Multi-junction solar cells use multiple solar cells each stacked on top of one another. Each of these solar cells in the stack are tuned to a specific wavelength of light to maximize the conversion efficiency of incoming light into electricity. The conversion of a wider solar spectrum into electricity is key to gaining higher solar cell efficiencies. Conventional single layer silicon solar cells will never be as efficient as a multi-junction semiconductor. Currently, most multi-junction concentrator solar cells consist of 3 cells but scientists are currently working on 4 and 5 junction solar cells to reach even higher efficiencies. It is forecasted that we will be able to use 50% efficient multi-junction solar cells in CHP systems by 2020!