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The extent of potential-induced degradation of crystalline silicon modules in an environmental chamber is estimated using in-situ dark I-V measurements and applying superposition analysis. The dark I-V curves are shown to correctly give the module power performance at 200, 600 and 1,000 W/m2 irradiance conditions, as verified with a solar simulator. The onset of degradation measured in low light in relation to that under one sun irradiance can be clearly seen in the module design examined; the time to 5% relative degradation measured in low light (200 W/m2) was 28% less than that of full sun (1,000 W/m2 irradiance). The power of modules undergoing potential-induced degradation can therefore be characterized in the chamber, facilitating statistical analyses and lifetime forecasting.

Dynamic Mechanical Loading (DML) of photovoltaic modules is explored as a route to quickly fatigue copper interconnect ribbons. Results indicate that most of the interconnect ribbons may be strained through module mechanical loading to a level that will result in failure in a few hundred to thousands of cycles. Considering the speed at which DML may be applied, this translates into a few hours of testing. To evaluate the equivalence of DML to thermal cycling, parallel tests were conducted with thermal cycling.

We deployed a 855 cm2 thin-film, single-junction gallium arsenide (GaAs) photovoltaic (PV) module outdoors. Due to its fundamentally different cell technology compared to silicon (Si), the module responds differently to outdoor conditions. On average during the test, the GaAs module produced more power when its temperature was higher. We show that its maximum-power temperature coefficient, while actually negative, is several times smaller in magnitude than that of a Si module used for comparison. The positive correlation of power with temperature in GaAs is due to temperature-correlated changes in the incident spectrum. We show that a simple correction based on precipitable water vapor (PWV) brings the photocurrent temperature coefficient into agreement with that measured by other methods and predicted by theory. The low operating temperature and small temperature coefficient of GaAs give it an energy production advantage in warm weather.

It is well known that photovoltaic performance is dependent on cell temperature. Although various methods have been explored to determine outdoor concentrating photovoltaic (CPV) cell temperature, no method has proven to work across all module technologies and result in desirable uncertainties. Menard (2012) has recently published results claiming accurate measurements of cell temperature using the wavelength shift of light emitted from the sub-cells of a Semprius CPV module. This work focuses on efforts to verify Menard's results using additional CPV technologies that are on-sun at NREL. Baseline electro-luminescence emission is recorded for modules under a low level forward bias and under isothermal conditions using thermal chambers. The same modules or sister modules are then placed on NREL's high accuracy two-axis tracker for outdoor measurements. Photo-luminescence emission peaks are measured for multiple modules at stable wind and irradiance conditions. Emission results from the sub-cells are compared to what is documented in the literature for the given semiconductor material. The signal to background ratio is analyzed and the possible broad applicability of this procedure is discussed.

GaInNAs solar cells grown by metal-organic, chemical vapor deposition (MOCVD) are known to demonstrate dramatic changes in performance upon annealing. GaInNAs necessarily experiences some in situ annealing when integrated into a multijunction cell, as when the upper four junctions of a six-junction (e.g., AlGaInP/GaInP/AlGaInAs/GaInAs/ GaInNAs/Ge) cell are grown on top of the GaInNAs subcell. Therefore, understanding the changes that occur during these inadvertent, in situ anneals is necessary to design an MOCVD growth process for high-performance six-junction cells. These six-junction cells have the potential for > 50% efficiency under the concentrated terrestrial spectrum and > 40 % under the air mass zero (AM0) spectrum. This paper shows how an in situ anneal at 650°C can cause movement of the junction in the GaInNAs, first improving and then ruining the performance of the cell. Similarly, in situ annealing depends on the annealing temperature, showing, for the conditions studied, optimal performance for an anneal at 675°C. Higher temperatures resulted in improved material quality, but the junction did not move, resulting in inferior performance compared with the samples annealed at lower temperatures. The performance of the best GaInNAs cells is summarized showing background carrier concentrations as low as 2 × 1015 cm-3, depletion widths as wide as ~ 0.6 μm, and AM0 photocurrents for operation under GaAs approaching 12 mA/cm2.

Acceleration factors are calculated for crystalline silicon photovoltaic modules under system voltage stress by comparing the module power during degradation outdoors with that in accelerated testing at three temperatures and 85% relative humidity. A lognormal analysis is applied to the accelerated lifetime test data, considering failure at 80% of the initial module power. Activation energy of 0.73 eV for the rate of failure is determined for the chamber testing at constant relative humidity, and the probability of module failure at an arbitrary temperature is predicted. To obtain statistical data for multiple modules over the course of degradation in situ of the test chamber, dark I–V measurements are obtained and transformed using superposition, which is found to be well suited for rapid and quantitative evaluation of potential-induced degradation. It is determined that shunt resistance measurements alone do not represent the extent of power degradation. This is explained with a two-diode model analysis that shows an increasing second diode recombination current and ideality factor as the degradation in module power progresses. Failure modes of the modules stressed outdoors are examined and compared with those stressed in accelerated tests.

