The adoption of photovoltaic (PV) modules for clean electricity relies on accurate measurements of their performance, which are essential for estimating their energy production potential. Herein, the calibration chain of PV cells and modules, with particular emphasis on primary reference cell calibrations, is discussed. Also, herein, the direct sunlight method the group has developed for these calibrations is presented and critical improvements and upgrades that lead to calibration uncertainty as low as 0.45% are discussed. The ultimate motivation behind this work is to provide low‐uncertainty performance measurements of PV modules, and lowering the calibration uncertainty of primary reference cells is a key first step toward achieving this goal. As the use of solar electricity continues to grow, the demand for primary reference cell calibrations inevitably increases beyond what the small handful of primary calibration laboratories can provide today. Therefore, this work can serve as a useful guide for implementing primary PV reference cell calibrations using the outdoor method, as well as outlining the critical elements required to make these calibrations highly accurate.

In this manuscript, we present an overview of our module self-reference (MSR) calibration results of PV electrical performance collected for over three years on a silicon module. Analysis and comparison of the results based on measurement uncertainty analysis enable the quantification of the major sources of uncertainty and an estimate of the total uncertainty of the MSR measurements. The control charts show that the mean values are consistent with uncertainty calculations for P-MAX, V-OC and I-SC.

Temperature and solar irradiance are among the most relevant parameters that affect the energy yield of photovoltaic cells and modules. A rise in the module temperature leads to a significant decrease in the open circuit voltage and a small increase in the short circuit current. Here we use a new tool to determine temperature coefficients as well as to assess power rating of multi c-Si, mono c-Si, CdTe, and CIGS-based PV modules in controlled indoor conditions per IEC 60891 and IEC 61853-1. We use the tool to explore how nonuniformities in module temperature affect the accuracy of the temperature coefficients.

Photovoltaic devices are characterized under standard testing conditions that include a defined reference spectrum and total irradiance. International standards for reference cell calibrations require that reported reference cell response (typically Isc) vs. total irradiance must be linear. How can linearity be efficiently determined? In 2006 NREL developed a test bed, based on the "two-lamp method" that provided a low cost, but low accuracy method for determining whether cell response was linear with irradiance. This paper describes very simple changes to NREL's historical method [1] for determining linearity that yield greatly improved results. It also describes a method that can be used to quantify and correct for non-linearity.

This paper presents a composite measurement approach that capitalizes on complimentary strengths of 3 different test beds to significantly reduce uncertainty in I-V parameters for secondary module calibration. This approach addresses PV manufacturers' need for reduced uncertainty in the calibration modules that provide the basis for their module power ratings. This new method enables NREL to reduce uncertainty for secondary module calibration in commercial c-Si modules from +/- 3.2% to +/- 0.7% for I-SC and from +/- 3.3% to +/- 1.1% for P-MAX. This new module self-reference, or MSR procedure is based on the sensitivity of module voltage to temperature plus the uniformity of outdoor sunlight. By calibrating module V-OC in thermal equilibrium on a flash simulator we provide an accurate gauge of junction temperature for subsequent measurements. By calibrating I-SC outdoors in natural sunlight we enable the module to serve as its own reference device in setting the intensity of a continuous simulator, effectively eliminating the impact of spatial non-uniformity and spectral mismatch on the I-V measurement. These procedures significantly reduce measurement errors due to temperature uncertainty, spatial non-uniformity, and spectral mismatch.

This paper presents an overview of the improvements made since 2014 to the primary reference cell calibrations performed at the National Renewable Energy Laboratory (NREL). Reducing the measurement uncertainty of the calibrations was the primary goal of the improvements, which were used during the recent calibrations conducted in the autumn of 2016. Changes and improvements have been made to the spectral irradiance and short-circuit current measurements, spectral irradiance modeling, and reference cell temperature corrections. Almost 6000 points were collected on 38 reference cells. Analysis of the results demonstrated reductions of expanded uncertainty from ±1.0-1.2% in previous years down to ±0.4-0.5% for crystalline-Si reference cells.

The calibration of the electrical performance of seven photovoltaic (PV) modules was compared between four reference laboratories on three continents. The devices included two samples in standard and two in high-efficiency crystalline silicon technology, two CI(G)S and one CdTe module. The reference value for each PV module parameter was calculated from the average of the results of all four laboratories, weighted by the respective measurement uncertainties. All single results were then analysed with respect to this reference value using the E-n number approach. For the four modules in crystalline silicon technology, the results agreed in general within +/- 0.5%, with all values within +/- 1% and all E-n numbers well within [-1,1], indicating further scope for reducing quoted measurement uncertainty. Regarding the three thin-film modules, deviations were on average roughly twice as large, i.e. in general from +/- 1% to +/- 2%. A number of inconsistent results were observable, although within the 5% that can be statistically expected on the basis of the En number approach. Most inconsistencies can be traced to the preconditioning procedure of one participant, although contribution of other factors cannot be ruled out. After removing these obvious inconsistent results, only two real outliers remained, representing less than 2% of the total number of measurands. The results presented show improved agreement for the calibration of PV modules with respect to previous international exercises. For thin-film PV modules, the preconditioning of the devices prior to calibration measurements is the most critical factor for obtaining consistent results, while the measurement processes seem consistent and repeatable. (C) 2017 The Authors and National Renewable Energy Laboratory. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.orgilicensesiby/4.0/).

