Nanocrystalline‐silicon hole contact layers enabling efficiency improvement of silicon heterojunction solar cells: Impact of nanostructure evolution on solar cell performance

Hydrogenated amorphous silicon (a‐Si:H) is a key enabler in high‐efficiency crystalline silicon solar cells known as the silicon heterojunction technology. Although efforts have been devoted to replacing doped a‐Si:H contact layer by hydrogenated nanocrystalline silicon (nc‐Si:H) to take advantage of its superior optoelectrical properties, it is still unclear whether the nc‐Si:H outperforms the a‐Si:H at the high efficiency level. Here, we show that boron‐doped ( p )nc‐Si:H prepared by plasma‐enhanced chemical vapor deposition (PECVD) acts as an efficient hole contact layer, providing not only a mitigation of the parasitic absorption loss but also improvements in passivation and electrical contact properties. This results in an efficiency increase by 0.3%–0.6% absolute compared to the reference cell with the ( p )a‐Si:H, and a best cell efficiency of 23.54%. We find that the critical thickness of the ( p )nc‐Si:H layers required for gaining high efficiency ( t c ~ 15–30 nm) is a factor of 3–6 greater than that of the ( p )a‐Si:H. UV Raman spectroscopy and electrical conductivity measurements reveal that the t c of the ( p )nc‐Si:H is associated with the layer growth needing for the surface coalescence of nanocrystals, determining the hole selectivity and the contact resistivity at the electrode/( p )nc‐Si:H interface. Our results suggest that such nanostructure evolution can be hastened by using a very‐high‐frequency PECVD process.