Integration of thin n-type nc-Si:H layers in the window-multilayer stack of heterojunction solar cells

N-type nanocrystalline silicon (nc-Si:H(n)) layers are good candidates to improve current and transport properties in heterojunction solar cells. In this work, we perform thickness series alongside PH3 doping series to unravel the desirable characteristics of nc-Si:H(n) along its growth direction. While increasing the PH3 flow is necessary to improve the conductivity of the layer, we observe that too high flows lead to an amorphization of the first 10–15 nm of the layers. In consequence, either high crystallinity are reached at intermediate PH3 flow, resulting in high doping at the n/TCO interface but a poorly doped nucleation zone, or the material becomes more amorphous at very high PH3 flow, allowing a higher doping in the nucleation zone but a less efficient screening of the barrier with the TCO due to the lower crystallinity. To overcome this trade-off, we integrate a 2.5 nm a-Si:H(n) layer underneath the nc-Si:H(n) layer. We also report on the impact of higher ITO doping as well as on the beneficial effect of an additional SiOx capping not only to form a double anti-reflective coating (DARC), but also to improve contact properties. We observe that each of those three features can improve passivation and selectivity as well as reduce strongly the front contact resistivity (ρc). Our best results are achieved by using a thin a-Si:H(n)/nc-Si:H(n) stack together with an IZrO/SiOx DARC, enabling front ρc lower than 15 mΩ cm2 and an efficiency of 23.7% on screen-printed 2×2 cm2 solar cells.