Viability of Intermediate Band Solar Cells Based on InAs/GaAs Submonolayer Quantum Dots and the Role of Surface Reconstruction

The effects of growth conditions on InAs/GaAs submonolayer-quantum-dot solar cells are still a little explored topic, and the literature shows contradictory results regarding the efficiency of these devices. Through electrical and optical characterizations (photoluminescence, current-voltage curves, and external quantum efficiency) and self-consistent Schro center dot dinger-Poisson simulations in the effective-mass approximation, we investigate how the reconstruction of the GaAs(001) surface prior to the deposition of InAs/GaAs submonolayer quantum dots in-fluences the properties of these nanostructures and the performance of solar cells. Current-voltage characteristics and external quantum efficiency curves show that the use of the (2 x 4) surface reconstruction-instead of the commonly used c(4 x 4) surface reconstruction-leads to higher short-circuit current density and improved performance at room temperature. The (2 x 4) surface reconstruction also leads to enhanced photoluminescence intensity at low temperatures compared to the c(4 x 4) surface reconstruction. The simulations-which are based on previous cross-sectional scanning tunneling microscopy data of InAs/GaAs submonolayer quantum dots-indicate that neither type of submonolayer quantum dot can confine electrons, as they are too small and their In content is too low. However, the electron ground state is closer to being confined in the SMLQDs grown with the (2 x 4) surface reconstruction, as such nanostructures are surrounded by a thick InGaAs layer having a lower In content than for the other surface reconstruction. The discussion presented herein elucidates a contradiction between different reports found in the literature regarding the conversion efficiency of InAs/GaAs submonolayer-quantum-dot solar cells and indicates possible ways forward for achieving 3D electron confine-ment in these devices.