  Harnessing low energy photons is of paramount importance for multi-junction
high efficiency solar cells as well as for thermo-photovoltaic applications.
However, semiconductor absorbers with bandgap lower than 0.8 eV have been
limited to III-V (InGaAs) or IV (Ge) semiconductors that are characterized by
high manufacturing costs and complicated lattice matching requirements in their
growth and integration with the higher bandgap cells. Here, we have developed
solution processed low bandgap photovoltaic devices based on PbS colloidal
quantum dots (CQDs) with a bandgap of 0.7 eV suited for both
thermo-photovoltaic as well as low energy solar photon harvesting. By matching
the spectral response of those cells to that of the infrared solar spectrum, we
report a record high short circuit current (JSC) of 37 mA/cm2 under full solar
spectrum and 5.5 mA/cm2 when placed at the back of a silicon wafer resulting in
power conversion efficiencies (PCE) of 6.4 % and 0.7 % respectively. Moreover,
the device reached an above bandgap PCE of ~6 % as a thermo-photovoltaic cell
recorded under a 1000 {\deg}C blackbody radiator.
