  Hybrid perovskites have received tremendous attention due to their
exceptional photovoltaic and optoelectronic properties. Among the two widely
used perovskite solar cell device architectures of n-ip and p-i-n, the latter
is interesting in terms of its simplicity of fabrication and lower energy
input. However this structure mostly uses PEDOT:PSS as a hole transporting
layer which can accelerate the perovskite solar cell degradation. Hence the
development of stable, inorganic hole extraction layers (HEL), without
compromising the simplicity of device fabrication is crucial in this
fast-growing photovoltaic field. Here we demonstrate a low temperature (~100
oC) solution - processed and ultrathin (~ 6 nm) NiO nanoparticle thin films as
an efficient HEL for CH3NH3PbI3 based perovskite solar cells. We measure a
power conversion efficiency (PCE) of 13.3 % on rigid glass substrates and 8.5 %
on flexible substrates. A comparison with PEDOT:PSS based MAPbI3 solar cells
(PCE ~ 7.9 %) shows that NiO based solar cells have higher short circuit
current density and improved open circuit voltage (1.03V). Apart from the
photovoltaic performance under 1 Sun, the efficient hole extraction property of
NiO is demonstrated for indoor lighting as well with a PCE of 23.0 % for NiO
based CH3NH3PbI2.9Cl0.1 p-i-n solar cells under compact fluorescent lighting.
Compared to the perovskite solar cells fabricated on PEDOT:PSS HEL, better
shelf-life stability is observed for perovskite solar cells fabricated on NiO
HEL. Detailed microstructural and photophysical investigations imply uniform
morphology, lower recombination losses, and improved charge transfer properties
for CH3NH3PbI3 grown on NiO HEL.
