First Author: Yong Cui Corresponding Authors: Feng Gao, Jianhui Hou DOI: https://doi.org/10.1016/j.joule.2021.03.029

Article Highlights

1. Explains and experimentally evaluates 5 common measurement errors in indoor solar cell testing.
2. Proposes 4 practical recommendations for precise indoor solar cell measurement.
3. Emphasizes that spectrometers are more reliable than traditional lux meters for measuring indoor light intensity.
4. By adopting the methods recommended in this article, PCE results can be reliably evaluated to ensure healthy development of solar cells in indoor applications.

Introduction

In May 2020, Joule magazine published the latest research on precise measurement methods for indoor solar cells by ICCAS researcher Jianhui Hou and Linköping University Professor Feng Gao. The study systematically investigated measurement error sources for organic solar cells (OPV), a promising candidate for indoor applications. The paper measured temporal stability and spatial uniformity of common light sources to evaluate their reliability. They found that spectrometers are more reliable than lux meters for measuring indoor light intensity. They also discovered that indoor light non-parallelism is a major cause of measurement error, with larger area cells being more suitable for indoor solar cell measurements. Additionally, stray light significantly affects indoor solar cell measurement accuracy, requiring careful elimination of scattered light from masks and other testing tools. Finally, the authors proposed a feasible measurement method to reliably evaluate PCE of OPV cells for indoor applications.

Background

In recent years, there has been increasing interest in exploring solar cells that effectively convert artificial indoor light into electricity, as they provide attractive opportunities for powering micro-power electronic devices for indoor applications. With the rapid development of mobile devices, wearable electronics, and the Internet of Things (IoT), billions of low-power indoor application electronic devices require substantial off-grid power. Solar cells have proven effective at converting low-intensity light in indoor environments into micro to milliwatt-level power, making them an ideal choice for powering low-power electronic devices. This has created new application opportunities for emerging solar cell technologies. Organic solar cells (OSC) and perovskite solar cells (PSC) have been proven to have higher power generation efficiency under low illumination, indoor light environments. With the rapid development in this field, developing reliable measurement standards is crucial for accurately evaluating solar cell conversion efficiency under indoor lighting. The paper analyzes 5 common sources of error:

1. Measurement errors caused by light source temporal stability
2. Measurement errors caused by light intensity measurement methods
3. Measurement errors caused by light source spatial uniformity
4. Measurement errors caused by solar cell edge effects
5. Measurement errors caused by stray light

More details are described in this article. Based on these error sources, the paper proposes 4 practical recommendations for accurate indoor solar cell measurement:

Light Source Requirements:

White LED and fluorescent tubes (FL) used for home lighting can be used as light sources for indoor solar cells only after careful evaluation of temporal instability and light intensity distribution uniformity meets requirements. Light source temporal instability and spatial distribution uniformity requirements can reference IEC 60904-9 AAA class solar simulator standards. The temporal instability of light sources for indoor solar cell measurement should be < 2%. Solar cell characteristics should be tested in illuminated areas with spatial distribution non-uniformity < 2%.

Masks Should Be Used for IV Testing:

Masks should be as thin as possible, equal to or larger than the transparent substrate size of the cell, and require anti-reflection treatment. The aperture area should be slightly smaller than the cell’s opaque metal electrodes. For example, a 9 x 9 mm² aperture is suitable for 10 x 10 mm² cells.

Use Spectral Irradiance Meters for Spectrum Measurement and Light Intensity Calibration:

Spectral irradiance and light intensity should be calibrated by precise spectrometers, not traditional lux meters which can produce large errors. Spectrometer operation should note:

1. Cosine collectors must not be contaminated
2. Probes should be placed at the sample test position for light intensity and spectral measurements
3. Probe plane should maintain the same horizontal level as the test cell
4. Spectrometers need annual (12-month) calibration to maintain test accuracy

Jsc(EQE) and Jsc(IV) Comparison Difference < 5% to Verify Test Results:

EQE is defined as the ratio of output electrons to incident photons. Jcal can be calculated through EQE curves and photon flux spectra. The formula is: Therefore, Jcal (Jsc(EQE)) can be obtained by integrating the indoor light source spectrum (measured by spectrometer) with EQE test results to verify Jsc(IV) from solar cell IV measurement under indoor light. The difference between the two should be less than 5%. Thus, the solar cell’s EQE curve and incident irradiance spectrum are necessary conditions for precise measurement.

