and military due to their flexibility and weight, which can be used especially for unmanned aerial vehicles (UAVs); and last but not least for concentrators, thanks to which solar cells can operate at very high temperatures. However, from a practical point of view, this type of solar cell is expensive for common use. may vary depending on the complexity of the technology—the number of junctions. The high price is influenced not only by the cost of the wafer but also by subsequent production—expensive equipment. Li et al. state that compared to silicon, the of GaAs cells are up to ten times higher [12]. In contrast, the of silicon cells are very affordable today. Since 1977, when the cost per watt was around 76 dollars, it is now approximately 36 cents [13].
Structure and Composition of GaAs Solar Cells
As mentioned in the introduction, not only have single-junction solar cells been developed for a long time, but multi-junction structures are being created to achieve the highest possible performance. The composition of these structures depends on the specific use. Thus, it is clear that, for example, the light of a different spectral range than on Earth will fall on the surface of Mars due to its atmosphere. Therefore, the Earth’s atmosphere filters not only harmful radiation for humans but also radiation that the solar cell can use. For multilayer structures, emphasis is placed on high crystal perfection in order to avoid recombination of generated minority carriers at cracks and other defects [14,15]. By default, production takes place by growing on a doped substrate. The specific substrate is chosen depending on the next layer that will grow on it to induce an ideal lattice within the epitaxy. The most typical materials are described in Table 1.
As shown in Table 1, temperatures at 300 K or even at 0 K are standardly presented. If necessary, the bandgap at any temperature can be determined empirically by fitting according to the Equation (1) by Varshni [18,19]:
where T is the desired temperature in Kelvin, E g ( 0 ) is the energy gap at 0 K of a given semiconductor, and α and β are specific material constants. So, if we want to calculate the GaAs energy bandgap, for example, at 273.15 K. where E g ( 0 ) = 1.52 eV, α = 0.5404 meV / K and β = 203 K. the result would be:
Well-established epitaxial crystal growth techniques include metal–organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE). Both methods originated in 1960 and have some differences [20].
MOVPE is used to deliver faster growth rates for bulk layers and low breakdown at high temperatures and low vacuum. MOVPE does not require significant bake times and can recover more quickly from equipment failures than MBE.
MBE is, unlike MOVPE, considered a method for superior quality and pure materials in ultra-high vacuum (UHV). It is easier to maintain and is able to grow thermodynamically forbidden materials [21].
There are also several grown concepts that can even be combined, as mentioned, for example, in the inverted metamorphic (IMM) solar cell in Section 3.3. This structure is currently relatively frequently used.
Metamorphic [24]—use the localization of defects in a buffer layer located between layers with different lattice constants.
Inverted [25]—this is an inverted growth of the structure, so materials with a higher bandgap grow here first. The structure is then rotated, and the substrate is removed. This leads to a better performance of the solar cell.
After the growing process, the solar cell is finished by layer bonding, an anti-reflection coating (ARC), and contact metallization [26]. Very thin contacts in the range of micrometer units are often used.
Applications of Solar Cells
As mentioned in the introduction, GaAs and multi-junction PV cells are used mainly in particular industries, where they are required to be highly efficient, durable, or lightweight. These are cutting-edge technologies for special purposes.
3.1. Aerospace and Military
Experimental high-altitude long-endurance UAVs are aircraft that are covered mainly with flexible solar cells because of stay in the air for up to months. They thus replace launching satellites into orbits, which are usually covered by considerable expenses. UAVs can then serve for mapping, surveillance, border patrol, or search and rescue. For civilian use, they are used in flying cell phone towers and communications. Experiments with UAVs and solar cells have been around for over 20 years, and there is constant progress [27,28,29]. Recent advances have been made since 2017 by Alta Devices, where their flexible solar cells exceed efficiencies of 30%, aerial densities of 170 g / m. and are 30 μ m thick. Their solar cells are widely used for aerospace purposes [30]. Microlink Devices Inc. also supplies solar cells to the UAV sector. For example, for Airbus Zephyr (Figure 2)—a solar high-altitude platform station operating in the stratosphere with 29% AM0 efficiency [31,32]. Last but not least is the Thales Stratobus airship capable of flying at an altitude of 20 km, which previously used a transparent envelope section that allows sunlight reflection in concentrator mirrors, which were directed to solar arrays inside the UAV. However, since 2018, this system has been abandoned and replaced by flexible multi-junction arrays installed on the top surface [33].
It is also worth mentioning other areas where flexible multilayer panels are, or have been, in use. These include Aquila by (discontinued) [35,36], Solara 50 by Google, formerly Titan Aerospace (discontinued) [37], HAWK30 by AeroVironment Inc. [38], Caihong (Rainbow) T-4 by the Chinese Academy of Aerospace Aerodynamics [39], PHASA-35 by BAE Systems (Figure 3) [40], Odysseus by Aurora Flight Sciences [41], etc. Even though GaAs flexible cells are constructed for most UAVs, these projects for the long-term sustainability of aircraft in the air are very demanding and have been evolving for a long time. Most of them are in experimental phases. In addition to Alta Devices, Sharp Corporation and SolAero Technologies Corp. are other significant manufacturers producing multilayer solar panels [42].
3.2. Solar Photovoltaic Concentrators
Together in the combination of GaAs PV cells, solar concentrators are widely used, i.e., devices consisting of various optical elements that concentrate light, most often sunlight, into one central point, which is a solar cell. Concentrator photovoltaics (CPV) are used to express the intensity of concentration in the number of Suns or ratios. By default, if the light intensity on the solar cell exceeds 10 Suns, it is already necessary to use passive cooling of the PV cell. This system is considered a low-concentration photovoltaic system (LCPV), and silicon solar cells can still be used here. If the light intensity exceeds 100 Suns, the solar cell must already be actively cooled by cooling fluid, and in that case, it can be considered high-concentration photovoltaics (HCPV). This is a nearly relative number and varies in the literature. GaAs and multilayer structures are already used exclusively for such performance concentrators.
