Optimization of inverted metamorphic multijunction solar cells for field-deployed concentrating PV systems

The inverted metamorphic multijunction (IMM) solar cell configuration allows significant increase of PV conversion efficiency over that of conventional InGaP/InGaAs/Ge triple junction (3J) devices. Recent activities have focused on tests of prototype IMM devices permanently mounted to a conductive substrate material. These devices exhibit no electrical or mechanical degradation after 750 thermal cycles of between -40°C and +110°C. Large area 3J-IMM devices of 1 cm² active area have been operated outdoors under concentrated sunlight. Stable performance of IMM devices operated at a geometric concentration ratio of 1090x have exhibited higher power output than conventional reference devices. At photocurrent densities of over 12 A/cm², V_oc improvement of greater than 400 mV has been obtained compared to conventional 3J devices. A model has been constructed to predict performance of IMM devices operated over a range of cell temperature and spectral input that is expected for outdoor systems. This work is applicable to the design and optimization of 4J-and 5J-IMM device architectures. Results of this analysis reveal a greater spectral sensitivity of such designs, underscoring the importance of subcell bandgap selection in maximizing performance over likely operating conditions.