Ultra-high stacks of InGaAs quantum dots for high efficiency solar cells

We report ultra-high stacks of quantum dots (QDs) for high efficiency solar cells fabricated by the intermittent deposition of InGaAs using molecular beam epitaxy. We obtained a 400-stack InGaAs/GaAs QD structure without using a strain balancing technique, in which the total number of QDs reaches 2 × 10¹³ cm^-2. Photoluminescence and cross-sectional scanning transmission electron microscope measurements indicate that the In_0.4Ga_0.6As QD structure exhibits no degradation in crystal quality, no dislocations and no crystal defects even after the stacking of 400 QD layers. The external quantum efficiency and the short-circuit current density of multistacked In_0.4Ga_0.6As QD solar cells increase as the number of stacked layers is increased to 150. Such ultra-high stacks and good cell performance have not been reported for QD solar cells using other material systems. The performance of the ultra-high stacked QD solar cells indicates that InGaAs QDs are suitable for use in high efficiency solar cells requiring thick QD layers for sufficient light absorption.