Relatively fast achievements
in the kesterite solar cell technology
have been made over the last decade, but the experimental efficiency
is still ∼13%. One proposed reason is an inappropriate band
alignment with Cu2ZnSnS4 (CZTS) and CdS that
results in strong interface recombination losses. Results of this
work show that the temperature and duration of air annealing of the
CZTS/CdS heterojunction are essential for device performance. Soft
annealing slightly improved the device efficiency due to the elemental
intermixing at the interface. On the other hand, extended annealing
increased absorber band gap energy, resulting in higher V
OC values, indicating the improved Cu–Zn ordering
in the CZTS structure, which also could be expected to have a beneficial
influence on the device performance. However, interface analysis revealed
that the CZTS absorber surface layer was Cu-rich, providing the reason
for the reduction in CZTS solar cell performance. The effect of annealing
on the interface defects was analyzed by the capacitance–frequency–voltage
(C–V–f) analysis combined with SCAPS
simulations. C–V–f-based loss maps
showed that air annealing modifies the density distribution of asymmetrical
interface states at the CZTS/CdS interface, which becomes fully symmetrical
for longer annealing times at 200 °C.
The reported results confirm that the positive effect of Ag in (Cu1−xAgx)1.85(Zn0.8Cd0.2)1.1SnS4 MGL solar cells appears only at very low Ag concentrations (x ≤ 1%).
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