Photovoltaic (PV) system performance depends on both the quality of the system and the weather. One simple way to communicate the system performance is to use the performance ratio (PR): the ratio of the electricity generated to the electricity that would have been generated if the plant consistently converted sunlight to electricity at the level expected from the DC nameplate rating. The annual system yield for flat-plate PV systems is estimated by the product of the annual insolation in the plane of the array, the nameplate rating of the system, and the PR, which provides an attractive way to estimate expected annual system yield. Unfortunately, the PR is, again, a function of both the PV system efficiency and the weather. If the PR is measured during the winter or during the summer, substantially different values may be obtained, making this metric insufficient to use as the basis for a performance guarantee when precise confidence intervals are required. This technical report defines a way to modify the PR calculation to neutralize biases that may be introduced by variations in the weather, while still reporting a PR that reflects the annual PR at that site given the project design and the project weather file. This resulting weather-corrected PR gives more consistent results throughout the year, enabling its use as a metric for performance guarantees while still retaining the familiarity this metric brings to the industry and the value of its use in predicting actual annual system yield. A testing protocol is also presented to illustrate the use of this new metric with the intent of providing a reference starting point for contractual content.

Potential-induced degradation in conventional p-type silicon-based photovoltaic solar cell modules is described as a failure mechanism involving positive ion migration, understood to be primarily Na+, drifting from the glass to the cells in negative-voltage arrays. Acceleration factors for this mechanism are determined for silicon photovoltaic modules by comparing the module power during degradation outdoors to that in accelerated testing at three temperatures and 85% relative humidity. A lognormal analysis is applied to the accelerated lifetime test data considering failure at 80% of the initial module power. Activation energy of 0.73 eV for the rate of failure is determined for the chamber testing at the constant relative humidity, and the probability of module failure at an arbitrary temperature is predicted. Estimation of module power in-situ in the environmental chamber is achieved using dark I-V measurements transformed by superposition. By this means, the power of the degrading module can be semi-continuously determined so that statistical data for multiple modules undergoing potential-induced degradation can be easily and accurately obtained.

The purpose of this study is to investigate CPV system performance in different climate conditions. The system data has been collecting since 2011. The same manufacturer's CPV systems have been operating in Aurora, CO, U.S. and Okayama, Japan. The relationship between the CPV performance and environment is considered from two different climate data sets. According to the energy yield analysis, the CPV array at the U.S. site generates more than twice the energy compared with Japan. The CPV array yield is better than the Fixed PV in U.S. site. In reverse, the Fixed PV is better than the CPV in Japan. The performance ratio (PR) of the CPV clearly decreases in winter while the conventional fixed flat PV increases at both sites. Based on spectral and temperature effects analysis, we conclude that the spectrum has a strong negative effect on the performance in winter. The spectral loss was up to 11.0% of its performance. The other factors: cf. soiling, alignment, tracker error, etc., had about 5% effect regardless of season. It was confirmed that these factors are non-negligible.

For solar cells dominated by radiative recombination, the performance can be significantly enhanced by improving the internal optics. Internally radiated photons can be directly emitted from the cell, but if confined by good internal reflectors at the front and back of the cell they can also be re-absorbed with a significant probability. This so-called photon recycling leads to an increase in the equilibrium minority carrier concentration and therefore the open-circuit voltage, Voc. In multijunction cells, the internal luminescence from a particular junction can also be coupled into a lower bandgap junction where it generates photocurrent in addition to the externally generated photocurrent, and affects the overall performance of the tandem. We demonstrate and discuss the implications of a detailed model that we have developed for real, non-idealized solar cells that calculates the external luminescent efficiency, accounting for wavelength-dependent optical properties in each layer, parasitic optical and electrical losses, multiple reflections within the cell and isotropic internal emission. The calculation leads to Voc, and we show data on high quality GaAs cells that agree with the trends in the model as the optics are systematically varied. For multijunction cells the calculation also leads to the luminescent coupling efficiency, and we show data on GaInP/GaAs tandems where the trends also agree as the coupling is systematically varied. In both cases, the effects of the optics are most prominent in cells with good material quality. The model is applicable to any solar cell for which the optical properties of each layer are well-characterized, and can be used to explore a wide phase space of design for single junction and multijunction solar cells.

Counter-intuitively, efficient external fluorescence is a necessity for achieving the highest possible solar cell efficiency. Why would a solar cell, which is intended to absorb light, benefit from emitting light? Although it is tempting to equate light emission with loss, paradoxically, light emission actually improves the opencircuit voltage, and the efficiency.

