This PDF file contains the front matter associated with SPIE Proceedings Volume 7412, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Photovoltaic (PV)-modules are exposed to solar irradiation, which includes Ultra-violet (UV) light. UV light is wellknown as degradation factor for polymeric materials, as used for encapsulation of PV-cells. Therefore they are protected by UV-filtering glass or UV protecting additives. The UV-stability is only tested on a very low level (total UV energy of 15kWh/m²) according to the actual type approval standards (IEC 61215, IEC61646, e.g.). An undefined acceleration is provided by the testing temperature of 60°C. The real UV-dose can reach more than 120 kWh/m² per year, however. The module-temperature during high UV-irradiation ranges between 40°C and 60°C, usually. The main reason for the inadequate test conditions is the lack of well-defined and inexpensive UV-light sources and therefore small test capacities. We developed an UV-radiation unit based on fluorescence tubes, which have the advantage of low visible and NIR irradiation avoiding overheating of the samples. The spectral irradiation is solar-like in the short-wavelength UV and lower in the long-wavelength UV, with a limited number of disturbing emission-lines. The design of the unit has been optimized for high UV-intensities up to 5X and usage on both sides. Our prototype has an area of 1.7m * 3m, which yields an usable testing area of 6m *1.7m. The unit is designed for usage in humid ambient in a temperature range up to 90°C for the future development of combined damp-heat and UV tests, in order to get the tests closer to reality.

The dynamic behaviour of modules with different designs and sizes is analyzed with different methods. Outdoor measurements of the deflection show their dynamic behaviour under wind loads and the correlation between wind velocity and deflection. Indoor tests were performed with acoustic excitation of the modules with monitoring the deflection. Numerical calculations, based on FEM-modelling, showed that their resonance frequencies are typically in the range from 1 to 100 Hz. Results of the indoor and outdoor measurements are reported and compared with the numerical results of the FEM-simulation.

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.

Environmental stress cracking (ESC) begins with crazes on the surface of the plastic. Plastic optics may corrode due to ESC. Usually ductile, plastic may become brittle, and subject to failure due to mechanical, physical, or chemical influences. Stress cracking is accelerated by temperature cycling, duration, temperature, chemicals, and cross-linking, orientation, or other characteristics within the plastic. Plastic optics for solar energy conversion include large-area Fresnel lens parquets commonly used in concentrator photovoltaics (CPV). Solar energy conversion takes place in a harsh environment. We look for evidence for ESC in plastics and glass optics for sunlight collection, measure stress, and discuss the possible impact of stress on longevity, optical efficiency, test methods, and manufacturing strategies.

Concentrator PV (CPV) systems have attracted significant interest because these systems incorporate the world's highest efficiency solar cells and they are targeting the lowest cost production of solar electricity for the world's utility markets. Because these systems are just entering solar markets, manufacturers and customers need to assure their reliability for many years of operation. There are three general approaches for assuring CPV reliability: 1) field testing and development over many years leading to improved product designs, 2) testing to internationally accepted qualification standards (especially for new products) and 3) extended reliability tests to identify critical weaknesses in a new component or design. Amonix has been a pioneer in all three of these approaches. Amonix has an internal library of field failure data spanning over 15 years that serves as the basis for its seven generations of CPV systems. An Amonix product served as the test CPV module for the development of the world's first qualification standard completed in March 2001. Amonix staff has served on international standards development committees, such as the International Electrotechnical Commission (IEC), in support of developing CPV standards needed in today's rapidly expanding solar markets. Recently Amonix employed extended reliability test procedures to assure reliability of multijunction solar cell operation in its seventh generation high concentration PV system. This paper will discuss how these three approaches have all contributed to assuring reliability of the Amonix systems.

The main objective to study the mechanisms of moisture penetration on glass/pvb/glass laminates comes from the request to improve PV module reliability that uses PVB as encapsulant film. In order to provide modeling, extensive experimental work was performed to understand the eventual preferential channels for moisture penetration. This covers different edge deletion methods (etching, sand blasting) and internal topographies (bus bars, laser scribes). The moisture content is determined by punctual IR Spectroscopy and the moisture penetration induced by standard Damp Heat.

