Bottom-Up and Top-Down Approaches for Identifying and Mitigating Electrical Losses in Silicon Heterojunction Solar Cells

Electrical losses in silicon heterojunction (SHJ) solar cells are difficult to identify and to control as multiple material layers and several entangled physical phenomena are involved. In this context, this contribution aims to accurately investigate and characterize the electrical losses affecting the collection of photogenerated carriers in SHJ solar cells and to provide means to mitigate them. In particular, the material properties controlling the physical coupling between the n-type hydrogenated silicon and the transparent conductive oxide (TCO) layers are studied. To this aim, the top-down and the bottom-up approaches are introduced and applied to develop different material layers and to study the final device performance once these layers are integrated inside actual solar cells. First, the bulk and interface layer properties required for an efficient carrier transport within the n-type carrier selective passivating contacts of SHJ devices are decoupled by using multilayers which combine thin n-type hydrogenated amorphous and nanocrystalline silicon layers. The multilayer characteristics yielding efficient transport are investigated for two different TCOs, which are indium-tin-oxide (ITO) and aluminium-zinc-oxide (AZO). Second, the passivation quality is studied at various process steps and the contact resistivity is investigated. Finally, these multilayers are further optimized to obtain low series resistance and high final passivation once coupled with both TCOs. As major outcomes, a $2\times 2$ -cm ² screen-printed SHJ solar cell with 82.33% fill factor and 24.24% efficiency was reached using AZO as rear TCO and the best certified $2\times 2$ -cm ² solar cell integrating ITO demonstrated a fill factor of 82.28% along with an efficiency of 24.21%.