Back Contact Passivation With Ga-Grading in Narrow Bandgap (Ag,Cu)(In,Ga)Se2 Bifacial Solar Cells on In2O3:Sn Back Contact

This study investigates the effect of Ga-grading in
narrow bandgap (Ag,Cu)(In,Ga)Se2 (ACIGS) bifacial solar cells
on In2O3:Sn (ITO) transparent substrates. It is found that the
performance of cells on ITO equals or supersedes the performance
of reference cells made on Mo. Up to 60% bifaciality is measured
for the cells on ITO. At the ACIGS/ITO interface a layer of approximately
3 nm GaOx is identified with scanning transmission
electron microscopy and is believed to have a passivating effect on
the back contact. An increase in the steepness of the Ga-grading
at the rear interface results in better overall performance for
front- and rear-side measurements,which likely indicates an added
rear-contact passivation effect. A steep Ga-grading is achieved
for absorbers of thicknesses ranging from 250 nm to 1 μm. The
importance of the Ga-grading for current collection under front
and rear illumination is shown by comparison with a flat profile,
where the graded absorber reached twice the current density of
the ungraded absorber under rear illumination and 12% higher
current density under front illumination.