Innovative Patent: AM1.5G Simulator Technology with Adjustable A+ Class Xenon Lamp Spectrum
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The tandem solar cell fabrication technology is an extremely important key element for perovskite-silicon tandem photovoltaic (PV) devices, greatly improving the prospects for solar cell development. However, accurately and effectively measuring the conversion efficiency or other electrical characteristics is a major challenge for testing these types of multi-junction solar cells.
To ensure the measurement results for tandem solar cells have credibility and comparability, researchers need to strictly control the device fabrication conditions and measurement methods, and follow international standards and regulations.
A paper published in Nature Photonics in 2015 pointed out that for the literature on new types of tandem solar cells published in Web of Science (from January 2009 to September 2014), 96% of the reported efficiency values were not measured according to relevant standards (Fig.1). Even though most articles have claimed record efficiencies, their characterization of the devices was insufficient or erroneous. Therefore, the authors proposed a clear and practical set of guidelines, based on existing standard test procedures, to pass on how to use calibrate measurement procedures to characterize these new types of tandem solar cells. See the previous article [Test Procedures and Key Points for Tandem Organic Solar Cells] for details.
According to the article, one of the main reasons for this negative factor could be the lack of proper experimental equipment to accurately characterize spectral mismatch. Measurement of new materials like perovskite-silicon tandem solar cells is even more difficult due to the lack of suitable reference cells.
In addition, some authors seemed unaware of or ignored the importance of measurement standards for multiple-junction tandem solar cells.
Principles of Precision Measurement
In 2017, the International Electrotechnical Commission released the standard IEC 60904-1-1 “Photovoltaic devices – Part 1-1: Measurement of current-voltage characteristics of multi-junction photovoltaic devices” for measuring the I-V characteristics of multi-junction photovoltaic devices under natural or simulated sunlight. Its aims to standardize the procedures for measuring the current-voltage characteristics of multi-junction photovoltaic devices under natural or simulated sunlight irradiation. Please refer to the previous article [“Comparison of the Differences Between the New and Old Versions of IEC 60904-1”] for details.
Since the sub-cells in multi-junction devices are connected in multiple layers, the measured I-V characteristic is a complex function of the photovoltaic current generated in each sub-cell. Therefore, the measurement conditions for multi-junction devices should strive to generate photovoltaic currents in each sub-cell that are similar to those under the short-circuit current condition with the AM1.5G reference spectral irradiance distribution (1 sun). This can usually be achieved through a test spectral irradiance distribution that is close to the AM1.5G reference spectral distribution (e.g. provided by natural sunlight under appropriate conditions) or a solar simulator with an adjustable spectral irradiance.
However, the imperfection measurement conditions exist and deviates from the reference conditions. IEC 60904-1-1 sets allowed deviations to obtain valid measurements. To summarize the key principles of precision measurement required by the test standard:
- When using a solar simulator for J-V characteristic measurements (current density vs. voltage), the spectral mismatch factor Zij calculated from the SR or EQE measurements of each sub-cell against the simulator spectrum needs to be controlled within 1% for each sub-cell junction.
- The current-limited junction under the test spectral irradiance distribution is the same as under the reference spectral irradiance distribution.
- The current balancing parameter Balij between all junctions is within ±5% of consistency under the simulator spectral irradiance distribution compared to the AM1.5G reference spectral irradiance distribution.
To obtain accurate efficiency results, a solar simulator with tunable spectrum must be used to adjust the spectrum and easily meet the above requirements, ensuring the spectral mismatch of each sub-cell meets the requirements before proceeding to IV measurement.
For more details on principles and explanations, please refer to [Cell Symposium: “Accurate Measurement of Perovskite and Organic Solar Cells” Enlitech; Next-Generation Materials for Energy Applications, Xiaman, 17th Nov. 2019; CellPress] and [Oriel: Precise Measurement of Perovskite-Silicon Tandem Solar Cells; 3rd Perovskite & Tandem Solar Cells Industry Development Forum].