As the photovoltaic industry has grown, the interest in comparative accelerated testing has also grown. Private test labs offer testing services that apply greater stress than the standard qualification tests as tools for differentiating products and for gaining increased confidence in long-term PV investments. While the value of a single international standard for comparative accelerated testing is widely acknowledged, the development of a consensus is difficult. This paper strives to identify a technical basis for a comparative standard., JRC.F.7-Renewables and Energy Efficiency

In high quality solar cells, the internal luminescence can be harnessed to enhance the overall performance. Internal confinement of the photons can lead to an increased open-circuit voltage and short-circuit current. Alternatively, in multijunction solar cells the photons can be coupled from a higher bandgap junction to a lower bandgap junction for enhanced performance. We model the solar cell as an optical cavity and compare calculated performance characteristics with measurements. We also describe how very high luminescent coupling alleviates the need for top-cell thinning to achieve current-matching.

A system dedicated to the optical transmittance characterization of Fresnel lenses has been developed at NREL, in collaboration with the UPM. The system quantifies the optical efficiency of the lens by generating a performance map. The shape of the focused spot may also be analyzed to understand change in the lens performance. The primary instrument components (lasers and CCD detector) have been characterized to confirm their capability for performing optical transmittance measurements. Measurements performed on SoG and PMMA lenses subject to a variety of indoor conditions (e.g., UV and damp heat) identified differences in the optical efficiency of the evaluated lenses, demonstrating the ability of the Scanning Lens Instrument (SLI) to distinguish between the aged lenses.

Many CPV cells are coupled to an optical component/coverglass with a polymer. We describe a screen test to identify field-induced failure modes in popular encapsulation materials. Results are presented for 36 months cumulative exposure.

The IEC Working Group 7 (WG7) is in the process of developing a draft procedure for an outdoor concentrating photovoltaic (CPV) module power rating at Concentrator Standard Operating Conditions (CSOC). WG7 recently achieved some consensus that using component reference cells to monitor/limit spectral variation is the preferred path for the outdoor power rating. To build on this consensus, the community must quantify these spectral limits and select a procedure for calculating and reporting a power rating. This work focuses on statistically comparing several procedures the community is considering in context with monitoring/limiting spectral variation.

The application of a simple point-charge model to NMR spin echo measurements of {sup 71}Ga in random and partially ordered films of Ga{sub 0.52}In{sub 0.48}P provides the first quantitative and accurate estimate of the average degree of local ordering (average order parameter {eta}) in a ternary III-V semiconductor. The value of {eta} extracted is consistent with theoretical predictions. {copyright} {ital 1996 The American Physical Society.}

We have applied reflectance-difference spectroscopy (RDS) to the study of optical anisotropy in spontaneously ordered Ga[sub 0.5]In[sub 0.5]P grown by metal-organic chemical-vapor deposition. The degree of order in Ga[sub 0.5]In[sub 0.5]P has been associated previously with a shift of the band-gap energy [Delta][ital E][sub 0] and a crystal-field valence-band splitting [Delta][sub [ital C]]. Theoretically, both quantities are, to first order, quadratic functions of the long-range order parameter [eta], which varies from 0 to 1 for disordered and perfectly ordered Ga[sub 0.5]In[sub 0.5]P, respectively. The main RD spectral feature in partially ordered Ga[sub 0.5]In[sub 0.5]P is a bulk-induced, asymmetric peak at [ital E][sub 0] with a long tail that extends well below [ital E][sub 0] and a sharp high-energy cutoff at [ital E][sub 0]+[Delta][sub [ital C]]. We find experimentally and theoretically that the intensity of this RD spectral feature is proportional to [radical]([Delta][ital E][sub 0]) and, therefore, is linear with the order parameter. This makes RDS particularly useful for measuring the optical anisotropy of high-band-gap Ga[sub 0.5]In[sub 0.5]P. We also compare heterostructures of GaAs and Al[sub 0.5]In[sub 0.5]P on Ga[sub 0.5]In[sub 0.5]P with uncoated Ga[sub 0.5]In[sub 0.5]P in an effort to separate bulk-, surface-, and interface-induced RD spectral features.

