Wafer bonding and layer transfer processes for 4-junction high efficiency solar cells

Abstract

A four-junction cell design consisting of InGaAs, InGeAsP, GaAs, and GaoslnosP subcells could reach 1xAMO efficiencies of 35.4%. but relies on the integration of nonlattice-matched materials. Wafer bonding and layer transfer processes show promise in the fabrication of InP/Si epitaxial templates for growth of the bottom InGaAs and InGaAsP subcells on a Si support substrate. Subsequent wafer bonding and layer transfer of a thin Ge layer onto the lower subcell stack can serve as an epitaxial template for GaAs and GaoslnosP subcelis. Present results indicate that optically active iliN compound semiconductors can be grown on both Ge/Si and InP/Si heterostructures. Currenk voltage electrical characterization of the interfaces of these structures indicates that both lnP/Si and Ge/Si interfaces have specific resistances lower than O.lncmz for heavily doped wafer bonded interfaces, enabling back surface power extraction from the finished cell structure.

Improved electrical properties of wafer-bonded p-GaAs/n-InP interfaces with sulfide passivation

“…We have focused on the fabrication of a multijunction solar cell wherein high band gap GaAs lattice-matched subcells ͑InGaP, GaAs͒ are combined to lower band gap InP latticematched subcells ͑InGaAsP, InGaAs͒ via direct wafer bonding. 8 In order to monolithically interconnect between the top and bottom subcells, the bonded InP/GaAs heterojunction must be a highly conductive Ohmic junction. The electrical properties of both n-GaAs/ n-InP and p-GaAs/ p-InP bonded heterojunctions have been investigated and analyzed based on thermionic emission models and tunnel effect across the energy barriers.…”

High efficiency InGaAs solar cells on Si by InP layer transfer

“…2 Additionally, a proposed four-junction solar cell, fabricated by joining subcells of InGaAs and InGaAsP grown on InP with subcells of GaAs and AlInGaP grown on GaAs through a wafer-bonded interconnect, would enable the independent selection of the subcell band gaps for well developed materials grown on lattice matched substrates. 3,4 Substitution of InP / Si substrates for bulk InP in the fabrication of such a four-junction solar cell could significantly reduce the substrate cost, as described below.…”

Direct-bonded GaAs∕InGaAs tandem solar cell

“…2,3 An alternative approach is to employ direct-bonded interconnects between subcells of a multijunction cell, which enables dislocationfree active regions by confining the defect network needed for lattice mismatch accommodation to tunnel junction interfaces. [4][5][6][7][8] We report here a direct-bond interconnected multijunction solar cell, a two-terminal monolithic GaAs/ InGaAs two-junction cell, to demonstrate a proof of principle for the viability of direct wafer bonding for solar cell applications.…”