posted on 2013-08-05, 00:00authored byConstantinos C. Stoumpos, Christos D. Malliakas, Mercouri G. Kanatzidis
A broad organic–inorganic
series of hybrid metal iodide perovskites with the general formulation
AMI3, where A is the methylammonium (CH3NH3+) or formamidinium (HC(NH2)2+) cation and M is Sn (1 and 2) or Pb (3 and 4) are reported. The compounds
have been prepared through a variety of synthetic approaches, and
the nature of the resulting materials is discussed in terms of their
thermal stability and optical and electronic properties. We find that
the chemical and physical properties of these materials strongly depend
on the preparation method. Single crystal X-ray diffraction analysis
of 1–4 classifies the compounds in
the perovskite structural family. Structural phase transitions were
observed and investigated by temperature-dependent single crystal
X-ray diffraction in the 100–400 K range. The charge transport
properties of the materials are discussed in conjunction with diffuse
reflectance studies in the mid-IR region that display characteristic
absorption features. Temperature-dependent studies show a strong dependence
of the resistivity as a function of the crystal structure. Optical
absorption measurements indicate that 1–4 behave as direct-gap semiconductors with energy band gaps
distributed in the range of 1.25–1.75 eV. The compounds exhibit
an intense near-IR photoluminescence (PL) emission in the 700–1000
nm range (1.1–1.7 eV) at room temperature. We show that solid
solutions between the Sn and Pb compounds are readily accessible throughout
the composition range. The optical properties such as energy band
gap, emission intensity, and wavelength can be readily controlled
as we show for the isostructural series of solid solutions CH3NH3Sn1–xPbxI3 (5). The charge
transport type in these materials was characterized by Seebeck coefficient
and Hall-effect measurements. The compounds behave as p- or n-type semiconductors depending on the preparation
method. The samples with the lowest carrier concentration are prepared
from solution and are n-type; p-type
samples can be obtained through solid state reactions exposed in air
in a controllable manner. In the case of Sn compounds, there is a
facile tendency toward oxidation which causes the materials to be
doped with Sn4+ and thus behave as p-type
semiconductors displaying metal-like conductivity. The compounds appear
to possess very high estimated electron and hole mobilities that exceed
2000 cm2/(V s) and 300 cm2/(V s), respectively,
as shown in the case of CH3NH3SnI3 (1). We also compare the properties of the title hybrid
materials with those of the “all-inorganic” CsSnI3 and CsPbI3 prepared using identical synthetic
methods.