Electrical Performance Measurements of Solar Photovoltaic Cells and Arrays

Summary

Renewable energy generation is required to achieve net-zero energy buildings. Solar photovoltaic (PV) arrays typically offer the best means for providing this energy source. The decision of which photovoltaic product to select and how each system is designed, operated, and maintained depends, in large part, on the electrical performance information provided to the decision makers (e.g., the PV array owner, facilities manager, financer). Furthermore, additional utilization of 2nd and 3rd generation photovoltaic devices in conditions that differ from conventional bright outdoor light, such as applications in indoor/low light environments for powering internet-of-things sensors, installation where lighting is diffuse, or situations where operation is improved with concentrated illumination, requires new measurement scales and characterization methods.  The creation of a NIST-designed and calibrated standard reference solar cell has established an SI-traceable reference instrument that will decrease the measurement uncertainty in electrical performance ratings of solar devices, thus giving more confidence to those specifying systems. This reference instrument is also a first step towards creating standard reporting conditions (SRC) other than the often-utilized “1-sun” or air mass 1.5 illumination condition. With the in-house development of the differential spectral responsivity method, performance of these NIST reference cells can be measured and calibrated under almost any lighting condition, enabling NIST to calibrate solar cells under unique conditions that no other laboratory in the world offers as of today. This effort also sets up NIST to lead a committee to write new standards on characterization of solar cells under non-standard reporting conditions. NIST will also complete work to capture high-quality performance data from field sites.  Two sites that have previously been instrumented will be maintained as will a meteorological station.  Data from the experiments will be published for use by outside researchers.  

Description

Objective - To develop and improve the measurement science to: (1) accurately characterize the electrical and optical performance of solar photovoltaic cells, (2) design a standard reference cell with appropriate calibrations under a standard reporting condition or an ad-hoc reporting condition as deemed necessary by the end user, and (3) explore the efficiency of new generations of PV technologies under a variety of lighting conditions or for energy harvesting to power internet of things (IoT) devices. 

What is the new technical idea? 
The technical ideas are to improve and implement state-of-the-art methods for characterizing PV cells and to develop standard reference instruments, measurement methods and new standards for the latest challenges in this field. NIST has been successful in developing (1) a hybrid monochromator + light-emitting diode (LED) based spectral response measurement technique, (2) a new combinatorial-based method for evaluating a cell’s photocurrent versus irradiance relationship (leading to a patent granted in 2018), (3) a variety of solar simulators and temperature dependent I-V measurement stations for obtaining the electrical performance of single junction, multijunction, and other non-traditional PV cells and modules, (4) a custom hyperspectral imaging system capable of performing electroluminescence imaging of solar cells from micron scale to dimensions of up to 150 mm, and (5) an approach for quantifying the spectral dependence of charge carrier lifetimes.  Progress has also been made on rounding out NIST’s eventual suite of PV cell characterization capabilities.  With regard to a measurement service, a reference solar cell has been fabricated and tested, and a comprehensive uncertainty budget has been developed for it. Initially, the majority of the progress noted above was achieved while focusing on applications to single-junction, monocrystalline silicon (mono-Si) PV cells. However, in the last few years significant progress has been made towards measuring and characterizing other emerging PV technologies such as multijunction solar cells, and this work will continue.  Additionally, novel PV materials that have improved performance under low light and non-standard reporting conditions are finding expanded use in powering sensors and controls for building operations, and work is needed to best capture their performance under the expected environmental conditions.  In all cases, steps will be pursued that minimize the measurement uncertainties.  

