Towards 28 %-efficient Si single-junction solar cells with better passivating POLO junctions and photonic crystals

We conduct numerical device simulations to study to what extend poly-Si on oxide (POLO)2 IBC solar cells can be optimized. In particular, we evaluate the benefit of the concept of photonic crystals (PCs) for “standard” cell thicknesses compatible with industrial wafer handling. We find that for our current surface passivation quality, implementing PCs and decreasing the wafer thickness down to 15 μm would increase the efficiency by „only“ 1% absolute due to limiting surface recombination losses. We deduce a high c-Si/SiOx interface state density Dit of 2.9 × 1012 eV−1cm−2 by analyzing special two-terminal IV measurements on small pads that contact the intact interfacial oxide between pinholes with our MarcoPOLO model. Consequently, we improve the hydrogenation process of our POLO junctions by an Al2O3/SiNx/Al2O3 rear-side dielectric layer stack. For n-type POLO (p-type POLO) J0 is reduced from 4 (10) fA/cm2 down to 0.5 ± 0.3 (3.3 ± 0.7) fA/cm2. For this improved surface passivation, our numerical device simulations predict an efficiency potential of 29.1% (27.8%) for POLO2 IBC cells with (without) PCs for a standard thickness of 150 μm. This shows that the “practical limit” for Si solar cells with poly-Si on oxide-based passivating contact schemes is above 27%, and, in general, that the efficiency potential of Si single-junction cells is still far from being exhausted. The first implementation of the improved POLO junctions into cell precursors confirms the predicted improvement on the level of suns - implied open-circuit voltage curves.