Many concentrator designs follow the concept of Fresnel lens, reflectors, parabolic mirrors, or luminescent concentrators. Notwithstanding, it always depends on their use. Kasaeian et al. summarized the parabolic and Fresnel-based photovoltaic thermal systems over several years, where GaAs cells have always given excellent performance compared to other conventional cells [44].
Solar cells, such as InGaP/GaAs/InGaAs inverted triple-junction, manufactured for the concentrator application, are also specially made for CPV, where Sasaki et al. achieved an efficiency of 45% [45]. In a similar way, concentrators can be created for a particular type of cell and used, for example, in space [46,47]. One such prototype was made by Warmann et al., which also served as ultralight multilayer optical coatings to increase the thermal emissivity of the concentrator and enhance radiative transfer. This unique parabolic concentrator was able to achieve a concentration of 15 Suns for the 1 mm wide cell [48].
One of the most applied and at the same time the oldest concentrators are Fresnel lenses, which are among the first concentrators to be used since 1979. Lenses are light and capable of achieving a short focal length and large aperture. They can be used in the construction in a shape of a circle focusing the light in a point like in Figure 4 (which is considered the most widespread) or in a cylindrical shape focusing the light in a line, resulting in a lower ratio concentration than in the previously mentioned construction. Their disadvantage is that the optical efficiency is limited by low or high temperatures and consequently by a change in the refractive index or deformation of the Fresnel structure by virtue of thermal expansion [49].
Application example of Fresnel lens optic made with Silicon-on-Glass (SoG) technology and designed by Fraunhofer ISE are FLATCON ® concentrator modules [51]. In 2003, the first module consisted of 16 c m 2 lenses and GaAs single-junction solar cells in 2–4 mm diameter. Later, Wiesenfarth et al. performed ten years of outdoor measurements, where triple-junction solar cells were used. Long-term stability was observed when the efficiency per year decreased by (−0.25 ± 0.18)%rel [52].
Steiner et al. measured the performance of 52 four-junction solar cells using FLATCON ® modules (Figure 5) for one month under concentrator standard operating conditions (CSOC) and concentrator standard test conditions (CSTC). The rated efficiency was 35.0% at CSOC and 36.7% at CSTC, and were calculated as mean values [51].
As another very popular concentrator type, and principally very powerful, where optical lenses are not used, is the parabolic concentrator [53]. It is usually utilized using two curved mirrors (Figure 6)—generally reminiscent of a parabolic antenna. The first larger mirror serves as a collector and the second as a focal point. However, various modifications exist where the focal point is already replaced by a solar cell. Like Fresnel lenses, they have a high ratio of around 500. These concentrators are often used in conjunction with thermal collectors (therefore, in the literature can be found for parabolic concentrators name collectors) and thus form a hybrid system. For example, in such a hybrid system, Widyolar et al. demonstrated the GaAs cell load of up to 365 ∘ C with a thermal efficiency of around 37% [54]. complex modern designs already count on a hybrid tubular thermoelectric generator, where the thermal model of the hybrid system with GaAs cells was studied [55].
The opposite case of very powerful parabolic concentrators is luminescent solar concentrators (LSC), which are basically composed of one or more glass or plastic plates. The light captured in these plates, which serve as a waveguide, is guided to one or more edges by total internal reflection (light bounces around the material) where the solar cell is located (Figure 7). High performance is not expected here, but silicon solar cells, as a result of their small bandgap, are no longer adequate for these needs, and GaAs multilayer structures are used for acceptable performance. The plates contain fluorescent dye or quantum dots, so they emit absorbed light at longer wavelengths. Their ratio concentration factor can be up to 10 and they are used mainly as transparent and semi-transparent materials for covering buildings, or as solar Windows. One such experiment was performed by Slooff et al., where multi-crystalline silicon (mc-Si), GaAs, and InGaP solar cells were investigated. The highest efficiency of 7.1% was achieved by GaAs solar cells when attached from four sides [56].
3.3. Probes, Satellites and Other Space Objects
Probably the most extensive use has been made of GaAs-based solar cells on space satellites, probes, and other objects, primarily because of the potential risk of gamma radiation, where GaAs also show higher resistance.
The first probes to carry GaAs-based solar cells were part of the Soviet Venera program used to explore the surface of Venus [57]. The probe Venera 2 was launched on 12 November 1965 and subsequently, after Venera 3 on 16 November 1965, from the Baikonur cosmodrome. Venus 3 is thought to have been the first human object to hit a foreign planet, but Leverington contradicts this claim due to a much earlier signal loss [58]. It is, therefore, uncertain whether the touchdown with the surface took place.
Another popular object using GaAs solar cells is the Hubble telescope, where the GaAs solar arrays with dimensions 7.1 × 2.6 m were installed in 2002 during Servicing Mission 3B. Solar panels replaced previous silicon ones [59].
Another exciting application is triple-junction solar cells by EMCORE Corporation for Orion Multipurpose Crew Vehicle (MPCV), which is a NASA spacecraft service module, and part of the Artemis 1 mission to travel around the Moon planned in November 2021 [60].
Many other solar system probes and other spacecraft utilize this type of solar cell and are active in space. Examples are the Venusian probe Akatsuki (InGaP/GaAs/Ge) [61], the robotic lander InSight (InGaP/InGaAs/Ge) to study the deep interior of Mars or the asteroid study probes Hayabusa2 and OSIRIS-REx [62]. Another current example is mission Mars 2020, which started at the end of July 2020. The Ingenuity helicopter (Figure 8) equipped with inverted metamorphic multi-junction solar cells specially tuned to Mars conditions by SolAero, which, together with the Perseverance rover, was part of the cruise stage. Its entire primary part, which was dropped just before the touchdown, was also covered by multi-junction GaAs solar cells. SolAero, which was mentioned in aeronautics applications, is a company that is also very involved in manufacturing and space applications [63].
Concentrators in space can also be used. However, there are some limitations. For example, near-Earth applications should use lower concentrations (5 Suns) in virtue of the more difficult heat dissipation [10]. However, concentrators in space have become very useful for far-Sun missions to increase low light intensities [65]. It is, hence, essential to know which light intensities can affect the cell.
