  Although methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$) perovskite has
attracted enormous scientific attention over the last decade or so, important
information on the charge extraction dynamics and recombination processes in
perovskite devices is still missing. Herein we present a novel approach to
evaluate the quality of CH$_3$NH$_3$PbI$_3$ layers, via in-situ monitoring of
the perovskite layer charge carrier dynamics during the thermal annealing
crystallization process, by means of time-resolved femtosecond transient
absorption spectroscopy (TAS). In particular, CH$_3$NH$_3$PbI$_3$ films were
deposited on two types of polymeric hole transport layers (HTL),
poly(3,4-ethylenedioxythiophene)-poly-(styrenesulfonate) (PEDOT:PSS) and
poly-(triarylamine) (PTAA), that are known to provide different carrier
transport characteristics in perovskite solar cells. In order to monitor the
evolution of the perovskite charge carrier dynamics during the crystallization
process, the so-formed CH$_3$NH$_3$PbI$_3$/HTL architectures were studied
in-situ by TAS at three different annealing temperatures, i.e. 90, 100 and 110
oC. It is revealed that the annealing time period required in order to achieve
the optimum perovskite film quality in terms of the decay dynamics strongly
depends on the annealing temperature, as well as, on the employed HTL. For both
HTLs, the required period decreases as higher annealing temperature is used,
while, for the more hydrophobic PTAA polymer, longer annealing periods were
required in order to obtain the optimum charge carrier dynamics. The
correlation of the TAS finding with the structural and morphological features
of the perovskite films is analysed and provides useful insights on the charge
extraction dynamics and recombination processes in perovskite optoelectronic
devices.
