Design and optimization of hole collectors based on nc-SiO :H for high-efficiency silicon heterojunction solar cells

Low activation energy (Ea) and wide bandgap (Eg) are essential for (p)-contacts to achieve effective hole collection in silicon heterojunction (SHJ) solar cells. In this work, we study Plasma-Enhanced Chemical Vapor Deposition p-type hydrogenated nanocrystalline silicon oxide, (p)nc-SiOx:H, combined with (p)nc-Si:H as (p)-contact in front/back-contacted SHJ solar cells. We firstly determine the effect of a plasma treatment at the (i)a-Si:H/(p)-contact interface on the thickness-dependent Ea of (p)-contacts. Notably, when the (p)nc-Si:H layer is thinner than 20 nm, the Ea decreases by applying a hydrogen plasma treatment and a very-high-frequency (i)nc-Si:H treatment. Such an interface treatment also significantly reduces the contact resistivity of the (p)-contact stacks (ρc,p), resulting in an improvement of 6.1%abs in fill factor (FF) of the completed cells. Thinning down the (i)a-Si:H passivating layer to 5 nm leads to a low ρc,p (144 mΩ⋅cm2) for (p)-contact stacks. Interestingly, we observe an increment of FF from 72.9% to 78.3% by using (p)nc-SiOx:H layers featuring larger differences between their optical gap (E04) and Ea, which tend to enhance the built-in potential at the c-Si/(i)a-Si:H interface. Furthermore, we observe clear impacts on ρc,p, open-circuit voltage, and FF by optimizing the thicknesses of (p)-contact that influence its Ea. In front junction cells, the vertical and lateral collection of holes is affected by ρc,p of (p)-contact stacks. This observation is also supported by TCAD simulations which reveal different components of lateral contributions. Lastly, we obtain both front and rear junction cells with certified FF well-above 80% and the best efficiency of 22.47%.