GaAs1−xNx with x=0.2% is grown by metal–organic chemical vapor deposition with growth rates between 2 and 7 μm/h and with two gallium sources. The GaAsN grown with trimethylgallium at high growth rates shows increased carbon contamination (>1017 cm−3), low photoluminescent lifetimes (∼0.2 ns), and high background acceptor concentrations (>1017 cm−3). The GaAsN is improved if it is grown with a lower growth rate or if triethylgallium is used, resulting in lower carbon contamination (∼1016 cm−3), longer photoluminescent lifetimes (2–9 ns), and slightly lower background acceptor concentrations (

Light-to-dark metastable changes in thin-film photovoltaic (PV) modules can introduce uncertainty when measuring module performance on indoor flash testing equipment. This study describes a method to stabilize module performance through forward-bias current injection rather than light exposure. Measurements of five pairs of thin-film copper indium gallium diselenide (CIGS) PV modules indicate that forward-bias exposure maintained the PV modules at a stable condition (within 1%) while the unbiased modules degraded in performance by up to 12%. It was also found that modules exposed to forward bias exhibited stable performance within about 3% of their long-term outdoor exposed performance. This carrier-injection method provides a way to reduce uncertainty arising from fast transients in thin-film module performance between the time a module is removed from light exposure and when it is measured indoors, effectively simulating continuous light exposure by injecting minority carriers that behave much as photocarriers do. This investigation also provides insight into the initial light-induced transients of thin-film modules upon outdoor deployment.

Chemical compounds were added into crystalline silicon cell mini modules, including in the encapsulant, interfaces, and glass, to determine their effect on potential-induced degradation (PID). Fe, either in the glass or at the glass/encapsulant interface, was found to be correlated with increased PID, but the difference in module power loss was not statistically significant compared to controls. Additions of Cu, Cr, Pb, Sn, Ag, and Na compounds to either the encapsulant or at the glass/encapsulant interface did not appear correlated with PID. Lock-in thermography on bare cells affected by PID removed from the mini modules show highly localized areas of junction breakdown, and SIMS analysis indicates localized impurities as well, though a spatial relation between the two was not established. Deposition of a conductive layer on the front surface of the cell, either with semitransparent Ta or Poly 3,4-ethylenedioxythiophene (PEDOT), eliminated PID when the cells were stressed at -1000 V bias, 50 degrees C, with the glass face grounded for 140 h.

A visual inspection data collection tool for the evaluation of fielded photovoltaic (PV) modules has been developed to facilitate describing the condition of PV modules with regard to field performance. The proposed data collection tool consists of 14 sections, each documenting the appearance or properties of a part of the module. This report instructs on how to use the collection tool and defines each attribute to ensure reliable and valid data collection. This tool has been evaluated through the inspection of over 60 PV modules produced by more than 20 manufacturers and fielded at two different sites for varying periods of time. Aggregated data from such a single data collection tool has the potential to enable longitudinal studies of module condition over time, technology evolution, and field location for the enhancement of module reliability models.

It is commonly understood that thermal cycling at high temperature ramp rates may activate unrepresentative failure mechanisms. Increasing the temperature ramp rate of thermal cycling, however, could dramatically reduce the test time required to achieve an equivalent amount of thermal fatigue damage, thereby reducing overall test time. Therefore, the effect of temperature ramp rate on physical damage in the CPV die-attach is investigated. Finite Element Model (FEM) simulations of thermal fatigue and thermal cycling experiments are made to determine if the amount of damage calculated results in a corresponding amount of physical damage measured to the die-attach for a variety of fast temperature ramp rates. Preliminary experimental results are in good agreement with simulations and reinforce the potential of increasing temperature ramp rates. Characterization of the microstructure and resulting fatigue crack in the die-attach suggest a similar failure mechanism across all ramp rates tested.

Long-term monitoring of systems installed in the field is the ultimate standard for evaluating photovoltaic components and systems. This study, which involves the long-term outdoor exposure in a hot and humid climate, intends to address the performance degradation and failure mechanisms which are difficult or impossible to simulate in the lab during time constrained accelerated tests. Experimental data including irradiance, temperature, DC/AC current and voltage has been collected on diverse generations of photovoltaic modules installed throughout the state of Florida. Long term module reliability and lifetime are evaluated using a two pronged approach. 1) Modules have been deployed outdoors for long time periods with systematic - 15 minutes interval- climatic and performance measurements 2) Real-time climatic and performance measurements of modules following long-term outdoor exposed. Visual, IR and electrical insulation inspections were performed are also presented in this paper. Multiple analytical methods are used to quantify energy production and power degradation over time, including Performance Ratio analysis, and PVUSA regression analysis. Real-time field measurements were reviewed for both overall return rates and compare them with the nameplate performance values and to identify the failure mechanism that caused the return.

