  We report thin-film InAs/GaAs quantum dot (QD) solar cells with $n-i-p{+}$
deep junction structure and planar back reflector fabricated by epitaxial
lift-off (ELO) of full 3-inch wafers. External quantum efficiency measurements
demonstrate twofold enhancement of the QD photocurrent in the ELO QD cell
compared to the wafer-based QD cell. In the GaAs wavelength range, the ELO QD
cell perfectly preserves the current collection efficiency of the baseline
single-junction ELO cell. We demonstrate by full-wave optical simulations that
integrating a micro-patterned diffraction grating in the ELO cell rearside
provides more than tenfold enhancement of the near-infrared light harvesting by
QDs. Experimental results are thoroughly discussed with the help of
physics-based simulations to single out the impact of QD dynamics and defects
on the cell photovoltaic behavior. It is demonstrated that non radiative
recombination in the QD stack is the bottleneck for the open circuit voltage
($V_{oc}$) of the reported devices. More important, our theoretical
calculations demonstrate that the $V_{oc}$ offest of 0.3 V from the QD ground
state identified by \emph{Tanabe et al., 2012}, from a collection of
experimental data of high quality III-V QD solar cells is a reliable - albeit
conservative - metric to gauge the attainable $V_{oc}$ and to quantify the
scope for improvement by reducing non radiative recombination. Provided that
material quality issues are solved, we demonstrate - by transport and rigorous
electromagnetic simulations - that light-trapping enhanced thin-film cells with
twenty InAs/GaAs QD layers reach efficiency higher than 28\% under
unconcentrated light, ambient temperature. If photon recycling can be fully
exploited, 30\% efficiency is deemed to be feasible.
