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

Abstract

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 $_{x}$ or Al 2O3/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.