Multi-Junction Solar Cell

A multi-junction solar cell is a photovoltaic device built with multiple p-n junctions made from different semiconductor materials, with each junction tuned to respond to a different band of light wavelengths so that the cell together captures a broader span of the solar spectrum than a single junction cell can. The materials are layered so that the layer with the largest bandgap, absorbing the shortest wavelengths, sits on top, letting longer wavelengths pass through to be absorbed by the layers beneath, which recovers energy that a single junction cell would otherwise lose. Laboratory examples have reached efficiencies above 46 percent under concentrated sunlight, well above the roughly 33 percent theoretical limit for a single junction cell, while commercial tandem cells reach about 30 percent efficiency under ordinary sunlight and about 40 percent under concentration. Because of their cost and complexity, multi-junction cells have mainly been used in aerospace, where their high power to weight ratio matters, including on Mars rovers, and they are increasingly used in terrestrial concentrator photovoltaic systems, with thin film tandem versions also appearing in niche roles such as roofing materials.