Enhanced Carrier Collection in Cd/In-Based Dual Buffers in Kesterite Thin-Film Solar Cells from Nanoparticle Inks

Increasing the power
conversion efficiency (PCE) of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells has remained challenging
over the past decade, in part due to open-circuit voltage (VOC)-limiting defect states at the absorber/buffer
interface. Previously, we found that substituting the conventional
CdS buffer layer with In2S3 in CZTSSe devices
fabricated from nanoparticle inks produced an increase in the apparent
doping density of the CZTSSe film and a higher built-in voltage arising
from a more favorable energy-band alignment at the absorber/buffer
interface. However, any associated gain in VOC was negated by the introduction of photoactive defects at
the interface. This present study incorporates a hybrid Cd/In dual
buffer in CZTSSe devices that demonstrate an average relative increase
of 11.5% in PCE compared to CZTSSe devices with a standard CdS buffer.
Current density–voltage analysis using a double-diode model
revealed the presence of (i) a large recombination current in the
quasi-neutral region (QNR) of the CZTSSe absorber in the standard
CdS-based device, (ii) a large recombination current in the space-charge
region (SCR) of the hybrid buffer CZTSSe–In2S3–CdS device, and (iii) reduced recombination currents
in both the QNR and SCR of the CZTSSe–CdS–In2S3 device. This accounts for a notable 9.0% average increase
in the short-circuit current density (JSC) observed in CZTSSe–CdS–In2S3 in comparison to the CdS-only CZTSSe solar cells. Energy-dispersive
X-ray, secondary-ion mass spectroscopy, and grazing-incidence X-ray
diffraction compositional analysis of the CZTSSe layer in the three
types of kesterite solar cells suggest that there is diffusion of
elemental In and Cd into the absorbers with a hybrid buffer. Enhanced
Cd diffusion concomitant with a double postdeposition heat treatment
of the hybrid buffer layers in the CZTSSe–CdS–In2S3 device increases carrier collection and extraction
and boosts JSC. This is evidenced by electron-beam-induced
current measurements, where higher current generation and collection
near to the p–n junction is observed, accounting for the increase
in JSC in this device. It is expected
that optimization of the heat treatment of the hybrid buffer layers
will lead to further improvements in the device performance.