Light Intensity Affecting Solar Cells in Space
In Earth’s orbit, the light intensity is E s = 1367 W / m 2. which is equal to solar constant. The factor of decrease in flux is, therefore 4.62 × 10 4 [66]. In the case of need to calculate the solar constant on Mars, the formula would be:
where the constant L ⊙ is the solar luminosity of 3.828 × 10 26 W and r is the distance of Mars from the Sun, which is 2.2794 × 10 11 m. The solar constant on Mars would therefore be 586 W/m 2 [67,68].
Because the Earth is in thermal equilibrium with this radiation equal to the solar constant, it must indeed emit the same amount. By adjusting this equality, we can approximate the effective temperature of the Earth as:
where T ⊕. T ⊙ and R ⊕. R ⊙ are the effective temperatures and radii of the Sun and the Earth, σ is the Stefan–Boltzmann constant, and a 0 is the distance of the Earth from the Sun [69].
Sunlight from the Earth is reflected or absorbed by the satellite and generates excess heat. The total irradiance E ABS absorbed by the solar cell on the satellite can be calculated as follows:
where T AR is the transmittance of the anti-reflective coating of the PV cell, η is the efficiency of the cell, A BULK is the absorbance of the bulk cell, and α is the albedo of the Earth (a diffuse reflection of solar radiation from the Sun) [50].
Stability and Degradation of Structures
From the text above, it is clear that GaAs cells are used in devices where the emphasis is on considerable performance and stability. For probes, it is assumed that GaAs cells will no longer be serviced or changed. For HCPV systems, their operation is expected even under extreme conditions, as they are highly stressed by temperature. Among other things, these conditions occur in space, not only in high temperatures but also in low temperatures.
An extensive study using several methods on a single-junction GaAs cell was conducted by Papež et al., which dealt with the degradation of GaAs cells over the past few years. Degradation after thermal processing [70,71], after cooling [6], after exposure to gamma radiation [50,72], and after exposure to broadband radiation was studied [73]. An unstressed sample was also observed, and defects and contamination after fabrication were examined [74].
During thermal heating, the samples were kept at 350 ∘ C for 240 min. The measurement was performed even with a short-term 30 min stress, when a temperature of up to 420 ∘ C was chosen. In both cases, the samples were shown to be functional, but the decrease in performance was noticeable, which can be seen from several parameters in Table 2. At a temperature of 420 ∘ C. there was already a considerable failure rate, and this could be considered a short-term limit value. The occurrence of surface defects and an evident change in morphology were obvious in Figure 9. However, it can be expected that the loss of solar cell performance is not only caused by a different surface structure but also by internal degradation processes [70,71,75].
On the contrary, after cooling in vacuum up to − 120 ∘ C. the changes on the GaAs-based PV cell surface were also measured in the form of reflectance. Reflectance was measured outside the vacuum chamber, where minimal differences were observed. It was mentioned that a significant decrease in the power of the solar cell could be affected by a negative thermal coefficient [6].
Papež et al., also in 2020, extensively studied the degradation of cells depending on gamma rays irradiation using a Cobalt-60 emitter when a dose of 500 k Gy was applied. The measurements took place within the electrical, optical, chemical, and structural characterization framework, which complemented each other. After a high irradiation dose, the solar cell worked without problems, but the efficiency decreased (fill factor decreased from 0.72 to 0.48). In addition to changes in morphology, it was discovered that after irradiation, elements that are part of the ARC diffused deeply into the material. The difference in the top thin layers is indicated in Figure 10. This phenomenon could cause a loss of cell performance [50,72]. Other extensive studies are underway by many authors on the radiation of either electrons [12,77] or protons [78,79].
Similarly, but on a smaller scale, Ti and Al atoms originating from anti-reflective layers migrated when the solar cell was spot irradiated with a supercontinuous laser with a power of 188 mW and a spectral range of 450 to 2400 nm. Here, the measurement was performed over a period of 67 days. The performance of the PV cell was also examined in real-time during the measurement of the sample. Interestingly, the degradation was not linear—there was a slight increase in efficiency at 42 days of irradiation in Figure 11, which could be due to the appearance of deep donor level centers (DX centers) [73]. The exact values from the measurement corresponding to Figure 11 are also added in Table 3.
Using the electron beam-induced current method (EBIC) Papež et al. also examined subsurface defects in the GaAs cell, where they found electrically active impurities affecting the pn junction during a cross-sectional view as illustrated in Figure 12. As the bias increased, there was gradual tunneling of electrons. However, this phenomenon did not have a permanent effect [74].
Many defects and impurities during imperfect fabrication can occur, and it is not always easy to eliminate them. It can be, for example, the fill factor and voltage loss caused by shunt or series resistance; interface recombination loss caused by lattice mismatching defects; bulk recombination loss caused by various defects, dislocations, and impurities; optical loss caused by poor ARC texture; or surface recombination loss caused by surface states. Thus, it is necessary to produce the best possible high-quality epitaxial growth, perfect lattice-matching layers, and ARC [80].
Conclusions
In this review, GaAs solar cells were discussed in many ways. In terms of use, their construction but also degradation were examined. As is known, these solar cells can be used in combination with several thin layers of other semiconductors with different bandgaps, such as AlGaAs, InP, GaInP, InGaAs, InGaP, and others. GaAs-based thin-film technology is over 50 years old and constantly evolving. To date, no successful challenger has been found to achieve such a high efficiency, which currently stands at 47.1% with the concentrator. Tripe-junction constructions have become a standard today, but experimentally, there are also constructions with seven or even eight layers. However, the question arises of the technical complexity, price, and meaning of using such a construction. How far can we go?
The solution may be to use different or new, more precise, and less demanding growing manufacturing processes and grown concepts. It has been reported from many publications that the most powerful solar cells use IMM. Another way may be to use concentrators, with which the record, as mentioned above was achieved. For this reason, part of this work was devoted to concentrators, as they are often combined with multilayer GaAs cells. Even here, there is a current development for excellent efficiency, hybridization, or miniaturization.
