Multi-junction solar cells are solar cells with more than one p–n junction. Each of these junctions is made of different semiconductor materials. Conventional solar PV cells comprise just one layer of semi-conductor cells, each with one p-n junction.
Yes, multi-junction cells are more efficient that conventional solar cells.
It is worthwhile to note that while single junction solar cells have a theoretical maximum efficiency of about 35%, multi-junction solar cells have a theoretical maximum of over 80%.
Just like many emerging solar PV cell technologies, what is limiting widespread adoption of this technology is cost. Multi-junction cells are expensive, mainly due to their high manufacturing costs.
Many companies have come up with interesting multi-junction cell products. One such company is Insolight which seems to have developed multi-junction cells having efficiencies of 42%.
In another effort, a team of researchers from MIT and Masdar Institute of Science and Technology claim to have found a way to fabricate multi-junction cells called ‘step cell’ due to the stepwise fashion in which the two layers are arranged. They have estimated practical efficiencies of 35%.
In a world that is looking beyond silicon modules, solutions based on multi-junction cells are likely to be increasingly adopted when they get cost competitive with conventional panels.