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How to precisely measure OPV/DSSC/Perovskite solar cells?

The efficiency of solar cells are evaluated in accordance with international standard called the Standard Test Conditions, STC (1000 W/m2 of AM 1.5G and a cell temperature of 25℃). With the rapid development of the photovoltaic industry and advancement of material researches in solar cells, a stricter measurement method is needed for new type of solar cells compared to conventional crystalline silicon solar cells. This article introduces the Maximum Power Measurement Method for OPV/DSSC/Perovskite(PVK) solar cells, which provides a standard procedure to make the measurements more accurate. Under Standard Testing Conditions, the bias error is computated by the spectrum of light source and the mismatch between the spectral responses of the solar reference cell and the test sample. Therefore, select an appropriate solar reference cell to calibrate the intensity of solar simulators which allows more accurate measurements.

1. Introduction

With the leap-forward development of OPV, DSSC and Perovskite(PVK) solar cells, many research teams have developed the solar cells with the conversion efficiency more than 10%. However, the measurement method of these new type of solar cells is different from the crystalline silicon solar cells. These solar cells have a slower reaction time to light compared to silicon solar cells. Also, one of key factors is the correction of spectral mismatch. Because OPV/DSSC/PVK and Si solar cells have different spectral responses, which is called spectral mismatch, the correction of spectral mismatch has to be done before adjustment of light intensity. Use an appropriate solar reference cell can minimise the spectral mismatch which enables to avoid the measurement errors.

2. Spectral Mismatch correction
It provides a method to analyze the characterization of solar simulator according to the cited standard IEC 60904-9. Solar reference cell is commonly used to adjust the intensity of solar simulator. Nevertheless, there is always a spectral mismatch caused by the bias error of the simulator spectrum from the standard spectrum AM 1.5G, it still has±25% bias error even for a class A solar simulator. When the spectral response between solar reference cell and test sample is different, the spectral mismatch factor is calculated to correct the radiance according to the IEC 60904-7 international standard.

The spectral response data of solar simulator and OPV solar cells is given in Figure 1.


Fig. 1. SR data of solar simulator and OPV solar cells

Calculation of spectral mismatch according to IEC 60904-7 as formula (1)
(1)

is referred to as reference spectral irradiance  AM 1.5G; is referred to as the measurement of the experimental spectrum of light source; is the spectral response of solar reference cell used for calibrating the intensity of solar simulator; is the SR of the test sample. The currents (I) is given by the spectral irradiance and spectral response of solar reference cell:

(1)

and is the coverage of the measurement wave-length range. The spectral mismatch factor (MM) is computed by the short-circuit current of reference spectrum AM1.5G with a test sample and the simulator spectrum with a test sample. As shown in equation (2):

(2)

Reference solar cell is used to adjust the intensity of a solar simulator, which ensures the short-circuit current produced by the reference cell through spectral mismatch correction is equivalent to its calibrated short-circuit current



Si reference cell is most commonly used to calibrate solar simulator and Si is the most popular material for solar cell, of which the spectral responses are very close and the mismatch factor can be ignored. Nevertheless, OPV/DSSC/PVK are new type of solar cells which have different spectral responses (as shown in the Fig. 2.), spectral mismatch correction is You can see the mismatch is huge if the Si solar reference cell is used to adjust the intensity of solar simulator. As for DSSC/OPV solar cells, you can choose KG5 window material. And since LBG-OPV has a wider spectrum response than OPV solar cell, KG3 is more recommended than KG5.

  Spectral Mismatch Factor MMF
  Cell Mono-Si DSSC OPV LBG-OPV PVK
Mono-Si 1.000000 1.115042 1.118173 1.074864 1.078538
KG5 0.895156 0.998136 1.000939 0.962171 0.965460
KG3 0.910652 1.015415 1.018266 0.978827 0.982173
Enli-PVK 0.930576 1.037632 1.040545 1.000243 1.003662
Table 1. Spectral mismatch factor for DSSC/OPV/PVK Solar Cells


Fig. 2. Spectral response of different materials

3. Spectral response of reference solar cell with different window materials
Spectral mismatch correction is required in order to reduce the measurement bias error, as mentioned earlier, using a solar reference cell can ensure a minimal spectral mismatch. In theory, the solar reference cell should be made from the same material as the test sample because of the same spectral response. However, given that most of materials are less stable and has the tendency of degradation, selecting the combination of Si solar cell as substrate and different window material to achieve its spectral response and minimise the spectral mismatch, as shown in the figure 3.


Fig. 3. Spectral response of different Windows


Using OPV as an example, you can select KG5 as the window material of solar reference cell which it is spectrally matched to the OPV samples and ensures a minimal spectral mismatch. The calculation of spectral mismatch factor for DSSC, OPV, Low Band-Gap OPV and PVK solar cells is given in Table 1.

4. Conclusion
Under Standard Testing Conditions, the bias error caused by the mismatch can vary greatly by the used light source spectrum and the spectral responses of the solar reference cell and the test sample. Select the Si solar cell which has a better stability with appropriate window material to minimise the spectral mismatch. For DSSC/OPV solar cells, use KG5 solar reference cell to adjust the intensity of solar simulator, and KG3 solar reference cell for LBG-OPV type.