There has been recent interest in the use of thermoplastic encapsulant materials in photovoltaic modules to replace chemically crosslinked materials, e.g., ethylene-vinyl acetate. The related motivations include the desire to: reduce lamination time or temperature; use less moisture-permeable materials; use materials with better corrosion characteristics or with improved electrical resistance. However, the use of any thermoplastic material in a high-temperature environment raises safety and performance concerns, as the standardized tests currently do not expose the modules to temperatures in excess of 85°C, though fielded modules may experience temperatures above 100°C. Here we constructed eight pairs of crystalline-silicon modules and eight pairs of glass/encapsulation/glass thin-film mock modules using different encapsulant materials of which only two were designed to chemically crosslink. One module set was exposed outdoors with insulation on the back side in Arizona in the summer, and an identical set was exposed in environmental chambers. High precision creep measurements (±20 µm) and performance measurements indicate that despite many of these polymeric materials being in the melt state during outdoor deployment, very little creep was seen because of their high viscosity, temperature heterogeneity across the modules, and the formation of chemical crosslinks in many of the encapsulants as they aged. In the case of the crystalline silicon modules, the physical restraint of the backsheet reduced the creep further.

The highest efficiency solar cells provide both excellent voltage and current. Of these, the open-circuit voltage (V oc ) is more frequently viewed as an indicator of the material quality. However, since the V oc also depends on the band gap of the material, the difference between the band gap and the V oc is a better metric for comparing material quality of unlike materials. To take this one step further, since V oc also depends on the shape of the absorption edge, we propose to use the ultimate metric: the difference between the measured V oc and the V oc calculated from the external quantum efficiency using a detailed balance approach. This metric is less sensitive to changes in cell design and definition of band gap. The paper defines how to implement this metric and demonstrates how it can be useful in tracking improvements in V oc , especially as V oc approaches its theoretical maximum.

State-of-the-art multijunction cell designs have the potential for significant improvement before going to higher number of junctions. For example, the V oc can be substantially increased if the photon recycling taking place in the junctions is enhanced. This has already been demonstrated (by Alta Devices) for a GaAs single-junction cell. For this, the loss of re-emitted photons by absorption in the underlying layers or substrate must be minimized. Selective back surface reflectors are needed for this purpose. In this work, different architectures of semiconductor distributed Bragg reflectors (DBR) are assessed as the appropriate choice for application in monolithic multijunction solar cells. Since the photon re-emission in the photon recycling process is spatially isotropic, the effect of the incident angle on the reflectance spectrum is of central importance. In addition, the DBR structure must be designed taking into account its integration into the monolithic multijunction solar cells, concerning series resistance, growth economics, and other issues. We analyze the tradeoffs in DBR design complexity with all these requirements to determine if such a reflector is suitable to improve multijunction solar cells.

Safety is a prime concern for the photovoltaics (PV) industry. As a technology deployed on residential and commercial buildings, it is critical that PV not cause damage to the buildings nor harm the occupants. Many of the PV systems on buildings are of sufficiently high voltage (300 to 600 Volts dc) that they may present potential hazards. These PV systems must be safe in terms of mechanical damage (nothing falls on someone), shock hazard (no risk of electrical shock when touching an exposed circuit element), and fire (the modules neither cause nor promote a fire). The present safety standards (IEC 61730 and UL 1703) do a good job of providing for design rules and test requirements for mechanical, shock, and spread of flame dangers. However, neither standard addresses the issue of electrical arcing within a module that can cause a fire. To make PV modules, they must be designed, built, and installed with an emphasis on minimizing the potential for open circuits and ground faults. This paper provides recommendations on redundant connection designs, robust mounting methods, and changes to the safety standards to yield safer PV modules.

The internal physics of a solar cell changes as it approaches the fundamental Shockley-Queisser limit. Photonic considerations overtake electronic ones, as an intense internal and external luminescence requires careful photon management. Counter-intuitively, maximizing light extraction increases voltage and therefore efficiency. Until 2010 the one-sun, single-junction efficiency record was set by a GaAs solar cell with an efficiency of 26.4% and an open-circuit voltage V OC = 1.03 V. Alta Devices recently improved the record with a GaAs cell that achieved 28.8% efficiency and V OC =1.12V, demonstrating the importance of photon management. Even with the best materials, the highest efficiencies cannot be achieved unless the solar cell is also designed to also be a good light emitting diode (LED). The physics of light extraction will be necessary in the next generation of high-efficiency solar cells.

In concentrating photovoltaics (CPV) cell assemblies, a large-area die-attach layer is subjected to thermal cycles, leading to thermomechanical fatigue. This causes cracking and the eventual failure of the CPV cell by thermal runaway. We define a damage metric representing lumped progress toward failure and present a numerical model for computing the accumulation of damage for arbitrary transient temperature conditions. The model is applied to a particular design with a solder dieattach layer. We show that accelerated-test thermal cycles with higher ramp rates cause more damage, both per cycle and per unit time. Outdoor exposure to one entire year in two geographic locations is also simulated, revealing that a year of exposure in Golden, Colorado is equivalent to 1.4 years of exposure in Oak Ridge, Tennessee.

