Effects of Temperature-Induced Bandgap Shift in 5-Junction Solar Cells

28th European Photovoltaic Solar Energy Conference and Exhibition; 101-106
The pursuit of higher efficiency photovoltaic cells for terrestrial concentrator applications leads naturally to consideration of 4-, 5-, and 6-junction cells. In this paper we investigate the semiconductor device mechanisms affecting thermal performance of multijunction cells with more than 3 junctions. The temperature dependences of 5-junction (5J) and 3-junction (3J) cells are modeled and compared, showing nearly identical relative changes in the Voc and efficiency of 5J and 3J concentrator solar cells with varying temperature. A small increase in Jsc as cell temperature rises is largely cancelled by a corresponding decrease in fill factor, both related to current imbalance caused by shifting subcell bandgaps as temperature increases, while changes in Voc dominate the multijunction cell efficiency temperature dependence. A formal treatment relating fractional change in Voc per °C to the average subcell bandgap for any multijunction cell is developed, and compared to measured temperature dependences of Voc for 1, 3, 4, and 5-junction cells, with inverted metamorphic (IMM), semiconductor bonding technology (SBT), and upright lattice-matched (LM) cell designs. Intentional changes in subcell current balance are studied, showing that 5J and 3J cells are remarkably insensitive to whether the subcells are designed to be current balanced at 0°C, 50°C, or 100°C.