We report a preliminary study on the influence of indium doping on ultra-thin film silicon solar cells. The design of the cell reported here is such that it should elucidate the impact of the indium dopant, which is concentrated in the thin film. Indium, a deep level in silicon (0.157 eV above the valence band), acts as a p-type dopant and a sensitizer. Absorption through sub-bandgap transitions is expected based on the previously reported Impurity PhotoVoltaic (IPV) Effect . It is proposed that the implementation of a novel vertical PN junction configuration together with the IPV effect enhances the efficiency of ultra-thin solar cells. The most efficient cell fabricated to date, in our research group, has a conversion efficiency of 4.3 % (active silicon thickness of 2.5 μm), a short-circuit current density of 14.9 mA/cm2 and an open-circuit voltage of 0.51 V under 1 sun illumination. The cell has not been optimized with any type of light trapping technique and 11.24 % of the cell surface is covered by the metal contacts. Numerical simulation indicates that for the geometry used, the maximum efficiency that may be expected is 9.8 % (compared to the 4.3 % measured).