These multi-junction voltaic cells are still far away from being introduced on rooftops, because of their high cost. Conventional photo-voltaic cells have a single layer of semiconductor tuned to a particular wavelength, leading to a narrower bandwidth. On the other hand, multi-junction photo-voltaic cells have various layers of semiconductors, each tuned a particular wavelength of light which leads to its higher efficiency and cost.
The key to the improved efficiency is a new method of bonding semiconductor layers. According to Frank Dimroth, Project Leader at Fraunhofer, “Besides improved materials and optimization of the structure, a new procedure called wafer bonding plays a central role. With this technology, we are able to connect two semiconductor crystals, which otherwise cannot be grown on top of each other with high crystal quality. In this way we can produce the optimal semiconductor combination to create the highest efficiency solar cells.”These cells also have one more drawback other than cost, that these cells require direct sunlight that means quiet sunny location and a good tracking system. These cells cannot generate significant electricity in indirect sunlight. But still there is a hope for solution of our energy problems.
