Enhanced
Carrier Collection from CdS Passivated Grains
in Solution-Processed Cu<sub>2</sub>ZnSn(S,Se)<sub>4</sub> Solar Cells
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Abstract
Solution
processing of Cu<sub>2</sub>ZnSn(S,Se)<sub>4</sub> (CZTSSe)–kesterite
solar cells is attractive because of easy manufacturing using readily
available metal salts. The solution-processed CZTSSe absorbers, however,
often suffer from poor morphology with a bilayer structure, exhibiting
a dense top crust and a porous bottom layer, albeit yielding efficiencies
of over 10%. To understand whether the cell performance is limited
by this porous layer, a systematic compositional study using (scanning)
transmission electron microscopy ((S)TEM) and energy-dispersive X-ray
spectroscopy of the dimethyl sulfoxide processed CZTSSe absorbers
is presented. TEM investigation revealed a thin layer of CdS that
is formed around the small CZTSSe grains in the porous bottom layer
during the chemical bath deposition step. This CdS passivation is
found to be beneficial for the cell performance as it increases the
carrier collection and facilitates the electron transport. Electron-beam-induced
current measurements reveal an enhanced carrier collection for this
buried region as compared to reference cells with evaporated CdS