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CdTe vs. CIGS Solar Panels: Differences, Performance & Applications

CdTe and CIGS solar panels are two prominent types of thin-film panels, with the former dominating this market segment while the latter follows right behind.

These two types of panels both hold distinct characteristics, advantages and disadvantages. In this article, we will compare them with regard to several aspects that users would be most concerned about.

Recapping the Basics: Composition, Structure and Manufacturing

Understanding the basics of CdTe and CIGS thin-film solar panels is crucial to comprehend how they perform in real-world conditions and what applications they are ideal for.

CdTe Solar Panels

Diagram: Different Layers of a Cadmium Telluride (CdTe) Solar Panel
Figure #1: Layers of a CdTe solar panel | Source: NREL

CdTe solar panels use cadmium telluride as the primary semiconductor material to convert sunlight into electricity. Akin to other panels, the parts of CdTe panels can be categorized into several layers; explained in detail as follows:

  • Substrate Layer: Typically made from glass, the substrate layer provides mechanical support and protection for upper layers.
  • Back Contact: This layer serves as the electrical contact that collects and conducts the electricity generated by the cells. It is generally made from materials like carbon paste infused with copper or other metals to create conduction in the panel.
  • Photovoltaic Layer: The core of the panel. It contains a p-doped cadmium telluride (CdTe) sublayer and an n-doped cadmium sulfide (CdS) or magnesium zinc oxide (MZO) sublayer, forming a p-n junction for energy conversion.
  • TCO Layer: Material like fluorine-doped tin oxide (SnO₂:F) or cadmium stannate (Cd₂SnO₄) is used to manufacture the transparent conductive oxide (TCO) layer. This transparent and conductive layer enables light to flow through while carrying electricity.
  • Encapsulation: Similar to other types of panels, the encapsulation layer provides physical protection for the entire assembly.

Techniques such as sputtering, chemical vapor deposition or close-spaced sublimation are employed to deposit different layers on the substrate.

The CdTe layer can be deposited by close-spaced sublimation, which involves heating the material until it sublimates and then condenses onto the substrate. During or after the deposition process, ‘impurities’ will be added to the CdTe and CdS layers to create p-type and n-type semiconductors, respectively, via doping. Alternatively, dopants like arsenic or phosphorus can be incorporated during the high-pressure synthesis of CdTe.

Annealing will be implemented after all key layers are assembled. It is a thermal process to heat these layers to a high temperature to improve crystallinity and promote interdiffusion between the CdS and CdTe layers.

Manufacturing techniques such as roll-to-roll processes on metal foils and the use of soluble CdTe nanocrystals are key in producing efficient and cost-effective CdTe panels.

CIGS Solar Panels

‘CIGS’ stands for Copper Indium Gallium Selenide, which is the semiconductor material used in these panels. Each layer of a CIGS solar panel serves a specific function, as shown below.

Layers of a CIGS Solar Panel
Figure #2: Layers of a CIGS solar panel | Source: NREL
  • Substrate Layer: This can be made from glass or a flexible polymer depending on the application. Most products on the market use flexible substrates; these panels can be bendable or even rollable.
  • Molybdenum (Mo) Back Contact: A thin layer of molybdenum (Mo) is typically sputtered onto the substrate. This layer acts as the rear electrode of the panel, collecting charge carriers and reflecting unabsorbed light back into the absorber layer.
  • Absorber Layer: The core of a CIGS panel is the absorber layer made of chemical composition containing copper, indium, gallium and selenium elements. It is the p-type semiconductor material that is responsible for absorbing sunlight and generating electron-hole pairs.
  • Buffer Layer: It is a layer of cadmium sulfide (CdS) deposited on top of the absorber layer. This is the n-type layer that forms the p-n junction of the panel, facilitating charge separation.
  • Window Layer: Generally consisting of a TCO, the window layer is fabricated by depositing a layer of intrinsic zinc oxide (i-ZnO) over the buffer layer, followed by the application of an AZO compound layer. This structure serves a dual purpose: protecting the underlying buffer layer from damage, while fulfilling its optical and electrical functions.
  • *Anti-Reflective Coating: Many premium CIGS panel products would add a proprietary anti-reflective coating on top to minimize the reflection of light, ensuring more photons reach the absorber layer.
  • Encapsulation: This layer provides protection and prevents electrical degradation of the lower layers and blocks moisture ingress, which is critical for maintaining the efficiency and longevity of the panel. 

The manufacturing of CIGS solar panels also involves various processes that combine the use of multiple deposition techniques.

Sputtering is the most widely used technique to deposit the back contact layer on the substrate. Then the CIGS absorber layer is formed using co-evaporation, sputtering or electrochemical deposition. The buffer layer is usually deposited using chemical bath deposition (CBD). The window layer is typically applied using methods like sputtering or atomic layer deposition (ALD). While the anti-reflective coating can be done with physical vapor deposition (PVD).

Recent years, scientists have also adopted sequential ‘selenization’ and sulfurization to deposit the CIGS material. Printing technology has been used and constantly improved to reduce the cost of CIGS panel manufacturing.

CdTe vs. CIGS Solar Panels: Efficiency

Efficiency is perhaps the most concerning criterion when considering between CdTe and CIGS solar technology.

Improvements in CdTe panel efficiency have experienced significant gains since 20101 (Figure #3). Commercial CdTe solar panels generally have an efficiency between 17% and 19%, which is roughly on par with that of average silicon-based solar panels. Until now, the recorded highest laboratory efficiency of CdTe panels is 22.1%.

