  It is currently possible to fabricate crystalline silicon solar cells with
the absorber thickness ranging from a few hundreds of micrometers (conventional
wafer-based cells) to devices as thin as $1\,\mu\mathrm{m}$. In this work, we
use a model single-junction solar cell to calculate the limits of energy
conversion efficiency and estimate the optimal absorber thickness. The limiting
efficiency for cells in the thickness range between 40 and $500\,\mu\mathrm{m}$
is very similar and close to 29%. In this regard, we argue that decreasing the
thickness below around $40\,\mu\mathrm{m}$ is counter-productive, as it
significantly reduces the maximum achievable efficiency, even when optimal
light trapping is implemented. We analyse the roles of incomplete light
trapping and extrinsic (bulk and surface) recombination mechanisms. For a
reasonably high material quality, consistent with present-day fabrication
techniques, the optimal thickness is always higher than a few tens of
micrometers. We identify incomplete light trapping and parasitic losses as a
major roadblock in improving the efficiency upon the current record of 25.6%
for silicon solar cells. Finally, considering the main parameters that impact
solar cell performance, we quantify the constraints and requirements for
achieving a specified energy conversion efficiency, which is important for a
proper design strategy of high efficiency silicon solar cells.
