In today's world, renewable energy is presented as a crucial solution for addressing environmental challenges and reducing dependence on fossil fuels. Among these, solar energy is recognized as one of the cleanest and most widely used renewable energy sources. Solar cells, which are the heart and brain of this technology, play a central role in converting solar energy into electricity.
The history of solar cell development dates back to the early 20th century. In 1839, French physicist Edmond Becquerel discovered the photovoltaic effect, which forms the basis of solar cell operation. Since then, various types of solar cells have been designed and developed, each with its unique features and applications. Solar cells can be classified into three generations, each with its specific characteristics. The first generation of solar cells, based on wafers, includes single-crystal silicon, polycrystalline silicon, and gallium arsenide cells. Silicon cells are among the most common and widely used types of solar cells. The theoretical efficiency of these cells is 29%, which has been practically achieved to about 25% at an industrial scale, but their production cost is also high. The second generation of solar cells, thin-film, is made using thin semiconductor materials such as amorphous silicon, cadmium telluride, and copper indium gallium selenide. Compared to silicon cells, these cells have lower efficiency but lower production costs. The third generation of solar cells includes organic and perovskite cells, among others, which are being developed with improved efficiency and reduced costs. Organic solar cells are made from organic materials and polymers. Although their efficiency is lower, their flexibility and light weight offer significant advantages for certain applications.
Perovskite solar cells have recently emerged as a promising option. These cells are made from perovskite materials with a specific crystal structure. Additionally, their relatively low production costs make them attractive for widespread applications.
In this article, we will examine the various types of solar cells and the characteristics of each. We will also discuss the advantages and challenges of each type of solar cell. This comprehensive review can help better understand solar energy's potential and future.
Generation One: Silicon Solar Cells
Silicon-based solar cells were the first generation of photovoltaic technology to enter the market, utilizing processing information and raw materials provided by the microelectronics industry. Silicon-based solar cells now account for over 80% of the global installed capacity and hold a significant market share. Due to their relatively high efficiency, these cells are the most commonly used.
The first generation of photovoltaic cells includes materials based on thick crystalline layers composed of silicon (Si). This generation is based on monocrystalline silicon, polycrystalline silicon (multi-crystalline), and gallium arsenide.
Features of First-Generation Photovoltaic Cells
- Monocrystalline Silicon Solar Cells (m-Si)
- Efficiency: 15% to 24%
- Lifespan: 25 years
- Advantages: Stability, high performance, long lifespan.
- Limitations: High manufacturing cost, greater temperature sensitivity, light absorption issues, deterioration of material quality over time.
- Read more: 6 Golden Tips About Monocrystalline Silicon Solar Cells
- Polycrystalline Silicon Solar Cells (p-Si)
- Efficiency: 10% to 18%
- Lifespan: 14 years
- Advantages: Simple production process, high profit margin, reduced silicon waste.
- Limitations: Lower efficiency, higher temperature sensitivity.
- Gallium Arsenide (GaAs) Solar Cells
- Efficiency: 28% to 32%
- Lifespan: 18 years
- Advantages: High stability, lower temperature sensitivity, better light absorption than m-Si, high efficiency.
- Limitations: Extremely expensive
General Method for Manufacturing First-Generation Solar Cells
As discussed in the article on solar cell structure, the first generation of photovoltaic cells is based on p-n junctions and primarily relies on monocrystalline or polycrystalline wafers. The production process for monocrystalline silicon solar cells involves growing polycrystalline silicon crystals from small monocrystalline silicon seeds and forming silicon ingots using the Czochralski process. These ingots are then sliced to create monocrystalline silicon wafers with a thickness of less than 180 microns. Finally, the production of solar cells uses these very thin wafers.
Monocrystalline materials are more widely used due to their higher efficiency compared to polycrystalline materials. Key technological challenges associated with monocrystalline silicon include stringent requirements for material purity, high material consumption during cell production, complex manufacturing processes, and the limited sizes of modules made from these cells.
The image below shows (a) the Czochralski process for producing monocrystalline ingots and (b) the directional solidification process for producing polycrystalline ingots.
