Silicon bottom subcell optimization for wafer-bonded III-V on Si multijunction solar cells

Combining III-V and Si subcells by wafer bonding is an interesting approach for III-V on Si multijunction solar cells since it allows direct connection through a permanent, electrically conductive, transparent interface. However, it is not possible to use conventional Si homo-junctions solar cells, which usually are formed by diffusion processes on square or pseudo-square Si substrates with textured front surface. Some of the requirements on the bottom Si subcell for high efficient III-V on Si photovoltaics by wafer bonding are: (i) high bulk minority carrier lifetime, (ii) low recombination at III-V/Si interface thanks to adequate emitter doping and/or III-V/Si interface passivation, (iii) low defective and smooth front Si surface, which often requires the use of chemo-mechanical polishing (CMP) and hence also impacts the emitter doping. In this paper, we compare diffusion, beamline ion implantation and plasma immersion ion implantation doping processes with different thermal activation treatments for the optimization of the Si bottom subcell. Resulting one-sun power conversion efficiency of III-V on Si two-terminal triple-junction (3J) devices increased from 25.2% for non-current matched GaInP/AlGaAs/Si reported in our previous work to >26 % for GaInP/GaAs/Si (28.6% under 30 suns), highlighting that the design of bottom Si subcell is extremely important for current matching in two-terminal tandem cell design.