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Cutting-Edge Photovoltaic Panels and Solar Cells: Advancements in Materials and Technology
Photovoltaic Panels and Solar Cells play a pivotal role in harnessing solar energy and converting it into electricity. Over the years, advancements in materials and technology have revolutionized the efficiency, durability, and affordability of solar cells, making them a key player in the transition toward a cleaner and greener future.
State-of-the-Art Solar Cells and Panels
We will delve into the cutting-edge developments in photovoltaic panels and solar cells, highlighting the innovative materials and technologies that have shaped their evolution.
1. Basic Principles of Design and Operation of Solar Cells
To comprehend the progression of solar cell design and efficiency, we’ll explore the fundamental principles and physical parameters involved in converting solar energy into electricity. In Fig.1, we see the simplified structure of a Silicon-based p-n junction solar cell. This crucial solar cell component consists of a p-n junction formed between p-type Si (Silicon doped with Boron) and n-type Si (Silicon doped with Phosphorus). The structure is created by using p-type Si as the substrate and adding the n-type layer on top through diffusion or deposition.
To increase efficiency, three major opportunities are identified:
- Reduce Surface Recombination: The loss of photogenerated carriers can be curbed by minimizing surface recombination, leading to improved efficiency.
- Increase Absorbed Light: Strategies to increase the amount of absorbed light in the solar cell will enhance its effectiveness.
- Improve Carrier Collection: Ensuring efficient extraction and use of photogenerated carriers at both sides of the solar cell will contribute to higher efficiency.
2. Streamline Materials and Manufacturing Techniques
Crystalline Silicon remains the top choice for both commercial and residential solar panels, despite several competing options. Two types of crystalline Silicon are utilized in solar cell manufacturing: Monocrystalline Si and Polycrystalline Si.
The primary method for growing Monocrystalline Si is the Czochralski method, illustrated in Fig. 2(a). This process involves pulling a Si ingot from the silicon melt and has been the most widely used approach for obtaining single crystals.
Fig.2 (a). Mono-Si growth by the Czochralski method.
On the other hand, Polycrystalline Si is predominantly manufactured using the Direct Solidification method, as shown in Fig. 2(b). In this method, poly-Si (also known as multi-Si or mc-Si) is produced from cast square ingots by cooling and solidifying molten silicon. The resulting cells contain multiple crystals, leading to a less perfect structure compared to its mono-Si counterpart. Due to the presence of defects, polycrystalline solar cells produce less electricity and are thus less efficient than monocrystalline silicon (mono-Si) cells.
Fig.2 (b). Poly-Si casting by Directional Solidification.
Fig. 2(c) Illustrates the main steps involved in manufacturing PV panels from the raw Silicon material involving the two methods shown above.
Fig.2 (c). Main steps from row Si to PV panels.
In our discussion of cutting-edge solar cell technologies, we will also explore other materials such as CdTe and Perovskite, which have their optical Bandgap optimized to harness the solar light spectrum effectively.
3. Solar Cell Designs and Technologies
Since Crystalline silicon (c-Si) solar cells represent the most prevalent type in the market today, most of the recent advancement was made in silicon-based solar cells, which include PERC, TOPCon, HIT, IBC, and Bifacial designs. Due to its cost parity and higher efficiencies, PERC Solar Cell Technology has become the official new industry standard for solar PV projects enjoying an almost 75% market share among new PV installations. To understand the progression of solar cell designs we will start with traditional cells that feature Aluminum Back Surface Field (Al-BSF). It will provide us with enough background to apprehend the evolution of solar cell technology.
3.1. Aluminum Back Surface Field (Al-BSF)
Back surface field (BSF) consists of creating a high-low junction, i.e. p-p+ or n-n+ at the rear side of the solar cell (depending on the doping type of the substrate) that builds a field-effect passivation (Fig. 3). For industrial standard screen–printed p-type Si solar cells, the state-of-the-art rear side passivation is realized by the p-p+ junction using aluminum BSF. The aluminum BSF is optimized for industrial solar cells since it provides good contact, field-effect passivation, and reflection on the rear side in a very simple step. For high-efficiency Si solar cells, the Al BSF is partially used such as the implementation of a passivated rear with local Al BSF (PERC-type solar cell).
Fig.3. Aluminum Back Surface Filed Si Solar Cell.
