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

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. 

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 minimize the spectral mismatch which enables to avoid the measurement errors. 

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. 

SR data of solar simulator and OPV solar cells

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


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

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


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


λ1 and λ2 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): 

 

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 response of different materials

Fig. 2. Spectral response of different materials


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.

Spectral response of different Windows

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. 

Spectral Mismatch Factor MMF

TEST SAMPLE

Mono-Si

DSSC

OPV

LBG-OPV

PVK

Reference Cell

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

1.048266

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

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. Guide for Solar reference Cell Selection