Exposure to elevated temperatures also causes degradation of
the perovskite layer. Understanding this point is crucial, as the
perovskite layer requires an annealing step and, ultimately, solar
modules will be subjected to elevated temperature during operation.
Phillippe et al. investigated the influence of elevated temperatures
on both CH3NH3PbI3 and CH3NH3PbI3xClx films [48].
The authors used hard X-ray photoelectron spectroscopy (PES),
rather than XRD, to explain the film decomposition, as this allows
for a determination of the chemical content regardless of the
crystallinity of the sample. In this experiment, the films were
heated in an analysis chamber under an ultra-high vacuum.
Removing the presence of water and air allowed for the isolation
of the effect of temperature on the film degradation. The films
were characterised using the I/Pb and N/Pb ratios, extracted from
PES. A reduction of these ratios indicates the conversion of the
perovskite into PbI2. The calculated ratios are displayed in Fig. 6.
Heating at 100 °C for 20 min led to a significant reduction of both
ratios. Further heating at 200 °C caused both ratios to drop to a
minimum, 2 and 0, respectively. This is indicative of a film composed
100% of PbI2. The authors suggest the following reaction for
the temperature induced decomposition