Water vapor is one of the major reasons for corrosion and aging within photovoltaic modules. We investigated different encapsulants for photovoltaic devices in respect of their water vapor transmission rate (WVTR), diffusion profile and saturation concentration for varied climatic conditions (temperature and relative air humidity). For measuring the WVTR a gravimetric testing procedure was used, the diffusion profile was detected by infrared-absorption-spectroscopy in the wavelength range of 1.7 μm to 2.9 μm. The tested materials are a fast-cure ethylene vinyl acetate (EVA fc), a poly vinyl butyral (PVB), a thermoplastic polyurethane (TPU), an ionomer (ION) and a thermoplastic silicone (TSI). It was ascertained that the thermoplastics foils (TPU and TSI) have the highest WVTR, the ionomer the lowest. The saturation concentration of PVB was the highest at all, followed by TPU and EVA. The silicone and the ionomer store practically no water.

To ensure the longevity and reliability of solar modules, the PV industry has adopted a series of accelerated aging tests. Among these, the damp heat test performed at 85°C, 85% relative humidity for 1000 hrs provides the most information on the degradation of encapsulant and backsheet materials. The purpose of this work is to define the proper accelerated test conditions that represent 25 years of real field life exposure of these polymers.

The plasma generated by dielectric barrier discharge(DBD) with the atmosphere of lasting modifying materials, gives modification to the surfaces of FFC backsheet, which is formed by coating FFC (a tetra-fluoro based material with high content of fluorine) on the double-surfaces of polyester(PET). The research on the character of FFC backsheet before and after DBD modification is hold through a series analyzing ways, such as measuring the surface contact angles and surface energy of FFC backsheet with different plasma modification time and different DBD power density, comparing the preservation of surface energy of FFC backsheet with different storage medium and storage period, observing the surfaces of FFC backsheet through scanning electron microscope(SEM), making use of Fourier transform attenuated total reflectance infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). Meanwhile, the soalr cell modules using FFC backsheet and other backsheets are tested under the condition of 85centigrade×85%RH to make comparison between FFC backsheet and other backsheets in various aspects, like the peel strength between backsheet and EVA and so on. All the tests show both the microscopic appearance and surface chemical composition of FFC backsheet is changed after the DBD plasma modification with the atmosphere of lasting modifying materials. After the DBD plasma modifications with a power density of 4.07W/cm² and different modification time, the water contact angle for FFC backsheet surface is reduced from 82° to 38°. Comparing with other types of backsheets as the solar cell modules encapsulant materials, FFC backsheet has obvious advantage in humit-heat aging resistant performance of the peel strength with EVA and other respects.

Performance of photovoltaic (PV) modules decreases as the operating temperature increases. This performance drop is typically higher for the crystalline silicon technologies (~0.5%/°C) as compared to thin film technologies (~0.2%/°C). The temperature of rooftop modules in hot climatic locations like Arizona could be as high as 95°C depending on the air gap between the modules and roof surface. There are several thermal models existing to predict the temperatures of open-rack PV modules but no comprehensive thermal models have been reported for the rooftop PV modules/arrays based on an extended field monitoring. The primary goal of this work is to quantitatively model the influence of air gap on the temperature of rooftop modules so that the system integrators could improve their designs to maximize the overall energy output (kWh/kW) of the rooftop PV systems. To predict the temperature of rooftop PV modules/arrays based on irradiance, ambient temperature and wind speed conditions, this paper presents five thermal models for each of the five air gaps (0, 1, 2, 3 & 4 inches) investigated in this work.

