The efficiency of a solar cell can be increased by stacking mul tiple solar cells with a
range of bandgap energies, resulting in a multijunction solar cell with a maximum the oretical
efficiency limit of 86.8%. III–V compound semiconductors are good candidates
for fabricating such multijunction solar cells for two reasons: they can be grown with
excellent ma te rial quality; and their bandgaps span a wide spectral range, mostly with
direct bandgaps, implying a high absorption coefficient. These factors are the reason for
the 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 the
ma te rials science of today’s high - efficiency multijunction cells and describes challenges
associated with new ma te rials developments and how they may lead to next - generation,
multijunction solar cell concepts.