The efficiency of a solar cell can be increased by stacking mul tiple solar cells with arange of bandgap energies, resulting in a multijunction solar cell with a maximum the oreticalefficiency limit of 86.8%. III-V compound semiconductors are good candidatesfor fabricating such multijunction solar cells for two reasons: they can be grown withexcellent ma te rial quality; and their bandgaps span a wide spectral range, mostly withdirect bandgaps, implying a high absorption coefficient. These factors are the reason forthe success of this technology, which has achieved 39% efficiency, the highest solar - to -electric conversion efficiency of any photovoltaic device to date. This ar ticle explores thema te rials science of today's high - efficiency multijunction cells and describes challengesassociated with new ma te rials developments and how they may lead to next - generation,multijunction solar cell concepts.