Electrical performance measurements and calibrations for photovoltaic cells and arrays

Summary

A significant source of generated power for residential, commercial and small-system applications these days are supplied by solar photovoltaic (PV) cells and arrays. PV offers a great choice for powering needs of small or large systems due to ease of integration, reliability, long safety track record and significantly reduced costs in recent years. Furthermore, recent industry-wide adoption of 2^nd and 3^rd generation photovoltaic devices in conditions that differ from conventional terrestrial insolation (i.e, air mass (AM) 1.5 conditions) such as in satellites for space commerce, indoor/low light environments for powering internet-of-things (IoT) sensors, or for portable power needs, requires new measurement scales and characterization methods. The creation of a NIST-designed and calibrated standard reference photovoltaic cell has established an SI-traceable reference instrument that will decrease the measurement uncertainty in electrical performance ratings of photovoltaic devices in all the situations described here, thus giving more confidence to those specifying systems. This reference instrument calibration service uses the differential spectral responsivity method, allowing the reference cells to be calibrated under almost any lighting condition. This effort also sets up NIST to lead a committee to write new standards on characterization of solar cells under non-standard reporting conditions. Significant effort will also be devoted to luminescence imaging of PV materials, particularly on the new class of perovskite solar cells, to study charge carrier transport phenomena and defects in solar cells. What will be learned from these measurements can also be applied to defect metrology for other types of important semiconductors such as wide band gap semiconductors for power applications and will be important for understanding reliability issues in these materials.   

Description

Objective 
To develop or improve the electrical and optical measurement science of photovoltaic cells to: (1) continue offering a calibration service to allow industry and academia to accurately assess device performance under a range of operating conditions, (2) offer a new measurement service for external quantum efficiency measurements of solar cells (3) explore the underlying physics of electroluminescence and photoluminescence in PV and other important semiconductor materials with the world-class NIST hyperspectral imager, and (4) continue characterizing defects and degradation effects of aged, rooftop PV modules. 

Technical Idea
The technical ideas are to improve and implement state-of-the-art methods for characterizing PV cells, understand charge carrier transport physics and the role of defects on overall device performance, and develop standard reference instruments, measurement methods and new standards for the latest challenges in this field. NIST has been successful in developing a (1) hybrid monochromator + light-emitting diode (LED) based spectral response measurement technique, (2) new combinatorial-based method for evaluating a cell’s photocurrent versus irradiance relationship (leading to a patent granted in 2018), (3) variety of solar simulators and temperature dependent current-voltage (I-V) measurement stations for obtaining the electrical performance of single junction, multijunction, and other non-traditional PV cells and modules, (4) custom hyperspectral imaging system capable of performing electroluminescence and photoluminescence imaging of solar cells and semiconductors for[WH1]  power electronics from micron scale to dimensions of up to 1.5 m, (5) optical cryostat for low temperature (cryogenic) measurements of solar cells[WH2] , and (6) time resolved photoluminescence system for measurements of charge carrier lifetimes in solar cell materials. Therefore, NIST’s suite of PV cell characterization capabilities places it in a strong position to carry out advanced research in photovoltaics.  Regarding a measurement service, a quality measurement system is in place and the NIST Standard Reference Instrument (SRI) 6014 (see  NIST SRI 6014. ) is now offered under 6 calibration reporting conditions.

Initially, most of the progress noted above was achieved while focusing on applications to single-junction, monocrystalline silicon (mono-Si) PV cells. However, in recent years the team has made significant progress towards measuring and characterizing other emerging PV technologies such as multijunction solar cells, gallium arsenide (GaAs), and more recently perovskite solar cells; this work will continue in FY25. In all cases, steps will be pursued that minimize the measurement uncertainties.

In past years, NIST has developed and made available to the public robust data sets of the field performance of multiple photovoltaic arrays on the NIST campus along with precise meteorological data (https://pvdata.nist.gov/).  The meteorological data have proven valuable for a range of research projects at NIST, including assessment of the performance of the Net-Zero Energy Residential Test Facility (NZERTF), evaluation of emissions from various materials and assessment of greenhouse gas measurements in the atmosphere. NIST will continue to operate a slimmed-down version of the old meteorological station and find ways to make the data available to researchers in the most efficient manner possible.  Additionally, modules from one of the field sites on the NIST campus that have been exposed for nearly 20 years provide an opportunity to assess the impact of aging on the modules.

