Cell-Level Thermal Management Issues in Concentrator III-V Multijunction Solar Cells

Concentrator solar cells are almost always characterized near room temperature (RT) conditions, but are operated at system-dependent elevated temperatures in the field. Cell designs that are optimized for RT can quickly lose their advantage at higher temperatures. Cell designers have traditionally considered thermal management to be the problem of system designers, but some initial consideration of these issues at the cell level can reduce the thermal resistances and heat-load that must be removed by system level active or passive cooling. We consider the location of heat generation within multijunction solar cells and its flow through thermal resistances caused by the substrates and mixed III-V alloy layers. We show advantages of inverted designs by rejection of much of the unused light below 1 eV, which makes up about 16% of the power flux. A 40% efficient IMM solar cell with a 1 eV bottom cell will therefore have about a 20% lower heat flux than a comparable 40% efficient Ge-based solar cell.