High
carrier recombination at the Cu2ZnSn (S, Se)4(CZTSSe)/CdS interface is the critical issue that results
in the low power conversion efficiency (PCE) of CZTSSe solar cells.
To reduce the recombination by optimizing the CZTSSe/CdS interfacial
structure, we fabricated a Zn doped CdS (Zn
x
Cd1 – x
S) thin
film with x of 0–0.32 and a CZTSSe solar cell
with the Zn
x
Cd1 – x
S as the buffer layer. It is found that Zn substitutes
for Cd in the x range of 0–0.26 and that some
of the Zn substitutes for Cd and another Zn locates in the interstitial
site of the CdS lattice in the x range of 0.26–0.32,
which make the lattice mismatch between CZTSSe and Zn
x
Cd1 – x
S decrease in x of 0–0.26 and increase in x of 0.26–0.32. The conduction band offset at the
CZTSSe/Zn
x
Cd1 – x
S interface is demonstrated by XPS to be a positive
″spike″-like type and increases from 0.11 to 0.43 eV
as x increases from 0 to 0.32. PCE is increased from
5.00 to 7.73% by optimizing x. The increased PCE
is attributed to increased open-circuit voltage (V
OC) and filling factor (FF), while the decreased PCE is
due to decreased V
OC, FF, and J
SC. By using quantitative analysis methods,
the increased V
OC and FF are mainly attributed
to the increased shunt resistance (R
sh) and decreased reverse saturation current density (J
0), and the decreased short-circuit current density (J
SC) is attributed to the increased conduction
band offset (CBO). The influence mechanism of Zn doping on R
sh, J
0, and CBO
is discussed.