Completed in December of 2007, the 14 MW photovoltaic power plant located at Nellis Air Force Base in Las Vegas, Nevada was the largest operational photovoltaic plant in North America, and as of June, 2009, it continues to be the largest plant in operation in North America. An analysis on system availability and weather variability was conducted to assess the impact of these factors on the overall system performance. Additionally, the plant is composed of two types of single-axis tracking systems and a variety of module technologies which has allowed for a direct comparison of their performance. Overall the plant performed above expectations for 2008, and through an analysis of the above factors, it has been concluded that they all affect the system performance on varying scales. However, the plant's availability is the largest controllable area for improvement in the plant's capacity.

The solar energy industry has reached the point where utilities are employing large multi-megawatt photovoltaic projects to provide power to the utility grid in a variety of configurations. The ability to deliver or connect solar energy to a commercial power grid with high efficiency and over a broad spectrum of environments is critical to the success of the solar utility industry. These analyses aid in evaluating the potential profitability of a project and aid in optimizing a power plant design by understanding the real life attributes of the components and subsystems. This paper presents a methodology for modeling system-level power production as a function of time based on concurrent reliability simulation of individual subsystems such as power inverters. Each subsystem model is also time-dependent and depends on understanding the probabilistic nature of the actual failure rates of the components and on endurance testing or field data for the subsystems. As an example of a detailed time-dependent predictive reliability model, the power inverter is used. The power inverter reliability and availability is essential for continuing improvement of solar power plant efficiency and cost effectiveness over the life of the installation. The authors discuss some of the common failure probability distributions, their application to components and how these affect such areas as the maintenance intervals and the number of expected spares needed. This affects the Levelized Cost of Energy (LCOE) of the system and the potential for profitable operation.

The heat dissipation performance of the junction boxes is directly related to the service life of solar modules. In this paper, the PT100 thermocouple and the MT2 infrared thermometer is used to conduct a preliminary study on the heat dissipation performance of different models of junction boxes. We selected three types of junction boxes from different manufacturers, and then produced a piece of laminating module without solar cells, using a piece of toughened glass which size is 1574*802*3.2mm, two-tier EVA film, and a sheet of TPT as the substrate. Junction boxes filled with silica gel A, silica gel B and no gel were fixed on the laminating module, which is a simple procedure on producing PV modules at present, and were evenly spread. All the junction boxes were connected in a series circuit. When every thing ready, we enforced respectively 5A and 8A DC current on the circuit for 5 hours apiece, and the environment temperature 25 °C was maintained. Simultaneously we tested the temperatures of the boxes inside and outside hourly. The investigation indicates that junction box filled silica gel shows better performance.

CdS/CdTe photovoltaic solar cells were made on two different transparent conducting oxide (TCO) structures in order to identify differences in fabrication, performance, and reliability. In one set of cells, chemical vapor deposition (CVD) was used to deposit a bi-layer TCO on Corning 7059 borosilicate glass consisting of a F-doped, conductive tin-oxide (cSnO₂) layer capped by an insulating (undoped), buffer (iSnO₂) layer. In the other set, a more advanced bi-layer structure consisting of sputtered cadmium stannate (Cd₂SnO₄; CTO) as the conducting layer and zinc stannate (Zn₂SnO₄; ZTO) as the buffer layer was used. CTO/ZTO substrates yielded higher performance devices however performance uniformity was worse due to possible strain effects associated with TCO layer fabrication. Cells using the SnO₂-based structure were only slightly lower in performance, but exhibited considerably greater performance uniformity. When subjected to accelerated lifetime testing (ALT) at 85 - 100 °C under 1-sun illumination and open-circuit bias, more degradation was observed in CdTe cells deposited on the CTO/ZTO substrates. Considerable C-V hysteresis, defined as the depletion width difference between reverse and forward direction scans, was observed in all Cu-doped CdTe cells. These same effects can also be observed in thin-film modules. Hysteresis was observed to increase with increasing stress and degradation. The mechanism for hysteresis is discussed in terms of both an ionic-drift model and one involving majority carrier emission in the space-charge region (SCR). The increased generation of hysteresis observed in CdTe cells deposited on CTO/ZTO substrates suggests potential decomposition of these latter oxides when subjected to stress testing.