Miniaturization of concentrators can be used (and already is used) in space technologies, where GaAs cells make the most sense in terms of their good resistance to radiation and their ability to withstand very high-temperature fluctuations. Therefore, it is essential to FOCUS not only on the effectiveness of the PV cell but also on its ability to resist degradation even in inhospitable conditions.
If we summarize the above overview of the past and present state, GaAs solar cells will not have a worthy challenger in many ways for some time to come. However, there are still many reasons to improve and drive their development forward.
Author Contributions
Conceptualization, N.P.; software, N.P.; validation, Ş.Ţ. and R.D.; formal analysis, J.K.; investigation, N.P.; resources, N.P., J.K. and R.D.; writing—original draft preparation, N.P. and R.D.; writing—review and editing, J.K.; visualization, N.P.; supervision, Ş.Ţ.; project administration, N.P. and Ş.Ţ.; funding acquisition, Ş.Ţ. All authors have read and agreed to the published version of the manuscript.
Funding
Research described in the paper was financially supported by the Ministry of Education, Youth and Sports of the Czech Republic under the project CEITEC 2020 (LQ1601), by the Internal Grant Agency of the Brno University of Technology, grant No. FEKT-S-20-6352. Part of the work was carried out with the support of CEITEC Nano Research Infrastructure supported by MEYS CR (LM2018110).
Conflicts of Interest
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
Sample Availability
Samples of single-junction GaAs-based solar cell are available on demand from Nikola Papež. E-mail: [email protected].
Abbreviations
The following abbreviations are used in this manuscript:
| AFM | Atomic Force Microscope |
| AM0 | Air Mass at zero atmosphere |
| ARC | Anti-Reflection Coating |
| CPV | Concentrator Photovoltaics |
| CSOC | Concentrator Standard Operating Conditions |
| CSTC | Concentrator Standard Test Conditions |
| EBIC | Electron Beam-Induced Current |
| HALE | High Altitude Long Endurance |
| HCPV | High-Concentration Photovoltaics |
| HEMT | High Electron Mobility Transistor |
| LCPV | Low Concentration Photovoltaic |
| MBE | Molecular Beam Epitaxy |
| MOVPE | Metal Organic Vapor Phase Epitaxy |
| MPCV | Multipurpose Crew Vehicle |
| MPP | Maximum Power Point |
| NASA | National Aeronautics and Space Administration |
| NREL | National Renewable Energy Laboratory |
| PV | Photovoltaic |
| SEM | Scanning Electron Microscope |
| SOE | Secondary Optical Element |
| SoG | Silicone-on-Glass |
| IMM | Inverted Metamorphic |
| IUPAC | International Union of Pure and Applied Chemistry |
| IR | Infrared |
| ISE | Institute for Solar Energy Systems |
| UAV | Unmanned Aerial Vehicles |
| UHV | Ultra-High Vacuum |
| UV | Ultraviolet |
References
Figure 1. (A) Stacks of discrete holographic elements (a single stack is described in part (C)) generate four spectral bands coupled into one of four dual-junction solar cells, including GaAs. Part (B) shows the volume phase hologram of thickness d with fringes representing the refractive index with periodicity L. tilted to the grating normal by angle ϕ, where incident light is split into diffracted orders S i [9].
Figure 1. (A) Stacks of discrete holographic elements (a single stack is described in part (C)) generate four spectral bands coupled into one of four dual-junction solar cells, including GaAs. Part (B) shows the volume phase hologram of thickness d with fringes representing the refractive index with periodicity L. tilted to the grating normal by angle ϕ, where incident light is split into diffracted orders S i [9].
Thin-Film Solar Panels: An In-Depth Guide | Types, Pros Cons
When talking about solar technology, most people think about one type of solar panel which is crystalline silicon (c-Si) technology. While this is the most popular technology, there is another great option with a promising outlook: thin-film solar technology.
Thin-film solar technology has been around for more than 4 decades and has proved itself by providing many versatile and unique applications that crystalline silicon solar cells cannot achieve. In this article, we provide you with a deep review of this technology, the types of solar panels, applications, and more.
Overview: What are thin-film solar panels?
Thin-film solar panels use a 2 nd generation technology varying from the crystalline silicon (c-Si) modules, which is the most popular technology. Thin-film solar cells (TFSC) are manufactured using a single or multiple layers of PV elements over a surface comprised of a variety of glass, plastic, or metal.
The idea for thin-film solar panels came from Prof. Karl Böer in 1970, who recognized the potential of coupling thin-film photovoltaic cells with thermal collectors, but it was not until 1972 that research for this technology officially started. In 1980, researchers finally achieved a 10% efficiency, and by 1986 ARCO Solar released the G-4000, the first commercial thin-film solar panel.
Thin-film solar panels require less semiconductor material in the manufacturing process than regular crystalline silicon modules, however, they operate fairly similar under the photovoltaic effect. This effect causes the electrons in the semiconductor of the thin-film PV module to move from their position, creating an electric flow, that can be harnessed into electricity through an external circuit.
Thin-film solar panels are manufactured using materials that are strong light absorbers, suitable for solar power generation. The most commonly used ones for thin-film solar technology are cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous silicon (a-Si), and gallium arsenide (GaAs). The efficiency, weight, and other aspects may vary between materials, but the generation process is the same.
What are the different types of thin-film solar technology?
There are several types of materials used to manufacture thin-film solar cells. In this section, we explain the different types of thin-film solar panels regarding the materials used for the cells.
Cadmium Telluride (CdTe) Thin-Film Panels
Cadmium Telluride (CdTe) thin-film solar technology was introduced to the world in 1972 by Bonnet, D. and Rabenhorst, H. when they evaluated a Cadmium sulfide (CdS)/CdTe heterojunction which delivered a 6% efficiency. The technology has been improved to reduce manufacturing costs and increase efficiency.
CdTe solar cells are manufactured using absorber layers comprising a p–n heterojunction, which combines a p-doped Cadmium Telluride layer and an n-doped CdS layer that can also be made with magnesium zinc oxide (MZO). To depose materials on the substrate, manufacturers use the vapor-transport deposition or the close-spaced sublimation technique.