3.2. Passivated Emitter and Rear Contact (PERC) Solar Cells
The PERC solar cell technology includes dielectric surface passivation that reduces electron surface recombination (Fig. 4). At the same time, the PERC solar cell reduces the semiconductor-metal area of contact and increases the rear surface reflection by including a dielectrically displaced rear metal reflector. This allows photons to be absorbed when going into the cell or out of it, and it also reduces heat absorption.
Fig.4. PERC Solar Cell structure.
The back capping layer need to cover the following qualities to increase the efficiency: optically, they need to work as a back reflector for long wavelength photons; mechanically, they need to be dense in order to act as a barrier for Al metal contact; electrically, they need to decrease the back surface recombination velocity and act as a negative charged back layer to reduce the well known parasitic shunting of p-type PERC cells. This highly efficient and improved version of c-Si technology results in PERC solar panels having a 0.86% or more increment in the efficiency of the solar cell. This provides several perks like reduced installation time, fewer space requirements, and cost reductions by requiring fewer wires, connectors, racks, and other components that you would require when installing the same PV capacity with traditional technology.
3.3 Tunnel oxide passivated contact (TOPCon) Solar Cell
Tunnel oxide passivated contact (TOPCon) solar cell technology is a new development with the potential to replace passivated emitter and rear contact (PERC) solar panels. The TOPCon solar cells are manufactured with an n-type crystalline silicon (c-Si) bulk layer because of its higher surface quality and it is coupled with a p+ emitter layer to create the p-n junction (Fig. 5).
Fig.5. TOPCon Solar Cell structure.
The TOPCon solar cell structure includes an ultra-thin silicon dioxide (SiO2) layer working as the tunnel oxide layer and replaces the back surface field layer with phosphorous-doped polycrystalline silicon (n+ Poly-Si) layer. These modifications have improved the efficiency by reducing the recombination process thanks to the passivation in the added layers.
3.4. Interdigitated Back Contact (IBC) Solar Cell
Interdigitated Back Contact (IBC) solar cell technology is one of the most innovative methods to have proven higher efficiencies using crystalline silicon (c-Si) cells. IBC technology surpasses PERC technology in its efficiency (26.7% vs 24.3%), but comes at a slightly higher price. The core component in the IBC solar cell is the n-type crystalline silicon (c-Si) wafer functioning as the absorbing layer (Fig. 6). Subsequently, an anti-reflective and passivation layer, often composed of SiO2, is applied to one or both sides of the solar cell.
Fig.6. IBC Solar Cell Structure.
A significant design enhancement for IBC solar cells involves incorporating a diffusion layer. This layer comprises interdigitated p-type emitter segments, allowing the implementation of rear-side metal contacts. The distinctive aspect of the IBC solar cell design is that all metal contacts are positioned on the rear side of the cell, thus eliminating shading materials on the front. This arrangement permits the installation of contacts across a broader area and reduces the series resistance of the cells. Furthermore, this setup enables separate optical and electrical optimizations, laying the groundwork for potential efficiency advancements in the future.
3.5. Heterojuniction (HJT) Solar Cell
Heterojunction (HJT) Solar Cell technology addresses prevalent constraints found in traditional photovoltaic (PV) modules. It mitigates issues such as recombination processes and enhances performance even in high-temperature environments. With a conversion efficiency of 26.7% for monofacial modules and exceeding 30% for bifacial modules, heterojunction stands out as one of the most proficient solar technologies available.
Fig.7. HJT Solar Cell Structure.
The absorber component within the heterojunction solar cell encompasses a layer based on c-Si wafer (Fig. 7 indicated in blue). This c-Si layer is sandwiched between two thin intrinsic (i) a-Si:H layers (depicted in yellow), while doped a-Si:H layers (colored in red and green) are positioned atop each respective a-Si:H (i) layer. The number of Transparent Conductive Oxide (TCO) layers depends on whether the HJT cell is monofacial or bifacial. In monofacial heterojunction cells, the backmost layer is a metal layer, serving as the conductor. During the light-absorption process, the initial photons that arrive are absorbed by the outermost a-Si:H layer, resulting in their conversion into electricity. The bulk of the incoming photons, however, undergo conversion within the c-Si layer, which boasts the highest solar conversion efficiency among the various materials within the cell. Eventually, the remaining photons reach the rear a-Si:H layer of the module, where they are ultimately transformed into electricity. This stepwise sequence ensures the high efficiency of up to 26.7% of monofacial HJT solar cells.