Enlitech Launches SS-PST100R Super A++ Class Spectrally Tunable Simulator
Current technologies of spectrum tunable solar simulators does exist their pros and cons [Pros and Cons of Dual-Source and LED Simulators]. To providing the better solutions between striking the balance of price, performance and practicality has always been the core value of Enlitech.
Targeting the characteristics of perovskite-silicon tandem solar cells, Enlitech has launched the SS-PST100R A++ Class Spectrum Adjustable Solar Simulator (A++ class output spectral mismatch < 6%; compare to A class is 25%) as a calibration and testing laboratory level solution for precisely measuring the IV conversion efficiency PCE of perovskite-silicon tandem solar cells. The uncertainty of measurement results can be within ≤ ± 2%.
Meanwhile, following the industrialization trend of perovskite solar, Enlitech offers large-area S-Series Spectrum Adjustable Solar Simulator with A+ class spectral mismatch (< 12.5% spectral mismatch; compare to A class is 25% ), using single xenon lamp sources with tunable spectrum, with irradiation areas up to 250mm (220mm A class uniformity).
How to Achieve Ultimate A Class Solar Simulation with Adjustable Spectrum Using A Single Xenon Lamp?
- Advanced optical design and fine coating technology
- Resourceful spectral control – Sophisticated thin-film interference principles
1.Advanced optical design and fine coating technology which creating ultimate A+ class AM1.5G solar spectrum
Xenon lamps were considered have fixed spectra determined by the gas energy level transitions inside the bulb, making it difficult if not impossible to modify or adjust the output spectrum since its invention.
The figure below shows the typical xenon lamp spectrum vs. the AM1.5G standard solar spectrum. We can see the visible spectrum of a xenon lamp (about 6000K) is the closest to 5500K color temperature of solar among artificial light sources. But there are significant differences in the wavelength of IR (> 800nm) compared to the AM1.5G standard spectrum.
Fig2. General xenon lamp spectrum and AM1.5G standard spectrum. The xenon lamp spectrum has the closest color temperature to the solar spectrum in the visible band (6000K vs 5500K), but has a larger difference from the AM1.5G standard spectrum in the IR wavelength.
To overcome these differences, Enlitech utilizes advanced optical simulation software to design the solar simulator’s optical system. And through fine coating technology on multi-layer to control the complex thin-film coating process by adjusting and optimizing the thickness of each layer, Enlitech`s SS-PST100R AM1.5G Spectrum Adjustable Solar Simulator achieves an AM1.5 spectrum beyond the A+ class.
Fig3. AM1.5G standard solar spectrum (Black line) and the irradiance spectrum output of (Red line) SS-PST100R AM1.5G Spectrum Adjustable Solar Simulator from Enlitech.
The comparison between the irradiance spectrum output of the SS-PST100R simulator and the AM1.5G standard solar spectrum shows excellent spectral match, not only in the visible wavelength, but also in the near-infrared (NIR) and short-wave infrared (SWIR) wavelength.
Regarding to the latest IEC simulator classification standard IEC 60904-9:2020, the SS-PST100R light source:
Achieves 100% spectral coverage (SPC)
Comply with A++ class (< 6%) standard in all spectral wavelengths
(A+ class: < 12.5%; A class: < 25%)
Has a spectral deviation (SPD) of 11.2% (0% is ideal)
Comply with SPD level (~3%) of the ideal dual-source simulator.
2.Resourceful spectral control – Angle of Incidence Control - Sophisticated thin-film interference principles
When incident light is irradiated on the interface between two different transmitter (e.g. air and glass), Snell’s Law indicates that the angle of the incident light will change when entering the second medium. The amount of change depends on the index of refraction of the two transmitter :
Fig 4. Snell’s Law
When thin-film interference occurs, the transmitted spectrum of the incident light changes with the angle of incidence (AOI), an effect known as “blue shift”. Meaning different wavelengths shift as AOI changes. This angular dependence can be described by:
The innovation of Enlitech`s single-xenon lamp adjustable spectrum solar simulators utilizes the coating design of optical components and controls the angle of incidence of the xenon light beam onto each optical component. Based on the significant spectral variation of reflection and transmission with the angle of incidence, adjusting the spatial position and angle of the optical components using a single lamp changes the output spectrum of the simulator. Related patent applications have been filed.