Polycrystalline Silicon Ingots are produced by melting high-purity silicon and crystallizing it in a large crucible using the directional solidification method. In this process, there is no seed crystal for orientation and crystal formation as in the Czochralski process. As a result, multiple silicon crystals with different orientations are produced. One of the base materials for solar cells is polycrystalline silicon doped with boron (p-type). Nowadays, n-type polycrystalline silicon doped with phosphorus is used more commonly for higher-efficiency solar cells, though it faces more technical challenges compared to the production of p-type polycrystalline silicon, such as achieving uniform doping throughout the silicon block.
In the production of crystalline solar cells, at least six consecutive steps must be performed. These steps include doping or injecting impurities to create P-Type or N-Type semiconductors, diffusion, oxide removal, surface texturing to reduce light reflection, anti-reflective coating, and placing busbars and fingers for current collection, among others.
At the end of the process, the cell efficiency and other parameters are measured. The efficiency of photovoltaic cells depends on the quality of the materials used and the precision of the manufacturing processes.
The theoretical maximum efficiency for first-generation solar cells is estimated to be around 29.4%, and achieving an efficiency relatively close to this number occurred about two decades ago. Since the advent of silicon solar cells, their efficiency has increased from 6% in the early years to the current record of 26.1%, marking an improvement of 20.1%. Significant advances in the production of these cells include the introduction of TOPCon technology to reduce recombination rates and facilitate charge transport at the rear surface of the cell, as well as HJT technology to enhance light absorption and reduce recombination.
Second Generation: Thin-Film Solar Cells
Thin-film photovoltaic cells are made from thin semiconductor materials such as amorphous silicon, cadmium telluride (CdTe), and copper indium gallium selenide (CIGS) to provide a more affordable alternative to crystalline silicon cells.
Thin-film solar cells offer improved mechanical properties that are ideal for flexible applications, but this advantage comes with the drawback of reduced efficiency.
While first-generation solar cells are derived from the microelectronics world, the evolution of thin-film technology requires new growth methods, including the incorporation of other fields such as electrochemistry.
Third Generation: Thin-Film Solar Cells
- Amorphous Silicon (a-Si) Solar Cells
Efficiency: 5% to 12%
Lifespan: 15 years
Advantages: Cheap, readily available and abundant raw materials, non-toxic, high absorption coefficient
Limitations: Low efficiency, difficulty in selecting doping materials, short minority carrier lifetime
The latest type of cells classified as second generation are those that use amorphous silicon. Amorphous silicon (a-Si) solar cells are significantly the most common thin-film technology. Some types of amorphous silicon (a-Si) include amorphous silicon carbide (a-SiC), amorphous silicon-germanium (a-SiGe), microcrystalline silicon (μ-Si), and amorphous silicon nitride (a-SiN). In this technology, hydrogen is required for doping the layers, resulting in hydrogenated amorphous silicon (a-Si:H).
Typically, the gas-phase deposition technique is used to form a-Si photovoltaic cells with metal or gas as the base material.
- Cadmium Telluride/Cadmium Sulfide (CdTe/CdS) Solar Cells
Efficiency: 16% to 22%
Lifespan: 20 years
Advantages: High absorption rate, less material required for production
Limitations: Low efficiency, highly toxic cadmium, limited amount of Te, higher temperature sensitivity
Cadmium Telluride (CdTe) solar cells are also part of the second generation of photovoltaic cells. Due to their high spectral efficiency, the thickness of the absorbing layer can be reduced to about 1 micrometer without significant loss in efficiency, though this process can be time-consuming.
These ultra-thin cells are particularly attractive for flexible applications, such as Building-Integrated Photovoltaics (BIPV), due to their lighter weight.
Another feature of CdTe photovoltaic cells is their transparency. The transparency of these cells ranges from about 10% to 50%, although increased transparency leads to reduced efficiency. Nevertheless, transparent panels can replace window glass in buildings, not only generating part of the building's electricity but also contributing to noise reduction and thermal insulation, as most cells are placed in double-glazed glass.
