Observation and Analysis for Evaluation of Solar Cells
In recent years, the reduction of fossil fuel consumption and the use of photovoltaics have become popular ways to prevent global warming and otherwise preserve the environment. The main advantage of solar cells is that an unlimited amount of energy can be obtained anywhere exposed to sunlight with no damage to the environment. For this reason and because the power generation efficiency does not depend on the scale of the equipment, the demand for photovoltaics is growing.
This section introduces basic knowledge of solar cells, including the operating principle and structure, and also introduces examples of high-resolution observation, highly accurate measurement and analysis, and quantitative evaluation with higher efficiency.
- Operating Principle, Structure, and Power Conversion Efficiency of Solar Cells
- Problems in Observation, Measurement, Analysis, and Evaluation of Solar Cells
- The Latest Application Examples of Our Microscope That Improves the Efficiency of Observation, Measurement, and Analysis and Enables Quantitative Evaluation of Solar Cells
- A 4K Microscope That Optimises Observation, Measurement, Analysis, and Evaluation of Solar Cells
Operating Principle, Structure, and Power Conversion Efficiency of Solar Cells
The major solar cells currently available are silicon solar cells or compound semiconductor solar cells. Although silicon solar cells are most commonly used, the market share held by compound semiconductor solar cells is increasing because they have an advantage in terms of production costs. The basic operating principle and structure of solar cells as well as typical materials and power conversion efficiencies of silicon solar cells and compound semiconductor solar cells are explained below.
Operating principle of solar cells
Silicon solar cells, which are commonly used, have an internal junction of two types of semiconductors, the p-type and n-type, each having different electric properties.
When a solar panel is exposed to sunlight, electrons (negative) and holes (positive) are generated. Holes are attracted to the p-type semiconductor, while electrons are attracted to the n-type semiconductor. For example, when a load, such as a light bulb, is connected to the electrodes of the front and back sides, electrical current passes through the junction as shown in the following figure.
- A: Anti-reflection film
- B: n-type silicon
- C: p-type silicon
- D: Electrodes
- E: Current
Structure of solar cells
The units and their names are explained below with a figure showing the structure and the unit names.
- A: Cell
- B: Module
- C: Array
- Cell
- A cell is the minimum unit.
- Cell string or cell array
- A single cell outputs a low voltage. A cell string contains multiple cells connected in series to generate the required voltage.
A cell array contains multiple cell strings connected in serial or parallel to further increase the output.
- Module
- A module, also called a solar panel, is a package containing multiple cell arrays for outdoor use. A module is covered with resin or reinforced glass to protect the internal cells and is attached with an outer frame to enhance its strength.
- Array
- Multiple modules are connected to form an array.
Power conversion efficiencies and materials of solar cells
An important performance indicator of solar cells is power conversion efficiency. Power conversion efficiency is a parameter that shows the fraction of incident sunlight energy converted into electric energy. Module power conversion efficiency and cell power conversion efficiency are two representative indicators for photovoltaic power conversion efficiency. The two power conversion efficiencies are explained below.
- Module power conversion efficiency
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Module power conversion efficiency is commonly used to indicate the power generation ability of a solar module (solar panel). Module power conversion efficiency is a percentage of electric energy converted from approximately 1 kW of light energy per 1 m² of solar module.
- Cell power conversion efficiency
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A cell is the minimum unit that makes up a solar module. Cell power conversion efficiency indicates the power conversion efficiency per solar cell. Cell power conversion efficiency can be derived with the following formula.
With continued research and development, module and cell power conversion efficiencies are improving year after year. However, modern technology cannot absorb 100% of the incident light energy and convert 100% of the absorbed light energy into electricity due to various factors such as light reflection on solar panels and the resistance of the cells.
- Materials and characteristics of solar cells
- Solar cells use various materials according to the application, the functions required by the application, and the expected costs. Power conversion efficiency varies accordingly. Therefore, manufacturers are working on research and development of materials and manufacturing processes to achieve higher power conversion efficiency and lower costs.
Below, solar cells are categorised into the silicon type and the compound semiconductor type, and the representative materials and their characteristics are explained for each type.
- Silicon solar cell
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- Monocrystal
- Solar cells using monocrystalline silicon are expensive but have high conversion efficiency and reliability.
- Polycrystal
- Polycrystalline silicon cells are most commonly used because polycrystalline silicon is cheaper than monocrystalline silicon.
- Amorphous silicon
- Because amorphous silicon is non-crystalline silicon, solar cells using amorphous silicon are cheaper than those using polycrystalline silicon but have low power conversion efficiency.
- Multi-junction cells
- Various types of solar cells, including amorphous silicon and thin film polycrystalline silicon, are laminated to make a tandem structure. This type of solar cell has high power conversion efficiency.
- Compound semiconductor solar cell
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- Copper indium selenide (CIS)
- CIS solar cells are made from copper, indium, and selenide. This type of solar cell can be manufactured at low cost but has relatively high power conversion efficiency.
- Copper indium gallium selenide (CIGS)
- With gallium added to the three elements used for CIS solar cells, CIGS solar cells use four elements. This type of solar cell has power conversion efficiency slightly higher than CIS solar cells.
- Cadmium telluride (CdTe)
- CdTe solar cells are made from cadmium and tellurium. This type is mainly used in Europe.
- Gallium arsenide (GaAs)
- GaAs solar cells are made from gallium and arsenide. This type of solar cell has high power conversion efficiency but is expensive. It is used in satellites and in similar applications.
