The hottest three-dimensional surface measurement

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Three dimensional surface measurement improves the efficiency of solar cells optical surface measurement through quantification, identification and monitoring, optical surface measurement instrument can improve the output of solar cells and reduce the overall production cost

Erik Novak, Stephen Hopkins, Andrew masters

soaring fuel prices, the government's energy advocacy program, and the growing worldwide demand to reduce greenhouse gases and carbon emissions are all driving the growing development of the solar industry. By 2050, the world's energy consumption will be twice that of today. At present, the production rate of photovoltaic solar cells is increasing by 40% every year. Like other industries, the key driver for the success of commercialization of solar cells is the total cost of end users. For solar cell manufacturers, this key driver is the cost of generating power per kilowatt hour over the life of the system

at present, in the growing solar energy market, there is fierce competition among a variety of photovoltaic technologies. Traditional solar cells are made of crystalline silicon. At present, this kind of cells still occupy a large market share in the manufacturing of solar cells worldwide. Amorphous thin film silicon can be used to manufacture solar cells that are lighter, easier to process, but relatively inefficient, so it is gaining more market share. Coupled with some other photovoltaic technologies, including CdTe, cuinga (SE) 2, and the most efficient iii/v triple junction battery, the current solar cell market is showing a competitive situation. Although each technology has its own merits, all these technologies require high-precision measurement of the surface to control the quality of materials. At present, many photovoltaic manufacturers use optical surface measuring instruments to quantify, identify and monitor various processing processes, so as to improve the output of solar cells and reduce the overall production cost

surface structure measurement

surface structure is one of the important surface parameters that affect the efficiency of solar cells. Depending on the wavelength of incident light, the reflectivity of polished monocrystalline silicon wafer (such as semiconductor grade bare wafer) is about 40%. The common technology to reduce the reflectivity is to structurize the surface so that the reflected photons have a certain degree of. Therefore, the jaws should often be swept and the probability of incident on another plane of the traditional industry of photovoltaic cell Internet empowerment, so as to improve the chance of photovoltaic reaction of photons in the emitting layer. However, if the surface is too rough, the average free path of electron/hole pair recombination will increase, so that the recombination probability that decreases after reaching a certain value will lead to the reduction of the total efficiency

using non-contact optical measurement, researchers have recently applied a set of strict theories and methods to develop an effective and repeatable method to establish the relationship between relative efficiency and surface properties. In the early research, researchers used Veeco NT Series optical surface measuring instruments to measure monocrystalline silicon solar cells under various amplification and scanning conditions. The researchers measured the three-dimensional surface characteristics of photovoltaic cells provided by three different manufacturers, and were able to correlate some surface structural characteristics with the output efficiency of the cells (see Figure 1)

Figure 1: different batteries have different surface structures, so their efficiency is different. The above figure shows the surface images of several photovoltaic cells with different surface slope (SSK). There is a relationship between the efficiency of these batteries and the surface slope (below)

surface texture differences between differently effective cells:

the surface structure of solar cells with different efficiency is different

a linear relationship between SSK and efficiency exists on these cells

there is a sexual relationship between the surface slope and efficiency of these cells

by optimizing the magnification and field of view, researchers established the relationship between surface structure and battery efficiency, The error between each battery is ± 0.5%

trace element and linewidth measurement

in addition to excellent vertical resolution and fast measurement time, the non-contact optical surface meter can segment the data to measure the important characteristics of different levels of the sample surface. For solar cell applications, this is commonly used for trace element and linewidth measurements. In the manufacturing of solar cells, it is necessary to accurately control the quality and quantity of silver or other elements used to ensure the performance of the panel, while reducing the fuzzy area caused by non photoelectric materials. Moreover, conductive trace elements that are too thick and too wide will increase the manufacturing cost (because the price of silver panels is relatively high) and reduce the efficiency of photovoltaic cells. In addition, especially in the process of film processing, expensive conductive ink is finally used to fill the scribed line, which can generate the required output power and current through "connecting" various active areas. If these scribed lines are too shallow or too deep, the width is incorrect or the location is incorrect, the performance of the battery board will be affected. Identifying this error before ink deposition can reduce ink waste

with the vision measurement software of Veeco company, the line width, line spacing, depth, volume and roughness of trace elements on the substrate can be automatically calculated, and all parameters can also be saved in the database for quality control in production (see Figure 2). The software can measure and analyze the surface with hundreds of features

