Quantitative evaluation of thermal runaway tolerance in space solar cells

In thin-film solar cells such as inverted metamorphic multijunction solar cells, a local shunt spot can cause thermal runaway because of low thermal conductivity along the in-plane direction of the junction. Since electrical performance can be greatly reduced by thermal runaway, an appropriate design of the solar cells is necessary to prevent this mechanism. However, quantitative analysis of the thermal runaway is difficult because its threshold is usually strongly affected by the testing conditions and the characteristics of the shunt spots. In this study, we proposed a method of analyzing the thermal runaway characteristics quantitatively. We intentionally induced a thermal runaway under a simulated space environment with an arbitrary artificial shunt spot by a laser beam. The thermal resistance of the shunt spots and the threshold temperature for the thermal runaway were estimated using electrical and thermal models. This method enables an optimized design of thin-film solar cells.