Failure analysis on lattice matched GaInP/Ga(In)As/Ge commercial concentrator solar cells after temperature accelerated life tests

Abstract

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.

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

“…This result suggests the need of some improvement in the CPV cell or in the module heat extraction for proper operation in Tucson. Fortunately, this does not seem a hard task because we have recently shown that the failure origin of these cells is related with the front metal grid and its external connection and not the semiconductor structure .…”

“…Therefore, if the failure associated with the front metal grid connection detected in the analysed solar cell was mitigated or solved, the remaining failure modes (which did not have time for appearing in the ALT) will appear later on, thus resulting in a higher warranty time. In consequence, improved versions of these commercial solar cells will increase substantially the real warranty times.…”

Temperature Accelerated Life Test and Failure Analysis on Upright Metamorphic Ga0.37In0.63P/Ga0.83In0.17As/Ge Triple Junction Solar Cells

“…The left column of Figure 13 shows that the solar cells with temperature soak alone ("Ref_Low", "Ref_Mid" and "Ref_High") experience two effects: a) a decrease of the shunt resistance and b) a rise of series resistance which increases with temperature. This impact of the temperature was not observed in commercial lattice-matched GaInP/Ga(In)As/Ge triple junction solar cells [3]. An interesting feature is observed when comparing the left (temperature soak only) with the right column of Figure 13, where the time evolution of the dark I-V curve of solar cells stressed by temperature but with periodical measurement of their dark I-V curve is presented: the deleterious effect of series resistance caused by the temperature soak in "Ref_IV_Low" and "Ref_IV_Mid" is counterbalanced by the current injection when measuring the dark I-V curve.…”

“…The average value of the calculated dV/dT was used then to obtain the junction temperature of the solar cells inside the chamber. This method has been successfully applied in previous ALTs [2,3] and is further explained in [11]. With this information the temperature of the climatic chambers was properly set in order to get 125, 145 and 165 °C as junction temperatures of the solar cells (Tcell) with current injection.…”

“…The deployment of Concentration Photovoltaic (CPV) systems based on Multijunction Solar Cell (MJSC) is currently dominated by systems integrating cell-on-carriers (CoCs) based on lattice-matched (LM) GaInP/Ga(In)As/Ge triple-junction concentrator solar cells .Their long term reliability has been assessed by means of temperature accelerated life tests (ALT) and failure analysis [1,2,3] proving a lifetime long enough in order not to limit the system one [3]. This type of LM solar cells is currently close to its practical efficiency limit [1], so no additional efforts are being paid to improve its performance, excepting those related to a further cost reduction.…”