Looking on the Bright Side – Advances in Solar Materials Testing and Analysis
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With a global eye on beating back climate change and a stronger push to create better solutions to feed our energy needs, the solar energy market is heating up. Not only does it hold the title of cheapest form of energy-generation on the planet, but the minds behind solar panel technology or photovoltaics (PV) are getting smarter – developing ever more adaptable technologies.
Recognizing that people need a variety of solutions depending on the environment, climate, people and culture, the industry boasts a growing array of options that can work just about anywhere under the sun. It’s no longer just about those giant solar farms we see in deserts – PVs could also be floating on water, gilding our buildings, riding on car roofs, hiding in road infrastructures and shading our farms.
As these new applications become a reality through material breakthroughs in glass, silicon and polymers, in turn scientists need to be able to rely on analytical solutions that can test for desired properties in evolving solar component materials.
Deposing the silicon king
A prime example of this is the use of aging analysis. It improves understanding of how novel materials coming into the PV mix will perform alongside crystalline silicon, which made up 95% of the PV market in 2020. Crystalline silicon has proven pretty effective too – there are silicon modules in the field that are over 40 years old but still producing 80% of their initial power. Not bad after four decades on the throne. However, there are signs that silicon’s grip on power might be starting to wane. A PV module is made up of a multilayer composite of a variety of different advanced materials. Each material may require a specific set of analytical techniques to characterize the material properties for optimal solar power generation and PV module longevity. Polymeric materials, which are used as cell encapsulants, for layer adhesion and as backsheets, underpin the durability and long-term use of cells. These can be characterized using a range of analytical techniques; from thermal analysis to Fourier-transform infrared spectroscopy (FTIR) and hyphenated solutions. The interaction of the different materials within the composite PV module has been determined to be a primary root cause for module degradation. As such, a combined instrumental approach using a variety of analytical techniques to characterize the chemical and physical interactions at the interface of these materials is crucial to predicting their long-term stability.
A well-established portion of PV modules on the market already include CdTe (cadmium telluride) and CIGS (copper indium gallium selenide). While these thin-film technologies maintain a toehold in the market, they will not likely replace crystalline silicon as the dominant material for standard PV modules. Emerging technologies such as organic photovoltaic (OPV) and metal halide perovskite (MHP) solar cells continue to gain momentum as potential alternatives. Due to being relatively new additions to the solar energy landscape, outdoor aging data on OPV and MHP solar cells is scant. As such, technological advances are heavily reliant on laboratory testing to understand the degradation mechanisms of the newer cell materials. So far, OPV cells have proven to be relatively inefficient and can cause serious stability issues. These types of cells have been under extensive research due to their simple preparation methodology, low toxicity, low cost and ease of production. MHP cells, on the other hand, prove problematic because they are less abundant and not as stable as silicon. However, the efficiency and versatility of MHP cells has been shown to be extremely promising. In addition, MHP cells can be used in combination with other existing technologies, increasing overall efficiency further. Silicon-perovskite tandem solar cells for example, use a silicon base cell with a secondary perovskite cell on top. The secondary cell has a different band gap allowing the cell to better exploit the incoming solar radiation.
The greatest challenge is to create long-term stability. As such, there is more work to be done before these emerging technologies can de-throne silicon. As previously mentioned, there are a variety of techniques at our disposal to understand material stability. To get the most data out of our samples, hyphenated techniques that analyze a single sample with multiple instruments are desirable. Thermogravimetric analysis (TGA) can be combined with FTIR and even gas chromatography-mass spectrometry (GC-MS) to provide a hyphenated solution. Configurations of hyphenated solutions include TGA-FTIR, TGA-GC-MS or even TGA-FTIR-GC-MS and are used to provide ultimate flexibility in testing, allowing different types of material characterization data to be obtained from a single sample. The advance of OPV relies on the development of polymer semiconducting materials. Differential scanning calorimetry (DSC) along with TGA are indispensable thermal characterization techniques for polymer research. In the case of new and existing solar cell technology, thermal stability and degradation is a big concern. DSC and TGA can be used to study the degradation mechanisms of the components of these PV modules.
Cell technology breakthroughs
By producing larger solar cell wafers and dividing them into halves or quarters, the overall active surface area can be increased, resulting in higher outputs and a reduction in serial resistance. This alteration means that wafer sizes are projected to increase exponentially in the next ten years. As with current solar materials, the purity and optical clarity of solar cells remain vital. Inductively coupled plasma-mass spectrometry (ICP-MS), is used for elemental purity analysis, such as testing for the purity of silicon, while ultra-violet-visible-near-infrared (UV-Vis-NIR) spectroscopy is used to analyze the reflective and absorptive characteristics of glass, glass coatings and encapsulants. UV-Vis-NIR spectroscopy can also be used to detect changes in ideal optical properties in aged and degraded samples.
The market is also seeing a seismic shift in cell technologies that will change PV module production. Passivated emitter and rear cell/contact (PERC) technologies are dominating as the standard cells are fading out. PERC aims to achieve higher energy conversion efficiency by adding a dielectric passivation layer on the rear of the cell. Heterojunction technology (HJT) is another method on the rise as well. HJT combines existing wafer-based PV technology with thin-film technology, making it highly efficient. It is predicted that PERC and HJT will become dominant cell technologies in the future and that half wafer cells will gain more market share.
Next to advances in cell technology, the interconnection of cells is advancing. There is a strong focus on using multiple thin wires to wire bond the cell. These thin wires put less stress on the cells, reducing the risk of breakage and creating a larger active area. Cell interconnection by shingling is another significant advance. This simple principle is based on what you see on most tiled roofs. Producers overlap the solar cells, which negates the need for any wires or ribbons for connectivity. Structured foils are also an emerging technology whereby all contacts, the anode and the cathode, are on the back like a printed circuit board. As with previous examples, it is vital to have a good understanding of how these new materials and technologies interact with each other to determine and predict degradation using a suite of materials characterization instrumentation.
A shining future ahead
Future market growth figures for solar energy are undoubtedly impressive. The PV electricity market is expected to grow from $76.6 billion (USD) in 2020 to $113.1billion by 2025, at a compound annual growth rate of 8.1%. More countries are also getting in on the action with the USA, Europe, Japan and a host of other countries joining China in the market.
Thanks to the efficiency of PVs, energy production is not really an issue, however, the real bottleneck is storage. How/where can we store the growing volumes of clean solar energy and how can we efficiently transfer the surplus? The answer lies in battery research and energy storage. Batteries of all kinds are entering exciting phases of development.
For the solar cells themselves – at the energy-generation level, researchers are increasingly looking to alternative non-silicon materials in order to make PV modules cheaper, smaller and lighter while maintaining durability and longevity and at the same time improving energy efficiency. The study of multilayer polymer composites encapsulating and reinforcing solar cells will continue to develop materials with ideal properties that allow for optimization of formulation as well as improve the overall production process of finished PV modules. Innovative testing and analysis technologies or approaches will continue to play a large role in making exciting visions for solar to become an even larger piece of global energy sustainability.
About the author:
Nicholas Lancaster is an application scientist in the R&D team at PerkinElmer Inc. with a specialty in materials characterization leveraging FTIR spectroscopy, UV-Vis spectroscopy, thermal analysis, chromatography and mass spectrometry.