posted on 2020-03-10, 12:03authored byBethan Charles, Mark T. Weller, Sebastian Rieger, Lauren E. Hatcher, Paul F. Henry, Jochen Feldmann, Daniel Wolverson, Chick C. Wilson
The mixed cation
lead iodide perovskite photovoltaics show improved
stability following site substitution of cesium ions (Cs+) onto the formamidinium cation sites (FA+) of (CH(NH2)2PbI3 (FAPbI3) and increased
resistance to formation of the undesirable ∂-phase. The structural
phase behavior of Cs0.1FA0.9PbI3 has
been investigated by neutron powder diffraction (NPD), complemented
by single crystal and power X-ray diffraction and photoluminescence
spectroscopy. The Cs-substitution limit has been determined to be
less than 15%, and the cubic α-phase, Cs0.1FA0.9PbI3, is shown to be synthesizable in bulk and
stable at 300 K. On cooling the cubic Cs0.1FA0.9PbI3, a slow, second-order cubic to tetragonal transition
is observed close to 290 K, with variable temperature NPD indicating
the presence of the tetragonal β-phase, adopting the space group P4/mbm between 290 and 180 K. An orthorhombic
phase or twinned tetragonal phase is formed below 180 K, and the temperature
for further transition to a disordered state is lowered to 125 K compared
to that seen in phase pure α-FAPbI3 (140 K). These
results demonstrate the importance of understanding the effect of
cation site substitution on structure–property relationships
in perovskite materials.