Manuel Vazquez

This paper presents the comparison of two temperature Accelerated Life Tests (ALTs) for space commercial solar cells. The tests are carried out in dark conditions to circumvent the inherent problems of illumination ALTs. In the first ALT the solar cell working conditions were emulated by means of forward bias (that emulates the electric performance under illumination) while in the second ALT, forward bias was used together with reverse bias (that emulates the shadowing at eclipse periods or any other shadowing event such as for example the shadows cast by antennas) in order to reproduce more adequately the real operation conditions of solar cells. Forward to reverse time ratio is 4:1. Furthermore, the high temperatures used in the second ALT (190, 210 and 230 °C) allow a considerable time reduction of the test with respect to the first ALT (150, 165 and 180°C). The evaluation of the Arrhenius activation energy for an illumination power degradation of 10% was 0.88 eV in the forward bias test while 1.06 eV in the forward-reverse test. Reverse bias promotes similar degradation mode than forward one, namely parallel resistance reduction, but probably with a different degradation mechanism that degrades solar cell more intensely, thus reducing reliability which is more than 7 times shorter in the forward-reverse test than in forward one. Despite this reliability reduction, the time for a 90% reliability in the forward-reverse ALT is 32 years of continuous operation assuming a nominal temperature of 80°C which is typical of many GEO missions. The solar cells used in both ALTs were commercial GaInP/Ga(In)As/Ge triple junction and perform very robustly and exhibiting a high reliability for many space applications. It should be noted that these reliability figures are related to the degradation caused exclusively by high temperatures and other stressors like radiation are not considered here.

The lifetime of ultraviolet high-power light-emitting diodes (UV HP-LEDs) is an open issue due to their high current density, high temperature, and UV radiation. This work presents a reliability study and failure analysis of three high-temperature accelerated life tests (ALTs) for 13,500 h with 3 W commercial UV LEDs of 365 nm at a nominal current in two working conditions: continuous mode and cycled mode (30 s on/30 s off). Arrhenius–Weibull parameters were evaluated, and an equation to evaluate the lifetime (B50) at any junction temperature and other relevant lifetime functions is presented. The Arrhenius activation energy was 0.13 eV for the continuous mode and 0.20 eV for the cycled mode. The lifetime at 50% survival and 30% loss of optical power as a failure definition, working at T_a = 40 °C with a multi-fin heat sink in natural convection, was over 4480 h for the continuous mode and 19,814 h for the cycled mode. The need to add forced convection for HP-LED arrays to achieve these high-reliability values is evidenced. The main source of degradation is the semiconductor device, and the second is the encapsulation silicone break.

Evaluating the probability of failure of a product during the warranty is key to estimating the cost of its entire life cycle. Photovoltaic (PV) modules are the most reliable elements of a PV system, and their high reliability translates into long warranty periods (typically 25 to 30 years). The number of PV modules that fail during warranty and when they failed is an important issue to estimate the life cycle cost of PV modules. In this paper, we propose a coupled degradation‐warranty model for PV modules. The proposed model is aimed to assess the probability of failure over time covered by warranty based on degradation data. This allows us to estimate the time of PV modules failure covered by the warranty. Although the warranty model has been applied to some forms of degradation and sales functions, our method can be applied to any real application. This allows the key parameters of the warranty to be obtained in a simple way. The required degradation parameters to fulfill warranty depend on the allowable ratio of warranted elements. However, the main conclusion of the influence of degradation rates on warranty is that it is not possible to fulfill 25 years of warranty if degradation rates are larger than 0.8%/year. The required degradation rates values, for 25 years of warranty, will be lower than 0.5%/year if the allowed warranted element is in the range of 1%.

