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

Abstract

Progressing beyond 3-junction inverted-metamorphic multijunction solar cells grown on GaAs substrates, to 4-junction devices, requires the development of high quality metamorphic 0.7 eV GalnAs solar cells. Once accomplished, the integration of this subcell into a full, monolithic, series connected, 4J-IMM structure demands the development of a metamorphic tunnel junction lattice matched to the leV GalnAs subcell. Moreover, the 0.7 eV junction adds about 2 hours of growth time to the structure, implying a heavier annealing of the subcells and tunnel junctions grown first. The final 4J structure is above 20 | _im thick, with about half of this thickness used by the metamorphic buffers required to change the lattice constant throughout the structure. Thinning of these buffers would help reduce the total thickness of the 4 J structure to decrease its growth cost and the annealing time. These three topics: development of a metamorphic tunnel junction for the 4th junction, analysis of the annealing, and thinning of the structure, are tackled in this work. The results presented show the successful implementation of an antimonide-based tunnel junction for the 4th junction and of pathways to mitigate the impact of annealing and reduce the thickness of the metamorphic buffers.

Semi‐quantitative temperature accelerated life test (ALT) for the reliability qualification of concentrator solar cells and cell on carriers

“…In order to design a semi‐quantitative temperature ALT, the following assumptions have to be taken into account: Life model : We assume that temperature accelerates the life of the solar cell/CoC following the Arrhenius model , as shown in previous temperature ALTs on solar cells/CoCs . where L ( T ) is a temporal measurable characteristic of the life of the device under test which depends on the temperature, k is the Boltzmann constant, E A is the activation energy of the mechanism which causes the failure and C is a parameter of the Arrhenius model which depends on the L ( T ) used. Working time : Concentrator solar cells work only for daylight, and their electricity production is variable, having a peak at about the solar noon.…”

“…Consequently, the following actions about β and E A are taken: Weibull shape parameter ( β ): Concentrator solar cells/CoCs are devices working at high temperature and current stress. Because of these working conditions, they typically have an increasing failure rate (namely, wear‐out mode, β > 1) , so in this analysis, we have studied two extreme values for this situation, β = 1 and β = 5. For β = 1, the devices have constant failure rate which corresponds with devices of low electric and thermal stress.…”

“…• Life model: We assume that temperature accelerates the life of the solar cell/CoC following the Arrhenius model [25], as shown in previous temperature ALTs on solar cells/CoCs [8,9,14].…”

“…It is of high interest to design an accelerated test that allows the qualification of the devices taking into account as reference the total warranty life of the device . The best technique to do this is a slower and quantitative ALT test .…”

“…In order to determine the right forward injection current level, we use our 3D distributed model for triple‐junction solar cells to obtain the equivalent current that has to be injected in darkness for several solar cell sizes. This model has been successfully used in previous reliability analysis of triple junction solar cells .…”