On top of the absorber layer, CdTe thin-film solar cells include a Transparent Conductive Oxide (TCO) layer usually made with fluorine-doped tin oxide (SnO2:F) or a similar material. The electrical contact for these cells is made with zinc telluride (ZnTe), and the materials are placed over a metal or carbon-paste substrate.
CdTe thin-film solar panels reached a 19% efficiency under Standard Testing Conditions (STC), but single solar cells have achieved efficiencies of 22.1%. This technology currently represents 5.1% of the market share worldwide, falling second only under crystalline silicon solar panels that hold 90.9% of the market. The cost for CdTe thin-film solar panels rounds the 0.40/W.
Copper Indium Gallium Selenide (CIGS) Thin-Film Panels
The first progress for Copper Indium Gallium Selenide (CIGS) thin-film solar cells was made in 1981 when the Boeing company created a Copper Indium Selenide (CuInSe2 or CIS) solar cell with a 9.4% efficiency, but the CIS thin-film solar cell was synthesized in 1953 by Hahn, H. In 1995, researchers at the National Renewable Energy Laboratory (NREL) embedded Gallium into the CIS matrix to create the first Copper Indium Gallium Selenide (CIGS) thin-film solar cell with a reported efficiency of 17.1%.
Manufacturing for Copper Indium Gallium Selenide (CIGS) thin-film solar panels has improved throughout history. Currently, CIGS thin-film solar cells are manufactured by placing a molybdenum (Mo) electrode layer over the substrate through a sputtering process. The substrate is usually manufactured with polyimide or a metal foil.
The absorbing layer is manufactured by combining a p-n heterojunction. The P-doped layer is made with copper indium gallium selenide (CIGS), placed above the electrode, and the CdS n-doped buffer is formed by chemical-bath deposition.
To protect the absorbing layer of the CIGS thin-film solar panel, a layer of Intrinsic Zinc Oxide (i-ZnO) is placed above the CdS buffer. The materials are finally covered with a thick AZO compound layer made with Aluminium doped Zinc Oxide (Al: ZnO), acting as the TCO layer to protect the cell.
The first CIGS thin-film solar panel manufactured by NREL reported a 17.1% efficiency, but the most efficient one ever created reported an efficiency of 23.4% and was made by Solar Frontier in 2019. The CIGS technology could be even more promising in the future since these materials can achieve a theoretical efficiency of 33%.
CIGS modules are not as popular for regular applications, being mostly used for space applications due to their resistance to low temperatures and great performance under low-intensity light conditions found in space. The cost is relatively more expensive than for other technologies, with a current price slightly above 0.60/W, but future manufacturing generations promise to reduce the cost for these panels.
While CIGS thin-film solar panels have not become as popular as CdTe panels in the market, CIGS technology still holds 2.0% of the PV market share. Considering that thin-film solar modules only hold around 10% of the market, This is still quite popular as a thin-film solar technology.
Amorphous Silicon (a-Si) Thin-Film Panels
The first observation of doping in Amorphous Silicon (a-Si) was achieved in 1975 by Spear and LeComber, a year later in 1976 it was demonstrated that Amorphous Silicon (a-Si) thin-film solar cells could be created. Great expectations have surrounded this technology, but the material represents several challenges like weak bonds, a relatively poor efficiency, and several others.
Unlike other thin-film solar panels, amorphous silicon (a-Si) modules do not include an n-p heterojunction, but a p-i-n or n-i-p configuration, which differs from the n-p heterojunction by adding an i-type or intrinsic semiconductor. There are two routes to manufacture amorphous silicon (a-Si) thin-film solar panels, by processing glass plates or flexible substrates. Efficiency for a-Si solar cells is currently set at 14.0%.
Disregarding the route taken to manufacture amorphous silicon (a-Si) thin-film solar panels, the following steps are part of the process:
First, the substrate is conditioned, the TCO and back reflector are placed under the deposition process, and then thin hydrogenated amorphous silicon (a-Si:H)-based layers are placed onto the electrodes, and the cells are connected in a monolithic series via laser scribing and silicon layers. The module is finally assembled and encapsulated, applying framing and electrical connections.
While manufacturing amorphous silicon (a-Si) requires an inexpensive material in low quantities, the price is relatively expensive, since the conductive glass for these panels is expensive and the process is slow, making the total cost of the panel to be set at 0.69/W. This technology currently holds 2.0% of the retail market for PV modules.
Gallium Arsenide (GaAs) Thin-Film Panels
The first Gallium Arsenide (GaAs) thin-film solar panel was made by Zhores Alferov and his students in 1970. The team persisted to create the gallium arsenide semiconductor, until they made a breakthrough in 1967, three years later they created the first gallium arsenide (GaAs) solar cell. Around 10 years later in 1980, the technology was being researched for specific applications like spaceships and satellites.
The manufacturing process for GaAs thin-film solar cells is more complex than for regular thin-film solar cells.
The first step is to grow the material. During this step, GaAs buffers are grown on Si substrates by being submitted to several temperature changes and different chemical processes, to finally create the layers for the cell.
After the GaAs buffer grows, the substrate is processed for the fabrication of the cell. The first step is to deposit a Platinum (Pt)/Gold (Au) layer (10/50 nm) which will serve as the bonding material and electrode for the GaAs solar cell, and then a bonding process is performed on the substrate.
After the bonding process is completed, the GaAs epitaxial layer that grew on the Si substrate is placed over the new substrate. To complete the assembly process a Pt/Titanium (Ti)/Pt/Au layer of 20/30/20/200 nm is deposited on the top contact layer through electron beam evaporation.
Since GaAs PV cells are multijunction III-V solar cells composed of graded buffers, they can achieve high efficiencies of up to 39.2%, but the manufacturing time, cost for the materials, and high growth materials, make it a less viable choice for terrestrial applications. The rated efficiency for GaAs thin-film solar cells is recorded at 29.1%.
