2021 Joule Magazine Published The Latest 4 Practical Suggestions for Accurate Indoor-Photovoltaic Measurement
- In the indoor photovoltaic cell test, the 5 common measurement errors are explained and experimentally evaluated.
- This article puts forward 4 practical suggestions for accurate indoor photovoltaic measurement.
- The article emphasizes that the spectrometer is more reliable than the traditional Lux meter in measuring indoor light intensity.
- By adopting the methods suggested in this article, the PCE results can be reliably evaluated, ensuring the healthy development of photovoltaic cells in indoor applications.
In May 2020, Joule magazine published the latest research results of the latest indoor photovoltaic precision measurement methods by researcher Jianhui Hou from the Institute of Chemistry of the Chinese Academy of Sciences and Professor Feng Gao from Linköping University in Sweden, who systematically studied the origin of measurement errors for organic photovoltaic (OPV) cells, a potential indoor photovoltaic candidate.
In this paper, the time stability and spatial uniformity of commonly used light sources are measured to evaluate their reliability. It is found that the spectrometer is more reliable than the lux meter in measuring the light intensity of indoor light. It is also found that the non-parallelism of indoor light is one of the main causes of measurement errors, and the larger area solar cells are more suitable for indoor photovoltaic measurement.
In addition, stray light also has a significant impact on the accuracy of indoor photovoltaic measurement, so the scattered light from the mask and other test tools must be carefully eliminated. Finally, the author proposes a feasible measurement method to reliably evaluate the PCE of OPV for indoor applications.
In recent years, people have paid more and more attention to exploring photovoltaic cells that efficiently convert artificial indoor light into electrical energy because they provide an attractive opportunity for driving micro-power electronic devices for indoor applications. With the rapid development of handheld devices, wearable electronic products and IoT, tens of billions of low-power indoor electronic devices, require a large amount of off-grid power. Many solar cells have been proven to effectively convert low-intensity light energy in indoor environments into microwatts to megawatts, and are recognized as the ideal choice for driving low-power electronic devices.
This also allows the emerging solar cell technology to get new application opportunities that are booming. Among them, organic solar cells (OSC) and perovskite solar cells (PSC) have been proven to have higher power generation efficiency under low illumination and indoor light environments. With the rapid development of this field, it is essential to develop a reliable measurement protocol so that the conversion efficiency of photovoltaic cells can be accurately evaluated under indoor lighting.
Five common sources of measurement error are analyzed in the article:
- Measurement error caused by the time stability of the light source.
- Measurement error caused by light intensity measurement method.
- Measurement error caused by the spatial uniformity of the light source.
- Measurement errors caused by edge effects of photovoltaic cells.
- Measurement error caused by stray light.
More details are described in the content of this article.
Based on the above-mentioned error source research, this article puts forward 4 practical suggestions for accurate indoor photovoltaic measurement:
1. Requirements of the light source.
White LEDs and fluorescent tubes FL used for home lighting can be used as indoor photovoltaic light sources only after careful evaluation of time instability and uniformity of light intensity distribution which meet the requirements. The time instability and spatial distribution uniformity requirements of the light source can refer to the IEC 60904-9 AAA-grade solar simulator standard. The time instability of the light source for indoor PV measurement should be less than 2%. PV characteristics should be tested in an illuminated area where the spatial distribution of light intensity is less than 2%.
2. A mask should be used for IV testing.
The mask should be as thin as possible. The size of the mask should be similar or larger than the transparent substrate of the solar cells. The anti-reflection treatment is required. The aperture area should be slightly smaller than the opaque metal electrode of the solar cells. For example, the 9 x 9 mm² aperture is suitable for 10×10 mm² solar cells.
3. Use an irradiance spectrometer to measure the spectrum and calibrate the light intensity.
Spectral irradiance and light intensity should be calibrated by an accurate spectrometer. It cannot be calibrated with a traditional illuminance meter, which will cause great errors. The operation of the spectrometer should pay attention to several key points:
(1) The cosine collector should not be contaminated.
(2) The probe should be placed at the position of the sample to be measured for light intensity and spectrum measurement.
(3) The plane of the spectrometer probe should be kept with the plane of the solar cell to be tested.
(4) The spectrometer needs to be calibrated once per year (12 months) to maintain the accuracy of the test.
4. The comparison difference between Jsc(EQE) and Jsc(IV) should be less than 5% to verify the test results.
EQE is defined as the ratio of the number of output electrons to the number of incident photons. Jcal can be calculated from EQE curve and photon flux spectrum of the indoor light source.
