In perovskite solar cells (PSCs),
the vertical inhomogeneities
which include uneven grains, voids, and grain boundaries are closely
linked to the underlying charge transport layer which controls the
nucleation and grain growth in the perovskite film. Herein, the vertical
inhomogeneity of perovskite films in the device structure is analyzed
by depth-dependent photoluminescence (PL) achieved with different
excitation wavelengths. An analytical representation between vertical
inhomogeneity and depth-dependent PL, parametrized with a factor,
b, is introduced to understand the relation between inhomogeneity
and charge recombination. Lower values of b correlate to lower vertical
inhomogeneity and hence reduced recombination. The analytical representation
is validated in two sets of devices that show remarkable differences
in perovskite film morphology, device based on mesoporous TiO2 and planar SnO2. By exploring the morphological
properties and the PL emission from different depths across the device
structures, we show that the lower vertical inhomogeneity leads to
more efficient charge carrier extraction in planar SnO2-based devices. Moreover, the SnO2-based devices exhibit
lower Urbach energy, which concurs with the slow transient photovoltage
decay, suggesting less defects and recombination losses. This work
provides a broader understanding of the impact of vertical inhomogeneity
on the charge extraction efficiency and presents a methodology to
study quantitatively the inhomogeneity of perovskite films in device
structures.
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