Quantification of pn-Junction Recombination in Interdigitated Back-Contact Crystalline Silicon Solar Cells

Interdigitated back-contact (IBC) solar cells based on diffused crystalline silicon comprise a series of pn-junctions which border at the rear surface of the wafer. In this work, it is established that the presence of these pn-junctions can induce significant additional charge-carrier recombination, which affect the conversion efficiency of IBC cells through a reduction in fill factor and open-circuit voltage. Using specialized test structures with varying length of pn-junctions per area of solar cell (i.e., with varying junction density), the magnitude of the recombination at the pn-junction was determined. For nonpassivated rear surfaces, a second-diode recombination current density per unit of junction density J
₀₂
of ~61 nA · junction
^-1
cm
^-1
was measured, whereas for surfaces which were passivated by either SiN or Al
₂
O
₃
/SiN
_x
, J
₀₂
was reduced to '0.4 nA junction
^-1
cm
^-1
. Therefore, passivation of defects at the rear surface was proven to be vital in reducing this characteristic recombination current. Finally, by optimizing the p- and n-type dopant diffusion process recipes,J
₀₂
recombination could be suppressed. The improved doping recipes led to an increase in conversion efficiency of industrial “mercury” IBC solar cells by ~1% absolute.