Fabrication of two-terminal metal-interconnected multijunction III–V solar cells

A novel approach has been developed to enable the creation of a fully lattice-matched two-terminal four-junction III-V solar cell is which an upper 1.85 eV GaInP/ 1.42 eV GaAs two-junction (2J) tandem cell is bonded to a lower Eg3 eV GaInAsP/ 0.74 GaInAs eV 2J tandem cell. In this configuration, the upper tandem is grown inverted and lattice-matched to a GaAs substrate, and the lower tandem is grown upright and lattice-matched to an InP substrate. Prove of concept devices have been fabricated using Au-Au bonding with either SiO₂ or GaInP₂ as a filler material. The bonding process is discussed in this paper as well as the result of an inverted GaAs cell bonded on a conducting GaAs wafer. The most complex device fabricated to date is a GaInP/GaAs 2J tandem cell bonded to a GaInAs cell using a GaInP₂ optical coupling layer, with a post-bonding V_oc of 2.7 eV.