A temperature accelerated life test on Upright Metamorphic Ga_0.37In_0.63P/Ga_0.83In_0.17As/Ge triple‐junction solar cells has been carried out. The acceleration has been accomplished by subjecting the solar cells to temperatures (125, 145 and 165°C) significantly higher than the nominal working temperature inside a concentrator (90°C), while the nominal photo‐current (500×) has been emulated by injecting current in darkness. The failure distributions have been fitted to an Arrhenius–Weibull model resulting in an activation energy of 1.39 eV. Accordingly, a 72 years warranty time for those solar cells for a place like Tucson (AZ, USA), was determined. After the ALT, an intense characterization campaign has been carried out in order to determine the failure origin. We have detected that temperature soak alone is enough to degrade the cell performance by increasing the leakage currents, the series resistance, and the recombination currents. When solar cells were also forward biased an increase of series resistance together with a reduction of short circuit current is detected. The failure analysis shows that: a) several metallization sub‐products concentrate in several regions of front metal grid where they poison the silver, resulting in a two times reduction of the metal sheet resistance; b) the metal/cap layer interface is greatly degraded and there is also a deterioration of the cap layer crystalline quality producing a huge increase of the specific front contact resistance, c) the decrease of short circuit current is mainly due to the GaInP top subcell degradation.

Qualification standards are one the driving forces for the commercialization and production of the Concentrator Photovoltaic (CPV) technology. After several years of preparation the Committee Draft of IEC 62787 has been finally submitted. The new standard IEC 62787 (Concentrator photovoltaic (CPV) solar cells and cell-on-carrier (CoC) assemblies – Qualification) fills the gap between the IEC TS 62789:2014 (Photovoltaic concentrator cell documentation) and the IEC 62108 Ed 2 (Concentrator photovoltaic (CPV) modules and assemblies - Design qualification and type approval). In the present article, apart from analyzing the objective, main guidelines, the innovative characteristics, and the life estimation from several tests of the qualification standard is also explained.

In a previous work the warranty time of commercial lattice-matched GaInP/Ga(In)As/Ge triple junction concentrator solar cells was evaluated under real climatic conditions. The solar cells had a size of 7x7 mm operating with an efficiency of 35% at 820×. For these particular solar cells the warranty time for three locations, Golden (CO-USA), Madrid (Spain), and Tucson (AZ-USA), exhibits a 4 to 1 ratio, which affects the LCOE (Levelized Cost of Electricity) in an important way. In this work, we go a step further evaluating the influence of concentration and solar cell size on the warranty for a specific thermal design.

This paper proposes a methodology for assessing the concentrator solar cell reliability in a real application for a given location provided the results from accelerated life tests. We have applied this methodology for the evaluation of warranty times of commercial triple junction solar cells operating inside real concentrator modules in Golden (Colorado, USA), Madrid (Spain) and Tucson (Arizona, USA) for the period 2012–2015. Warranty times in Golden and Madrid, namely, 68 and 31 years, respectively, for the analysed period, indicate the robustness of commercial triple junction solar cells. Nevertheless, the warranty time of 15 years for Tucson suggests the need of improvement in the heat extraction of the solar cell within the concentrator module. Therefore, the influence of the location on the reliability of concentrator solar cells is huge, and it has no sense to supply general reliability values for a given concentrator product. The influence of these warranty times for the three locations on the levelised cost of electricity has been analysed. Cost of €c10–12/kWh can be achieved nowadays, while after 1 GW_p cumulative installed power, a dramatic reduction to levels of €c2–3/kWh is achievable. Copyright © 2017 John Wiley & Sons, Ltd.

Accelerated life tests are frequently used to provide reliability information in a moderate period of time (weeks or months), and after that, a failure analysis is compulsory to detect the failure origins. In this paper, a failure analysis has been carried out after a temperature accelerated life test on lattice matched GaInP/Ga(In)As/Ge triple junction commercial solar cells. Solar cells were forward biased in darkness inside three climatic chambers in order to emulate the photo‐generated current under nominal working conditions (a concentration level of 820 suns). After the accelerated aging test, a characterization of the resulting cells by means of quantum efficiency, dark and illumination I–V curves, electroluminescence, scanning electron microscope, energy dispersive X‐ray, scanning transmission electron microscope and X‐ray photoelectron spectroscopy has been carried out.Current is identified as the cause of degradation while temperature just dominates the accelerating factor of the aging test. Current promotes the front metal damage produced by the chemical evolution of the electroplating impurities together with those of the tab soldering process. Semiconductor structure does not seem to be responsible of any failure. Therefore, this kind of lattice matched GaInP/Ga(In)As/Ge triple junction solar cells, that as of 2016, are the workhorse of CPV technology, exhibits as a very robust device if the front metal connection is properly accomplished. Copyright © 2016 John Wiley & Sons, Ltd.