- Copper Indium Gallium Selenide (CIGS) Solar Cells
Efficiency: 19% to 23%
Lifespan: 12 years
Advantages: Less material required for production
Limitations: Very expensive, unstable, higher temperature sensitivity, highly unreliable
In the world of solar cell production, a key aspect that needed improvement was the reduction of heavy reliance on semiconductor material resources. This driving force led to the emergence of the second generation of thin-film photovoltaic cells, including CIGS. The record efficiency of CIGS is 23.4%, which is comparable to the best efficiency of silicon cells.
Although the efficiency of CIGS solar cells is very promising in a research environment, it should be noted that this efficiency does not necessarily translate to achievable efficiency on an industrial scale. This difference is due to the challenges of large-scale processing.
However, the reality is that today, the efficiency of CIGS modules in the industry has reached around 20%. This remarkable achievement indicates that the industry has gradually overcome scalability barriers. In recent years, there has been a significant increase in the efficiency of these cells, and it is expected that this trend of improvement will continue. These advancements are promising and suggest that CIGS solar cells could become an attractive and competitive option in the solar energy industry.
Third Generation: Tandem, Perovskite, Dye-Sensitized, Organic, and Emerging Concepts Solar Cells
Third-generation solar cells (including tandem, perovskite, dye-sensitized, organic, and emerging concepts) represent a wide range of approaches, from low-cost systems with lower efficiency (dye-sensitized, organic cells) to high-cost systems with high efficiency (III-V multi-junction cells) for applications ranging from building-integrated to space applications.
Third-generation photovoltaic cells are sometimes referred to as "emerging concepts" due to their weak market penetration, although some of these types of solar cells have been studied for over 25 years. The most advanced studies in technology development and efficiency improvement are now focused on third-generation solar cells. One current method for increasing the efficiency of photovoltaic cells involves creating additional energy levels in the semiconductor bandgap (IPV) and increasing the use of ion implantation in the production process. Other innovative third-generation cells that are considered "emerging" with less recognized commercial technologies include:
- Organic or Organic Solar Cells (OSC)
- Perovskite Solar Cells (PSC)
- Dye-Sensitized Solar Cells (DSSC)
- Multi-junction Solar Cells
- Quantum Dot Solar Cells (QD)
Organic Solar Cells
- Efficiency: 19.2%;
Advantages: Low processing cost, lighter weight, flexibility, thermal stability.
Limitations: Low efficiency.
These solar cells are made from organic and carbon-based materials instead of minerals like silicon. In these cells, conductive polymers and organic molecules are used as the active materials. These materials absorb sunlight and produce free electrons.
Organic Solar Cells (OSCs) hold promising potential for the future of solar energy. This innovative technology benefits from the unique advantages of organic semiconductors. Lightweight, low manufacturing costs, flexibility, semi-transparency, and the capability for large-scale roll-to-roll processing make these cells an attractive option for diverse solar energy applications.
These features allow OSCs to be used in a wide range of applications—from building-integrated solar coatings to portable electronic devices. This diversity of applications demonstrates the potential of this technology to revolutionize clean energy. With ongoing advancements in OSC efficiency and stability, organic solar cells could become a cost-effective and efficient alternative to traditional solar technologies. The bright future of this technology promises a more sustainable and greener future.
Organic solar cells that absorb near-infrared (NIR) radiation have been studied worldwide. NIR-absorbing organic solar cells have also attracted attention for their potential use in next-generation optoelectronic devices, such as semi-transparent solar cells and NIR detectors, due to their potential for industrial applications. As research in this field progresses, the energy conversion efficiency of these cells is also increasing. Therefore, organic solar cells are expected to be an attractive option for solar power generation in the future.
Perovskite Solar Cells: A Revolutionary Innovation in Solar Energy
- Efficiency: 26%
- Advantages: Low-cost and simple structure, lightweight, flexibility, high efficiency, low manufacturing cost.
- Limitations: Unstable.
Perovskite Solar Cells (PSCs) represent a revolutionary new concept in the photovoltaic field. This technology is based on metal halide perovskites (MHPs), such as methylammonium lead iodide and formamidinium lead iodide (MAPbI3 or FAPbI3).
