We report PIN-type narrow-gap (E-g < 1.5 eV) a-SiGe:H single-junction solar cells containing integrated n-type hydrogenated microcrystalline silicon oxide (mu c-SiOx:H) layers that exhibit exceptionally high FFs in this paper. In addition to effectively enhancing the long-wavelength response of the a-SiGe:H single-junction solar cells, the integration of an n-type mu c-SiOx:H layer with a small oxygen content to fine-tune the leakage current characteristics can enormously improve the FFs. This opens a new way to improve narrow-gap a-SiGe:H solar cells, in addition to traditional optimization strategies such as material quality optimization and band gap profiling. The highest FF of 70.62% achieved with the proposed approach was for PIN-type a-SiGe:H single-junction solar cells with an average band gap of 1.48 eV. The omnipotent properties of mu c-SiOx:H layers, including optical enhancement of the long-wavelength response, independent control of longitudinal and lateral conductivity, effective elimination of the leakage current, and so on, make them likely to be extended to other types of solar cells such as microcrystalline silicon (mu c-Si:H) or polycrystalline chalcopyrite Cu(In,Ga)Se-2 (CIGS) cells, opening up new opportunities for acquiring solar cells with higher performance. (C) 2015 Elsevier B.V. All rights reserved.