The long-term performance and reliability of photovoltaic (PV) modules and systems are critical metrics for the economic viability of PV as a power source. In this study, the power degradation rates of two identical PV systems deployed in Florida are quantified using the Performance Ratio analytical technique and the translation of power output to an alternative reporting condition of 1000 W m−2 irradiance and cell temperature of 50 °C. We introduce a multi-pronged strategy for quantifying the degradation rates of PV modules and arrays using archived data. This multi-pronged approach utilizes nearby weather stations to validate and, if needed, correct suspect environmental data that can be a problem when sensor calibrations may have drifted. Recent field measurements, including I-V curve measurements of the arrays, visual inspection, and infrared imaging, are then used to further investigate the performance of these systems. Finally, the degradation rates and calculated uncertainties are reported for both systems using the methods described previously. Copyright © 2012 John Wiley & Sons, Ltd.

In the past three years, at least 15 different concentrating photovoltaic (CPV) companies have sent modules to NREL for on-sun testing and evaluation. While the data set is intended primarily for other purposes, it is useful to present the various failures and performance issues that have occurred over these three years. It is emphasized that many of these modules were prototypes that had not completed design qualification. The observations presented here elucidate potential design challenges the CPV industry generally faces, and in one case an additional test is suggested for the design qualification standard.

FEM simulation and accelerated thermal cycling have been performed for the CPV die attach. Trends in fatigue damage accumulation and equivalent test time are explored and found to be most sensitive to temperature ramp rate. Die attach crack growth is measured through cycling and found to be in excellent agreement with simulations of the inelastic strain energy accumulated. Simulations of an entire year of weather data provides for the relative ranking of fatigue damage between four cities as well as their equivalent accelerated test time.

Many concentrating photovoltaic (CPV) systems use a polymeric encapsulant to couple and optical component and/or coverglass to the cell. In that location, the encapsulation improves the transmission of concentrated optical flux through interface(s), while protecting the cell from the environment. The durability of encapsulation materials, however, is not well established relative to the desired service life of 30 years. Therefore, we have initiated a screen test to identify the field-induced failure modes for a variety of popular PV encapsulation materials. An existing CPV module (with no PV cells present) was modified to accommodate encapsulation specimens. The module (where nominal concentration of solar flux is 500x for the domed-Fresnel design) has been mounted on a tracker in Golden, CO (elevation 1.79 km). Initial results are reported here for 18 months cumulative exposure, including the hottest and coldest months of the past year. Characteristics observed at intervals during that time include: visual appearance, direct and hemispherical transmittance, and mass. Degradation may be assessed from subsequent analysis (including yellowness index and cut-on frequency) relative to the ambient conditions present during field exposure. The fluorescence signature observed of all the silicone specimens is examined here, including possible factors of causation -- the platinum catalystmore » used in the addition cured materials as well as the primer used to promote adhesion to the quartz substrate and superstrate.« less

Performance of III-V multi-junction solar cells depends on spectral conditions according to which junction limits the photocurrent. Specifically, the response of concentrating multi-junction solar cells depends on the illumination at the cell surface. Because the illumination condition depends on alignment, it is important to characterize the CPV performance not only for a mono-module but also for a system or an array. In this paper the spectral effect on the CPV system and the mono-module consisting of III-V multi-junction solar cells from three different manufactures will be discussed.

Two CdTe and two copper indium gallium (di)selenide (CIGS)-type modules were tested for potential-induced degradation (PID) with positive and negative 1000 V biases applied to the active cell circuit in an 85°C, 85% relative humidity environmental chamber. Various degradation mechanisms could be seen with signatures such as shunting, transparent conductive oxide (TCO) corrosion, charge carrier lifetime reduction, and dead active layer at edges along with resulting cell mismatch. All modules tested exhibited degradation by system voltage stress in chamber, but only one module type has degraded in parallel field tests. I−V curve data indicated that one CdTe-type module sequentially exhibited shunting followed by a recovery and then series resistance losses. This module type showed TCO delamination from the glass in the environmental chamber tests and also exhibited power degradation within 5 weeks in field tests. Relative rates of Coulomb transfer from the voltage-biased active cell circuit to ground are compared for the modules in chamber tests to those placed outdoors under system voltage stress to extrapolate the anticipated time to failure in the field. This analysis correctly indicated which module type failed in the field first.

The band gap and microstructure of Ga0.5In0.5P have been shown to vary with deposition conditions. However, growth on (511)B GaAs substrates has been reported to give Ga0.5In0.5P with band gaps close to that of disordered material. It is shown here, that with appropriate selection of the growth parameters, Ga0.5In0.5P can be grown with low band gap and significant ordering on even the (511)B substrates, implying that surface steps play an important role in the ordering process. For the lattice‐matched composition, a band gap of 1.83 eV was obtained using low growth temperature (575 °C), low growth rate (0.55 μm/h), and high phosphine pressure (5 Torr).

