Gallium arsenide phosphide nitride shows promise for developing high-efficiency tandem solar cells on low-cost silicon substrates. To date, the highest efficiency conversions have been reached by using III–V (that is, compounds of elements from groups III and V of the periodic table) monocrystalline multijunction solar cells (MJSCs) under concentrated sunlight. Soitec and the Fraunhofer Institute have pushed the solar cell record to 44.7% for terrestrial applications. 1 They achieved this record with a wafer-bonded four-junction gallium indium phosphide/gallium arsenide//gallium indium arsenide phosphide/gallium indium arsenide (GaInP/GaAs// GaInAsP/GaInAs) solar cell under concentration of 297 suns (that is, light 297 times as bright as sunlight). Soitec and Fraunhofer also announced on the Soitec website very recently the achievement of 46% efficiency under concentration of 508 suns. Moreover, Solar Junction has demonstrated a III–V triple junction coherently grown (lattice-matched) onto a gallium arsenide (GaAs) substrate with 44% efficiency under 942 suns (AM1.5D spectra, representing the direct-radiation—that is, without the diffuse radiation and albedo—annually averaged solar spectrum at ground level at mid-latitudes). 2 It contains a highly sought-after 1eV gallium indium arsenide nitride an-timonide (GaInAsNSb) diluted-nitride compound. However, maintaining the GaAs or germanium substrates to build these high-efficiency III–V solar cells is costly. With the strategic challenge cost of e0.25–0.5 per watt of peak power in mind, we investigated using silicon, which is the most abundant element on earth and inexpensive. Indeed, true monolithic integration of III–V compound semiconductor heterostructures with silicon would enable both highly efficient Figure 1. Internal quantum efficiency (IQE) of a PIN junction made up of a p-doped gallium phosphide (GaP) on top of a 1 layer of an intrinsic gallium arsenide phosphide nitride (GaAsPN) absorber on top of an n-doped GaP layer, grown on a (001)-oriented GaP substrate, where (001) represents a particular crystal plane. and low-cost photovoltaic (PV) production and is the subject of great research interest. A tandem solar cell, made of a 1.7eV III–V top and a 1.1eV crystalline silicon (c-Si) bottom cell, would theoretically reach an efficiency of 37%, under AM1.5G illumination (that is, the global incident solar radiation, including the direct, diffuse, and albedo radiation). 3 However, MJSC efficiency is very sensitive to structural defects, such as misfit dislocations, which appear during metamorphic growth. They dramatically reduce the carrier lifetime and, thus, current extraction and solar cell performance. A perfect lattice-matched epitaxial PV structure of III–V and silicon technologies on silicon substrate would significantly increase efficiency, as well as reducing the overall cost of mul-tijunction PV cells.