Improvements in CdTe Panel Efficiency Over the Years
Figure #3: Laboratory efficiencies of the CdTe solar cell | Source: Erteza Tawsif Efaz et al., 2021

The efficiency of CIGS panels has also seen big improvements over the last decade2. In comparison to CdTe panels, the average efficiency of CIGS panels is slightly lower, falling within the range of 12%-16%. The prototype panel created by a research team at Uppsala University has been marked as the latest world record, with an efficiency of 23.64%.

CdTe vs. CIGS Panels: Cost

Cost is another important criterion when weighing up these panels.

Thanks to comparatively lower material costs as well as the advancement in manufacturing techniques and material science, the price of CdTe solar panels has been driven down, ranging from $0.20 to $0.35 per watt. While the price of CIGS panels can range from $0.30 to $0.50 per watt.

As a benchmark, the price of silicon-based panels, including both mono- and poly-crystalline panels, range from $0.30 to $0.70 (or higher) per watt.

Overall, the price of both CdTe and CIGS products can also vary more widely based on technology maturity and manufacturing terms apart from the scale of deployment.

CdTe vs. CIGS Panels: Real-World Performance

Although silicon-based panels still hold the lion's share of the market, the market shares of both CdTe and CIGS panels are witnessing continuous growth.

CdTe panels are well-known for their lower temperature coefficient, ranging from approximately -0.20%/ºC to -0.30%/ºC. Some premium products can reach below -0.20%/ºC. The temperature coefficient of CdTe panels outperforms those of CIGS and silicon-based panels which both share similar figures. This strength implies better temperature tolerance, and makes CdTe panels maintain good performance in high-temperature conditions.

While the coefficient for CIGS panels is not as favorable as that of CdTe panels, CIGS panels are renowned for their excellent flexibility and lightweight. Unlike traditional silicon-based panels and most CdTe panels, these panels can be bendable and even rollable, offering outstanding flexibility when deployed onto irregular or curved surfaces. Their lightweight property also provides greater possibilities to suit various installations.

Notably, the primary materials used to manufacture CdTe and CIGS panels are of direct-bandgap. This means these panels are easier to capture and convert energy in low-light conditions, in contrast to traditional silicon-based panels with indirect-bandgap materials.

Pros and Cons of CdTe and CIGS Panels

Besides a couple of shared features, CdTe and CIGS solar panel technologies have distinct pros and cons. Here is a quick roundup to help to better comprehend them before guiding a decision.

Pros of CdTe Panels

  • Cost-Effective: CdTe panels are easier to manufacture and are one of the most affordable options on the market. They generally cost less than CIGS panels.
  • High Efficiency: CdTe panels boast a moderately higher efficiency than that of CIGS panels, and are comparable to some traditional silicon-based products.
  • High Temperature Tolerance: Their lower temperature coefficient enables a more stable performance and a satisfying yield under hot weather.
  • Thin-Film Technology: As one type of thin-film technology, CdTe panels are lighter and more flexible than traditional panels.

Cons of CdTe Panels

  • Environmental & Health Concerns: Cadmium is a toxic material that can be hazardous to humans and the environment if not handled and disposed of properly.
  • Regulatory Restrictions: Due to the toxicity of cadmium, CdTe products could be subject to regulations in some regions.
  • Space Requirements: Still being not able to outshine crystalline silicon panels regarding efficiency, CdTe requires more space to produce expected energy. 

Pros of CIGS Panels

  • Flexible & Lightweight: CIGS panels are more flexible and lightweight than CdTe products, leading to broader applications no matter for mobile or stationary setups.
  • Superior Aesthetics: The invisible electrical contacts and excellent flexibility make CIGS panels ideal for applications where aesthetics and adaptability are important.
  • Low Environmental Impact: CIGS technology uses less cadmium compared to CdTe panels, reducing environmental and health risks associated with the unfriendly element.

Cons of CIGS Panels

  • Slightly Lower Efficiency: Despite that their efficiency is slightly lower than CdTe products, their top-notch flexibility makes them the second-to-none choice for many projects.
  • Slightly Higher Price Tag: They might in general cost more than CdTe panels due to their complex manufacturing process, material properties and process control.

CdTe and CIGS panels also share good performance in low-light conditions, which somehow mitigates some of their cons.

What's more, even though CdTe and CIGS panels have been limited by their relatively short lifespan and susceptibility to environmental factors, ongoing development has led to significant improvements through the incorporation of advanced materials and innovative techniques.

Final Words: Ideal Applications for CdTe and CIGS Panels 

CdTe solar panels are particularly suitable for large-scale solar projects, offering a compelling combination of cost-effectiveness, good efficiencies that are closer to those of silicon-based products, and a simpler manufacturing process that enables rapid mass production. 

Remarkably, CdTe solar panels are also especially ideal for high-temperature environments. They have better performance in high-temperature environments compared to CIGS panels and some types of silicon-based panels.

On the other hand, CIGS panels can be manufactured to have aesthetically pleasing designs and are hence good for building-integrated photovoltaics (BIPV) projects. Their flexibility and customization options make them suitable for integration into building materials like glass facades and roofs in either commercial or residential projects.

Moreover, CIGS panels are the go-to clean power solution for applications in the consumer market, where portability is underscored. The end products can be mobile charging stations and camping equipment, to name a few.

*Cover Image: "Thin-film flexible solar PV wire management" (cropping) by ken fields is licensed under CC BY-SA 2.0.

  1. A review of primary technologies of thin-film solar cells (Erteza Tawsif Efaz et al., 2021) ↩︎
  2. State of the art review on the Cu(In,Ga)Se2 thin-film solar cells (Mohammad Anwar Omid et al., 2020) ↩︎

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