A. Anderberg

Degradation analysis of weathered crystalline-silicon PV modules

We present an analysis of the results of a solar weathering program that found a linear relationship between maximum power degradation and the total UV exposure dose for four different types of commercial crystalline Si modules. The average degradation rate for the four modules types was 0.71 % per year. The analysis showed that losses of short-circuit current were responsible for the maximum power degradation. Judging by the appearance of the nondegraded control modules, it is very doubtful that the short-circuit current losses were caused by encapsulation browning or obscuration. When we compared the quantum efficiency of a single cell in a degraded module to one from an unexposed control module, it appears that most of the degradation has occurred in the 800-1100 nm wavelength region, and not the short wavelength region.

Performance test of amorphous silicon modules in different climates - year four: Progress in understanding exposure history stabilization effects

In a round robin outdoor exposure experiment carried out in three different climates, we have previously demonstrated that amorphous silicon (a-Si) PV modules reach higher stabilized performance levels in warmer climates. The four-year experiment involved three identical sets of thin-film a-Si modules from various manufacturers deployed outdoors simultaneously in three sites with distinct climates. Each PV module set spent a one-year period at each site before a final period at the original site where it was first deployed. The experiment aimed to determine the light-induced degradation and stabilization characteristics of a-Si regarding specific history of exposure, and to compare degradation rates in different climates. We propose that after the initial sharp degradation associated with the Stabler-Wronski effect (SWE) has passed, the subsequent stabilized performance levels attained will depend largely on light exposure and a characteristic temperature associated within a coherent time-scale. PV modules which were first deployed at the lowest-temperature site for one year, reaching a stabilized state, and were then further deployed at higher temperature sites for two more years, experienced considerable recovery in output parameters (Pmax and FF). However, when further deployed back at the original, lowest-temperature site, performance degraded back to the first year, original level.

Striving for a standard protocol for preconditioning or stabilization of polycrystalline thin film photovoltaic modules

Polycrystalline photovoltaic (PV) modules containing cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS) thin film materials can exhibit substantial transient or metastable current-voltage (I-V) characteristics depending on prior exposure history. Transient I-V phenomena confound the accurate determination of module performance, their reliability, and their measured temperature coefficients, which can introduce error in energy ratings models or servicelifetime predictions. Indeed, for either of these two technologies, a unique performance metric may be illusory without first specifying recent exposure or state—even at standard test conditions. The current standard preconditioning procedure for thin-film PV modules was designed for amorphous silicon (a-Si), and is likely inadequate for CdTe and CIGS. For a-Si, the Staebler-Wronski effect is known to result from defects, created via breaking of weak silicon bonds or light-activated trapping at the device junction, occurring rapidly upon light-exposure. For CdTe and CIGS devices, there is less agreement on the causes of metastable behavior. The data suggests that either deep-trapping of charge carriers, or the migration and/or electronic activation of copper may be responsible. Because these are quite disparate mechanisms, we suspect that there may be a more practical preconditioning procedure that can be employed prior to accurate performance testing for CdTe and CIGS modules. We devise a test plan to examine and compare the effects of light soaking versus forward-biased dark exposure at elevated temperatures, as parallel strategies to determine a feasible standard protocol for preconditioning and stabilizing these polycrystalline PV technologies, and report on the results of our tests.

Stability of CIS/CIGS modules at the outdoor test facility over two decades

We examine the status and question of long-term stability of copper indium diselenide (CIS) photovoltaic (PV) module performance for numerous modules that are deployed in the array field, or on the roof of, the outdoor test facility (OTF) at NREL, acquired from two manufacturers. Performance is characterized with current-voltage (I–V) measurements obtained either at standard test conditions (STC) or under real-time monitoring conditions, taken over the course of many years. We present and scrutinize I–V characteristics for degradation modes. Analysis yields that CIS PV modules can exhibit either moderate (2% to 4% per year) to negligible or small (less than 1% per year) degradation rates, and that the predominant loss mode appears to be fill factor diminution, often associated with increases in the series resistance in some of the modules. A secondary mode of degradation observed comprises metastable changes to the open-circuit voltage. The featured modules are deployed on three separate testbeds.