3.6. Bifacial Solar Cell
A Bifacial solar cell is one of the newest solar power innovations. It functions as a dual-sided energy generator, converting sunlight into electrical energy on both its upper and lower surfaces. The term “bifacial” originates from “bi-” (meaning two) and “facial” (pertaining to the face). Bifacial photovoltaic panels and solar cells are designed with solar cells on both the upper and rear sides of the structure (Fig. 8). The front captures incident sunlight while the back absorbs reflected light. More captured sunlight means greater solar cell efficiency compared with traditional solar cells.
Fig.8. Bifacial Solar Cell Structure.
The upper surface of each solar module is shielded by protective glass, while the opposite side can feature either glass or a transparent back sheet. This stands in contrast to conventional solar panel setups that employ opaque backings. As a result, bifacial solar cells can increase efficiency by 11% compared to a conventional solar panel system.
3.7. CdTe Thin Film Solar Cell
Following crystalline silicon, CdTe thin film solar cells constitute the second most prevalent photovoltaic (PV) technology within the global market, presently accounting for 5% of the overall share. CdTe thin-film solar cells offer a rapid and cost-effective manufacturing process, serving as a viable option to traditional silicon-based technologies. The peak efficiency achieved by a laboratory CdTe solar cell stands at 22.1%, credited to First Solar.
CdTe thin-film solar cells offer the following advantages: Effective Absorption due to adjustable CdTe bandgap energy within the range of 1.4 to 1.5 (eV), and Economical Production due to employed high-speed manufacturing technique.
Fig.9. CdTe Thin Film Solar Cell Structure.
The most common CdTe solar cells (Fig. 9) consist of a p-n heterojunction structure containing a p-doped CdTe layer matched with an n-doped cadmium sulfide (CdS) or magnesium zinc oxide (MZO) window layer. Typical CdTe thin-film deposition techniques include vapor-transport deposition and close-spaced sublimation. CdTe absorber layers are generally grown on top of a high-quality transparent conductive oxide (TCO) layer—usually fluorine-doped tin oxide (SnO2:F). Cells are completed using a back electrical contact—typically a layer of zinc telluride (ZnTe) followed by a metal layer or a carbon paste that also introduces copper (Cu) into the rear of the cell.
3.8. Perovskite Solar Cell
Perovskite solar cells have demonstrated remarkable advancements in recent years, experiencing rapid efficiency growth, evolving from approximately 3% in 2009 to surpassing 25% today. Perovskites hold promise of creating solar panels that could be easily deposited onto most surfaces, including flexible and textured ones. These materials would also be lightweight, cheap to produce, and as efficient as today’s leading photovoltaic materials, which are mainly silicon.
Despite their swift efficiency improvements, perovskite solar cells still face several hurdles that must be overcome before they can emerge as a commercially competitive technology.
Fig.10. Perovskite Thin Film Solar Cell Structure.
The most common Perovskite thin film solar cells (Fig. 10) consist of a Perovskite flat layer sandwiched between n-type (Electron conductor) and p-type (Hole conductor) layers. After light absorption, both charge generation, as well as charge extraction, occur in the Perovskite layer.
Perovskites exhibit the capacity to be adjusted in response to distinct colors across the solar spectrum through alterations in material composition. Various formulations have showcased impressive performance levels. This adaptability permits the pairing of perovskites with divergent absorber materials, yielding increased power output from the same device. This configuration is termed a tandem device architecture (Fig. 11). By incorporating multiple photovoltaic materials, tandem devices can potentially achieve power conversion efficiencies exceeding 33%, surpassing the theoretical cap of a single junction PV cell.
Fig.11. Perovskite on Silicon Tandem Solar Cell Structure.
Perovskite materials can be tailored to exploit segments of the solar spectrum that silicon PV materials struggle to utilize effectively. Consequently, they serve as excellent candidates for hybrid-tandem collaborations. Moreover, it’s feasible to blend two perovskite solar cells with distinct compositions to create a perovskite-perovskite tandem. Such tandems hold strong potential in sectors such as mobility, disaster response, and defense operations, as they can be shaped into lightweight, flexible devices with remarkable power-to-weight ratios.
