Despite the rapid
development and enormous success of organic–inorganic
hybrid halide perovskites (AB′X
3), such as CH3NH3PbI3 as absorbers for perovskite-based solar cells (PSCs), the commercial
applications of photovoltaic techniques still face several challenges,
such as decomposition when exposed to light and moisture, and lead
toxicity. On the other hand, the double perovskites (A
2
B′B″X
6) are derived from the AB′X
3 when half of the octahedrally coordinated B′-cations are partially replaced by the suitable B″-cations. They are attracting attention due to
a new design strategy to replace Pb2+ ions with the couple
of a monovalent M
+ ion and a trivalent M
3+ ion, leading to a new family of quaternary
double perovskites. In this way, we aim to synthesize and characterize
Cs2AgSbCl6 powdered samples, designed for solar
cell applications. The crystalline phase and morphological features
are investigated by X-ray powder diffraction (XRPD), neutron powder
diffraction (NPD), scanning electron microscopy (SEM) in complement
with UV–vis spectroscopy, showing a suitable band gap of 2.7
eV. The solution synthesis method proved to be efficient in obtaining
polycrystalline-Cs2AgSbCl6 samples in a cubic
ordered phase. DFT calculations also provided insights on the vibrational
properties of Cs2AgSbCl6, corroborating the
experimental data and elucidating the optical activity of Raman and
infrared modes.
Methylammonium lead iodide
(MAPbI3) is a very promising semiconducting material for
photovoltaic applications. Despite extensive research and tremendous
progress, basic charge transport properties are still being debated.
Combining first-principles calculations and macroscopic and local
measurements, we have investigated the structural, optical, thermal,
and electrical transport (ac/dc) properties of MAPbI3 hot-pressed
pellets through the tetragonal-to-cubic phase transition. Thermal
analysis and X-ray diffraction experiments confirm the tetragonal-to-cubic
phase change around T
S = 56 °C, which
is often close to the working temperature of photovoltaic devices.
The ac/dc electrical resistivities of the tetragonal phase indicate
a metallic-like behavior as a function of temperature followed by
an abrupt decrease in the cubic phase just above T
S. In contrast to the abrupt changes observed in the electrical
properties, the bandgap energy is barely affected across the phase
transition. Similarly, local measurements obtained by means of nuclear
magnetic resonance confirm a continuous variation in the lattice parameters
and site symmetry (207Pb and 127I) across the
structural phase transition. Density functional theory calculations
combined with electrical characterizations indicate that iodine and/or
unintentionally incorporated hydrogen interstitials influence decisively
the charge transport activation energy in the cubic phase. In light
of these findings, the unusual electrical resistivity behavior across
the phase transition is discussed taking the grain boundary effects
into consideration.
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