High Efficiency Multijunction Tandem Solar Cells with Embedded Short-Period Superlattices

Abstract

We propose a 1cm2 tandem solar cell with different lattice-matched materials based on a 2eV/1.42eV/0.66eV energy gap sequence. The top unit is a p-n-n AlAs/GaAs cell, connected in series with a bottom cell which is a bulk GaAs/Ge p-n-n cell; a narrow GaAs/Ge superlattice region embedded in the middle region. Transition of carriers between the two units is possible via a tunnel junction connecting the two units. More specifically, the upper cell is a 20 μm bulk p-n-n cell tuned to the visible range of the solar spectrum, producing short circuit currents near 30mA/cm2, and open-circuit voltage (OC) of 1.04V; the bottom cell is an 80 μm bulk p-n-n GaAs/Ge with an embedded GaAs/Ge superlattice tuned at 1eV. The bottom cell produces short circuit current density at18.5 mA/cm2 in the bulk; however a 20-period GaAs/Ge embedded short superlattice provides an additional 10 mA/cm2 thermionic current density, so that total bottom current reach 28.5mA/cm2, in close matching (5%) with the top currents, and an OC voltage of 0.968V. The tandem cell’s basic parameters are (a) average fill factor of (FF) 85% (b) short circuit current 28.5 mA/cm2 and (c) OC voltage 2.008V (due to the series connection); for 100mW/cm2 standard solar radiation, collection efficiency of such a device is depicted in excess of 47% under one sun. Such small area cells are useful for CPV for their minimized size and material requirements.