  Photon impingement is capable of liberating electrons in semiconductors. When
the electron transport is primarily governed by temperature gradients, high
irreversibilities will result, thus lowering converters' efficiencies. A
fundamental study in the absence of photovoltaics\cite{1} has achieved the
reduction of these irreversibilities by considering entropy changes due to
electron flows. Here we present an unreported mechanism that integrates
photovoltaic conversion and electron tunneling. Photon-excited electrons that
occupy energy levels beyond windowed limits are first imprisoned inside the
cathode, then given opportunities to rapidly re-thermalize, and eventually
allowed to enter the tunnel. Energies wasted by both the irreversibility and
the recombination are minimized with respect to the transmission energy and the
transmission window that characterize the tunnel. Upon application of this
mechanism to high-concentration solar cells, the proposed hybrid model
outperforms others. It further provides a guide for elevating efficiencies in
future photon-to-electron converters typified by third-generation photovoltaic
systems.