Figure Analysis

Figure 1. Solar Cell Measurement and Light Source Comparison Diagram (A) Typical setup diagram for PCE measurement. (B) Illuminance chart of 6,500 K LED bulb and 6,500 K FL tube operating continuously for 3 hours. Initial illuminance value controlled at 500 lux by adjusting distance between light source and high-precision spectrometer, with continuous illuminance monitoring. (C) Illuminance comparison between three lux meters and spectrometer under 6,500 K LED bulb and 6,500 K FL. Lux meter and spectrometer sensors placed at same position. Figure 2. Light Power Distribution (LPD) LPD of 6,500K LED bulb. Measurement center is directly below light source center. H is distance between light source and horizontal plane (X and Y directions). D is LED bulb diameter. Spectrometer manually moved in 1 cm² steps in X and Y directions within 20 x 20 cm² horizontal plane for testing. Figure 3. Device Cross-section and Incident Light Path Diagram Device’s transparent substrate thickness is significantly greater than cell thickness. Pink rectangles represent transparent electrodes; green rectangles represent active layers; interface layers omitted for clarity; silver rectangles represent metal electrodes. Right side shows (1) reflected light from mask (red lines); (2) reflected light from test clips (blue lines); (3) reflected light from test box (green lines). Figure 4. Device EQE Spatial Distribution Diagram (D) EQE spatial distribution diagram of 9.80 mm² device without mask. EQE value in white area is approximately 85%. (E) EQE spatial distribution diagram of 1.07 cm² device without mask. EQE value in white area is approximately 85%. Figure 5. OPV Device Architecture and Mask Alignment Diagram Figure 6. Device EQE Curve and Jsc Deviation Comparison (A) OPV cell EQE curve. Inset shows photon flux spectrum of 6,500 K LED bulb at 500 lux. EQE measurement system’s xenon lamp has ≤ ±0.5% instability per 3 hours. EQE value uncertainty ≤ ±2.8% in 400-940 nm range. (B) Effect of cell active area on Jsc. (C) Effect of aperture area on Jsc. (D) Effect of mask reflection on Jsc. Test box reflections blocked by black barrier. All standard deviations approximately ± 0.5. (E) Test box diagram. Measurements conducted in dark room test box to eliminate other light source effects. (F) Effect of stray light on Jsc. From left to right, standard deviations are ± 0.5, ± 0.6, and ± 0.5. Figure 7. OPV Cell Performance Parameters Under 6500K LED at 500lux Corresponding Pin is 167 μW/cm². Also shows Jcal, derived from EQE and LED irradiance spectrum integration calculation. Jcal or Jsc(EQE) is obtained from EQE spectrum, which is integration of EQE spectrum with indoor lamp irradiance spectrum. EQE curves all measured by Enlitech’s QE-R Quantum Efficiency Optical Instrument. The QE-R Quantum Efficiency Optical Instrument’s precision performance matches well with Jsc(IV) in this table. Jsc(IV) and Jsc(EQE) deviation is 2%~3.2%, meeting journal acceptance requirement of 5%.

Summary

This paper designed a series of experiments to analyze indoor solar cell measurement error sources. It demonstrates that just checking:

1. Temporal stability of common LED or FL light sources
2. Light intensity spatial non-uniformity

is sufficient for solar cell measurement. Spectral irradiance and light intensity should be calibrated by precise spectrometers, not lux meters. After evaluating cell area and aperture-to-cell area ratio, the paper recommends cells of 1 cm² or larger with slightly smaller apertures for solar cell measurement. To minimize stray light effects, light reflection and scattering from the environment should be carefully eliminated in solar cell measurement. The paper proposes a practical method for evaluating PCE of solar cells for indoor applications. By adopting the recommended methods, readers can reliably evaluate PCE results, ensuring healthy development of solar cells in indoor applications.

Reference Information

Accurate Photovoltaic Measurement of Organic Cells for Indoor Applications Yong Cui, Ling Hong, Tao Zhang, Haifeng Meng, He Yan, Feng Gao, Jianhui Hou DOI: https://doi.org/10.1016/j.joule.2021.03.029

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