Figure 2: three dimensional and multi area display of the scribed line measurement results on thin-film solar panels can provide information such as overall roughness, scribed area roughness, line width and scribed depth

surface Stats: surface statistics

multiregion analysis: multi region analysis

similarly, optical surface measuring instruments can also measure the surface properties of materials under various conditions. For example, the Department of materials science and engineering at the University of Illinois measured the effect of grain boundary on the growth and photoelectric efficiency of CIGS twins using Veeco's optical surface measurement instrument (see Figure 3). Through high-precision and rapid quantitative measurement, optical surface measuring instrument can help researchers improve the performance of solar cells

Figure 3: the influence of the junction of different substrate crystal orientation on the growth of CIGS material can be measured by optical surface measuring instrument. The above figure shows the different grain structures on both sides of the boundary

optimization and control

in the processing instruments used in solar energy manufacturing industry, people have long used interferometric optical systems for quality control and processing development/improvement. With the advanced self-control ability of optical or probe measuring instruments, the etching rate and precipitation rate along the wafer can be calculated quickly. The feature height of different positions can be quickly measured along the substrate. These data can provide feedback for processing control and optimization. A typical measurement application is to measure the height change of step characteristics within the range of 8-inch wafer during precipitation. This measurement and analysis can be carried out at various locations. The obtained data can be used to improve the uniformity and average height of important features

The Veeco optical surface measuring instrument can also integrate other features for quantitative defect detection and analysis. The user can set the volume or height threshold, and the software can automatically identify defects and report the maximum range such as height, diameter, volume and X or Y direction. By quantifying these surface defects, the system user can decide in which process the defects appear, so as to optimize the processing process and eliminate the defects

film thickness

the thickness of different substrate layers, whether transparent or not, needs to be properly measured, especially for CIGS devices. The contact method adopted by the probe measuring instrument provides a fast and accurate method to measure the film thickness at the boundary, and can easily determine the film substrate spacing. The contact force of Veeco dektak analyzer is very small, so it can carry out nondestructive measurement of materials, even soft polymers. More importantly, due to the contact method, the probe surface measuring instrument is not sensitive to the difference of material properties. These differences will lead to the deviation of optical technology when the material is very thin or the absorption is different. More importantly, because these information can be obtained in only a few seconds, it becomes practical in frequently detecting the processing quality

due to their different capabilities, probe measuring instruments and optical surface measuring instruments are usually used together in film thickness control. For example, nt9100s optical surface measuring instrument can be a useful supplement to dektak probe measuring instrument in many important aspects, and can measure the surface of transparent film samples with a thickness greater than 2 m. The optical system can make faster area based measurements, but if there is a height difference caused by optical characteristics, the dektak meter can quickly calibrate the film. Then, the analysis software can automatically compensate in the next optical measurement. In addition, the NT measuring instrument can provide the roughness and defect information of the upper and lower surfaces of the film respectively, so it can analyze the projection properties of the film. Therefore, these two kinds of measuring instruments can work together well to ensure that the thickness and surface quality of the film can be fully described at the same time, so as to improve and ensure how to solve the problem of how to maintain the highest performance of many electronic change experimental machines

meet the rapid progress of photovoltaic technology

with the increasing demand for improving efficiency and reducing costs, various solar cell processing technologies have developed rapidly. Among them, accurate measurement of key surface features is the core of this development. By measuring the thickness and surface of the sample with sub nanometer accuracy, we can provide the necessary data to improve the development and production process of solar cells. Texture, jump height, trace, reticle width, film thickness and defect detection are all important to the actual production line. At the same time, researchers can study material effects, environmental effects and fatigue, and make complex measurements to better understand the impact of various variables in processing equipment on end products. (end)

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