The cost for these III-V thin-film solar cells rounds going from 70/W to 170/W, but NREL states that the price can be reduced to 0.50/W in the future. Since this is such an expensive and experimental technology, it is not mass-produced and is mainly destined for space applications, holding the lowest market share.
Thin-film vs. Crystalline silicon solar panels: What’s the difference?
Before comparing the different types of thin-film solar panels against crystalline silicon solar panels (c-Si), it is important to remark that there are two main types, monocrystalline silicon (mono c-Si) and polycrystalline silicon (poly c-Si) solar panels.
In this section, we compare several aspects of both types of crystalline silicon solar panels against the different types of thin-film solar panels.
Thin-film solar panels have many interesting applications, and they have been growing in the last decade. Below you will find some of the most popular applications for thin-film.
Building-Integrated Photovoltaics
One application starting to become widely popular worldwide is the Building-Integrated Photovoltaic (BIPV) highly dependent on thin-film solar technology. There are two main branches of this technology, solar shingles or solar roof tiles, and solar Windows or solar glass.
The goal for both applications is to provide the means to keep aesthetics for homes and buildings while allowing the possibility of solar power generation. This technology integrates thin-film solar technology to provide a certain generation efficiency, which can be used just like with regular c-Si solar panels.
Space applications
One of the most important applications for thin-film solar technology, specifically Copper Indium Gallium Selenide (CIGS) and Gallium Arsenide (GaAs) technology is the space applications. The technology provides many advantages like being extremely lightweight, highly efficient, having a wide temperature of operation range, and even the damage resistance against radiation, making it ideal for these applications.
Rooftop of vehicles and marine applications
One common application for thin-film solar panels is the installation of flexible PV modules on vehicle rooftops (commonly RVs or buses) and the decks of boats and other vessels. This application allows the installation of modules on curved surfaces, provides solar power generation while keeping practicality and aesthetics for the vehicles and vessels.
Portable applications
An advantage of thin-film solar technology is its portability and size. The technology has been installed for years in calculators, but with much improvement, now there is a possibility of having solar power in remote locations with foldable solar panels, solar power banks, solar-powered laptops, and more.
Large-scale applications
Due to its versatility, an important FOCUS of thin-film solar technology is commercial applications. While c-Si solar modules hold the largest market share, efficiency for thin-film solar panels is growing and manufacturing processes are becoming cheaper, which could lead to thin-film solar panels becoming the norm for most installations.
Another important FOCUS for thin-film solar panels is the industrial level applications, specifically at the utility scale. Since thin-film solar panels degrade at a much slower pace, they offer a potential alternative to the traditional c-Si solar panels, sometimes providing a better investment over time.
Rounding up: Pros and cons of thin-film solar panels
Thin-film solar panels have many pros, while only holding a few cons to them. These are the most important pros and cons of this technology.
Pros
- Higher resistance to degradation.
- Lower thermal losses at extreme temperatures due to the low-temperature coefficient.
- High efficiency for most technologies (CdTe, CIGS, and especially GaAs)
- Ideal for portable and BIPV applications.
- Promising research and development with much more ground to cover.
- Requires less material to create PV modules.
- Thin-film solar panels are lighter than c-Si PV modules.
Cons
- Higher retail cost.
- Less availability in the market.
- installation space is required to achieve the same generation capacity as c-Si modules (Except for GaAs PV modules).
Final Words
Thin-film solar technology might not be as popular as crystalline silicon, but it has an incredibly promising future. This technology opens possibilities that are not available for c-Si panels, like BIPV applications, portable modules, and even high-efficiency space applications with CIGS and GaAs PV modules.
While c-Si technology will probably keep having the largest market share due to its currently high rated efficiency, low manufacturing prices, and other pros attached to it, thin-film technology is still a valuable option to consider. As a matter of fact, the market share for thin-film solar has grown in the last decade, and it could keep it up in the following one.
With further research and breakthroughs for thin-film solar cells, this technology could be adapted to even more applications in the future and potentially increase its market value not only in large-scale applications but also in small commercial and residential sectors in the next 10 years.
Thin-film Solar Panels: What You Need to Know！
What are Thin-film Solar Panels?
Thin-film solar panels are pretty thin with each layer of only 1 micron thick, they are even thinner than hair.
Like traditional monocrystalline or polycrystalline solar panels, thin-film solar panels can convert light energy into electricity through the photovoltaic effect. However, a thin-film solar cell is much more lightweight and flexible to install. Its surface is composed of many light-absorbing films that combine to form a film that is about 300-500 times thinner than standard silicon, which is the outstanding advantage of thin-film solar panels.
It is no exaggeration to say that thin-film solar panels are the lightest panels available today, with each cell consisting of three main components: the photovoltaic material, the conductive sheet, and the protective layer. In addition to the slim internal design, thin-film solar panels are also extremely durable, as we will expand on in the next article.
How Thin-film Solar Cells Are Made?
Thin-film solar panels are manufactured through a process that involves depositing one or more thin layers of photovoltaic (PV) material onto a supporting substrate. This substrate can be made of various materials such as plastic, glass, or metal. The production of thin-film solar panels typically consists of three main components:
Photovoltaic Material:
The photovoltaic material is the active layer responsible for converting sunlight into electricity. It is typically composed of various semiconductor materials, such as amorphous silicon (a-Si), cadmium telluride (CdTe), or copper indium gallium selenide (CIGS). Each of these materials has unique properties that affect the efficiency and performance of the thin-film solar panel.
Conductive Sheet:
The conductive sheet serves as the electrical contact for the thin-film solar panel. It enables the flow of electricity generated by the photovoltaic material. Commonly, transparent conductive oxide (TCO) layers made of materials like indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) are used as the conductive sheet.
Protective Layer:
The protective layer safeguards the thin-film solar panel from external factors such as moisture, dust, and physical damage. It acts as a barrier, preventing the degradation of the photovoltaic material and ensuring the longevity and durability of the panel. Materials like glass or transparent polymers are often utilized as the protective layer.