We report progress on the development of an advanced four-subcell IMM CPV solar cell that is designed for extremely high conversion efficiency under realistic concentrator operating conditions. Practical considerations allowing the design to mitigate problems related to Al-containing alloys, lattice mismatch, non-ideal short-wavelength response, and reflection losses are described. Performance modeling is used to guide the choice of optimal subcell band gaps for the new IMM cell. Early experimental efforts to develop and implement the new design are described and discussed. Copyright (C) 2015 John Wiley&Sons, Ltd.

This paper develops the mathematical foundation for a translation of solar cell short-circuit current from one thermal and spectral irradiance operating condition to another without the use of ill-defined and error-prone temperature coefficients typically employed in solar cell metrology. Using the partial derivative of quantum efficiency with respect to temperature, the conventional isothermal expression for spectral mismatch corrections is modified to account for changes of current due to temperature; this modification completely eliminates the need for short-circuit-current temperature coefficients. An example calculation is provided to demonstrate use of the new translation.

The changes in short-circuit current of photovoltaic (PV) cells and modules with temperature are routinely modeled through a single parameter, the temperature coefficient (TC). This parameter is vital for the translation equations used in system sizing, yet in practice is very difficult to measure. In this paper, we discuss these inherent problems and demonstrate how they can introduce unacceptably large errors in PV ratings. A method for quantifying the spectral dependence of TCs is derived, and then used to demonstrate that databases of module parameters commonly contain values that are physically unreasonable. Possible ways to reduce measurement errors are also discussed.

The inverted metamorphic solar cell has highly tunable bandgaps, in part due to the metamorphic subcells. Using phosphide-based compositionally graded buffers, we show a wide variety of GaInAs solar cells, ranging in bandgap from 1.2 to 0.7 eV. These metamorphic subcells are all high quality and can be used for a wide variety of multijunction designs. GaInAs solar cells with 0.70 eV bandgaps are developed using an InAsP buffer that extends beyond the InP lattice constant, allowing access to an additional 2 mA/cm2 of photocurrent at AM1.5D and 25 °C. This subcell is implemented into a four-junction inverted metamorphic solar cell combined with an appropriate antireflective coating, which increases the series-connected multijunction current by 0.5 mA/cm2 with respect to designs using 0.74-eV GaInAs. However, the optimal design depends on the spectrum and operating temperature. We show how the device flexibility can be used to fine-tune the design for various spectra in order to maximize energy yield for a given operating condition. One-sun devices achieve 35.3 ± 1.2% efficiency under the AM0 spectra and 37.8 ± 1.2% efficiency under the global spectra at 25 °C. Concentrator devices designed for elevated operating temperature achieve 45.6 ± 2.3% peak efficiency under 690× the direct spectrum and 45.2 ± 2.3% efficiency at 1000× and 25 °C. Device optimization is performed for the direct spectrum on 1-sun devices with 2% shadowing, which achieve 39.8 ± 1.2% efficiency under the direct spectrum at 1 sun, highlighting the excellent performance and bandgap tunability of the four-junction inverted metamorphic solar cell.

We present an integrated measurement and modeling approach based on the effective irradiance ratio, which is simply the ratio of the short-circuit current of a photovoltaic (PV) device under operating conditions to its short-circuit current under standard test conditions. Using a PV reference device to measure effective irradiance with respect to a standard spectrum, this approach handles device-specific and reference-device-specific temperature and spectral irradiance effects that are significant factors in performance measurements, such as IEC 61853-1, as well as in the diode-based equivalent circuit performance models that are calibrated from such measurements. Avoiding the use of spectrally dependent short-circuit current temperature coefficients, this approach uses a temperature-dependent spectral correction function that is a direct extension of the standardized spectral correction parameter used by numerous PV calibration laboratories. When a matched reference device is used, this function becomes identically one, and it need not be computed. This approach should be useful in the advancement of PV performance testing and modeling that use reference devices to monitor irradiance.

