Improving sub-bandgap carrier collection in GaAs solar cells by optimizing InGaAs quantum wells using strain-balancing and distributed Bragg reflectors

Bulk carrier collection in GaAs solar cells is limited by the GaAs bandgap of 1.42eV, leaving low-energy photons below 1.42eV uncollected. The addition of low bandgap materials allows for the collection of these ‘unreachable’ photons in a GaAs solar cell.In_(x)Ga_(1-x)As can have bandgaps between 0.354-1.42eV based on indium composition, allowing for tunable collection at longer wavelengths. Including thin 9.2nm In_(x)Ga_(1-x)As quantum wells in the i-region of a GaAs n-i-p structure enhances photon collection past the 870nm GaAs band edge, increasing current production and potentially leading to higher efficiencies. Increased current production in GaAs solar cells is especially valuable since GaAs is used as a mid-junction cell in triple junction InGaP/GaAs/Ge structures, where current-matching between the three structures is vital for reaching efficiencies above 40\% under solar concentration. To prevent accumulation of strain due to the differences in the GaAs and InGaAs lattice constants, tensile GaAs_(1-y)P_(y) barriers are added on either side of the InGaAs wells. The effects of utilizing InGaAs-GaAsP superlattices in the i-region on device performance and material quality are investigated in this work. It was found that strain-balancing quickly became necessary in devices with as few as three wells. Distributed Bragg reflectors were incorporated in the structural design to allow a second pass of photons through the wells and are shown to double the well current production without adversely affecting Voc. Decreasing the local strain between the InGaAs wells and GaAsP barriers by dropping the phosphorus composition from 32% to 10%, along with optimizing the growth conditions of the superlattice, dramatically improved the interface quality of devices grown on 2 deg. and 6 deg. offcut substrates in addition to recovering Voc above 1.00V with a 12xSBQW superlattice. Current production of the superlattice was also shown to increase from 13 microamps/well using 32%P barriers to over 30 microamps/well using 10%P barriers