Molybdenum oxide (MoO_x) is attractive for applications as hole-selective contact in silicon heterojunction solar cells for its transparency and relatively high work function. However, the integration of MoO_x stacked on intrinsic amorphous silicon (i)a-Si:H layer usually exhibits some issues that are still not fully solved resulting in degradation of electrical properties. Here, we propose a novel approach to enhance the electrical properties of (i)a-Si:H/MoO_x contact. We manipulate the (i)a-Si:H interface via plasma treatment (PT) before MoO_x deposition minimizing the electrical degradation without harming the optical response. Furthermore, by applying the optimized PT, we can reduce the MoO_x thickness down to 3.5 nm with both open-circuit voltage and fill factor improvements. Our findings suggest that the PT mitigates the decrease of the effective work function of the MoO_x (WF_MoOx) thin layer when deposited on (i)a-Si:H. To support our hypothesis, we carry out electrical simulations inserting a dipole at the (i)a-Si:H/MoO_x interface accounting the attenuation of WF_MoOx caused by both MoO_x thickness and dipole. Our calculations confirm the experimental trends and thus provide deep insight in critical transport issues. Temperature-dependent J-V measurements demonstrate that the use of PT improves the energy alignment for an efficient hole transport.

Electrical simulations show that the dipole formed at (i)a-Si:H/MoOx interface can explain electrical performance degradation. We experimentally manipulate this interface by a plasma treatment (PT) to mitigate the dipole strength without harming the optical response. The optimal PT + MoOx stack results in strongly improved electrical parameters as compared to the one featuring only MoOx and to the silicon heterojunction reference cell. Optical simulations and experimentally measured currents suggest that the additional PT is responsible of very limited parasitic absorption overcompensated by the thinner MoOx used (3.5 nm) and by the lower losses in the (i)a-Si:H layer underneath.