The specific manufacturing process may vary depending on the type of thin-film technology employed, but the fundamental principle remains the same. Through precise deposition techniques and careful layering of materials, thin-film solar panels are created, offering a flexible and lightweight alternative to traditional crystalline silicon solar panels.
Types of Thin-film Solar Panels
They’re four common types of thin-film solar panels typically used in outdoor applications: copper gallium indium diselenide (CIGS), cadmium telluride (CdTe), amorphous silicon (a-Si), and gallium arsenide (GaAs) solar panels. Now, let’s take a look at these different thin-film solar panels one by one.
Copper indium gallium selenide (CIGS) solar panels
CIGS solar panel is versatile manufactured by various processes, and implemented in different forms. In addition, the CIGS solar panel is more appealing as a highly efficient alternative to large commercial solar modules. Some companies are using this for commercial use, which has proved to work well. BougeRV has also released CIGS solar panels for you to install easier and use more happier.
CIGS solar panels are not popular for conventional applications and are mainly used in space applications because of their low-temperature resistance and their excellent performance under low-intensity light conditions in space. The cost is relatively high compared to other technologies, currently priced at slightly more than 0.60/W, but future manufacturers promise to reduce the cost of these panels. Although CIGS thin-film solar panels are not yet as popular in the market as CdTe panels, CIGS technology still accounts for 2.0% of the PV market share. This is still quite popular as thin-film solar technology, considering that thin-film solar modules hold only about 10% of the market share.
Composition of Copper Indium Gallium Selenide (CIGS) thin-film solar cell. Source: SOLAR ENERGY TECHNOLOGIES OFFICE
Cadmium telluride (CdTe) solar panels
CdTe is the second most common photovoltaic material after silicon, and CdTe cells can be manufactured using low-cost manufacturing processes. While this makes CdTe solar panels a cost-effective alternative, they are still not as efficient as silicon solar panels.
CdTe solar cells are fabricated using an absorber layer containing a p-n heterojunction that combines a p-doped CdTe layer with an n-doped CdS layer, which can also be made from magnesium zinc oxide (MZO). To deposit the material on the substrate, manufacturers use vapor-phase transport deposition or proximity sublimation techniques.
Composition of CdTe thin-film solar cells. Source: SOLAR ENERGY TECHNOLOGIES OFFICE
Amorphous silicon (a-Si) solar panels
Amorphous silicon (A-SI) is a kind of allotropic amorphous silicon, which is the perfect thin-film technology so far. Thin film silicon substitutes traditional wafer (or block) crystalline silicon. A-si is attractive as a solar cell material because it is an abundant, non-toxic material. It requires lower processing temperatures and is capable of scalable production on flexible, low-cost substrates that require little silicon.
Although a small amount of inexpensive material is required to manufacture amorphous silicon (a-Si), it is relatively expensive because the conductive glass for these panels is expensive and the process is slow, making the total cost of the panels set at 0.69/W. This technology currently accounts for 2.0% of the retail PV module market.
Schematic of amorphous silicon (a-Si) cell structure. Source: Inorganic photovoltaic cells: Operating principles, technologies, and efficiencies. review by Karzazi, Y., and Arbouch, I.
Gallium arsenide (GaAs) solar panels
Gallium arsenide (GaAs), an III-V direct bandgap semiconductor, is a very common material used in monocrystalline thin-film solar cells.GaAs solar cells have been among the highest-performing thin-film solar cells due to their superior thermal performance and high efficiency. As of 2019, single-crystal GaAs cells have the highest solar cell efficiency of all single-junction solar cells, with an efficiency of 29.1%.
Because GaAs PV cells are multi-junction III-V solar cells consisting of a hierarchical buffer layer, they can achieve high efficiencies of up to 39.2 percent, but manufacturing time, material costs, and high growth materials make them a less viable option for terrestrial applications. GaAs thin-film solar cells are rated at a record 29.1 percent efficiency.
The cost of these III-V thin-film solar cells ranges from 70/watt to 170/watt, but NREL says the price could be reduced to 0.50/watt in the future. Because this is such an expensive and experimental technology, it is not in mass production and is used primarily for space applications, where it has the lowest market share.
Schematic diagram of the GaAs SJ solar cell. Source: Single-material zinc sulfide bi-layer antireflection coatings for GaAs solar cells by Woo, J. et al
Features of Thin-film Solar Panels
Thin-film solar panels possess unique features that set them apart. One notable characteristic is their high flexibility and lightweight design. These panels are manufactured by depositing one or more thin layers of photovoltaic material onto a substrate, resulting in a unified appearance.
Compared to conventional silicon panels, thin-film solar panels offer easier installation and require less effort. Their lightweight nature simplifies the mounting process, making them a convenient choice for various applications.
Another advantage of thin-film solar panels lies in their reduced emissions during the manufacturing process, attributed to their low silicon content. This environmental benefit aligns with the increasing demand for sustainable energy solutions.
One significant use case for thin-film solar panels is their suitability for uncommon and uneven surfaces, such as RVs and yachts. Unlike regular rigid solar panels, thin-film variants can easily adapt to these non-traditional installations. This flexibility makes them highly sought after in such scenarios.
In conclusion, thin-film solar panels offer the combined benefits of being lightweight, easy to install, and ideal for use on uneven surfaces like RVs and boats. Their flexibility and environmental advantages make them a compelling choice for those seeking versatile and sustainable solar solutions.
What are Thin Film Solar Panels Used for?
Thin-film solar panels find diverse applications in areas where traditional photovoltaic cells may not be suitable. Their flexibility allows them to be used on curved surfaces of buildings, cars, and even clothing, enabling the generation of power in unique ways.
In institutional and commercial buildings with wide roofs and open areas, thin-film solar panels are an excellent option due to their requirement for a larger mounting area. Unlike heavy rigid solar panels, thin-film variants can be installed on roofs that may not have the capacity to bear the weight of traditional panels, offering a viable alternative.
Wooded regions also benefit from the use of thin-film solar panels. Their durability ensures that they continue to function even in the presence of panel penetration or damage. This feature makes them well-suited for RV owners and adventurers who venture into forested areas, providing peace of mind knowing that the panels can withstand potential challenges.
