Abstract
We have examined the crystal structures and structural phase transitions of the deuterated, partially deuterated and hydrogenous organic-inorganic hybrid perovskite methyl ammonium lead iodide (MAPbI3) using time-of-flight neutron and synchrotron X-ray powder diffraction. Near 330 K the high temperature cubic phases transformed to a body-centered tetragonal phase. The variation of the order parameter Q for this transition scaled with temperature T as Q ∼ (Tc−T)β, where Tc is the critical temperature and the exponent β was close to ¼, as predicted for a tricritical phase transition. However, we also observed coexistence of the cubic and tetragonal phases over a range of temperature in all cases, demonstrating that the phase transition was in fact first-order, although still very close to tricritical. Upon cooling further, all the tetragonal phases transformed into a low temperature orthorhombic phase around 160 K, again via a first-order phase transition. Based upon these results, we discuss the impact of the structural phase transitions upon photovoltaic performance of MAPbI3 based solar cells.
Highlights
To understand the origin of their PV performance, and accelerate the search for new materials, it is essential to first understand the crystal structures of the hybrid perovskites, characterized by structural phase transitions, considerable static or dynamic disorder, and unknown concentrations of various defects such as halogen anion or organic cation vacancies
The structure was found to be similar to d3-CH3ND3PbBr3 at 11 K16 and h6-MAPbI3 at 100 K14, but the lower temperature resulted in less thermal motion than observed in the 100 K data[16]
Some diffuse scattering was visible in the background even at 10 K but a fully un-constrained refinement with anisotropic displacement parameters (ADPs) could still be carried out
Summary
To understand the origin of their PV performance, and accelerate the search for new materials, it is essential to first understand the crystal structures of the hybrid perovskites, characterized by structural phase transitions, considerable static or dynamic disorder, and unknown concentrations of various defects such as halogen anion or organic cation vacancies. To improve our understanding of the crystal structures and phase transitions in MAPbI3, we performed detailed structural studies using both time-of-flight neutron and synchrotron X-ray powder diffraction. There have been several studies of MAPbI3 using reactor-based neutron powder diffraction[14,15], those studies did not utilize deuterated samples to eliminate the strong incoherent scattering from hydrogen, nor did they fully address the structural complexity introduced by the MA cations in this perovskite. There have not been any temperature-dependent studies of MAPbI3 that took advantage of the higher resolution of synchrotron X-ray powder diffraction to study the structural phase transitions in detail. The current studies showed some surprising features having implications for the use of MAPbI3 as a PV absorber
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