  Maximum efficiency of ideal single-junction photovoltaic (PV) cells is
limited to 33% (for one sun illumination) by intrinsic losses such as band edge
thermalization, radiative recombination, and inability to absorb below-bandgap
photons. This intrinsic thermodynamic limit, named after Shockley and Queisser
(S-Q), can be exceeded by utilizing low-energy photons either via their
electronic up-conversion or via thermophotovoltaic (TPV) conversion process.
However, electronic up-conversion systems have extremely low efficiencies, and
practical temperature considerations limit the operation of TPV converters to
the narrow-gap PV cells. Here we develop a conceptual design of a hybrid TPV
platform, which exploits thermal up-conversion of low-energy photons and is
compatible with conventional silicon PV cells by using spectral and directional
selectivity of the up-converter. The hybrid platform offers
sunlight-to-electricity conversion efficiency exceeding that imposed by the S-Q
limit on the corresponding PV cells across a broad range of bandgap energies,
under low optical concentration (1-300 suns), operating temperatures in the
range 900-1700K, and in simple flat panel designs. We demonstrate maximum
conversion efficiency of 73% under illumination by non-concentrated sunlight. A
detailed analysis of non-ideal hybrid platforms that allows for up to 15% of
absorption/re-emission losses yields limiting efficiency value of 45% for Si PV
cells.