In conclusion, by optimizing light absorption, charge carrier generation, and carrier collection, solar cells can achieve higher efficiency while keeping manufacturing costs down. These advancements play a crucial role in harnessing solar energy more effectively and sustainably.
4. State-of-the-Art Photovoltaic Panels and Technologies
Solar photovoltaic (PV) panels vary in type and size based on their composition (mono-crystalline, polycrystalline, thin film) and solar cell structures (PERC, TOPCon, IBC, HJT, thin film CdTe, or Perovskite). In addition, these panels need to meet specific criteria for optimal functionality, including maximizing power output, reliability during varying weather conditions that could cause shading or damage, and even aesthetic considerations. As a result, distinct panel categories have emerged to address these specific requirements, which we’ll discuss here.
4.1. Half-Cut Solar PV Panels
Half-cut cell panels provide several benefits over traditional solar cells. Most important of them are increased partial shading tolerance and reduced resistivity losses. Performance-wise, half-cut cells can increase panel efficiencies by a few percentage points. Generally, modules with 60 solar cells include three substrings of 20 cells in series (Fig. 12). The equivalent half-cut solar cell modules have 120 solar cells, divided into six substrings of 20 cells. Each side of the half-cut solar panel has three substrings in parallel, with both sides also connected in parallel.
Fig.12. Half-Cut Solar PV Panels vs Standard PV Panels.
This type of wiring allows panels built with half-cut cells to lose less power when a single cell is shaded because a single-shaded cell can only eliminate a sixth of the total panel power output. As a result, Half-cut cells are more resistant to the effects of shade than traditional solar cells. Also, by cutting solar cells in half, the current generated from each cell is halved, and lower current flowing leads to lower resistive losses and subsequently to higher power output.
Solar cells with improved efficiency like PERC and bifacial can be used to manufacture half-cut solar cells to push forward the efficiency of the PV panels. The first half-cut cell solar panels were introduced in 2014 by REC Solar. Aside from REC, many manufacturers have introduced half-cell modules. Trina Solar, Q CELLS, JinkoSolar, and LONGi Solar are just some of the large solar panel manufacturers.
4.2 Bifacial Solar PV Panels
Bifacial Solar PV Panels have been designed to harness the advantages of bifacial solar cells by allowing light to penetrate from both sides. The mounting hardware for these panels is meticulously engineered to minimize shading, featuring slender support rails and vertical supports only at the corners. In a bifacial solar panel system, the upper solar cells are oriented toward the sun, directly capturing specific wavelengths of incident sunlight. Meanwhile, the lower solar cells absorb light that is reflected from the ground. Optimal reflection occurs with lighter colors such as white or silver, which can be replicated by applying these shades to surfaces like roofs or concrete areas beneath the panels.
Fig. 13a. Angled Installation of Bifacial Solar PV Panel.
Fig. 13b. Vertical Installation of Bifacial Solar PV Panel.
In contrast to monofacial solar panels, which are typically positioned parallel to surfaces like rooftops, bifacial panels exhibit higher energy output when angled away from the roof or ground at varying degrees (Fig. 13a). In such tilted setups, considerable reflection takes place. Bifacial solar panels excel at capturing sunlight from diverse angles, as sunlight reflects off objects at numerous angles. This attribute empowers them to perform effectively even on overcast days, a scenario where monofacial solar cells face greater limitations. An alternative installation approach involves placing bifacial panels vertically (Fig. 13b), resulting in two energy production peaks each day.
Fig. 13c. Bifacial Solar PV Panel’s Daily Power Distribution.
During these peaks, sunlight’s reflection on the opposite side of the panels further contributes to energy generation (Fig. 13c). Notably, vertical setups are less susceptible to obstruction by snow or blown sand during inclement weather. Bifacial solar panels stand out due to their dual glass covers, which enhance their durability compared to conventional solar panel systems. Consequently, warranties for bifacial panels often extend for more than 5 years beyond those of conventional panels, providing a lifespan of 30+ years as opposed to the typical 20–25 years. Some of the leading Bifacial Solar PV Panels manufacturers today are Trina, REC Solar, Canadian Solar, Longi, Jinko, and others.