The band gap of Ga0.5In0.5P is reported as a function of doping level and growth rate. The lowest band gaps are obtained for hole concentrations of about 2 × 1017 cm−3. For samples doped p-type above 1 × 1018 cm−3, the band gap increases dramatically, regardless of growth rate. This effect is shown to be the result of disordering during growth rather than a change in the equilibrium surface structure with doping. The doping level dependence of the band gap of Ga0.5In0.5P samples grown at higher and lower growth rates differs for selenium and zinc doping even though the effects of high doping are the same for both dopants.

The influence of substrate misorientation on the electronic structure of ordered Ga 0.5 In 0.5 P is investigated by piezomodulated reflectivity, photoluminescence and photoluminescence excitation spectroscopy. A “U”-shaped dependence of the band gap on the misorientation angle is observed. In addition, we have investigated the dependence of the crystal field and spin-orbit splitting, photoluminescence linewidth and the Stokes shift on substrate misorientation in these materials. The dependence of the crystal field and spin orbit splitting on the band gap of the alloy is found to be independent of substrate misorientation and is shown to be in reasonable agreement with a theoretical model relating these quantities to the long range order parameter in the alloy. The PL linewidth and Stokes shift both decrease with increasing band gap.

Use of thermoplastic materials as encapsulants in photovoltaic (PV) modules presents a potential concern in terms of high temperature creep, which should be evaluated before thermoplastics are qualified for use in the field. Historically, the issue of creep has been avoided by using thermosetting polymers as encapsulants, such as crosslinked ethylene-co-vinyl acetate (EVA). Because they lack crosslinked networks, however, thermoplastics may be subject to phase transitions and visco-elastic flow at the temperatures and mechanical stresses encountered by modules in the field, creating the potential for a number of reliability and safety issues. Thermoplastic materials investigated in this study include PV-grade uncured-EVA (without curing agents and therefore not crosslinked); polyvinyl butyral (PVB); thermoplastic polyurethane (TPU); and three polyolefins (PO), which have been proposed for use as PV encapsulation. Two approaches were used to evaluate the performance of these materials as encapsulants: module-level testing and a material-level testing.

The initial qualification standards for photovoltaic modules were designed to help develop a product that is safe, and able to survive reasonably long time periods when deployed in the field. To accomplish this, TC-82 of the International Electro-Technical Commission (IEC), developed and published module qualification standards (IEC 61215 for crystalline Si, IEC 61646 for thin films and IEC 62108 for concentrating modules) and a module safety standard (IEC 61730 -1 and 2). As PV has developed and the technology has become better understood, the properties of materials used in the module package play an increasingly important part in achieving long-term durability and safety. Certain basic properties are required of the materials in order for the modules to be safe and to be able to survive in the field for 25 years or more. Therefore Working Group 2 (Modules) of TC-82 began work to develop new material-level standards for PV that will utilize existing standards, whenever available, but tailored for characterizing the properties that are important for PV modules and modified to take into account the environmental conditions specific to PV applications. The goal is to provide a uniform approach to characterizing candidate materials, providing the necessary information to designers selecting materials for use in their PV products as well as to certification bodies assessing the quality and safety of the products made from these materials. This paper will describe the details of the effort underway to determine what PV material standards are necessary and the progress on developing those standards.

Over the past decade, degradation and power loss have been observed in PV modules resulting from the stress exerted by system voltage bias. This is due in part to qualification tests and standards that do not adequately evaluate for the durability of modules to the long-term effects of high voltage bias experienced in fielded arrays. High voltage can lead to module degradation by multiple mechanisms. The extent of the voltage bias degradation is linked to the leakage current or coulombs passed from the silicon active layer through the encapsulant and glass to the grounded module frame, which can be experimentally determined; however, competing processes make the effect non-linear and history-dependent. Appropriate testing methods and stress levels are described that demonstrate module durability to system voltage potential-induced degradation (PID) mechanisms. This information, along with outdoor testing that is in progress, is used to estimate the acceleration factors needed to evaluate the durability of modules to system voltage stress. Na-rich precipitates are observed on the cell surface after stressing the module to induce PID in damp heat with negative bias applied to the active layer.

Long-term, stable performance of photovoltaic (PV) modules will be increasingly important to their successful penetration of the power grid. This paper summarizes more than 150 thin-film and more than 1700 silicon PV degradation rates (R d ) quoted in publications for locations worldwide. Partitioning the literature results by technology and date of installation statistical analysis shows an improvement in degradation rate especially for thin-film technologies in the last decade. A CIGS array deployed at NREL for more than 5 years that appears to be stable supports the literature trends. Indoor and outdoor data indicate undetectable change in performance (0.2±0.2 %/yr). One module shows signs of slight degradation from what appears to be an initial manufacturing defect, however it has not affected the overall system performance.