Problems in Observation, Measurement, Analysis, and Evaluation of Solar Cells
In the photovoltaic industry, with the need to preserve the environment and the growing demand for renewable energy, manufacturers are researching and developing solar cells that provide higher power conversion efficiency at lower costs and are competing with each other to capture new markets. Also, high levels of quality assurance and control are necessary to provide stable photovoltaic and power storage products and maintain reliability in the aftermarket.
Solar cells have surface irregularities to increase the surface area. Each section has a mix of various materials with varying colours and glossy surfaces. This makes it difficult to accurately observe, measure, and analyse microscopic parts—such as electrodes—of defective products and prototypes, making these operations time intensive.
A lot of time and effort and a high level of expertise are required for observation, measurement, analysis, and other related tasks using optical microscopes. On the other hand, when a scale is used for visual measurement, measured values can vary from operator to operator.
Preparation takes a lot of time and effort when using a scanning electron microscope (SEM) for cross-section measurement. It is also difficult to identify materials and foreign particles in defective areas because SEMs do not support colour observation.
The Latest Application Examples of Our Microscope That Improves the Efficiency of Observation, Measurement, and Analysis and Enables Quantitative Evaluation of Solar Cells
The recent technological progress of digital microscopes eliminates the problems faced by optical microscopes and greatly improves the efficiency of observation, measurement, and analysis. Our latest digital microscope has an automatic assist function that easily enables observation using high-resolution images, highly accurate 2D and 3D measurements, and particle counting on the details of solar cells.
With high functionality accessible with simple operations, KEYENCE’s VHX Series 4K digital microscope provides clear images and accurate dimensional measurement using high-resolution HR lenses, a 4K CMOS image sensor, and lighting and image processing technologies, greatly improving with a single unit the efficiency and speed of the series of tasks from observation, measurement, and analysis to report creation for solar cells.
Read on for an introduction to examples of observation, measurement, and analysis of solar cells using the VHX Series.
3D shape measurement of electrodes
To increase the power conversion efficiency of solar cells, it is necessary to minimise the width and also the height of the electrodes. When an expensive material, such as gold, is used for electrodes, minimising the volume can reduce the cost.
It is difficult to accurately measure fine electrode shapes using optical microscopes, making it impossible to measure 3D shapes quickly.
With the VHX Series 4K digital microscope, 3D shapes can be measured with micrometre level precision using a magnified high-resolution image. The combination of a colour map that visualises height data and profile measurement at multiple specified locations makes it easy to compare microscopic part shapes.
Cross-section observation of defective areas
When a polished cross section of embedded resin is observed at high magnifications using an optical microscope, even subtle irregularities left on the surface make it impossible to bring the entire surface into focus, preventing clear observation. Preparation such as completely or almost completely evacuating the sample chamber takes a long time when observing a cross section using a scanning electron microscope (SEM). It is also difficult to detect changes in materials and to identify foreign particles mixed in the cross section because SEMs do not support colour observations.
The VHX Series 4K digital microscope has a 4K CMOS image sensor and a large depth of field achieved with a newly developed optical system. These features enable observation using clear 4K colour images that are fully focused throughout the field of view, without being affected by surface irregularities of samples.
With the seamless zoom that automatically switches the observation magnification level from 20x to 6000x without replacing the lenses, the magnification can be switched quickly using a mouse or handheld controller, enabling high-resolution observation of cross-section samples quickly with simple operations.
Observation of solar modules (panels)
It is difficult to observe solar modules (panels) with optical microscopes because various materials having different colours and glossy surfaces are mixed on the surfaces and because subtle irregularities and scratches on the surfaces have low contrast.
The VHX Series 4K digital microscope is equipped with the High Dynamic Range (HDR) function, which combines together multiple images captured at varying shutter speeds to acquire an image with high colour gradation, enabling observation using high-contrast images that emphasise textures. Even during tilted observation using the free-angle observation system, which enables observation at any angle, samples can be observed in images focused throughout the entire depth using the depth composition function.
Particle counting on wafer surfaces
With functions that assist automatic operations such as the Multi-lighting function, which just requires the operator to select an image suitable for the observation from multiple images captured under automatically controlled omnidirectional lighting, the VHX Series 4K digital microscope can simplify lighting condition determination for observation and reduce the required time. Of course, past settings can be easily reproduced for other samples.
Additionally, area measurement and particle counting can be performed automatically in an area specified with simple operations. In this area, targets that are not required can be excluded, overlapping targets can be separated, and other useful functions can also be used.
These functions allow any operator to quickly obtain highly accurate analysis results without numeric errors attributable to the operator’s experience or level of expertise.
A 4K Microscope That Optimises Observation, Measurement, Analysis, and Evaluation of Solar Cells
The VHX Series 4K digital microscope enables reliable observation with simple operations and the clear image quality only available with high-resolution 4K images. This microscope uses observation images for 2D and 3D (3D shape) measurement and automatic area measurement/count, enabling quick acquisition of numeric data and quantitative evaluation without human errors.
The VHX Series is a powerful tool that solves various problems faced by optical microscopes and SEMs with a single unit, greatly improving work efficiency. Additionally, spreadsheet software can be installed directly on the VHX Series, like PCs, which enables even automatic report creation by outputting images and measured values to a template prepared in advance. These features make workflows more accurate and fast, which is indispensable to prompt research and development of products superior to those of your competitors, quick implementation of quality assurance, and reductions in the time required for quality control.
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