Long-term performance reliability is essential for any photovoltaic module to become established in the PV market. Reliability is characterized based on many factors, one of the most important being the capability of the module to be resistant to moisture at elevated temperatures. This work continues our efforts to search for a high-performance and high-stability transparent conducting oxide (TCO) window layer for CuInGaSe₂ (CIGS) devices. In this experimental study, we compared the optical, electrical, and structural stability of various TCOs deposited on glass, including single-layer Al-doped ZnO (AZO), bilayer intrinsic-/Al-doped ZnO (BZO), B-doped ZnO (ZnO:B), amorphous In₂O₃:SnO₂ (ITO), and amorphous In₂O₃:ZnO (IZO). The samples were exposed to damp heat (DH) at 85°C and 85% relative humidity (RH) and were characterized periodically. The results showed that all ZnO-based TCOs are more sensitive to moisture with substantial electrical degradation and apparent optical changes than the ITO and IZO. The amorphous IZO showed peculiar behavior in electrical property, and exhibited structural change with the appearance of some finite crystallinity after DH >220 h. The results from this experimental series will assist in determining the best-performing TCO for CIGS solar cells.

Various transparent conducting oxide (TCO) films are used as window layers to permit light transmission, photocurrent generation, and for electrical current collection in thin-film photovoltaic technologies. Bilayer composites (BZO) of intrinsic ZnO (i-ZnO) and Al-doped ZnO (AZO) have been used on NREL's high-efficiency CuInGaSe₂ (CIGS) solar cells. Previously, we demonstrated that, when tested in damp heat (DH) condition at 85°C and 85% relative humidity (RH), the stability trend of some TCOs was in decreasing order of SnO₂:F > In₂O₃:SnO₂ (ITO) > ZnO-based films of AZO, BZO and Al-doped Zn_1-xMg_xO (ZMO). We also observed that the degradation rate of AZO, BZO, and ZMO was influenced by additional factors such as film thickness, deposition conditions, and exposure history. This work continued our efforts in searching for a high-performance and high-stability window layer TCO, as well as in finding mitigation methods to protect the ZnO layer, either i-ZnO or BZO, for use on the CIGS solar cells. The current study, which involved the third experimental set of TCOs deposited on glass, further examined in DH test conditions the thickness effect on single-layer AZO films, the glass substrate effect on BZO, the stability and protective effect of amorphous In₂O₃:ZnO (InZnO or IZO) as a conducting window layer for the underlying i-ZnO, and the stability and protective power of a protective transparent metal oxide (PTMO) coating for all three types of ZnO (AZO, BZO, and i-ZnO). The samples were periodically characterized with optical, electrical, and structural measurements during the course of DH exposure. The results show that the DH stability of AZO increased as the film thickness increased, BZO on Corning^® Eagle 2000™ glass degraded somehow faster than on Corning^® 7059™, and both the IZO and PTMO showed generally high DH stability and good protective power for the ZnO layers underneath. However, the results of decreased (002) peak intensity of ZnO from X-ray diffraction analysis indicated that both IZO and PTMO still allowed certain levels of moisture penetration.

Thin-film photovoltaics (PV) are sensitive to lateral nonuniformities (LN) that manifest themselves in spatial variations of the device local characteristics and in the variability of the measured parameters between nominally identical devices. LN affect all the aspects of device operations and stability and appear as a hidden cost of the otherwise inexpensive technology. They are omnipresent as originating from multiple factors typical of thin-film PV: deposition geometry, wet and heat treatments, dispersion in grain and amorphous phase parameters, and fluctuations in metal-semiconductor barriers. LN are seen in the device mappings, including that of PL, Voc, OBIC, EBIC, thermography, and electroluminescence. Stresses localized on certain vulnerable spots drive the entire device degradation. We present a general summary of physical processes related to LN, including modeling aspects, characteristic length and variability scales, statistics, degradation mechanisms, and superadditive effects between different device components, such as a negative correlation between the resistive and LN related loss, and a positive correlation between LN and device shunting failures under stress. We then review the known practical techniques of mitigating LN effects patented by different groups from 1970s to nowadays and show how nonuniformity treatments play the key role in the existing technologies.