PV customers wish to have confidence in PV modules they purchase. Today, no test can quantify a module’s lifetime with confidence, but stress tests are routinely used to differentiate PV product designs. We suggest that the industry would be strengthened by using the wisdom of the community to develop a single set of tests that will help customers quantify confidence in PV products. This paper evaluates the need for quality assurance (QA) standards and suggests a path for creating these. Two types of standards are needed: 1) QA of the module design and 2) QA of the manufacturing process., JRC.F.7-Renewable Energy

Long-term reliability is critical to the cost effectiveness and commercial success of photovoltaic (PV) products. Today most PV modules are warranted for 25 years, but there is no accepted test protocol to validate a 25-year lifetime. The qualification tests do an excellent job of identifying design, materials, and process flaws that are likely to lead to premature failure (infant mortality), but they are not designed to test for wear-out mechanisms that limit lifetime. This paper presents a method for evaluating the ability of a new PV module technology to survive long-term exposure to specific stresses. The authors propose the use of baseline technologies with proven long-term field performance as controls in the accelerated stress tests. The performance of new-technology modules can then be evaluated versus that of proven-technology modules. If the new-technology demonstrates equivalent or superior performance to the proven one, there is a high likelihood that they will survive versus the tested stress in the real world.

This paper discusses why it is necessary for new lower cost PV modules to be tested using a reliability test sequence that goes beyond the Qualification test sequence now utilized for modules. Today most PV modules are warranted for 25 years, but the Qualification Test Sequence does not test for 25-year life. There is no accepted test protocol to validate a 25-year lifetime. This paper recommends the use of long term accelerated testing to compare now designs directly with older designs that have achieved long lifetimes in outdoor exposure. If the new designs do as well or better than the older ones, then it is likely that they will survive an equivalent length of time in the field.

Semprius has developed a novel microcell based, highly scalable HCPV module that addresses performance, cost and reliability requirements for utility scale solar installations. Semprius has fabricated dual junction cell based engineering prototype modules with 1000X concentration based on this technology. A 1 kW HCPV system using these modules was installed in Tucson to validate the technology and acquire on‐sun data. Eight months of on‐sun results from this system are presented.

Quantum dot (QD) enhanced GaInP/InGaAs/Ge solar cells are presented and characterized under flash and continuous solar simulators. InAs QD within the middle sub‐cell increase the carrier generation due to absorption in the range 900–940 nm. These QD‐enhanced solar cells routinely achieve production efficiencies of ∼40%, and this set of research samples obtain a peak efficiency of >38% under flash solar simulators. Continuous solar simulator testing is performed to test the QD‐enhanced solar cells under thermal loads similar to concentrated photovoltaic systems, in which cells demonstrate excellent reliability. Numerical simulations of the QD‐enhanced solar cells are performed using an effective medium to model the additional absorption due to the QD layers. Temperature dependence of the QD‐enhanced solar cells are modeled, in which temperature‐dependent bandgap narrowing changes the dark current and the semiconductor absorption profiles. Comparison between the experimental results and numerical model show...

A 1‐axis carousel tracker equipped with four 3‐sun low‐concentration mirror modules has now been under test outdoors at the University of Nevada in Las Vegas (UNLV) for three years. There are three unique features associated with this unit. First, simple linear mirrors are used to reduce the amount of expensive single crystal silicon in order to potentially lower the module cost while potentially maintaining cell efficiencies over 20% and high module efficiency. Simple linear mirrors also allow the use of a single axis tracker. Second, the azimuth carousel tracker is also unique allowing trackers to be used on commercial building rooftops. Third, an experiment is underway comparing aluminum based mirrors with novel 3M Company multilayer polymeric mirrors which are potentially very low cost. Comparing the data from March of 2008 through March of 2011 shows that the aluminum mirror degradation to date is negligible and that the carousel tracker has been operating continuously and reliable. Also, no degradation has been observed for the 3M brand cool mirrors after one year in use.

An extended 3D distributed model based on distributed circuit units for the simulation of triple‐junction solar cells under realistic conditions for the light distribution has been developed. A special emphasis has been put in the capability of the model to accurately account for current mismatch and chromatic aberration effects. This model has been validated, as shown by the good agreement between experimental and simulation results, for different light spot characteristics including spectral mismatch and irradiance non‐uniformities. This model is then used for the prediction of the performance of a triple‐junction solar cell for a light spot corresponding to a real optical architecture in order to illustrate its suitability in assisting concentrator system analysis and design process.