  The intermediate band solar cell (IBSC) and quantum ratchet solar cell (QRSC)
have the potential to surpass the efficiency of standard single-junction solar
cells by allowing sub-gap photon absorption through states deep inside the band
gap. High efficiency IBSC and QRSC devices have not yet been achieved, however,
since introducing mid-gap states also increases recombination, which can harm
the device. We consider the electronically coupled upconverter (ECUC) solar
cell and show that it can achieve the same efficiencies as the QRSC. Although
they are equivalent in the detailed balance limit, the ECUC is less sensitive
to nonradiative processes, which makes it a more practical implementation for
IB devices. We perform a case study of crystalline-silicon based ECUC cells,
focusing on hydrogenated amorphous silicon as the upconverter material and
highlighting potential dopants for the ECUC. These results illustrate a new
path for the development of IB-based devices.