Furthermore, thin-film solar panels are ideal for powering small devices such as fan blades and Wi-Fi modems. Whether on an RV or a yacht, these panels can be mounted on the roof, enabling the generation of electricity for off-grid living or during outdoor excursions.
Considering their prevalence in RV and yacht applications, several types of thin-film solar panels are commonly seen in these contexts.
Types of photovoltaic cells
Photovoltaic cells or PV cells can be manufactured in many different ways and from a variety of different materials. Despite this difference, they all perform the same task of harvesting solar energy and converting it to useful electricity. The most common material for solar panel construction is silicon which has semiconducting properties. [2] Several of these solar cells are required to construct a solar panel and many panels make up a photovoltaic array.
There are three types of PV cell technologies that dominate the world market: monocrystalline silicon, polycrystalline silicon, and thin film. Higher efficiency PV technologies, including gallium arsenide and multi-junction cells, are less common due to their high cost, but are ideal for use in concentrated photovoltaic systems and space applications. [3] There is also an assortment of emerging PV cell technologies which include Perovskite cells, organic solar cells, dye-sensitized solar cells and quantum dots.
Monocrystalline Silicon Cell
The first commercially available solar cells were made from monocrystalline silicon, which is an extremely pure form of silicon. To produce these, a seed crystal is pulled out of a mass of molten silicon creating a cylindrical ingot with a single, continuous, crystal lattice structure. This crystal is then mechanically sawn into thin wafers, polished and doped to create the required p-n junction. After an anti-reflective coating and the front and rear metal contacts are added, the cell is finally wired and packaged alongside many other cells into a full solar panel. [3] Monocrystalline silicon cells are highly efficient, but their manufacturing process is slow and labour intensive, making them more expensive than their polycrystalline or thin film counterparts.
Figure 2. An image comparing a polycrystalline silicon cell (left) and a monocrystalline silicon cell (right). [4]
Polycrystalline Silicon Cell
Instead of a single uniform crystal structure, polycrystalline (or multicrystalline) cells contain many small grains of crystals (see figure 2). They can be made by simply casting a cube-shaped ingot from molten silicon, then sawn and packaged similar to monocrystalline cells. Another method known as edge-defined film-fed growth (EFG) involves drawing a thin ribbon of polycrystalline silicon from a mass of molten silicon. A cheaper but less efficient alternative, polycrystalline silicon PV cells dominate the world market, representing about 70% of global PV production in 2015. [3]
Thin Film Cells
Figure 3. A thin film solar panel composed of non-crystalline silicon deposited on a flexible material. [5]
Although crystalline PV cells dominate the market, cells can also be made from thin films—making them much more flexible and durable. One type of thin film PV cell is amorphous silicon (a-Si) which is produced by depositing thin layers of silicon on to a glass substrate. The result is a very thin and flexible cell which uses less than 1% of the silicon needed for a crystalline cell. [3] Due to this reduction in raw material and a less energy intensive manufacturing process, amorphous silicon cells are much cheaper to produce. Their efficiency, however, is greatly reduced because the silicon atoms are much less ordered than in their crystalline forms leaving ‘dangling bonds’ that combine with other elements making them electrically inactive. These cells also suffer from a 20% drop in efficiency within the first few months of operation before stabilizing, and are therefore sold with power ratings based on their degraded output. [3]
Other types of thin film cells include copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe). These cell technologies offer higher efficiencies than amorphous silicon, but contain rare and toxic elements including cadmium which requires extra precautions during manufacture and eventual recycling. [3]
High Efficiency Cells
Other cell technologies have been developed which operate at much higher efficiencies than those mentioned above, but their higher material and manufacturing costs currently prohibit wide spread commercial use.
Silicon is not the only material suitable for crystalline PV cells. Gallium arsenide (GaAs) is an alternative semiconductor which is highly suitable for PV applications. Gallium arsenide has a similar crystal structure to that of monocrystalline silicon, but with alternating gallium and arsenic atoms.
Due to its higher light absorption coefficient and wider Band gap, GaAs cells are much more efficient than those made of silicon. Additionally, GaAs cells can operate at much higher temperatures without considerable performance degradation, making them suitable for concentrated photovoltaics. GaAs cells are produced by depositing layers of gallium and arsenic onto a base of single crystal GaAs, which defines the orientation of the new crystal growth. This process makes GaAs cells much more expensive than silicon cells, making them useful only when high efficiency is needed, such as space applications. [3]
The majority of PV cells, including those discussed above, contain only one p-n junction of semiconductor material which converts energy from one discreet portion of the solar spectrum into useful electricity. Multi-junction cells have 2 or more junctions layered on top of each other, allowing energy to be collected from multiple portions of the spectrum. Light that is not absorbed by the first layer will travel through and interact with subsequent layers. Multi-junction cells are produced in the same way as gallium arsenide cells—slowly depositing layers of material onto a single crystal base, making them very expensive to produce, and only commercially viable in concentrated PV systems and space applications. [3]
Emerging Cell Technologies
Electricity can be produced through the interaction of light on many other materials as well. Perovskite solar cells, named after their specific crystal structure, can be produced from organic compounds of lead and elements such as chlorine, bromine or iodine. They are relatively cheap to produce and can boast efficiencies close to those of commercially available silicon cells but they are currently limited by a short lifespan. Organic solar cells consist of layers of polymers and can be produced cheaply at high volumes. These cells can be produced as a semi-transparent film, but suffer from relatively low efficiencies. Dye-sensitized solar cells can be produced using semiconducting titanium dioxide and a layer of ‘sensitizer’ dye only one molecule thick. These cells boast modest efficiencies but cannot withstand bright sunlight without degrading. Quantum dots utilize nanotechnology to manipulate semiconducting materials at extremely small scales. ‘Nanoparticles’ consisting of a mere 10,000 atoms can be tuned to different parts of the solar spectrum according to their size, and combined to absorb a wide range of energy. Although theoretical efficiencies are extremely high, laboratory test efficiencies are still very low. [3]