We present recent improvements to the 4-junction inverted metamorphic solar cell. The device now includes a (Ga)InAsP buffer that transitions to lattice constants greater than InP, which allows access to GaInAs subcells with bandgaps < 0.74 eV and an additional 2 mA/cm2 of bottom junction photocurrent at AM1.5D. However, the optimal design depends on the spectrum and operating temperature. We show how the device flexibility can be used to fine-tune the design for various spectra in order to maximize energy yield for a given operating condition. 1-sun devices achieve 35.3% efficiency under the AM0 spectrum and 37.8% efficiency under the global spectrum at 25°C. Concentrator devices achieve 45.7% peak efficiency under 234x the direct spectrum and maintain over 45% efficiency at 700x at 25°C. Other device improvements include a 4-layer anti-reflection coating with low power loss, and reduced series resistance.

The emission of light from each junction in a series-connected multijunction solar cell both complicates and elucidates the understanding of its performance under arbitrary conditions. Bringing together many recent advances in this understanding, we present a general 1-D model to describe luminescent coupling that arises from both voltage-driven electroluminescence and voltage-independent photoluminescence in nonideal junctions that include effects such as Sah-Noyce-Shockley (SNS) recombination with n not equal 2, Auger recombination, shunt resistance, reverse-bias breakdown, series resistance, and significant dark area losses. The individual junction voltages and currents are experimentally determined from measured optical and electrical inputs and outputs of the device within the context of the model to fit parameters that describe the devices performance under arbitrary input conditions. Techniques to experimentally fit the model are demonstrated for a four-junction inverted metamorphic solar cell, and the predictions of the model are compared with concentrator flash measurements.

We present results for quadruple-junction inverted metamorphic (4J-IMM) devices under the concentrated direct spectrum and analyze the present limitations to performance. The devices integrate lattice-matched subcells with rear heterojunctions, as well as lattice-mismatched subcells with low threading dislocation density. To interconnect the subcells, thermally stable lattice-matched tunnel junctions are used, as well as a metamorphic GaAsSb/GaInAs tunnel junction between the lattice-mismatched subcells. A broadband antireflection coating is used, as well as a front metal grid designed for high concentration operation. The best device has a peak efficiency of (43.8 +/- 2.2)% at 327-sun concentration, as measured with a spectrally adjustable flash simulator, and maintains an efficiency of (42.9 +/- 2.1)% at 869 suns, which is the highest concentration measured. The V-oc increases from 3.445 V at 1-sun to 4.10 V at 327-sun concentration, which indicates high material quality in all of the subcells. The subcell voltages are analyzed using optical modeling, and the present device limitations and pathways to improvement are discussed. Although further improvements are possible, the 4J-IMM structure is clearly capable of very high efficiency at concentration, despite the complications arising from utilizing lattice-mismatched subcells.

In this paper, we present an empirical error correction procedure for efficiency measurements of series-connected, multijunction concentrator solar cells. The error arises from the use of unfiltered xenon flash solar simulators with excess infrared radiation (wavelengths > 900 nm) into lowerbandgap subcells, and always results in an artificial increase of the measured efficiency. The efficacy of the procedure is demonstrated by comparing unfiltered efficiency data against other data from a flash simulator in which the spectral irradiance was properly adjusted.

The behavior of Cu(In,Ga)Se-2 (CIGS) solar cells under low flux concentration is being investigated for two complementary reasons. First, pairing photovoltaic devices with inexpensive low flux optical concentration elements can be a rational pathway towards systems that yield a low levelized cost of electricity (LCE). Second, characterization of photovoltaic films under elevated flux levels can be an effective tool for helping to understand the nature of recombination and other parasitic loss mechanisms in these device structures. In this paper we report on a CIGS device that achieved a record efficiency of 23.3% at 14.7 Suns optical concentration and outline a strategy for future work intended to use characterization of devices under elevated flux to explore the physical mechanisms currently limiting the performance of CI(G)S solar cells.

ABSTRACTSo‐called “air mass functions” of photovoltaic modules are used to approximate the effects of spectral responsivity and to correct short‐circuit current to or from a reference condition. These empirical functions are determined from outdoor measurements with test modules mounted on two‐axis solar trackers and then calculated from plots of normalized calibration value (short‐circuit current divided by total irradiance) versus optical air mass. Because they are incorporated into a number of photovoltaic system modeling and sizing software programs, the accuracy of the functions has direct implications for system costs. We discuss the assumptions associated with these functions that are generally not considered or ignored, and study their variability with respect to atmospheric constituents. The variability study included a 6‐month outdoor measurement on a crystalline‐Si module and a software simulation of the same module using a solar spectral irradiance model. We conclude that air mass functions depend on the measurement location and time, and therefore are not unique to a particular device. Also, using these functions introduces two distinct errors, the magnitudes of which are unknown without knowledge of spectral irradiance conditions. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.