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Why the latest IEC 60904-9:2020 standard set A+ grade spectrum for a solar simulator classification?

How the solar simulator with A+ spectrum can more accurately evaluate perovskite solar cells?

IEC (International Electrotechnical Commission) officially released the latest solar simulator evaluation standard, IEC 60904-9:2020, in 2020. Compared with the previous version, IEC 60904:2007, there are three main differences:

  1. It adds 300nm~400nm spectral range to spectrum evaluation.
  2. It changes the wavelength range of each spectral band.
  3. It adds the New A+ spectrum grade for solar simulator spectrum.

Table 1 shows the new global reference solar spectral irradiance distribution given in IEC 60904-3 contribution of wavelength intervals to total irradiance. According to these contribution potions, IEC 60904-9:2020 defines the classifications of a solar simulator to 4 grades, A+, A, B, and C, in Table 2. Among that, A+ grade is the new grade in IEC 60904-9:2020 compared to the previous standard IEC 60904-9:2007.

Table 1. The AM1.5G reference solar spectral irradiance distribution contribution of different intervals to total irradiance. [IEC 60904-9:2020]

Table 2. Definition of solar simulator classifications. [IEC 60904-9:2020]

We can find that the requirement for the better simulator spectrum is one of the main focuses of this new revision. Mainly in response to the vigorous development of high-efficiency solar cells, such as perovskite solar cells, more accurate spectrum can be used to characterize the conversion efficiency of solar cells more accurately. The more accurate spectral meaning includes a wider evaluation range and a more detailed grading band, which can be closer to the AM1.5G spectrum.

Enlitech has accumulated more than a decade of constructing artificial light source to develop SS-X series, a new generation of solar simulators, which complied with the latest IEC 60904-9:2020 standard. SS-X series solar simulators have better spectral match to AM1.5G spectrum. The spectral grade is classified to A+ according to IEC 60904-9:2020. The better spectrum feature can help users to get more accurate IV curves and better PV conversion efficiency, which can accelerate users’ RD achievement.

A+ and A Grades Spectra

Figure 1 and Table 3 show Enlitech’s SS-X solar simulator spectrum and the other manufacture’s spectrum.

Figure 1. The irradiance spectra of Enlitech’s SS-X and other brand’s solar simulators. AM1.5G spectrum is also shown for comparison.

Table 3. The spectrum grades of SS-X and other brand’s solar simulators, which are graded according to IEC 60904-9:2020.

It shows that SS-X has better spectrum grade in each wavelength band according to the new IEC standard. The maximum spectral error of each band is less than 9% (A+ grade requirement is less than 12.5%). Compared with other brands, although each band meets the class A requirement (25% error), in 772nm~919nm band the error is as high as 21%. This IR band covers the absorption band edge of many new type of solar cells, such as perovskite solar cells and organic solar cells, which will affect the IV and conversion efficiency results. Therefore, the spectral match is quite important for accurate PV characterizations.

Verification of the Real Solar Cell Device

A stable device which has similar absorption band edge and EQE curve to organic and perovskite solar cells is chosen. The EQE curve is shown in Figure 2, which is measured by Enlitech’s QE-R EQE system. The QE-R system is currently the best EQE test system in the world. It is adopted by many international PV research laboratories.  In the past five years, more than 1,000 SCI journals have cited QE-R by name, especially the Jsc (EQE) test results, which have been recognized by experts and scholars all over the world.

Figure 2. The External quantum efficiency (EQE) spectrum of the device under test (DUT) which has similar EQE response to perovskite solar cells and organic solar cells.

How to verify the accuracy of the IV results obtained by different simulators?

We will adopt the most common and important method when submitting a manuscript to the scientific journal: Jsc (SS) and Jsc(EQE) comparison. Jsc(SS) represents the short-circuit current density tested under a solar simulator. Jsc(EQE) is the integrated short-circuit current density from EQE curve with AM1.5G spectrum. The method of Jsc(EQE) is described detailly in IEC 60904-7 standard. The difference between Jsc(SS) and Jsc(EQE) is viewed as the judgment of reliable characterization results in PV society and journal referee. Generally, if the difference between Jsc (SS) and Jsc (EQE) is within 5%, reviewers of journals will consider it to be an accurate and trustable result. We will use this methodology to verify the differences between A+ and A solar simulator spectra. Experimental steps are described as follows:

  1. One WPVS type reference solar cell, which has absorption band edge at 850nm and calibrated by NREL, is used to calibrate the light intensity of the solar simulators.
  2. IVS-KA6000 IV software is used to control the SMU Keysight B2901 to read the current-voltage curve of the device under test.
  3. Pt temperature sensor is used to monitor the temperature variation of the device under test to minimize the error caused by temperature variation.

The IV curves and solar cell performances is shown in Figure 3. The short-circuit current densities are 22.388 mA/cm2 and 21/482 mA/cm2 from Enlitech’s SS-X50 and other brand solar simulators, respectively. The Jsc(SS) under two solar simulators are quite close, but which one is more accurate? We can compare to the Jsc(EQE) which will show that which result is more accurate. From Figure 2, the Jsc(EQE) is integrated from EQE curve by AM1.5G spectrum and equals to 22.44 mA/cm2.

Figure 3 The IV curves and solar cell performance parameters.

Table 4. Jsc(SS) and Jsc(EQE) comparison table.

The comparison table shows the Jsc(SS) and Jsc(EQE) in Table 4. From Table 4 we can find that Jsc(SS) under SS-X50 solar simulator is much close to Jsc(EQE). The difference is 0.11% which much smaller than difference (2.18%) between Jsc(SS) under other brand’s solar simulator and Jsc(EQE). This is because that SS-X50’s spectral grade is A+which has much better spectral match to AM1.5G spectrum. Therefore, it can provide more accurate PV testing result without any spectral mismatch correction. The other brand’s solar simulator’s spectrum is A grade. However, the spectrum at 712nm~919nm band is negatively deviates from AM1.5G spectrum by more than 20%, which is the main reason why the Jsc(SS) has bigger difference (2.18%) from Jsc(EQE).

From the above results, if the testing procedure is proceeded appropriately, the A+ and A grade solar simulators both can provide the difference smaller than 5% between Jsc(SS) and Jsc(EQE). However, with increasingly fierce competition in modern high-efficiency solar cell development, it is quite important to take the 0.3% PCE difference, which may create the new word record. Therefore, it is necessary and valuable to select a A+ spectrum grade solar simulator.

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