Progress toward a stabilization and preconditioning protocol for polycrystalline thin-film photovoltaic modules

Cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) thin-film photovoltaic (PV) modules can exhibit substantial variation in measured performance depending on prior exposure history. We studied the metastable performance changes in these PV modules with the goal of establishing standard preconditioning or stabilization exposure procedures to mitigate measured variations prior to current-voltage (IV) measurements. We present the results of our case studies of module performance vs. exposure: light-soaked at 65°C, exposed in the dark under forward bias at 65°C; and finally longer-term outdoor exposure. We find that stabilization can be achieved using either light or dark bias methods. Additionally, we performed and present capacitance-voltage profile measurements on the modules to examine the changes in depletion widths or its hysteresis plus electronic carrier concentrations as a function of exposure.

Long-Term Performance Data and Analysis of CIS/CIGS Modules Deployed Outdoors

The long-term performance data of copper indium diselenide (CIS) and gallium-alloyed CIS (CIGS) photovoltaic (PV) modules are investigated to assess the reliability of this technology. We study and report on numerous PV modules acquired from two manufacturers (A and B), deployed at NRELs outdoor test facility (OTF) in various configurations in the field: some are free standing, loaded with a fixed resistance and periodically tested indoors at STC; other modules are connected to data acquisition systems with their performance continuously monitored. Performance is characterized using current-voltage (I-V) measurements obtained either at standard test conditions or under real-time monitoring conditions: the power parameters plus other factors relating to quality like diode quality factors or series resistance are analyzed for changes against time. Using standard diode analysis to determine the sources of degradation indicates that CIS modules can exhibit between moderate and negligible degradation, with the dominant loss mode being fill factor declines along with decreases in open-circuit voltage, for illumination intensities near 1-sun. At lower intensities, current losses can appear appreciable. The real-time performance data also indicate that fill factor loss is the primary degradation mode, generally as a result of increases in series resistance.

PV cell and module performance measurement capabilities at NREL

The Photovoltaic (PV) Cell and Module Performance Characterization team at NREL supports the entire photovoltaic community by providing: secondary calibrations of photovoltaic cells and modules; efficiency measurements with respect to a given set of standard reporting conditions; verification of contract efficiency milestones; and current versus voltage (I-V) measurements under various conditions of temperature, spectral irradiance, and total irradiance. Support is also provided to in-house programs in device fabrication, module stability, module reliability, PV systems evaluations, and alternative rating methods by performing baseline testing, specialized measurements and other assistance when required. The I-V and spectral responsivity equipment used to accomplish these tasks are described in this paper.

New data set for validating PV module performance models

A new publicly available data set was completed for use in validating models that estimate the performance of flat-plate photovoltaic (PV) modules. The data were collected for one-year periods at three climatically diverse locations (Cocoa, Florida; Eugene, Oregon; and Golden, Colorado) and for PV modules representing all technologies available in 2010 when the work began. The same makes and models of PV modules were tested at all locations and common data acquisition systems were used with calibrations performed at the National Renewable Energy Laboratory. For use in determining model parameters and coefficients, baseline and post-deployment measurements were performed indoors with solar simulators, including per the requirements of IEC 61853 Part 1: Irradiance and Temperature Performance Measurements and Power Ratings. Outdoors, the PV modules were characterized per the requirements of the Sandia array performance model. A users manual describes the contents of the data set and how to access the data.

Trust but verify: procedures to achieve accurate efficiency measurements for all photovoltaic technologies

The measurement of the photovoltaic (PV) performance with respect to reference conditions requires measuring the performance with respect to a given tabular reference spectrum, junction temperature, and total irradiance. This paper discusses the procedures implemented by NRELs PV Cell and Module Performance Characterization Group to achieve the lowest practical uncertainty. This paper describes the process of trusting and verifying software, hardware, calibrations, procedures, and results. As an ISO 17025 accredited calibration facility, the quality system that is in place is designed to assure customers that the results are valid within specified uncertainty limits and are traceable. The process of trusting performance claims but desiring an independent verification permeates the PV business and society.