The effect of using indium, a deep-level acceptor in silicon, as a dopant for an ultra-thin film ( 2.5 mu m thick) single-crystalline silicon solar cell has been investigated. Indium acts as a p-type dopant in silicon with an energy level 0.157 eV above the valence band and the use of this deep-level acceptor has been proposed as a method to enable sub-bandgap transitions via the impurity photovoltaic ( IPV) effect. In the current work, a maximum conversion efficiency under 1 sun illumination, air mass 1.5, of 4.74 +/- 0.2% is measured for a solar cell using indium as a p-type dopant. An equivalently processed cell using boron in place of indium has a maximum efficiency of 4.16 +/- 0.2%. Similar relative increases in efficiency have been observed for nine individual devices. The area of the cells is 0.5 x 0.5 cm(2) with 7% covered by the metal contacts. A dual-junction analytical model is used to show that for all indium-doped cells in this study there exists an enhancement in the generation of charge carriers compared to boron-doped cells. External-Quantum-Efficiency measurements of both boron and indium doped cells shows an enhancement in optical to electrical conversion for wavelengths of similar to 470-1000 nm when indium is used as a dopant. No significant conversion is observed for wavelengths >1100 nm, in contradiction of an IPV effect existing in these devices. (C) 2016 Elsevier B.V. All rights reserved.