  In the present study, we report the synthesis and characterization of a
low-temperature solution-processable monodispersed nickel cobaltite (NiCo2O4)
nanoparticles via a combustion synthesis using tartaric acid as fuel and
demonstrate its performance as hole transport layer (HTL) for Perovskite Solar
Cells (PVSCs). NiCo2O4 is a p-type semiconductor consisting of environmentally
friendly, abundant elements and higher conductivity compared to NiO. We show
that the combustion synthesis of spinel NiCo2O4 using tartaric acid as fuel can
be used to control the NPs size and provide smooth, compact and homogeneous
functional HTLs processed by blade coating. Study of PVSCs with different
NiCo2O4 thickness as HTL reveal a difference on hole extraction efficiency and
for 15 nm optimized thickness enhanced hole carrier collection is achieved. As
a result, p-i-n structure of PVSCs with 15 nm NiCo2O4 HTLs showed reliable
performance and power conversion efficiency values in the range of 15.5 % with
negligible hysteresis.
