In this study, we demonstrate the Al0.1Ga0.9N/GaN heterojunction solar cells with a Mn (manganese)-doped active layer. Under a 1-sun AM1.5 G illumination condition, the devices exhibited an improved conversion efficiency by a magnitude of 5 compared to the cells without Mn doping in the active layer. This dramatic increase in conversion efficiency is attributed to the fact that the Mn-related energy states cause subband gap photon absorption and thereby contribute an extra photocurrent. On the basis of electroluminescence spectra and the spectral responses obtained from the Mn-doped devices, it is shown that the Mn-related energy states form a partially filled intermediate band (IB) within the bandgap of the Al0.1Ga0.9N active layer; the IB induces subband gap absorption, leading to an increase of short-circuit current in the Mn-doped Al0.1Ga0.9N/GaN heterojunction solar cells compared to the devices without Mn doping in the active layer. (C) 2016 Elsevier B.V. All rights reserved.