Fig 5. single xenon lamp Spectrum Tunable Solar Simulator from Enlitech
To meet the current mismatch requirement of <5% for perovskite-silicon tandem solar cells, the simulator does not need arbitrary spectral adjustment, only tuning the two sub-cell absorption wavelength (300nm-750nm and 750nm-1200nm) is sufficient to achieve the spectral mismatch factor Zij requirements of IEC 60904-1-1:2017.
In order to make the spectral mismatch factor Zij of each junction on sub-cell of the tandem solar cell more compatible and have less spectral mismatch to reduce the uncertainty of the IV curve test of the tandem cell, it is necessary to be able to adjust the spectrum of the mono-xenon lamp solar simulator.
Perovskite-crystalline silicon tandem solar cells within the two absorption wavelengths of sub-cells, the “top cell” (sub-cells of perovskite) absorption wavelength of 300 nm ~ 750 nm and “bottom cell” ( sub-cells of crystalline silicon) 750 nm ~ 1200 nm the two absorption bands.
Therefore, the development of Spectrum Adjustable Solar Simulator for perovskite-crystalline silicon tandem solar cells does not require the adjustment of arbitrary spectral, but only the spectral adjustment of the absorption wavelength of the two sub-cells (300nm~750nm and 750nm~1200nm), which can meet the requirements of IEC 60904-1-1:2017 for each junction sub-cell, with the spectral mismatch factor Zij less than 1%. The spectral mismatch factor Zij of each junction cell is less than 1% as required by IEC 60904-1-1:2017.
In addition to controlling the Zij mismatch factor, adjusting only these two sub-cell wavelength (300nm-750nm and 750nm-1200nm) greatly simplifies complexity on spectral tuning. Compared to LED simulators which need to adjust of 20+ different wavelength of LED from 350nm-1000nm and hundreds of possible combinations to minimize Zij through repeatedly calculations, which the detailed formulas and methods for adjusting each wavelength and calculating sub-cell mismatch factors Zij and current balance factors Balij can be found in [Precise IV Characterization of Multi-Junction Solar Cells – Spectral Adjustment and Mismatch Factor Calculations].
Fig7. Spectral Correction and Mismatch Factor Calculation for Accurate IV Characterization of Multi-Junction Solar Cells
Spectral Tunability of LED Multi-Light Simulators
Although LEDs provide “full spectrum” adjustability, but Practically with many concerns. e.g.
Myths – Is it really “cheaper” to use an LED simulator, considering the time cost of adjusting the spectrum and reducing the mismatch?
- The aging period of each LED color is different, so an additional illumination spectrometer is needed to check the spectral consistency of LEDs.
- LED the temperature coefficient of the grains of each light color is different, with the usage of time, and temperature changes in the center wavelength and intensity, and there will be drift, resulting in **Zij mismatch factor ** change caused the repeatedly checks.
- The inconsistency of the aging period in each light color caused concern on uniformity change.
- The intensity adjustment of each light color will also cause the spectral changes on the position of irradiation area.
- The cognitive difference between the lifetime of 10,000 hours and the Output light intensity of 1 sun
Therefore, we believe that the “ideal and perfect LED solar simulator” perhaps, applicable for laboratories with professional calibration and test the uncertainty analysis capabilities for various types of multi-junction solar cell testing, because they are familiar with how to accurately measure the spectrum and mismatch factors of the entire IEC specification of the superposition calculation, and have the relevant measurement tools for solar simulators, such as 300nm ~ 1200nm spectral range amplitude spectrometer, irradiance uniformity distribution measurement tools.
Most of the scientific research laboratories have a background in device physics, chemical synthesis or materials, but not familiar with optics and spectroscopy, moreover, most of laboratories do not have the corresponding equipment to verify the performance of the simulators mentioned above.
How to expeditiously and automatically adjust the measurement conditions for graduate students and researchers in most science laboratories for the purpose of accurate measurement of perovskites-silicon tandem solar cells, which strive Enlitech to develop the single spectrum tunable solar simulator and optimize continually.
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