4.3. All-Black Solar PV Panels
All-Black Solar PV Panels‘ biggest advantage is the aesthetic (Fig. 14). Traditional panels use white backsheets and silver frames, while All-Black modules use black backsheets and black frames. They’re manufactured the same way through the same processes, except black adhesives may be used around junction boxes and other electronics on All-Black modules. The major difference between the two is their efficiency ratings. All-Black modules run a bit hotter and offer fewer opportunities for reflected light absorption, so their efficiencies are about 0.5% lower.
Fig. 14. All-Black Solar PV Panel.
The use of higher-efficiency solar cells in All-Black modules evens out this major difference. To lower the nominal operating cell temperature all busbars and electrical connections are moved to the back of the solar cells. Along with a flexible, conductive backsheet it allows a more effectively spread of the heat produced by a module.
Among the leading manufacturers of All-Black Solar PV Panels are Aptos Solar Technology, Silfab Solar, SunPower, REC Solar, Canadian Solar, and some others.
4.4. Thin Film Solar Panels
Cadmium Telluride (CdTe) -based solar PV Panels have taken the forefront as the predominant commercially adopted thin film photovoltaic technology (Fig. 15a). They inherently exhibit superior temperature coefficients, energy yield, and degradation rates when compared to silicon (Si) technologies.
A common strategy for thin film photovoltaic (PV) technologies, involves the use of monolithic integration of assembling cells into modules. In this method, thin layers with electronic capabilities are applied onto cost-effective substrates like glass. The interconnected cells are then created through follow-up processes involving back contacts and scribing. This stands in contrast to wafer-based approaches, such as silicon (Si) technologies, where individual wafers are separately treated to form cells, which are later joined through soldering and assembled into modules.
Fig. 15a. CdTe Thin-Film Solar PV Panel.
The prevailing technology for CdTe-based modules presently centers around a thin film absorber layer of p-type doped CdTe or graded CdSe1-xTex (CdSeTe) (Fig. 15b). These polycrystalline layers possess a minimal bandgap, approximately 1.5 eV for CdTe and ∼1.4 eV for CdSeTe, respectively. These absorber layers are produced in a superstrate arrangement, situated on glass to allow light to pass through. To ensure enduring reliability and reduced degradation in most commercial modules, an additional sheet of glass and edge seals are used to hermetically encase the module. This configuration presents a significant opportunity for the development of bifacial modules featuring transparent back electrodes.
Fig. 15b. CdTe Thin-Film Solar PV Panel Structure.
Although the other thin-film PV panel technologies exist based on CIGS, amorphous Si, and organic solar cells, each having its own niche in the application, the CdTe thin-film technology was commercialized on an unbeatable scale due to First Solar’s extensive investments and installation. With over 30 GW peak (GWp) of installed CdTe-based modules globally, numerous companies are engaged in their production such as CTF Solar, Toledo Solar, and some others. These modules are being dispatched with efficiencies of up to 18.6%, while laboratory cell efficiency surpasses 22%.
5. Cutting Edge Solar Panels and their Manufacturers
| Company | Modules | Technology | Efficiency | Output, W |
| SunPower | SunPower M-Series Blk | IBC, All Black | 22.00% | 425 |
| Panasonic | EVERVOLT® SOLAR MODULE SERIES | HIT Half-Cut | 22.16% | 410 |
| Canadian Solar | TOPBiHiKu7 | TOPCon Bifacial N-type Dual Cell | 21.57% | 670 |
| REC Solar | REC ALPHA Pure-R Series | HIT, Bifacial, All Black, Half-Cut | 22.23% | 430 |
| Trina | 210R N-TOPCon Glass Bifacial | TOPCon, Bifacial, Half-Cut | 21.82% | 590 |
| Longi | Hi-MO 5 | PERC, Bifacial, Half-Cut, Ga-doped Wafer | 21.52% | 550 |
| Jinko | EAGLE 72 G6B | TOPCon, Bifacial, Half-Cut | 22.45% | 580 |
| Q Cells | Q.PEAK DUO BLK ML-G10+ | PERC, Half-Cut (Dual Cell) All Black | 20.63% | 405 |
| First Solar | Series 7 TR1 | Thin film CdTe | 19.31% | 540 |
The evolution of photovoltaic panels and solar cells over the years has been truly remarkable. Advancements in materials and technology have significantly improved the efficiency, durability, and versatility of solar cells, making them an indispensable part of our renewable energy future. As research and innovation continue, we can expect even more groundbreaking developments, driving us closer to a cleaner and sustainable energy landscape powered by the sun.
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