MHPs integrate desirable features in photovoltaic absorbers, including direct band gaps with high absorption coefficients, long carrier lifetimes and diffusion lengths, low defect densities, and the ability to tune compositions and band gaps. This unique combination of properties makes PSCs a highly attractive option in solar energy. With ongoing improvements in efficiency and stability, they are expected to soon become a cost-effective and efficient alternative to traditional solar technologies.
In simpler terms, these solar cells use perovskite materials as the active component. Perovskite materials are crystalline compounds with a cubic structure that can efficiently absorb light and produce free electrons.
The main advantage of perovskite cells is their high energy conversion efficiency and low manufacturing cost. Additionally, these cells are made from inexpensive and simple materials. This revolutionary innovation in solar energy promises a brighter and more sustainable future, with PSCs playing a pivotal role.
Dye-Sensitized Solar Cells
- Efficiency: 6 to 13%
- Advantages: Lower cost, good performance in low light and wider angle range, lower internal temperature performance, increased durability and lifespan.
- Limitations: Temperature stability issues, presence of toxic and volatile materials.
Dye-Sensitized Solar Cells (DSSCs) represent an innovative and interesting technology in the field of solar energy. Their concept is inspired by the photosynthesis process in plants. The diagram below shows a representation of dye-sensitized solar cells.
In fact, dye-sensitized solar cells (DSSCs) are among the best nanotechnology materials for energy harvesting in photovoltaic technologies. This hybrid organic-inorganic structure includes a porous nanocrystalline layer of titanium dioxide (TiO2) as an electron conductor in contact with an electrolyte solution containing organic dyes.
In other words, instead of using conventional semiconductor materials like silicon, these cells use a porous semiconductor material (usually titanium dioxide). A thin layer of natural or synthetic dyes is coated on this semiconductor. These dyes are responsible for absorbing sunlight and generating free electrons. The freed electrons are collected by the semiconductor layer and directed towards the electrodes.
The main advantage of these cells compared to silicon solar cells is their lower manufacturing cost and the possibility of using cheaper materials. Additionally, these cells offer greater flexibility and can be used in various design applications.
The main challenges in commercializing this technology include low efficiency and cell stability. While the theoretical maximum efficiency is 32%, the highest reported practical efficiency is 13%. Efforts are ongoing to improve efficiency through optimization of the redox processes, dye absorption, and electrode improvements.
Multi-Junction Solar Cells
- Efficiency: 30% and higher.
- Advantages: High performance.
- Limitations: Complex, expensive.
Imagine a solar cell as a cake. In conventional solar cells, this cake is made of just one thin layer of silicon. However, in multi-junction solar cells, this cake is composed of several layers of different materials.
Each layer in these cells has a specific function. Some layers absorb light, others direct electrons and holes toward the electrodes, and some serve as protective layers. In fact, these cells are made from multiple semiconductor layers with different energy bands, allowing them to absorb a broader spectrum of sunlight and achieve higher energy efficiency.
In other words, this multi-layer structure enables multi-junction cells to have higher conversion efficiency compared to single-layer cells. Each layer can optimally absorb light and direct electrons toward the electrodes.
For this reason, heterojunction (multi-junction) solar cells are a very attractive option for high-efficiency solar power generation. With technological advancements, these cells are expected to be a suitable replacement for silicon solar cells in the future.
Quantum Dot Photovoltaic Cells
- Efficiency: 11% to 19%
- Advantages: Low production cost, low energy consumption.
- Limitations: High environmental toxicity, high degradation rate.
Quantum Dot Solar Cells represent an emerging technology in the field of solar energy. These cells are made from nanocrystalline semiconductor materials known as "quantum dots."
Quantum dots are very small particles where electrons and holes are confined in a limited space. This confinement causes the energy of the electrons and holes to occupy discrete levels, as opposed to continuous levels in conventional materials.
This unique feature of quantum dots allows for the tuning of their electronic and optical properties. By changing the size of the quantum dots, the light absorption spectrum and energy band can be controlled.
This tunability makes quantum dot solar cells a popular technology for converting solar energy into electricity. With ongoing improvements in efficiency and production costs, this technology has the potential to be a suitable alternative to silicon solar cells in the future.