High-Pressure Single-Crystal Structures of 3D Lead-Halide Hybrid Perovskites and Pressure Effects on their Electronic and Optical Properties

We report the first high-pressure single-crystal structures of
hybrid perovskites. The crystalline semiconductors (MA)­PbX3 (MA = CH3NH3+, X = Br– or I–) afford us the rare opportunity of understanding
how compression modulates their structures and thereby their optoelectronic
properties. Using atomic coordinates obtained from high-pressure single-crystal
X-ray diffraction we track the perovskites’ precise structural
evolution upon compression. These structural changes correlate well
with pressure-dependent single-crystal photoluminescence (PL) spectra
and high-pressure bandgaps derived from density functional theory.
We further observe dramatic piezochromism where the solids become
lighter in color and then transition to opaque black with compression.
Indeed, electronic conductivity measurements of (MA)­PbI3 obtained within a diamond-anvil cell show that the material’s
resistivity decreases by 3 orders of magnitude between 0 and 51 GPa.
The activation energy for conduction at 51 GPa is only 13.2(3) meV,
suggesting that the perovskite is approaching a metallic state. Furthermore,
the pressure response of mixed-halide perovskites shows new luminescent
states that emerge at elevated pressures. We recently reported that
the perovskites (MA)­Pb­(BrxI1–x)3 (0.2 < x < 1)
reversibly form light-induced trap states, which pin their PL to a
low energy. This may explain the low voltages obtained from solar
cells employing these absorbers. Our high-pressure PL data indicate
that compression can mitigate this PL redshift and may afford higher
steady-state voltages from these absorbers. These studies show that
pressure can significantly alter the transport and thermodynamic properties
of these technologically important semiconductors.