Research Plan
In FY25, we intend to continue work related to the performance of indoor photovoltaics (IPV), with emphasis on diffused lighting effects. IPV is a very fast growing area of PV and the proposed work will be important in interlab comparisons and increased confidence within the community in reporting of device parameters. This work may also present a future opportunity for offering a new PV measurement service. Most of the diffuse light effects will be investigated using the new robotic arm that we procured and programmed in FY 23. The arm can now be used for angular measurements of device performance in almost any spatial configuration and such capability has proved extremely valuable to our work, so much so that we have begun a collaboration effort with a European manufacturer of organic IPV modules to better understand the diffuse light effects on the performance of these devices.

Regarding the current PV calibration service, the plan is to maintain the service, perform regular calibrations of the equipment, and publicize the NIST PV reference cells to a greater degree, highlighting the great selection of reference PV materials that are now available for customer selection such as GaAs solar cells. We intend to work with the NIST Public Affairs Office to underline advantages of the NIST reference cells over other technologies, participate in writing roadmaps or book chapters and collaborate with other researchers who use our reference cells for their research.

An area that we plan to visit in FY25 is the use of the multijunction solar simulator apparatus for performing I-V measurements of multijunction (or tandem) perovskite/inorganic solar cells. These cells have gone through tremendous improvements in recent years, reaching a certified power conversion efficiency of 34 % under the standard reporting condition. More detailed characterization of these devices using the NIST apparatus could shed light on issues that currently remain as roadblocks to higher efficiency cells and may help us understand the role of defects that lead to interfacial degradation in these material systems.

In FY 20, our new hyperspectral imaging system was used extensively for the first time to measure PV luminescence data on (mostly) multijunction solar cells in electroluminescence mode. The results of these early measurements were encouraging and we performed baseline calibration measurements to obtain absolute electroluminescence data. In FYs 21 through 24, we focused on performing extensive electroluminescence and photoluminescence measurements using the hyperspectral imager on a variety of PV materials including GaAs and CdTe PV films or devices. With the delivery of a new optical cryostat in FY 21, we have been able to carry out these measurements as a function of temperature down to 77 K using liquid nitrogen. Measurements at such low temperatures have revealed novel, previously unexplored effects in some devices that could be important for understanding reliability issues for these materials in cold conditions such as deployment in space satellites. We intend to continue these temperature dependent measurements into FY 25 with new perovskite solar cells provided to us by two important collaborators, The University of Toledo and the Korea Research Institute of Chemical Technology. These new measurements will increase our scientific understanding of defects and shunts in these materials and will shed light on charge carrier transport phenomena in these devices.

One of the major efforts in FY22 was to build and set up an indoor irradiance monitoring system inside the NZERTF for the purpose of building a comprehensive database of ambient light energy in a residential setting. Although this system was dedicated to collect large amounts of data throughout the second half of FY22 and into FY23, we have recently realized that additional routine calibrations, adjustments and monitoring will be needed to arrive at a high-quality dataset. For example, we changed the location of one of the sensors earlier this year to be mounted directly under a ceiling LED light. During the early months of FY24, we performed major upgrades to the setup to increase the accuracy of these measurements including re-calibration of all three measurement units, deployment of a reference IPV cell for an independent second measurement system to monitor the irradiance levels, and including correction factors for the diffuse light measurements. The data that has been collected in the last 6 months is of the highest quality and we will continue measuring and maintaining this system at least until the end of the calendar year. Future plans include publishing a curated dataset and teaming up with other researchers to perform energy production modeling.

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 working with existing internal and external collaborators to complete collection, compilation, reduction, and dissemination of quality data for various research and modeling needs. The meteorological station was dismantled in FY21 to allow for a re-roofing of the building on which it sat.  The system will be reinstalled in FY25 and we will initially focus on deploying two spectroradiometers for solar irradiance monitoring.

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REFERENCES:

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[WH2]Semiconductors?

Major Accomplishments

• The photovoltaic reference cell calibration service. See SRI 6014
• 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."