In this article we discuss the development of various solar panel technologies and demonstrate the analysis and repeatability results of a typical CIGS solar cell using the non-destructive Energy-Dispersive X-ray Fluorescence analytical technique.
Developments in research and manufacturing have pushed copper gallium indium selenide (CIGS) solar panel to the forefront in the adoption of photovoltaic technology for energy generation. CIGS is a highly stable, high performance, and mature thin film PV technology. While the composition of these PV material has not been substantially altered since 1986, its popularity is due to improvements in manufacturing to lower costs and expand installation opportunities such built-environments, portability and into diffuse lighting and high-heat installations.
Silicon crystalline solar panels
Over the past few decades monocrystalline and polycrystalline silicon solar panels have experienced the majority PV installations. Silicon is an abundant material and has proven to be an easy and relatively inexpensive product to produce and sell. Monocrystalline panels offer the greatest energy conversion efficiency, but are more expensive than polycrystalline panels, which are composed of melted fragments of silicon joined to form the wafers for the panel. Owning to manufacturing processes, monocrystalline panels are the most expensive material on the market. Improvements in polycrystalline technologies have made them more widely adopted, but due to lower efficiency more panels are needed to cover a given area.
Thin film solar panels
Thin film cells exhibit lower efficiencies than crystalline panels and require the most space for the same amount of power. Since they are becoming the cheapest panels to produce because of the low material costs for thin film they are quickly becoming the more economically efficient panel types. Their adaptability to structures such as building siding or rooftop shingles and improved aesthetic appearance compensates for space requirements. Thin film panels are generally flexible and lighter weight, adding to their versatility in installations.
Amorphous solar panels
Like conventional solar panels, amorphous solar panels are made from silicon, but they are constructed by depositing non-crystalline silicon on a substrate like glass, plastic, or metal. Unlike many other thin-film panel options, amorphous silicon panels use very little toxic materials. When compared mono- or poly-crystalline solar panels, amorphous panels use much less silicon. However, this technology is the least efficient.
Cadmium telluride (CdTe) panels
Another common photovoltaic technology is cadmium telluride (CdTe) panels. CdTe thin film panels are made from several thin layers: one main energy producing layer made from the compound cadmium telluride, and surrounding layers for electricity conduction and collection. Slightly higher in efficiency than amorphous solar panels, the biggest drawback to CdTe is that cadmium, while abundant, is one of the most toxic materials known, making manufacturing and disposal an environmental concern.
Copper gallium indium selenide (CIGS) solar panels
Over the past years, Copper gallium indium selenide (CIGS) has become the fastest growing thin film PV technology. Sandwiched between conductive layers and deposited on substrates such as glass, plastic, steel, and aluminum, CIGS layers are thin enough to be allowed full-panel flexibility. While also using the toxic chemical cadmium, its implementation is at a lower percentage than CdTe panels.
Due to its promise as the highest efficiency of thin film PV technologies, considerable public and private research has gone into making its production cheaper and more efficient compared to other solar technologies.
Some of the technological advantages of CIGS panels are:
- Low temperature coefficient: CIGS efficiency does not decrease as quickly as silicon panels when in high temperatures – making it ideal for installation in the fast-growing solar markets. Coupled with superior performance in diffuse light conditions, CIGS PV remains a high yield technology even in less than ideal environments.
- High absorption: This direct-bandgap material can absorb a significant portion of the solar spectrum, enabling it to achieve the highest efficiency of any thin-film technology.
- Tandem design: A tunable bandgap allows the possibility of tandem CIGS devices.
- Protective buffer layer: The grain boundaries form an inherent buffer layer, preventing surface recombination and allowing for films with grain sizes of less than 1 micrometer to be used in device fabrication.
Japan’s Solar Frontier is currently the largest CIGS producer, with 1 GW of production capacity and 5 GW of modules deployed globally. New, large-scale investments in CIGS manufacturing from major energy and industrial players is currently underway, primarily in China. Around 600 MW of CIGS production capacity was added in 2018 with expansion plans for multiple gigawatts of production. CIGS research institutes and endeavors in countries including Germany, France, Switzerland, the Netherlands, Sweden, and Spain make Europe the leading international center for CIGS technology development. When this fundamental expertise is combined with the established network of advanced production equipment suppliers, Europe provides a promising ecosystem for CIGS technology development – with laboratory developments readily transferable into scale production machinery and solutions.
Analyzing a CIGS solar cell
In a recently published application note, we used Energy-Dispersive X-ray Fluorescence (EDXRF) to examine a glass substrate coated with a Mo layer on which CIGS is deposited and sealed with layers of CdS, ZnO and Al doped ZnO. EDXRF as a truly nondestructive technique is ideal for the characterization of CIGS solar cells, to determine both layer composition and thickness. Total measurement times of 1-minute live time provide excellent repeatability values, both for the layer composition and thickness. The fundamental parameters-based thin film software allows measurement of thickness, mass and composition of up to 6 layers containing any number of elements.