SiN_x and AlO_x Nanolayers in Hole Selective Passivating Contacts for High Efficiency Silicon Solar Cells

The recombination of photogenerated charge carriers at metal-semiconductor interfaces remains a major source of efficiency loss in photovoltaic cells. Here, we present SiN _x and AlO _x nanolayers as promising interface dielectrics to enable high efficiency hole selective passivating contacts. It is demonstrated that SiN _x deposited via direct plasma enhanced chemical vapour deposition can be grown controllably at thicknesses of 2 nm. The valence band offsets between crystalline silicon and ultrathin AlO _x and SiN _x nanolayers are measured as 3.5 and 1.4 eV, respectively. This predicts a larger tunnelling current for holes, compared to SiO _x used typically. Resistivity measurements show that SiN _x and AlO _x nanolayers have lower contact resistivities compared to SiO _x , with values as low as 100 mΩ·cm ² . Analysis of the current transport mechanisms confirmed that tunnelling dominates the conduction through SiN _x , while a mixture of tunnelling and pinholes are present in the AlO _x structure. Lifetime measurements gave initial indications of the passivation quality of the films, with just 10 cycles of AlO _x achieving 260 μ s after annealing and 1.9 ms with extrinsic field effect passivation added. Finally, the intrinsic built-in charge in the dielectrics was determined using surface photovoltage measurements and simulations are used to estimate the influence of nanolayer built-in charge in both poly-Si and dopant-free passivating contacts to enable future high efficiency solar cells.