Solar panels are photovoltaic devices composed of multiple solar cells to harness sunlight and generate electricity when the photons interact with a semiconductor material like silicon. A variety of types of solar panels are available in the market namely Monocrystalline, Polycrystalline, Thin-film and Passivated Emitter and Rear Contact cells (PERC).
Thin film solar cells are made by depositing more than a single layer of photovoltaic materials on a substrate like glass, metal, or plastic.
Cadmium Telluride (CdTe) is a second-generation solar cell used in thin solar panel technology that maximizes the efficiency of converting solar radiation into electricity. In 1972, Bonnet and Rabenhorst were the first to develop the CdS/CdTe, heterojunction that eventually led to the manufacturing of CdTe solar cells.
CdTe solar panels are different from other types of solar panels in terms of efficiency, availability of materials, manufacturing process, cost, and applications.
It would be interesting to know the science behind the CdTe solar cells by knowing their advantages and disadvantages in detail…
What is the Science Behind Cadmium Telluride Solar Cells (CdTe)?
The photovoltaic effect is the basic science behind the working of CdTe solar cells. Light energy gets absorbed when sunlight strikes the CdTe solar cell and gets transferred to negatively charged particles called electrons. This additional energy allows electrons as electrical current to flow through the material.
The key components of CdTe solar cells include a p-n heterojunction structure containing a p-doped Cadmium Telluride layer combined with an n-doped cadmium sulphide (CdS) or Magnesium Zinc Oxide (MZO) window layer. Typically, the deposition of CdTe thin film includes vapour transport deposit and closed space sublimation.
The high-efficiency and low-cost thin film polycrystalline solar cells prove CdTe as an ideal absorber with a direct bandgap material possessing an energy gap of 1.5 eV. In the visible region, the absorption coefficient is around 104 cm-1 which indicates a thick layer of a few micrometres is acceptable to absorb 90% of incident photons. Therefore, the CdTe is an efficient material for solar energy conversion.
Manufacturing Process of cadmium telluride Solar Cells
The CdTe solar cells are made by using semiconductors using p–n heterojunctions of the Cadmium Telluride (CdTe) layer doped at the p-type junction and Cadmium Sulphide (CdS) layer doped at the n-type junction.
There are three main steps in the production process of CdTe solar cells. Initially, polycrystalline materials are used to prepare the CdTe-based solar cells, using glass as the base material. The second stage is layer deposition, which involves various techniques of multiple coating of CdTe solar cells using various cost-effective methods such as sputtering, close-spaced vapour transport (CSVT), chemical spray pyrolysis, and electrodeposition. However, the highest efficiency is obtained by the chemical bath method to reduce the layer thickness.
The manufacturing cost has been less expensive making it an ideal choice for investors with the price of $0.46/Watt which is less than the crystalline panels.
Advantages of cadmium telluride Solar Cell Technology
There are several advantages of CdTe solar cell technology you need to know:
1. Efficiency: CdTe solar cells can achieve an efficiency of 22% in laboratory settings, and the average efficiency of such panels exists up to 19%.
2. Overall Costs: The inexpensive overall cost can prove an alternative to the costly silicon-based solar cells, and these panels can perform well in low light conditions. Mainly, the reduction in the CdTe absorption layer thickness reduces the material consumption and diminishes the production cost of CdTe components.
3. Suitable for solar scale farms: Due to its robust nature and high chemical stability, the CdTe cells are suitable for solar scale farms. For instance, the CdTe solar cells are used in the world’s largest PV power stations, like the Topaz Solar Farm.
Disadvantages of Cadmium Telluride Solar Cell Technology
Similar to a two-sided coin, CdTe solar cell technology has certain disadvantages:
1. Concerns about Cadmium: The primary material of CdTe solar panels is Cadmium, which is toxic and leads to environmental concerns like production and disposal. Earlier, Greenpeace cautioned about the toxicity and contamination levels of Cadmium. It highlighted that the toxic materials of CdTe panels contain 6g/m2, and they produce cadmium emissions equal to approximately 0.5g/GWh, approximately.
2. Lifecycle performance: Despite the lifecycle of CdTe photovoltaics having the smallest carbon footprint, lowest water usage and shortest energy payback compared to any other photovoltaic technology performance, these panels account for 49.3% of the total fossil fuel use. As a result, the materials are highly resistive because it doesn’t allow excellent charge collection. It leads to a short minority carrier lifetime when the electron-hole pairs recombination occurs at the defect centres in CdTe layers, as well as at the junction between CdS and CdTe.
3. Recycling Issues: The uncertainty in the recycling process of CdTe solar cells raises issues about the separation of materials like glass, Cadmium, and Tellurium, which is a challenging process to minimize lifecycle emissions and energy usage.
At the same time, industries are addressing these challenges with various strategies such as:
The ongoing projects are striving to gain higher cell efficiencies by enhancing the quality of crystals, improving the process of doping and increasing the lifetime of minority charge carriers.
