Invited: Advances in III-V/Active-Silicon Multijunction Photovoltaics: Progress Toward a Si-Plus Architecture

Multijunction solar cells consisting of III-V sub-cells monolithically integrated with Si growth substrates, which also serve as active sub-cells, hold the potential for the achievement of high conversion efficiencies that are on par with pure III-V multijunction structures, but at substantially lower costs. In addition to high-efficiency photovoltaics, the III-V/Si materials system is of interest to a range of other applications, including Si-based light emitters and detectors and III-V enhanced microelectronics. Although a “ Si-plus ” architecture such as this has been a goal within the photovoltaics research community for decades, progress toward this end has been effectively hampered by issues related to heterovalent (polar/nonpolar) epitaxy at the GaP/Si interface. However, work in recent years on the epitaxy of high-quality GaP on Si(100) substrates with substantial control and suppression of nucleation-related defects, including antiphase domains, stacking faults, and microtwins, via both molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD), has provided realistic pathways for the development of these structures. Combined with an active GaP/Si 1.12 eV bottom junction, near-ideal 2- and 3-junction monolithic structures are possible via integration with metamorphic GaAsP top-cell and GaInP/GaAsP top-/middle-cell stacks using semi-transparent compositionally-graded GaAs y P 1-y buffers to bridge the lattice mismatch. For the Si-plus architecture to succeed, every aspect of the multijunction structure must be fully optimized, as any weak point will reduce the overall performance accordingly. This includes not only the metamorphic III-V sub-cells and associated tunnel junctions, but also the Si sub-cell. Achievement of high performance from the metamorphic III-V sub-cells requires minimization of detrimental defects resulting from the lattice-mismatched heteroepitaxy (i.e. threading dislocations), as well as optimization of device structures and growth conditions at these relatively unexplored compositions. Sub-cell interconnection requires high-performance tunnel diodes (optically transparent, low electrical resistance), made additionally difficult by the wider bandgaps and metamorphic nature of the III-V system of interest. And finally, the Si sub-cell must be re-designed and optimized for multijunction application and integration with III-V materials, which, despite Si PV’s general high level of maturity, is a non-tr