UV-stable surface passivation for crystalline silicon cells in solar modules with UV light transmitting encapsulation materials

We investigate the ultraviolet (UV) stability of dielectric passivation layers on n
⁺
-type industry-typical phosphorus- diffused emitters with saturation current densities J
_0e
in the range of 36 fA cm
^-2
to 67 fA cm
^-2
. We prepare symmetrical silicon wafer test structures with various types of passivation layers and derive their saturation current densities from carrier lifetime measurements after exposure to different types of UV lamps. Our results reveal that UV illumination of emitters with a typical industrial silicon nitride (SiNx) passivation layer strongly increases the surface recombination. The illumination by narrow band lamps with intensity peak at 312 nm for a UV dose of 80 kWh m
^-2
significantly increases the J
_0e
for these samples from 67 fA cm
^-2
to 507 fA cm
^-2
. In contrast, a passivation layer stack consisting of a thermally grown silicon oxide and SiNx improves the UV stability of the samples. For this passivation layer stack, the J
_0e
only marginally increases from 36 fA cm
^-2
to 46 fA cm
^-2
. The application of this thermal SiOy/SiNx passivation layer to bifacial passivated emitter and rear cells (PERC+) results in a stable conversion efficiency after exposure to a UV dose of 24 kWh m
^-2
. Our development of UV-stable PERC+ cells with SiOy passivation layers enables to apply UV light transmitting encapsulation materials for solar modules in order to increase the annual energy yield in the field.