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Innovative CdTe Solar Technology: Transparent Panels (BIPV)

The CdTe (Cadmium Telluride) solar panel is an important branch of thin-film solar technology. Some of its advantages compared to traditional c-Si panels have led to its ever-growing adoption in industrial, commercial, as well as residential segments, representing around 5-6% of the global panel market share.

It is remarkable that several distinctive properties of CdTe technology are contributing significantly to the advancement of revolutionary 'transparent solar panels'.

Let’s dive into why & how CdTe technology is applied to transparent panels, and their pros and cons.

Standing in Front of CdTe-Based Transparent Solar Panels

Starting with the Basics of CdTe

Going over the basics of CdTe technology is crucial for you to understand the transparent panels built upon it.

CdTe cells use cadmium telluride as the semiconductor material to convert sunlight into electricity. Somewhat similar to the structure of CIGS cells, CdTe cells typically use a thin layer of CdS as the n-type layer, which is placed above the CdTe layer to create a p-n heterojunction, i.e., the photovoltaic material. Other layers comprising a CdTe cell structure also include:

  • Encapsulant: A layer that protects the underneath layers of the cell from moisture and degradation.
  • Transparent Conductive Oxide (TCO) Layer: It is generally made by depositing SnO2:F or a similar material. This layer serves as the front electrode, allowing light to enter while conducting electricity.
  • Back Electrical Contact: It is commonly a layer of zinc telluride (ZnTe). This layer also requires the deposition of a metal layer or carbon paste that introduces copper (Cu) into the rear of the cell.
  • Substrate: This is the cell base on which other layers are deposited, providing structural support. It can be glass or other flexible materials.

The Development of Transparent Solar Panels

Transparent solar panels are considered a groundbreaking technology that aims to harvest solar energy without obstructing sunlight from entering the interior, serving the dual roles of a clean energy generator and a window for the structure.

While research into this panel technology has evolved over the past few decades, it wasn’t until 2014 that the research team at Michigan State University (MSU) invented what is recognized as the first version of 'transparent solar panels.' This milestone began to procure tremendous traction for the technology and spur significant strides in its development.

Although it is acceptable to call them ‘transparent solar panels’, they are not authentically  transparent photovoltaic glass panels. 

The formal term for MSU’s technology is transparent luminescent solar concentrator (TLSC). Physically speaking, it utilizes small innovative organic molecules to pick up the invisible wavelengths of sunlight in the ultraviolet and near-infrared, ‘glow’ them at another wavelength in the infrared, then guide it to the edge of the panel where it is converted to electricity through thin photovoltaic strips.

The initial conversion efficiency of this technology is about 1%. Through ongoing optimization on the design of TLSCs, MSU’s transparent panel technology has recorded efficiencies above 5%.

Wiring Newly Installed Transparent Solar Panels
Wiring newly installed transparent solar panels. | Source: MSU

However, achieving substantial breakthroughs in the efficiency of highly transparent solar panels appears to be challenging, largely due to the inherent constraint imposed by their core principle: converting invisible waves while letting go of visible waves. 

This, on the flip side, is opening up another route within the ‘transparent roadmap’: semi-transparent solar panels.

Simply put, semi-transparent solar panels make some compromises on transparency to make room for efficiency improvements. By striking a balance between transparency and efficiency to suit different use cases, these panels can vary in transparency, with some reaching near transparent.

Why Is CdTe Technology Applied to Transparent Solar Panels?

With the expanded connotation of transparent panel technology, researchers are exploring the use of other materials to create these panels.

One of these is the CdTe cell technology.

Excellent Thickness

In the first place, CdTe cells are renowned for their thinner absorber layers, falling within the range of 2-10 micrometers (μm). This thickness is considerably thinner than that of traditional c-Si cells, with a thickness range of 100-200 μm

Researchers found that CdTe cells manufactured with a certain absorber thickness can result in better performance. In some cases, altering the concentration of certain elements in the absorber layer can lead to thickness approaching or even below the lower end of the average range, but this will sometimes sacrifice efficiency.

In other words, the excellent thickness makes CdTe technology another good fit for creating transparent solar panels. Experts are able to manufacture these panels with varying transparency percentages by applying trade-offs between thickness (transparency) and efficiency.

High Efficiency in Lower Light

Secondly, CdTe cells demonstrate excellent performance under low light conditions. This property well adapts to the light conditions and environmental variables of the locations where transparent panels are installed.

In urban settings, buildings and other structures can create shadows while relatively higher levels of pollution and particulate matter can absorb and scatter sunlight. Both of these two factors can reduce the amount of direct sunlight that reaches transparent panels throughout the day. CdTe cells’ performance in low light can somewhat partially offset these negativeness.

Lower Temperature Coefficient

Thirdly, urban areas often go through the "urban heat island" effect, where temperatures are higher than in surrounding rural areas. Higher temperatures can reduce the efficiency of solar cells, referring to the temperature coefficient of panels; CdTe cells are not exempt.

However, CdTe cells have a lower temperature coefficient typically in the range of -0.20%/ºC to -0.25%/ºC as opposed to that of c-Si cells, typically in the range of -0.30%/ºC to -0.50%/ºC. This is also considered to help partially mitigate the impact of their lower efficiency.

Proven Tech With Low Product Cost

Last but not least, CdTe solar technology ranks second after the c-Si as one of the most common PV technologies. The manufacturing process for CdTe panels is simpler and less expensive than c-Si panels and other types of the thin-film panel family. This not only bolsters the competitiveness of these CdTe-based transparent solar panels, but also lays the foundation for future large-scale commercial use in urban areas.

CdTe-based transparent panels are generally made by getting the CdTe panel sandwiched between two pieces of tempered glass.

These transparent solar panels can also be tailored made, either be formed into different shapes and sizes or dyed with colors, to accommodate the unique appeal of the sites where they are installed, such as museums and exhibition centers.

Final Verdict

Utilizing mature CdTe solar technology in transparent solar panel manufacturing unlocks significant potential, providing a wise and tangible solution for ‘solarification’ in cities where large open ground and rooftop spaces are limited yet glass surfaces abound.

It is noteworthy that there are close to ten billion square meters of glass surfaces in the United States alone!

Further development of this new technology encompasses advancing efficiency while ensuring outstanding transparency, fostering the custom design and implementation of a comprehensive ‘transparent solar power generation system’, and building a well-rounded recycling chain to handle not-friendly elements.

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