Solar Cell Analysis Shines

Alternative energy sources have received much attention and government funding in the past decade, and solar cell, or photovoltaic (PV), technology is no exception. The research and production of solar cells relies upon a variety of analytical instrumentation, including atomic spectroscopy, GC, molecular spectroscopy and surface science techniques. Raman, FTIR and UV-Vis-NIR spectroscopy, as well as scanning electron microscopy (SEM) are among the analytical techniques employed to improve solar cell efficiency and ensure quality control.

It is only in the last five years that PV technology has become a fast-growing commercial source of energy. The development of more efficient solar cell technology and interest in minimizing carbon emissions has led to its advancement. According to 2010 data from the US Department of Energy (DOE), worldwide solar capacity increased 75% in 2008 to 13.9 GW, and the PV industry has had a five-year compound annual growth rate of 56% through 2008. According to a 2009 report by the European Commission, worldwide solar capacity is estimated to reach 16.0 GW this year. One of PV’s major hurdles, its high price tag, is also being overcome at an increasing rate. A 2009 study by the Lawrence Berkeley National Laboratory found that the cost of installed solar cells have fallen by more than 30% over the past decade.

The rapid growth of the PV industry has translated into increased demand for analytical instrumentation for solar applications. One of the primary applications for analytical instruments in this sector is materials characterization. The most important types of materials characterization applications include contamination, surface, structural and defect analyses. Research utilizing materials characterization techniques are focused on improving the cost efficiency of crystalline silicon (first-generation) and thin film (second-generation) cells by decreasing wafer thickness and investigating the use of more efficient and cheaper materials for cells. For cell manufacturing, materials characterization applications are necessary to test the raw silicon when it is received and as it is being processed into a cell, as well as to test the finished product.

The increased commercialization of cells has put the emphasis on materials characterization during manufacturing. As many manufacturers rush to produce cells, they often seek to improve the efficiency of the produced cells to meet the cells’ theoretical efficiency levels by making changes during the manufacturing process. “When the [manufacturers] started to get into [PV], they kind of skipped the step of the research and went straight into process monitoring. At the moment, there’s definitely a time-to-market issue going on,” said Emmanuel Roy, Raman Development technical support manager for Horiba. According to him, this is due to the relatively low number of PV manufacturers that are in the market. In an effort to create more efficient cells more quickly, many cell manufacturers are improving the material properties of their cells during manufacturing. “To cut corners, [manufacturers are] saying ‘we’re not going to do any research in the lab.’ The first thing is to get the process control online and determine if the material complies with what they think they’re making,” said Mr. Roy.

The nondestructive nature of detection done by molecular spectroscopy, such as Fourier transform infrared (FTIR) and Raman spectroscopy, has made these techniques popular for analyzing the quality of cell materials and for analyzing finished cells for contamination and structural integrity. Using Raman and FTIR, impurities in silicon can be detected and quantified. Impurities in PV cells are generally any unwanted material inadvertently introduced during the manufacturing process, and include inorganics, such as trace metals, and organics, such as chemicals and gases used in the production line. In addition, Raman and FTIR techniques can analyze structural parameters, such as the crystallinity of finished crystalline silicon. “For silicon, the crystallinity ratio is the parameter that people fine tune to improve efficiency or cost. Crystalline silicon is efficient, but it’s really expensive. So if you mix crystalline and amorphous silicon you can get much better quality to price ratio,” said Andrew Whitley, PhD, vice president of Sales at Horiba. For thin film cells, the stoichiometry and phase of the material are the structural parameters most commonly monitored.

