Recent efforts to include a quantum dot array within the intrinsic region of a pin GaAs solar cell have focused on minimising the open circuit voltage (Voc) loss relative a control device without quantum dots . Strategies include the addition of strain balancing (e.g. GaP, GaAsP)  or high energy (e.g. AlGaAs) barrier layers . In this work we demonstrate a significant improvement of up to 260 mV in Voc by controlling only the size of the quantum dots at the nanometre scale using precise molecular beam epitaxial wafer growth. High resolution scanning transmission electron microscope (STEM) imaging is used to determine the dimensions of individual quantum dots, providing valuable input to a theoretical model. The modelling suggests that the performance improvement is a direct consequence of opening a clear energy gap between the conduction band and the quantum dot ensemble ground state energy e0. With appropriate quantum mechanical design this energy gap can be up to ~90 meV, giving rise to intermediate band behaviour rather than quantum dot solar cell behaviour at room temperature. Current-voltage measurements under air mass 1.5 conditions indicate an efficiency (active area) of 18.4% (19.7%) at 5-suns concentrations. Higher concentration measurements confirm the quality of the material with diode ideality factors as low as 1.16 and Voc ≈ 1 V at 500 suns.
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