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We report thin-film InAs/GaAs quantum dot (QD) solar cells with $n-i-p{+}$deep junction structure and planar back reflector fabricated by epitaxiallift-off (ELO) of full 3-inch wafers. External quantum efficiency measurementsdemonstrate twofold enhancement of the QD photocurrent in the ELO QD cellcompared to the wafer-based QD cell. In the GaAs wavelength range, the ELO QDcell perfectly preserves the current collection efficiency of the baselinesingle-junction ELO cell. We demonstrate by full-wave optical simulations thatintegrating a micro-patterned diffraction grating in the ELO cell rearsideprovides more than tenfold enhancement of the near-infrared light harvesting byQDs. Experimental results are thoroughly discussed with the help ofphysics-based simulations to single out the impact of QD dynamics and defectson the cell photovoltaic behavior. It is demonstrated that non radiativerecombination in the QD stack is the bottleneck for the open circuit voltage($V_{oc}$) of the reported devices. More important, our theoreticalcalculations demonstrate that the $V_{oc}$ offest of 0.3 V from the QD groundstate identified by \emph{Tanabe et al., 2012}, from a collection ofexperimental data of high quality III-V QD solar cells is a reliable - albeitconservative - metric to gauge the attainable $V_{oc}$ and to quantify thescope for improvement by reducing non radiative recombination. Provided thatmaterial quality issues are solved, we demonstrate - by transport and rigorouselectromagnetic simulations - that light-trapping enhanced thin-film cells withtwenty InAs/GaAs QD layers reach efficiency higher than 28\% underunconcentrated light, ambient temperature. If photon recycling can be fullyexploited, 30\% efficiency is deemed to be feasible.

Light-trapping enhanced thin-film III-V quantum dot solar cells fabricated by epitaxial lift-off