One of the important issues involved while taking a process from lab environment to pilot plant scale is the yield of the process. Mechanical scribing that is used for making integral interconnects in CIGSeS thin film solar cells can be used to test the mechanical properties of the absorber film. Hence, it is necessary that the process delivers cohesive absorber films that exhibit good adhesion to molybdenum back contact and high efficiencies that are amenable to mechanical scribing. Optical and scanning electron microscopy can be used to study the effect of scribing on the absorber film and the morphology of the scribe lines.

Despite significant growth in photovoltaics (PV) over the last few years, only approximately 1.07 billion kWhr of electricity is estimated to have been generated from PV in the US during 2008, or 0.27% of total electrical generation. PV market penetration is set for a paradigm shift, as fluctuating hydrocarbon prices and an acknowledgement of the environmental impacts associated with their use, combined with breakthrough new PV technologies, such as thin-film and BIPV, are driving the cost of energy generated with PV to parity or cost advantage versus more traditional forms of energy generation. In addition to reaching cost parity with grid supplied power, a key to the long-term success of PV as a viable energy alternative is the reliability of systems in the field. New technologies may or may not have the same failure modes as previous technologies. Reliability testing and product lifetime issues continue to be one of the key bottlenecks in the rapid commercialization of PV technologies today. In this paper, we highlight the critical need for moving away from relying on traditional qualification and safety tests as a measure of reliability and focus instead on designing for reliability and its integration into the product development process. A drive towards quantitative predictive accelerated testing is emphasized and an industrial collaboration model addressing reliability challenges is proposed.

In the testing of photovoltaic materials and modules, failure analysis provides insights into the specific mechanism of performance breakdown and offers opportunities to improve performance by materials or process modification. We review various analytical methods applied to photovoltaic modules and test structures to better understand the nature of failure, including several methods not previously discussed in failure analysis literature as applied to photovoltaic devices. Included in this discussion will be the use of environmental scanning electron microscopy (ESEM) and x-ray microtomography to investigate the failure mechanism in electrical impulse testing of a candidate PV module.

Photovoltaic modules are exposed to extremely harsh conditions of heat, humidity, high voltage, mechanical stress, thermal cycling and ultraviolet (UV) radiation. The current qualification tests (e.g. IEC 61215) do not require sufficient UV exposure to evaluate lifespans of 30 years. Recently, photovoltaic panel manufacturers have been using glass that does not contain Cerium. This has the advantage of providing about 1.3% to 1.8% more photon transmission but potentially at the expense of long term stability. The additional transmission of light in the 300 nm to 340 nm range can cause delamination to occur about 3.8 times faster. Similarly, UV radiation will cause polymeric encapsulants, such as ethylene vinyl-acetate (EVA), to turn yellow faster losing photon transmission. Silicones do not suffer from light induced degradation as hydrocarbon based polymers do, therefore if silicone encapsulants are used, a 1.6% to 1.9% increase in photon transmission can be obtained from removal of Ce from glass, with no tradeoff in long term stability. Additionally antimony can be added to non-Ce containing glass to further improve photon transmission (principally in the IR range) by an additional 0.4% to 0.7%; however, this does not significantly affect UV transmission so the same UV induced reliability concerns will still exist with common hydrocarbon-based encapsulants.

A program is underway at Sandia National Laboratories to predict long-term reliability of photovoltaic (PV) systems. The vehicle for the reliability predictions is a Reliability Block Diagram (RBD), which models system behavior. Because this model is based mainly on field failure and repair times, it can be used to predict current reliability, but it cannot currently be used to accurately predict lifetime. In order to be truly predictive, physics-informed degradation processes and failure mechanisms need to be included in the model. This paper describes accelerated life testing of metal foil tapes used in thin-film PV modules, and how tape joint degradation, a possible failure mode, can be incorporated into the model.

