AlGaAs quantum-well solar cell junctions on beryllium telluride grown on silicon

A bandgap combination of 1.7 eV and 1.1 eV offers the highest theoretical efficiency for a series-connected tandem-junction solar cell. The monolithic structure of aluminum gallium arsenide grown on silicon is a natural implementation, but has long-standing crystal-quality challenges such as lattice mismatch and island growth of AlGaAs. We address the growth issues by use of an interlayer of BeTe on silicon. AlGaAs grown on BeTe has a strong tendency for island formation, which is suppressed by low-temperature growth initiation. A barrier for electrical transport at the p-BeTe/p-AlGaAs interface is also reduced by low-temperature growth, and BeTe anneal under arsenic rather than tellurium flux. Al$_{0.15}$Ga$_{0.85}$As-GaAs multiple quantum-well p-i-n junction structures were grown on both Si/BeTe and GaAs substrates for electrical characterization. In preliminary results, the short-circuit photocurrent J$_{SC}$ and open-circuit voltage V$_{OC}$ is lower in the Si/BeTe based junction than the GaAs based junction, with about twice the fractional reduction of V$_{OC}$ than of J$_{SC}$. A graded-bandgap emitter structure with different n+GaAs top contact layer thicknesses exhibited J$_{SC}$ reduction less than 15{\%}.