With regard to collecting field data, a very high quality set of PV and meteorological data has been collected over the last several years, and factors such as measurement redundancy, measurement resolution (i.e., at the module, string, and/or circuit levels), sampling frequency, data capture rates, and curation of the deployed instruments have been considered to produce a detailed and reliable data set. Such an approach allows the greatest utility for using the data for effectively evaluating and improving PV system simulation models, for providing datasets that can be confidently used when learning how to use commercially-available PV modeling tools, for analyzing the impact of local PV on the electrical grid and how to better estimate the local PV generation several minutes to a full day in advance, and for quantifying the impact of using data from more typical PV field monitoring systems when investigating issues such as fault detection and service lifetime predictions. A large amount of this dataset has been published to a public website, (https://pvdata.nist.gov/), and more of that data will be evaluated and made ready for online dissemination or directly to collaborators over the next year. 

What is the research plan?
Efforts will continue to establish a NIST measurement service for calibration of 20 mm x 20 mm, silicon-based reference solar cells under the standard reporting conditions (SRC) through the use of the spectral response measurement system. The focus in FY 21 will be on developing a proposal for a new measurement service and submitting it to the Measurement Services Council for approval and subsequent development. Also, additional improvements to the current measurement system will be made such as a better quantification of the repeatability component of the uncertainty budget and use of the new automated scanning stages to improve the calibration procedure and methodology. The scanning stage can also facilitate other types of diagnostic measurements such as mapping out the uniformity of the external quantum efficiency response of solar cells. Measuring the uniformity can help reduce measurement uncertainties and this topic will be explored in more detail.

In addition to the reference cell work, steady progress has also been made on multijunction solar cell measurements. In FY 20, a complete custom multizone solar simulator was designed, fabricated and tested, complete with an XY programmable moving stage. A comprehensive LabVIEW based program was written for measuring irradiance levels using multiple reference cells, adjusting the synthesized spectra and performing I-V curve measurements. The FY 21 efforts will focus on comprehensive measurement of various multijunction solar cells using this new set up and verifying that it can accurately measure the performance of multijunction solar cells. Test cells for inter-lab comparisons have already been acquired and will be used to further validate the efficacy of this system.

In FY 20, our new hyperspectral imaging system was used extensively for the first time to measure PV data on both multijunction and single junction solar cells using in the electroluminescence mode. The results of these early measurements were extremely encouraging and we performed some baseline calibration measurements to obtain absolute electroluminescence data. In FY 21, we intend to continue such measurements on more variety of solar cells. Furthermore, a new optical cryostat will be delivered to NIST in the fall of 2020, which will allow us to perform temperature dependent measurements, providing additional data for modeling and understanding of various PV phenomena. We will also be using the newly added photoluminescence capability with laser illumination to study a range of effects that were previously inaccessible to us. Finally, we will collaborate with Division 731 to study formation of defects or degradation phenomena in field-induced or artificially-degraded solar PV modules.

Energy harvesting from ambient lighting conditions for the purpose of providing power for IoT sensors and devices is a new area that will continue to be explored in more depth in FY21, building on the success of the FY19 Exploratory Project. In FY 20, we published a paper outlining steps for a new method for measuring the performance of solar cells under indoor ambient lighting. We also performed an inter-lab measurement comparison with an international partner that exposed certain deficiencies in the early standards and test protocols. We intend to address these issues further in FY21 with more testing of solar cells under various lighting conditions and by doing more inter-lab measurements. Other issues to consider are temperature effects, angular effects and some modeling to address discrepancies between experimental results and various diode-based current-voltage models. We will also be setting up apparatus to measure and record irradiance and temperature data within a residential or commercial building for the purpose of forming a complete database that can be used for modeling and predicting available light energy inside buildings over an extended time.

Finally, as the PV field data collection efforts begin to ramp down, the team will maintain some core capabilities such as the weather station and roof-top I-V and irradiance measurements while it will work with existing internal and external collaborators to complete collection, compiling, reducing, and making available quality data for various research and modeling needs.

Major Accomplishments

Outcomes:

• One year of high-fidelity data collected that quantifies the performance of three photovoltaic systems installed in the NIST campus
• Draft revision of ASTM Standard E1021-06, "Standard Test Method for Spectral Responsivity Measurements of Photovoltaic Devices."