Progress in adapting inverted metamorphic solar cell technology for terrestrial concentrator applications

Lattice-matched (LM) InGaP/InGaAs/Ge triple-junction (3J) devices are rapidly approaching their practical efficiency limits. Fortunately, there are alternative device structures under investigation in which much higher practical efficiency is predicted. Inverted metamorphic multijunction (IMM) devices hold promise for an additional 4 to 5% conversion efficiency to be achievable over that of LM-3J devices, and a significant effort is presently underway to bring this technology to manufacturing for CPV applications. Because in the IMM configuration the diffused Ge junction (bottom cell) is replaced with a higher bandgap epitaxial In0.3G0.7As junction, improved current matching, lower reverse saturation current, and higher open-circuit voltage can be obtained. High-intensity pulsed solar simulator measurements (HIPSS) measurements of large area (1 cm²) IMM devices under high concentration demonstrate a Voc increase to over 3.5 V from 3.2 V obtained from their LM counterparts. In addition, the tunnel diodes used for low-resistivity, ohmic interconnection between subcells appear to be suitable for device operation at 1200 suns. Recent performance results for IMM concentrator solar cells are described, as well as well as discussion on what milestones remain to be achieved in order for this technology to demonstrate both high efficiency and manufacturability.