Application of infrared reflecting (IRR) coverglass on multijunction III-V solar cells

It is well known that the Ge subcell in multijunction GaInP/GaAs/Ge based solar cells produces a significantly higher photogenerated current (nearly 2x) than the other two subcells connected in series. The excess current is converted into heat, and as a result, increases the cell operating temperature. Because the solar cell efficiency decreases with higher temperatures, it is desirable to maintain a lower cell operating temperature. This can be achieved by rejecting a part of the incident sunlight that would otherwise be absorbed and converted into heat by the Ge subcell. For many space applications, coverglass incorporated with infrared reflecting (IRR) coatings can be applied to these solar cells for the purpose of lowering the cell operating temperature and/or improving the power output from the solar arrays. Achieving higher power output requires an appropriate IRR coating design that carefully balances the reduction in the cell absorptance against the Ge subcell current output. In this paper, this key issue is discussed. Also, preliminary IRR coating designs have been evaluated by applying them on high efficiency 3-junction solar cells, and the performance data are used to help predict optimal designs.