The formula is as follows:
Therefore, the EQE test results can be used to integrate the spectrum of the indoor light source (measured by the spectrometer) to obtain Jcal, or called Jsc (EQE), to verify the comparison of the Jsc (IV) measured by the IV under the indoor light. The difference between them should be less than 5%. Therefore, the EQE curve of PV and the illuminance spectrum of incident light are necessary for accurate measurement in indoor PV testing.
Figure 1. PV measurement and light source Diagrams
(A) Schematic diagram of typical settings for PCE measurements. (B) is the illumination graph of 6,500 K LED bulb and 6,500 K FL fluorescent tube continuously working for 3 hours. Adjust the distance between the light source and the high-precision spectrometer to control the initial illuminance value to 500 lux, and continuously monitor the illuminance value. (C) Comparison of the illuminance of three lux meters and spectrometers at 6,500 K LED bulbs and 6,500 K FL. The sensors of the lux meter and the spectrometer are placed in the same position.
Figure 2. Light Power Distribution (LPD)
LPD of 6,500K LED bulbs. The measurement center is located directly below the center of the light source. H is the distance between the light source and the horizontal plane (X and Y directions). D is the diameter of the LED bulb. The spectrometer is manually moved in the X and Y directions within a horizontal plane of 20 x 20 cm2 in steps of 1 cm2 for testing.
Figure 3. Schematic diagram of the cross-section of the device and the path of the incident light.
The thickness of the transparent substrate of the device is significantly greater than the thickness of the solar cell. The pink rectangle represents the transparent electrode; the green rectangle represents the active layer; the interface layer is omitted for clarity; the silver rectangle represents the metal electrode. Shown on the right are (1) reflected light from the mask (red line); (2) reflected light from the test clip (blue line); (3) reflected light from the test box (green line).
Figure 4. EQE spatial distribution diagram of device.
(D) EQE spatial distribution diagram of 9.80 mm² device without mask. (E) EQE spatial distribution map of 1.07 cm2 device without mask. The EQE value in the white range are all approximately 85%. This high resolution EQE spatial distribution diagram is measured by Enlitech’s LSD system. Enlitech’s LSD system is a laser scanning photocurrent spatial distribution measuring system, which can resolve the EQE distribution with 10 um spatial resolution. If you are interested in LSD system, please contact Enlitech.
Figure 5. Schematic diagram of the OPV device architecture and the alignment of the mask.
Figure 6. Comparison of device EQE curve and Jsc deviation.
(A) EQE curve of OPV. The inner graph shows the photon flux spectrum of a 6,500 K LED bulb at 500 lux. The xenon lamp of the EQE measurement system has an instability of ≤ ±0.5% every 3 hours. The uncertainty of the EQE value is ≤ ±2.8% in the range of 400-940 nm. (B) The influence of solar cell active area on Jsc. (C) The effect of aperture area on Jsc.
(D) The effect of light reflected by the mask on Jsc. The reflected light of the test box is blocked by the black baffle. All standard deviations are approximately ± 0.5. (E) Schematic diagram of the test box. To eliminate the influence of other light sources, the measurement is carried out in a dark test box. (F) The effect of stray light on Jsc. From left to right, the standard deviations are ± 0.5, ± 0.6, and ± 0.5.
Figure 7. OPV cells performance parameters under 6500K LED at 500lux.
The corresponding Pin is 167 uW/cm2. This also shows the Jcal which is from the integrating calculation from EQE with the LED irradiance spectrum.
The Jcal, or Jsc(EQE), is obtained from the EQE spectra, which is integrated the EQE spectra with the indoor lamp irradiance spectra. The EQE curves are all measured by Enlitech’s QE-R quantum efficiency system. The accuracy performance of QE-R quantum efficiency system shows excellent coincidence with Jsc(IV) from this Table. The deviation between Jsc(IV) and Jsc(EQE) are 2%~3.2% which achieves the journal acceptance requirement 5%.
This paper designs a series of experiments to analyze the sources of indoor photovoltaic measurement errors. The article proves that as long as the two factors,
- the time stability of the commonly used LED or FL light source and
- the spatial unevenness of the light intensity,
are checked before the measurement, it is sufficient for PV measurement. Spectral irradiance and light intensity should be calibrated by an accurate spectrometer, and an lux meter should not be used. After evaluating the solar cell area and the ratio of the aperture size to the cell area, the authors recommends that cells with a slightly smaller aperture size of 1 cm2 or larger are suitable for PV measurement. In order to minimize the influence of stray light, the light reflection and scattering caused by the environment should be carefully eliminated in the PV measurement. This paper proposes a practical method to evaluate the PCE of photovoltaic cells for indoor applications. By adopting the methods suggested in the article, readers can reliably evaluate PCE results and ensure the healthy development of photovoltaic cells in indoor applications.
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