Silicon Heterojunction Solar Cells With Nanocrystalline Silicon Oxide Emitter: Insights Into Charge Carrier Transport

We recently demonstrated how the short-circuit current density of an a-Si:H/c-Si heterojunction solar cell can be significantly improved to above 40 mA/cm² by replacing the standard a-Si:H(p) emitter by a silicon oxide emitter containing p-doped silicon nanocrystallites. While we could obtain a conversion efficiency of 20.3%, the cell suffered from a lower fill factor of 72.9%, compared with 77.0% for our standard process. In this paper, we address this issue both theoretically and experimentally. We found that a thin (~3 nm) highly doped nanocrystalline silicon layer on top of the emitter can greatly improve the fill factor. Using 1-D device simulation, we explain the prevalent loss mechanism, which originates mostly from poor tunnel recombination at the transparent conducting oxide/emitter interface rather than in the bulk of the emitter. We suspect that have their origin in the lower effective dopant concentration of the nanocrystalline silicon oxide emitter. From the model, implications for further developments can be derived.