With
a judicious selection of the nickel complex (precursor) and
stabilizer, that is, nickel formate and enthylenediamine, respectively,
a conventional sol–gel method followed by a sintering process
at 450 °C, nanoporous NiO
x
(np-NiO
x
) thin films have been successfully prepared.
However, the alcohol solvent used in the sol–gel process is
critical that only low-viscosity ones, such as methanol, ethanol,
2-methoxyethanol, and iso-propanol, afford np-NiO
x
instead of plate/flake-like compact NiO
x
(cp-NiO
x
). Some stabilizers, such
as 1,3-diaminopropane, diethylenetriamine, and N,N-dimethylethylenediamine, provide nothing but seemly amorphous
NiO
x
fragmentary thin films, which are
unsuitable for the hole-transporting material (HTM) in the inverted
perovskite solar cells (PVSCs). SEM or AFM study has a clear differentiation
between np-NiO
x
and cp-NiO
x
. Moreover, we have characterized that the np-NiO
x
thin films show a better conductivity, a
higher hole mobility, a stronger perovskite photoluminescence quenching,
and a superior transparency than those of cp-NiO
x
thin films. Using MAPbI3, inverted PVSCs were fabricated
with either np-NiO
x
or cp-NiO
x
as HTM. A high power conversion efficiency approaching
20% was achieved by np-NiO
x
-based PVSCs,
which was better than our best PVSC (17.95%) based on cp-NiO
x
. Electrochemical impedance spectroscopy (EIS) has
observed a lower charge recombination resistance but a higher charge
transport resistance for the cp-NiO
x
device
comparing with those of each np-NiO
x
devices.
Moreover, np-NiO
x
MAPbI3 PVSCs
exhibit a less hysteresis effect and an extended lifetime in terms
of shelf stability compared with those of cp-NiO
x
MAPbI3 PVSCs. The SEM and XRD have demonstrated
the larger grain size and less gain boundary of MAPbI3 thin
films on np-NiO
x
HTM, where MAPbI3 is in fact more hydrophobic, evident by the water contact
angle.
