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

Abstract

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 PH 3 doping series to unravel the desirable characteristics of nc-Si:H(n) along its growth direction. While increasing the PH 3 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 PH 3 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 PH 3 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 SiO x 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 ( ρ c ). Our best results are achieved by using a thin a-Si:H(n)/nc-Si:H(n) stack together with an IZrO/SiO x DARC, enabling front ρ c lower than 15 mΩ cm 2 and an efficiency of 23.7% on screen-printed 2 × 2 cm 2 solar cells. • Development of nc-Si :H(n) layers for front application. • Trade-off between nucleation zone and high crystallinity evidenced. • a-Si:H(n)/nc-Si:H(n) bilayer strategy allow for ρ c ¡ 15 mΩ cm 2 . • Decreases dependency upon TCO lateral conductivity. • Good passivation (pFF = 85%) and transparency maintained.