Viabilidade e desenvolvimento de células solares de banda intermediária baseadas em GaAs com pontos quânticos de submonocamada de InAs/GaAs

Abstract

Our objective is to assess the viability of GaAs-based intermediate band solar cells grown by molecular beam epitaxy using InAs/GaAs submonolayer quantum dots to create intermediate bands. This work is divided into two parts. In the first one, given that epitaxial III-V solar cells are a scarce technology in Brazil and that the photovoltaics research program is new in our research group, we developed conventional GaAs solar cells. We combined theoretical analyses using SCAPS and OpenFilters software with experimental analyses using current-voltage method, spectrophotometry (reflectance) and spectroscopic ellipsometry. This allowed us to produce a device with 17.2 % efficiency operating under 298 K and AM1.5G illumination. We discussed in detail the design of a TiO 2 /Al 2 O 3 antireflective coating that led our best device to achieve a short-circuit current density of 23,6 mA/cm 2 . In the second part, we discussed the workings of quantum dot solar cells. We observed that InAs/GaAs submonolayer quantum dots led to more efficient solar cells relative to those having Stranski-Krastanov quantum dots, as the latter cause a much higher open-circuit voltage degradation than the former. However, the submonolayer quantum dots did not lead to devices more efficient than conventional ones. Through Schrödinger-Poisson calculations in the effective-mass approximation with NextNano software, we concluded that this shortfall is probably due to the submonolayer quantum dots not being able to confine electrons in three dimensions as required by the intermediate band solar cell model. The calculations indicate that the nanostructures need to be larger and have a higher In concentration to allow such electron confinement. Lastly, using atom probe tomography, we identified growth conditions that lead to a higher In concentration in submonolayer quantum dots relative to the ones we applied to our devices. Such growth conditions might be explored in the future to achieve true intermediate band solar cells.