Determination of Thermal Expansion Coefficients and Locating the Temperature-Induced Phase Transition in Methylammonium Lead Perovskites Using X‑ray Diffraction

Lead
halogen perovskites, and particularly methylammonium lead
iodine, CH3NH3PbI3, have recently
attracted considerable interest as alternative solar cell materials,
and record solar cell efficiencies have now surpassed 20%. Concerns
have, however, been raised about the thermal stability of methylammonium
lead iodine, and a phase transformation from a tetragonal to a cubic
phase has been reported at elevated temperature. Here, this phase
transition has been investigated in detail using temperature-dependent
X-ray diffraction measurements. The phase transformation is pinpointed
to 54 °C, which is well within the normal operating range of
a typical solar cell. The cell parameters were extracted as a function
of the temperature, from which the thermal expansion coefficient was
calculated. The latter was found to be rather high (αv = 1.57 × 10–4 K–1) for
both the tetragonal and cubic phases. This is 6 times higher than
the thermal expansion coefficient for soda lime glass and CIGS and
11 times larger than that of CdTe. This could potentially be of importance
for the mechanical stability of perovskite solar cells in the temperature
cycling experienced under normal day–night operation. The experimental
knowledge of the thermal expansion coefficients and precise determination
of the cell parameters can potentially also be valuable while conducting
density functional theory simulations on these systems in order to
deliver more accurate band structure calculations.