To mitigate the toxicity concerns and absence of materials, manufacturers are working to improve the reuse and recycling processes.
The improvement in solar technology over the past few years has led to an innovation in the fabrication of CdTe solar cells by various redesigning processes.
Many ongoing research aim to optimize the separation and recovery of glass, Cadmium, and Tellurium by minimizing the life cycle emissions.
CdTe vs Other Solar Panels: An interesting comparison to be known
The CdTe technology has numerous unique characteristics that vary from crystalline silicon and other thin solar film panels like Amorphous and CIGS solar panels in terms of:
1. Efficiency: Silicon-based solar cells outperform CdTe solar cells with monocrystalline having an efficiency above 20% and polycrystalline with 15 % to 20%. Whereas CdTe solar cells in the laboratory can achieve upto 22% and commercially offer 11 % to 16% efficiencies. However, Amorphous silicon panels are lower than CdTe with 6% to 7%. Similarly, CIGS solar panels are less with 12 to 14% and the highest can be achieved under laboratory settings of 20%.
2. Cost: CdTe solar panels are lower in cost at $0.46/Watt, and it is 70% cheaper than crystalline ($0.70 to $1.50/Watt), Amorphous( $0.50 to $1.50/Watt) and CIGS ( $2.8 to $2.9/ Watt)
3. Temperature Coefficient: The CdTe solar panels are less affected by huge variations in temperatures and have a low-temperature coefficient of -0.21%/℃ than these panels:
Crystalline: -0.3% / ℃ to -0.5% / ℃ for Monocrystalline and -0.3% / ℃ to -1% / ℃ for Polycrystalline.
Amorphous: −0.26 % / ℃ to -0.45 % / ℃
CIGS: -0.36%/°C
4. Environmental Impact: CdTe solar cells have a small impact on the environment with a lower carbon footprint, less water usage and the fastest energy payback time compared to crystalline, amorphous and CIGS solar panels. But CIGS solar panels are less polluting than CdTe. This is because CdTe panels consist of a higher amount of toxic metals, and they produce high cadmium particles. You can also learn about the environmental impacts of solar energy in detail.
Applications of Cadmium Telluride Solar cells
The CdTe solar cells can be used in a variety of applications such as:
Large-scale solar farms due to cheaper costs and higher stability to chemical reactions.
Commercial applications like portable photovoltaic modules to withstand higher temperatures, BIPV (Building Integrated Photovoltaics), solar shingles, flexible PV modules for multiple applications, and several others.
Used for spacecraft, military vehicles and to accomplish important missions in space.
At the same time, you need to know that the CdTe solar panels are not the best choice for residential purposes due to 1 % to 6% less efficiency than the crystalline silicon panels and higher toxicity. However, they are ideal for various commercial and industrial applications.
Several factors make these applications of CdTe solar panels suitable which include:
Affordable costs for large-scale installations and less expensive to manufacturer compared to other types of solar panels.
Highly robust as it helps to withstand harsh environmental conditions making it suitable for space applications.
Higher flexibility to integrate into the buildings.
Future of Cadmium Telluride Solar Cells
CdTe (Cadmium Telluride) solar cells have very quickly started to gain popularity in the market. Also, they are well recognized as one of the leading thin-film solar technologies that is currently available. They have a lot of advantages which lead to their popularity.
Economical Cost: CdTe solar cells are well-known for their cost-effective manufacturing compared to traditional crystalline silicon solar cells.
Quick Manufacturing: CdTe thin-film solar cells can be manufactured very rapidly, making them a competitive alternative to silicon-based technologies. This lightning-fast manufacturing capability leads to their cost-efficiency and growth potential. There is a lot more to know about how solar panels are made.
Market Position: CdTe solar cells make up the largest share of commercial thin-film solar panel manufacturing in the entire world. The United States is a leader in CdTe photovoltaic production, and lots of companies are dedicated to producing and shipping CdTe modules across the globe.
Researchers and manufacturers are actively focusing on new technologies to further enhance the performance and efficiency of CdTe solar cells. Some of the most important areas of innovation include:
Alloyed CdSexTe1-x (CST) absorbers: Alloying CdTe with selenium (CdSexTe1-x) is said to have proven promising results in further improving the efficiency of CdTe solar cells. This approach is based on changing the composition of the absorber layer to optimally optimize its performance.
The ultra-thin glass substrates: This is an appealing alternative to sturdily constructed glass substrates.
New materials: A decrease in the dependence on rare materials through the creation of new materials with the same convenience of processing.
Defect passivation: Researchers are looking into methods that aim to passivate the defects in CdTe solar cells, such as serendipitous selenium defect passivation. These efforts are intended to further enhance the performance and stability of the cells.
Ray is an avid reader and writer with over 25 years of experience serving various domestic and multinational private and public energy companies in the USA.
