On the effects of impurities and defects on GaAs-based photovoltaics*

In the field of photovoltaics, GaAs is one of the most studied materials not only for its convenient properties—namely, high absorption coefficient and high mobility—but also because it is a useful framework for the development of new technologies. A notorious example is the InAs/GaAs Stranski-Krastanov-quantum-dot solar cell (SKQD-SC), an instance of intermediate band solar cells. Despite numerous efforts, highly efficient InAs/GaAs SKQD-SCs have not yet been demonstrated due to the difficulties involved in achieving the Fermi-level splitting between the quantum dot levels and the conduction and valence bands, which results in substantial (> 15%) open-circuit voltage degradation (V_oc). The absence of Fermi-level splitting is partially explained by the presence of recombination centers localized at the interface between SKQDs and their surrounding matrix. In this presentation, we will show that quantum dots grown by the submonolayer (SML) technique lead to a much smaller V_oc degradation (< 5%) when compared to SKQDs as a consequence of the lower density of defects resulting from the planar growth technique. The quality and growth parameters of the SMLQDs will be discussed by means of photoluminescence and cross-sectional scanning tunneling microscopy results.