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In 2021, Joule Magazine published the latest four practical suggestions for accurate indoor photovoltaic measurement. Latest accurate indoor photovoltaic measurement method.
First Author: Yong Chui
Corresponding Authors: Feng Gao, Jianhui Hou
Contents
Highlights
1. In the indoor photovoltaic cell test, the five common measurement errors are explained and experimentally evaluated.
2. This article puts forward four practical suggestions for accurate indoor photovoltaic measurement.
3. The article emphasizes that the spectrometer is more reliable than the traditional Lux meter for measuring indoor light intensity.
4. By adopting the methods suggested in this article, the PCE results can be reliably evaluated, ensuring the healthy development of photovoltaic cells for indoor applications.
Summary
In May 2020, Joule magazine published the latest research results on indoor photovoltaic precision measurement methods. The research was conducted by Jianhui Hou from the Institute of Chemistry of the Chinese Academy of Sciences and Professor Feng Gao from Linköping University in Sweden. They 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 were measured to evaluate their reliability. It was found that the spectrometer is more reliable than the lux meter in measuring the light intensity of indoor light. It was also discovered that the non-parallelism of indoor light is one of the main causes of measurement errors, and larger area solar cells are more suitable for indoor photovoltaic measurement.
In addition, stray light also significantly impacts the accuracy of indoor photovoltaic measurement, so the scattered light from the mask and other testing tools must be carefully eliminated. Finally, the authors propose a feasible measurement method to reliably evaluate the PCE of OPV for indoor applications.
Background
In recent years, there has been increasing interest in exploring photovoltaic cells that efficiently convert artificial indoor light into electrical energy. This is because they offer an attractive opportunity to power micro-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 substantial amount of off-grid power. Many solar cells have proven effective in converting low-intensity light energy in indoor environments into microwatts to megawatts and are recognized as the ideal choice for powering low-power electronic devices.
This also provides emerging solar cell technology with new application opportunities that are booming. Among them, organic solar cells (OSC) and perovskite solar cells (PSC) have 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.
The article analyzes five common sources of measurement error:
- Measurement error caused by the time stability of the light source.
- Measurement error caused by the 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 research into the above-mentioned sources of error, this article offers four practical suggestions for accurate indoor photovoltaic measurement:
Requirements for the Light Source
White LEDs and fluorescent tubes (FL) used for home lighting can only be used as indoor photovoltaic light sources after a 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%.
A Mask Should Be Used for IV Testing
The mask should be as thin as possible. The size of the mask should be similar to or larger than the transparent substrate of the solar cells. Anti-reflection treatment is required. The aperture area should be slightly smaller than the opaque metal electrode of the solar cells. For example, a 9 x 9 mm² aperture is suitable for 10×10 mm² solar cells.
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. They cannot be calibrated with a traditional illuminance meter, as this will cause significant errors. Operating the spectrometer requires 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 align with the plane of the solar cell being tested.
(4) The spectrometer needs to be calibrated once per year (every 12 months) to maintain the accuracy of the test.
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 the EQE curve and the 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, also known as Jsc (EQE), to verify the comparison of the Jsc (IV) measured by the IV under 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.
Key Results
Figure 1. PV Measurement and Light Source Diagrams
(A) Schematic diagram of typical settings for PCE measurements. (B) is the illumination graph of a 6,500 K LED bulb and a 6,500 K FL fluorescent tube continuously working for 3 hours. The distance between the light source and the high-precision spectrometer is adjusted to control the initial illuminance value to 500 lux, and the illuminance value is continuously monitored. (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 the device.
(D) EQE spatial distribution diagram of a 9.80 mm² device without a mask. (E) EQE spatial distribution map of a 1.07 cm2 device without a mask. The EQE values 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 a 10 um spatial resolution. If you are interested in the 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 the solar cell active area on Jsc. (C) The effect of the 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 with the indoor lamp irradiance spectra. The EQE curves are all measured by Enlitech’s QE-R quantum efficiency system. The accuracy performance of the QE-R quantum efficiency system shows excellent coincidence with Jsc(IV) from this Table. The deviation between Jsc(IV) and Jsc(EQE) is 2%~3.2%, which achieves the journal acceptance requirement of 5%.
Conclusion
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, they are sufficient for PV measurement. Spectral irradiance and light intensity should be calibrated by an accurate spectrometer, and a 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 recommend 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.
Article Information
Accurate Photovoltaic Measurement of Organic Cells for Indoor Applications
Yong Cui, Ling Hong, Tao Zhang, Haifeng Meng, He Yan, Feng Gao, Jianhui Hou
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