At present the failure modes and mechanism of PV modules are not well understood. The current accelerated tests cannot duplicate the various field failures. It is very important to continue to carry out accelerated testing of PV modules in order to reduce the infant mortality of new technology PV modules as well as to improve the production techniques of the PV modules. However, the accelerate tests need to be complemented with actual field deployment of PV modules and specifically designed tests in real world conditions or preferably in harsh climates. In this work the inclusion of outdoor monitoring of PV modules and high voltage bias testing of PV modules in real world climatic conditions in the current best practices for PV module reliability testing is being proposed. One of the objectives of this paper is to show the importance of carrying out continuous monitoring of field deployed PV modules as well as high voltage bias testing of PV modules over an extended period of time.

This study is focused on testing methods determining quality of solar cells. Nowadays the development of solar cells is much faster and there is still necessary to increase their quality by removing causes of materials defects and also defects in a process of their production. Non-destructive methods are used for correct determination of defects by using of recombination effect of charge carrier in PN junction. Due to these methods can be the solar cell diagnosed and described. By using of various temperatures during the testing we can receive more objective results thanks to simulated operation conditions. Peltier cells are used for graditional change of temperature. Cooling system with liquid nitro - LN2 is used to reach the very low temperature. Diagnostic and testing methods described in this study are based on emission of light and the recombination processes in PN junction. It is especially electroluminescence and photoluminescence method. For comparison it is used the observation of emitted light from microplasma method. Described methods detect materials and process defects due to use of lownoise and very sensitive CCD camera.

There are many factors which are assumed to be concerned in the PV module deterioration. We have to understand phenomena, modes, factors and mechanism, and then we have to build a stress acceleration method. As the first step of this study, we conducted the forward voltage applying test and reverse biased breakdown test of c-Si PV cells. Then, we extended the method into single-cell PV modules. After the breakdown test, we measured the I-V characteristics, and then it was found that the shape of I-V curves are categorized into four types as follows: 1) decrease of shunt resistor, 2) linear, 3) suspected normal and 4) the others. We will report some important experimental results and we propose an idea for new acceleration method to PV module deterioration such as cell burn or interconnector failure.

Both the economic viability and energy payback time of photovoltaic (PV) systems are inextricably tied to both the electrical performance and degradation rate of PV modules. Different module technologies exhibit different properties in response to varying environmental conditions over time. The purpose of this study is to quantify the effects of those differences on the life-cycle economical cost and energy payback time of two fielded PV systems; one system comprised of polycrystalline silicon (c-Si) modules and one featuring hydrogenated amorphous silicon (a-Si) modules. The DC operating current, DC operating voltage, AC power, and conversion efficiency of each system have been monitored for a period of over four years, along with plane-of-array (POA) irradiance, module temperature, and ambient temperature. Electrical performance is evaluated in terms of final PV system yield (Y_f), reference yield (Y_r), and performance ratio (PR), which are derived from the primary international standard used to evaluate PV system performance, IEC 61724¹. Degradation rates were evaluated over the four year period using regression analysis. The empirically determined trends in long-term performance were then used to approximate the energy produced by both system types under the same environmental conditions; most importantly, the same levels of solar irradiation. Based on this modeled energy production and economic conditions specific to the state of Florida, comparisons have been carried out for life-cycle costs and energy payback time.

The continued exponential growth of photovoltaic technologies paves a path to a solar-powered world, but requires continued progress toward low-cost, high-reliability, and high-performance PV systems. High reliability is an essential element in achieving low-cost solar electricity by reducing operation and maintenance (O&M) costs and by extending system lifetime and availability, but these attributes are difficult to verify at the time of installation. Utilities, financiers, homeowners, and planners are demanding this information in order to evaluate their financial risk as a prerequisite to large investments. Reliability research and development (R&D) is needed to build market confidence by improving product reliability and by improving predictions of system availability, O&M cost, and system lifetime. Universities, industry, National Labs, and other research entities can be most effective by working together and in complementary ways. The Department of Energy supports a variety of research projects to improve PV-system reliability. These projects and current reliability issues for each PV technology are surveyed.