Self-organized InGaAs/GaAs quantum dot arrays for use in high-efficiency intermediate-band solar cells

We have investigated the material properties of multi-layer stacked InGaAs/GaAs quantum dots (QDs) grown on GaAs (3 1 1) B substrates. Symmetrical lens-shaped QDs were observed along [0 1 -1], while their shape was asymmetric along the [-2 3 3] azimuth surrounded by two different dominant facets. Further, QDs were vertically aligned in the [3 1 1] direction when viewed along [0 1 -1], while the alignment was inclined with respect to the growth direction when viewed along [-2 3 3]. The inclination angle of vertical alignment QDs became monotonically smaller from 22 degrees to 2 degrees with decreasing spacer layer thickness from 40 to 20 nm. Time-resolved photoluminescence measurements showed that multi-stacked QDs with thinner spacer layers resulted in increased PL decay times. We believe that an electronically coupled QD state or an intermediate band was formed, if the spacer layer thickness was reduced below 20 nm in this material system. For an InGaAs/GaAs QD solar cell grown on a GaAs (3 1 1) B substrate, the external quantum efficiency showed a clear increase in the longer wavelength range due to an additive contribution from the QD layers. Furthermore, photocurrent production due to two-step absorption of sub-bandgap photons, which is a key element that is necessary to be demonstrated for an increase in the efficiency of a single-junction solar cell beyond the Shockley-Queisser, was observed at room temperature under one sun condition. This photocurrent production increased under a forward-bias regime as the QDs were partially filled with carriers under the bias.