The recommended model for your device as shown in the following table 2:

Your Device Recommended Model
c-Si/mc-Si Si
CIGS Si
a-Si Si + KG5 Filter
OPV Si + KG5 Filter
Low Band Gap OPV Si + KG3 Filter
DSSC Si + KG5 Filter
Perovskite Si + PVK Filter
Table 2. GUİde for Solaf reference Cell Selectİon

What are QE and IPCE?

Quantum Efficiency, or IPCE (Incident Photon-to-electron Conversion Efficiency), is the conversion ratio of the incident photon to electron by a photovoltaic cell. For instance, if there are 100 photons irradiating on the surface of the solar cell and 80 electrons are generated, the Quantum Efficiency of the cell is 80%. Furthermore, because Quantum Efficiency is typically plotted as a curve related to wavelengths, it can also reflect the characteristics under variouw wavelengths, whichhelp diagnose the process to improve the efficiency of the solar cell.

In the figure above, the yellow line represents the perfect Quantum efficiency of this solar cell in an ideal case. However, the real QE will be affected by various factors. For example, Quantum Efficiency in the short wavelength range will be reduced by the front surface recombination; in the infrared range, it can be decreased by the rear surface passivation and the diffusion length.

Therefore, Quantum Efficiency is a critical indicator to evaluate the performance of the photovoltaic device. While research laboratories need QE to examine the characteristics of their materials, increasing number of solar cell manufacturers implement QE into their quality control standards, substantiating the importance of acquiring a precise Quantum Efficiency measurement system.


What is PL?

PL (Photoluminescence) is a non-destructive testing method, in which a substance absorbs high energy incoming photons and then re-emits photons of lower energy. By analyzing the photons emitting from the sample, many key information, such as material characteristics, carrier transmission route, or the carrier lifetime, can be elucidated when examining the PL spectrum. High-efficiency PL measurement systems are critical to the development of the nanomaterials, especially in the Photonics area.

Enlitech adopts both high-sensitivity spectrum measurement system and high-speed/accuracy scanning device on our latest product –Rapid PL Mapper(PLM), a high-performance and cost-effective PL measurement system which can examine samples with a large surface area. PLM can be used to analyze the epitaxial process of various wafers in the R&D stage, and the critical information, such as the spectrum and spatial distribution, can be applied in a wide range of applications from incoming inspection to quality control.


The PLM scan result of a 8” III-V wafer

Enlitech’s PL imaging device—the PL Imager, adopts a high-sensitivity image detector, which is capable to obtain the PL image in a very short time, and has been successfully applied to the III-V group and solar cell materials. Compared to EL images, PL images can be collected from the sample without completing the electrodes, and the results can be analysis can be finished within a few seconds .

The example of PL image

What is Repeatability?

Repeatability is one of the most critical factors of the measurement capability of an equipment. In general, the sample will be tested several times, and the result will be calculated from those data thus generate a repeatability, which is shown in percentage. Repeatability shows the stability of the equipment, ensuring every test result is precise without being affected by the environmental noise. Enlitech’s QE/IPCE measurement system has outstanding performances both in stability and repeatability exceeding 99.5%, and has been acknowledged and approved by many leading research centers and solar cell manufacturers.


What are the electrical standards around the world?

With the rapid development of the electronic industry, the number of electronic products has significantly progressed. The standardization of product quality benefits the international trade of products. For this reason, the IEC (International Electrotechnical Commission) is a global organization for standardization consisting all national electrotechnical committees (IEC National Committees). The objective of the IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. Enlitech is proud to be one of the testing equipment suppliers, please

refer to

In addition, there are different reliability standards extended by nations or industries. For example, JIS (Japanese Industrial Standards) specifies the standards used for industrial activities in Japan. The standardization process is coordinated by Japanese Industrial Standards Committee and published through Japanese Standards Association. ASTM (American Society for Testing and Materials) also plays a significant role in the industry standard, which is dedicated to the standardization of the global market to meet the demands.

Therefore, Enlitech develops a variety of inspection instruments for the photovoltaic industry in accordance with IEC, JIS, ASTM, etc, to ensure our product reliability.

What are the authoritative laboratories for solar energy in the world?

The National Renewable Energy Laboratory (NREL) is the primary American laboratory for renewable energy and energy efficiency research and development (R&D). ,NREL established the Solar Energy Research Institute since 1977, which publishes the best research-cell efficiency records obtained worldwide twice a year. NREL plays an important role in the world's PV measurement field.


Best Research-Cell Efficiencies(Source: Lawrence Kazmerski, NREL)

In addition, the Fraunhofer Institute for Solar Energy Systems (ISE) in Germany is also dedicated to the development of the PV industry. The institute has remarkable results on R&D of solar cells and also plays an equivalent role as with NREL in the PV measurement field.

Enlitech’s solar cell measurement equipments possess the same quality with NREL and Fraunhofer ISE. We help customers achieve the same measurement standard with NREL and Fraunhofer ISE.