2 research outputs found
Impact of Na Dynamics at the Cu<sub>2</sub>ZnSn(S,Se)<sub>4</sub>/CdS Interface During Post Low Temperature Treatment of Absorbers
Cu<sub>2</sub>SnZn(S,Se)<sub>4</sub> (CZTSSe) solar cells based on earth abundant and nontoxic
elements currently achieve efficiencies exceeding 12%. It has been
reported that, to obtain high efficiency devices, a post thermal treatment
of absorbers or devices at temperatures ranging between 150 and 400
°C (post low temperature treatment, PLTT) is advisable. Recent
findings point toward a beneficial passivation of grain boundaries
with SnO<sub><i>x</i></sub> or Cu-depleted surface and grain
boundaries during the PLTT process, but no investigation regarding
alkali doping is available, even though alkali dynamics, especially
Na, are systematically reported to be crucial within the field. In
this work, CZTSSe absorbers were subjected to the PLTT process under
different temperatures, and solar cells were completed. We found surprisingly
behavior in which efficiency decreased to nearly 0% at 200 °C
during the PLTT process, being recovered or even improved at temperatures
above 300 °C. This unusual behavior correlates well with the
Na dynamics in the devices, especially with the in-depth distribution
of Na in the active CZTSSe/CdS interface region, indicating the key
importance of Na spatial distribution on device properties. We present
an innovative model for Na dynamics supported by theoretical calculations
and additional specially designed experiments to explain this behavior.
After optimization of the PLTT process, a Se-rich CZTSSe solar cell
with 8.3% efficiency was achieved
Complex Surface Chemistry of Kesterites: Cu/Zn Reordering after Low Temperature Postdeposition Annealing and Its Role in High Performance Devices
A detailed study
explaining the beneficial effects of low temperature
postdeposition annealing combined with selective surface etchings
for Cu<sub>2</sub>ZnSnSe<sub>4</sub> (CZTSe) based solar cells is
presented. After performing a selective oxidizing surface etching
to remove ZnSe secondary phases typically formed during the synthesis
processes an additional 200 °C annealing step is necessary to
increase device performance from below 3% power conversion efficiency
up to 8.3% for the best case. This significant increase in efficiency
can be explained by changes in the surface chemistry which results
in strong improvement of the CdS/CZTSe heterojunction commonly used
in this kind of absorber/buffer/window heterojunction solar cells.
XPS measurements reveal that the 200 °C annealing promotes a
Cu depletion and Zn enrichment of the etched CZTSe absorber surface
relative to the CZTSe bulk. Raman measurements confirm a change in
Cu/Zn ordering and an increase in defect density. Furthermore, TEM
microstructural investigations indicate a change of grain boundaries
composition by a reduction of their Cu content after the 200 °C
annealing treatment. Additionally, insights in the CdS/CZTSe interface
are gained showing a significant amount of Cu in the CdS buffer layer
which further helps the formation of a Cu-depleted surface and seems
to play an important role in the formation of the pn-heterojunction