Concentrated photovoltaic technology is on the rise and PV standards revision is urgent

High conversion efficiency solar energy is the general trend, which brings great challenges to measurement standards and related testing equipment. Faced with manufacturers' innovation in high conversion efficiency solar cell technology, the International Standards Organization has also followed the industry's footsteps and released updated measurement and safety standards to promote the vigorous development of the entire industry.

In 2004, Germany's high subsidy policy set off a solar energy (PV) boom. After more than half a century of dormancy, solar cell manufacturers finally felt the warmth of spring for the first time. After crazy investment and rapid development in various countries, prices and technology fluctuated between 2009 and 2010 due to the imbalance between supply and demand. In 2011, the German government finally announced the suspension of solar energy subsidies due to concerns about the stability of the power grid caused by the high proportion of renewable energy installed, which almost announced the arrival of another cold winter for the solar energy industry.

Solar industry players must face the challenges of the future. Should they move to countries that still have subsidies, or must they further reduce costs or develop technologies with higher conversion rates? However, as a country's subsidy policy is revised, cost reduction is also limited. Developing technologies with higher conversion rates seems to be the only way to go in the era without subsidies.

High conversion rate solar cells are the general trend

While the solar energy industry is shrouded in uncertainty due to fluctuations in production, competition and policies, manufacturers with production disadvantages are also actively developing more new forms of solar cells. Since conversion efficiency is limited as yield increases, elements with higher conversion efficiency combined with different structures are gradually becoming the direction of efforts of various R&D teams.

The theory of quantum efficiency and the law of conservation of energy define the limit of photoelectric conversion efficiency of each interface form. Therefore, the current methods to improve the overall conversion efficiency of each unit are mainly to increase the unit light intensity and improve efficiency (Figure 1), and to increase the solar spectrum sensing frequency band to increase the utilization rate of sunlight. There are also hybrid modes that combine the two in the hope of achieving a multiplier effect. Theory needs to be proven by experiments. The conversion rate is directly equated with profit. In order to avoid different opinions, the verification of conversion rate has become the graduation test for these new technologies. The fairness and universality of the test naturally become the focus of disputes between buyers and sellers.

Figure 1 Schematic diagram of focusing light to increase light intensity

Quality and efficiency verification is crucial

The most direct way to verify solar cells is to put the sample under the sun and measure the data. However, the sun itself is not a standard. As the distance between the earth and the sun changes, the sunspot activity of the sun itself, etc., the sun cannot be an ideal stable light source. In addition, the weather, air composition and angle of illumination of the earth itself will further deteriorate the consistency of sunlight, making outdoor sunlight a cheap but completely impractical solution.

Simulating the sun becomes the only effective solution. The IEC60904-9 proposed by the International Electrotechnical Commission (IEC) and the G159 of ASTM have both established so-called standard spectra to facilitate the definition of the quality of simulated light sources. The IEC60904 series has become a standard that simulator manufacturers follow.

For large-area solar cells (length and width of more than 1 meter) without focusing, in order to simulate uniform sunlight, simulators of point-shaped light sources mostly use a multi-angle superposition system with a long distance to achieve uniformity. However, the above two types of new solar energy technologies are not suitable for direct measurement with current flat-panel solar simulators in the new solar energy battlefield where every penny counts. The main reasons are as follows:

High-concentration system testing is full of challenges

The increase of focusing ratio can increase the input and achieve the immediate effect of increasing the output, so the increase of focusing ratio has also become the goal of focusing industry. From the common low focusing of 5 to 10 times to the high focusing of 200 to 400 times, German industry tends to adopt low focusing ratio, while American industry tends to adopt high focusing ratio. However, the problem brought by high focusing ratio is not just the problem of optical system. Even if the optical system becomes perfect, the high irradiation energy brought by high focusing can also cause danger. If the tracking system is not accurate enough, the offset beam energy can almost burn the circuit or insulation material around the battery. Therefore, the accuracy of the tracking system often needs to reach the range of ±0.1 degrees.

Even if the accuracy of the sun tracking system can meet the requirements, the high-concentration system will face more stringent tests. If the focusing ratio is higher than 300 times, even if the focus is correct, the long-term accumulation of light and heat that cannot be converted may directly burn the battery body.

On the other hand, the curse of high concentration also shrouds the battery R&D and testing team. Since there is no direct light source with a light intensity of 200 to 400 that can test solar cells, the current conversion efficiency calibration cannot be carried out directly, and it is also necessary to achieve the requirement of increasing the unit light intensity of the tested area through concentration. However, the sunlight characteristics generally received by the concentrating module are not just the difference in light intensity, but also the direct characteristics of light must be considered. The operating environment received by the lens of the concentrating module is basically direct parallel light, and the simulated light source is mostly a point light source. Therefore, if it is irradiated on the concentrating module without reaching parallel direct light, it will not produce the same measurement results as outdoor exposure. At the same time, due to the consideration of reducing manufacturing costs and increasing the concentration ratio, the cell wafer of the concentrating cell will be made smaller. How can it be determined that the consistency can be maintained on small areas of different sizes after refraction, and the measurement must be performed before assembly? In the case that the lens of the assembler does not necessarily exist, the simulator may also have to simulate the light characteristics after the lens is refracted, so it seems difficult.

In addition, in the past, high-concentration photovoltaic cell manufacturers often used transient (2 millisecond flash rate) solar simulators as power calibration equipment. Although they can quickly generate IV curves, they ignore the time difference effect of light path refraction. Even if the transient time can be extended to 10 milliseconds, it is impossible to generate enough accumulated heat energy to observe the thermal effect of the battery cell. As a result, the tragedy of the high-concentration system being highly efficient but unusable results. Therefore, in addition to transient simulators, steady-state simulators are also a necessity for high-concentration solar cell manufacturers.

Multi-junction battery testing requires comprehensive consideration

Due to the characteristics of the elements, there is currently no product that can achieve full spectrum response to sunlight with a single junction. Therefore, the current method of increasing the sensing frequency band must be carried out by using multiple junctions. The goal of sharing full spectrum light energy is achieved by utilizing the different response characteristics of different junctions. However, light will be lost when passing through each junction, and the energy reaching the lower junctions will be lower. Under the cutoff current requirements of the photoelectric effect, it is also necessary to pay attention to whether the light beam reaching the target junction still has the intensity to start the photocurrent. In addition, since the photocurrent of multiple junctions is the sum of each junction, the quality of different junctions will affect the overall efficiency. In order to ensure the quality analysis of different stages and different junctions, the multi-junction solar cell must also be able to control the transmitted spectrum and penetration depth in order to confirm the effective position and effective response of the tested area.

Table 1 of IEC60904-9 shows the grading requirements for the difference between the simulator spectrum and the standard spectrum. Even the highest level A still has a 25% irradiance difference. In the case of multi-segment spectrum response of multi-junction systems, the band in which the 25% difference exists will significantly affect the power calibration results. Therefore, further improving the compliance of the light source spectrum will be another important challenge for multi-junction solar simulators.

New standards are being formulated non-stop

In view of the increasingly prosperous and progressive solar cell technology, which has a declining profitability, the high-priced concentrator and multi-junction technologies will sooner or later leave the laboratory and enter mass production and price reduction. The International Standards Organization is aware of the need for standardization of measurement methods to promote the sound development of new industries. Under the existing IECTC-82 working group framework, it also carries out the formulation of calibration standards for high conversion efficiency cells and starts the specification standardization of simulators for concentrator solar cells. The current work and goals of IECTC-82 on high conversion efficiency are shown in Table 2.