Neftalí Núñez

Degradation of AlInGaP red LEDs under drive current and temperature accelerated life tests

AlInGaP LEDs are widely used in illumination applications as automotive and signalization due their low consumption and high durability. In order to verify the high durability data it is necessary to consider not only catastrophic failures but also degradation. In this work LEDs degradation at different temperature and drive current accelerated tests have been analyzed. In all the tests we have carried out an exponential degradation trend have been observed. Temperature and drive current influence in degradation rate and reliability have been evaluated.

Evaluation of AlGaInP LEDs reliability based on accelerated tests

High efficiency LEDs are replacing incandescent bulbs in many applications particularly those requiring durability and low power consumption. In some of these applications, as automotive and traffic signals, LEDs work in outdoor environment and in some cases in extreme temperature and humidity conditions. Main failure mechanisms have been identified for the different stress conditions (temperature/humidity accelerated tests). MTTF of 127,000 h have been evaluated for real working conditions by means of applying Arrhenius and Peck model values obtained in this paper.

Degradation mechanism analysis in temperature stress tests on III-V ultra-high concentrator solar cells using a 3D distributed model

A temperature stress test was carried out on GaAs single-junction solar cells to analyze the degradation suffered when working at ultra-high concentrations. The acceleration of the degradation was realized at two different temperatures: 130 °C and 150 °C. In both cases, the degradation trend was the same, and only gradual failures were observed. A fit of the dark I–V curve at 25 °C with a 3D distributed model before and after the test was done. The fit with the 3D distributed model revealed degradation at the perimeter because the recombination current in the depletion region of the perimeter increased by about fourfold after the temperature stress test. Therefore, this test did not cause any morphological change in the devices, and although the devices were isolated with silicone, the perimeter region was revealed as the most fragile component of the solar cell. Consequently, the current flowing beneath the busbar favors the progression of defects in the device in the perimeter region.

Novel accelerated testing method for III–V concentrator solar cells

Accelerated testing is a necessary tool in order to demonstrate the reliability of concentration photovoltaic solar cells, devices which is expected to be working not less than 25 years. Many problems arise when implementing high temperature accelerated testing in this kind of solar cells, because the high light irradiation level, at which they work, is very difficult to achieve inside a climatic chamber. This paper presents a novel accelerated testing method for concentrator solar cells, under simulated electrical working conditions (i.e. forward biasing the solar cells at the equivalent current they would handle at 700 suns), that overcomes some of the limitations found in test these devices inside the chamber. The tracked power of the solar cells to 700×, experiences a degradation of 1.69% after 4232 h, in the 130 °C test, and of 2.20% after 2000 h in the 150 °C one. An additional test has been carried out at 150 °C, increasing the current to that equivalent to 1050 suns. This last test shows a power degradation of 4% for the same time.

Preliminary temperature accelerated life test (ALT) on lattice mismatched triple-junction concentrator solar cells-on-carriers

A temperature accelerated life test on concentrator lattice mismatched Ga0.37In0.63P/Ga0.83In0.17As/Ge triple-junction solar cells-on-carrier is being carried out. The solar cells have been tested at three different temperatures: 125, 145 and 165°C and the nominal photo-current condition (500X) is emulated by injecting current in darkness. The final objective of these tests is to evaluate the reliability, warranty period, and failure mechanism of these solar cells in a moderate period of time. Up to now only the test at 165°C has finished. Therefore, we cannot provide complete reliability information, but we have carried out preliminary data and failure analysis with the current results.

Evaluation of the reliability of commercial concentrator triple-junction solar cells by means of accelerated life tests (ALT)

A temperature accelerated life test on commercial concentrator lattice-matched GaInP/GaInAs/Ge triple-junction solar cells has been carried out. The solar cells have been tested at three different temperatures: 119, 126 and 164 °C and the nominal photo-current condition (820 X) has been emulated by injecting current in darkness. All the solar cells have presented catastrophic failures. The failure distributions at the three tested temperatures have been fitted to an Arrhenius-Weibull model. An Arrhenius activation energy of 1.58 eV was determined from the fit. The main reliability functions and parameters (reliability function, instantaneous failure rate, mean time to failure, warranty time) of these solar cells at the nominal working temperature (80 °C) have been obtained. The warranty time obtained for a failure population of 5 % has been 69 years. Thus, a long-term warranty could be offered for these particular solar cells working at 820 X, 8 hours per day at 80 °C.

Reliability of commercial triple junction concentrator solar cells under real climatic conditions and its influence on electricity cost

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 GWp cumulative installed power, a dramatic reduction to levels of €c2–3/kWh is achievable. Copyright

Functional Analysis in Long-Term Operation of High Power UV-LEDs in Continuous Fluoro-Sensing Systems for Hydrocarbon Pollution

This work analyzes the long-term functionality of HP (High-power) UV-LEDs (Ultraviolet Light Emitting Diodes) as the exciting light source in non-contact, continuous 24/7 real-time fluoro-sensing pollutant identification in inland water. Fluorescence is an effective alternative in the detection and identification of hydrocarbons. The HP UV-LEDs are more advantageous than classical light sources (xenon and mercury lamps) and helps in the development of a low cost, non-contact, and compact system for continuous real-time fieldwork. This work analyzes the wavelength, output optical power, and the effects of viscosity, temperature of the water pollutants, and the functional consistency for long-term HP UV-LED working operation. To accomplish the latter, an analysis of the influence of two types 365 nm HP UV-LEDs degradation under two continuous real-system working mode conditions was done, by temperature Accelerated Life Tests (ALTs). These tests estimate the mean life under continuous working conditions of 6200 h and for cycled working conditions (30 s ON & 30 s OFF) of 66,000 h, over 7 years of 24/7 operating life of hydrocarbon pollution monitoring. In addition, the durability in the face of the internal and external parameter system variations is evaluated.

Influence of concentration and solar cell size on the warranty time of triple junction solar cells

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 AZUSA), 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.

Statistical calculation of the main reliability functions of GaAs concentrator solar cells

This paper presents some of the results of a method to determine the main reliability functions of concentrator solar cells. High concentrator GaAs single junction solar cells have been tested in an Accelerated Life Test. The method can be directly applied to multi-junction solar cells. The main conclusions of this test carried out show that these solar cells are robust devices with a very low probability of failure caused by degradation during their operation life (more than 30 years). The evaluation of the probability operation function (i.e. the reliability function R(t)) is obtained for two nominal operation conditions of these cells, namely simulated concentration ratios of 700 and 1050 suns. Preliminary determination of the Mean Time to Failure indicates a value much higher than the intended operation life time of the concentrator cells.