Compared to FTIR, Raman’s spatial resolution distinguishes the technique, making it the choice for a number of PV materials characterization applications. Dr. Ian Mowat, director of Sales and Customer Service at Evans Analytical Group, an independent testing laboratory that specializes in materials characterization, stated: “We use Raman to look at things that are too small for FTIR. So, for example, if you have a defect or residue that is much smaller than 10 or 20 µm, that goes a little past the limit of FTIR’s X and Y focus capabilities.” Raman is also the centerpiece of Horiba’s line of analytical instruments for PV materials characterization. Mr. Roy attributed the technique’s popularity for PV applications to its flexibility. “Raman has been the first technique to get into the factory, and then we push the other techniques. On the research side of things, a lot of techniques are being used, but Raman seems to be the one, or one of the ones, that is the most compatible to go online, so that’s why it’s popular,” he said. Comparing Raman’s use in the PV industry to its use in the pharmaceutical industry, Dr. Whitley noted that it is proficient in both materials development and process control. “The advantage of Raman [in both industries] is that because it’s a visible light technique, it’s very easy to use some sort of fiber-optic probe to throw the technique onto a process line,” he said.

FTIR’s appeal to the PV market is its price, broad application coverage and ability to do specialized measurements. FTIR is unique compared to other spectroscopy techniques for PV materials characterization in that it can determine the oxygen content of silicon. As Dr. Mowat explained, “lots of the silicon manufacturers use FTIR in house for specialized silicon measurements, with oxygen content being one of them.” Other specialized measurements include the measurement of substitutional carbon, which is linked to negative electrical and physical effects. Evans Analytical Group primarily uses FTIR for the analysis of cell packaging, in order to measure polymer, encapsulant and plastics levels. Kevin McLaughlin, senior manager of Marketing Communications at Shimadzu, also highlighted degradation analysis of packaging materials, such as polymers, as an application for FTIR.

UV-Vis-NIR is another important spectrometry technique for PV materials characterization. But, unlike FTIR and Raman, UV-Vis-NIR is used for absorbance and reflectance measurements of cells. Such measurements are used to examine the effectiveness of texturing and treatments on the surface of PV cells. Solar cells undergo texturing and treatments to form indentations on a cell’s surface that increase the absorbance of light. UV-Vis-NIR monitors cell transmission, absorbance and reflectance in order to determine if a cell’s texturing is maximizing its efficiency. “Solar cell manufacturing plants are constantly seeking higher conversion efficiency. For this reason, the evaluation of transmission and reflectance by a spectrophotometer at a specific wavelength region is required,” explained Mr. McLaughlin. “For crystalline silicon, the key applications include verification of the effectiveness of antireflective film, and reflectance and transmission measurements in UV, visible and NIR ranges. For thin film cells, the key analysis is the transmission measurement of substrate materials.” Currently, 80% of Shimadzu’s sales to the PV market are of UV-Vis-NIR instruments. In the past four years, Shimadzu’s sales of analytical instruments to the PV market in general have increased threefold, according to Mr. McLaughlin.

SEM is used to analyze cell structure, the interface of cell layers and defects in completed cells. One of the main benefits of SEM is that it can analyze a PV cell in multiple modes. SEM is used to image cells, scan at a higher magnification using cathodoluminescence, perform electron back-scattering detection to determine grain orientation, and analyze cross sections of thin films to examine various cell properties. Dr. Mowat described some of the thin film properties that can be analyzed with SEM. “If you have a thin film structure and you want to look at that film structure and the uniformity of that film structure, then doing a cross-section [of the thin film] would be the way you want to go with SEM, so you can determine the layer structure, layer thickness and layer uniformity.” Cross-sectioning thin films requires sample preparation prior to SEM analysis. JEOL, a provider of SEMs, noted increased interest in its SEM sample preparation products for PV applications. “We’ve actually seen a lot of growth for our sample preparation equipment in [the PV] market. We have two sample preparation products. One is the cross-section polisher and the other is a much more familiar product, the dual-beam focused-ion-beam (FIB). The cross-section polisher was introduced about five years ago and in the past year or two, it has seen a lot of growth in this market,” said Natasha Erdman, PhD, SEM/FIB product manager at JEOL. According to Dr. Erdman, one of the other factors driving growth in the use of SEM for PV analysis is the need for tools to analyze the newer materials being developed. “I think with the way that solar technology has been going, the materials have been shrinking significantly in size. Materials have become much more complex, and a lot of composite type of materials are now involved. People are now looking for high-resolution SEM technology, which really only became available in the last five or so years,” she stated.

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