Ruddlesden–Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors

The hybrid two-dimensional
(2D) halide perovskites have recently
drawn significant interest because they can serve as excellent photoabsorbers
in perovskite solar cells. Here we present the large scale synthesis,
crystal structure, and optical characterization of the 2D (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 (n = 1, 2, 3, 4, ∞) perovskites, a family of layered
compounds with tunable semiconductor characteristics. These materials
consist of well-defined inorganic perovskite layers intercalated with
bulky butylammonium cations that act as spacers between these fragments,
adopting the crystal structure of the Ruddlesden–Popper type.
We find that the perovskite thickness (n) can be
synthetically controlled by adjusting the ratio between the spacer
cation and the small organic cation, thus allowing the isolation of
compounds in pure form and large scale. The orthorhombic crystal structures
of (CH3(CH2)3NH3)2(CH3NH3)­Pb2I7 (n = 2, Cc2m; a = 8.9470(4), b = 39.347(2) Å, c = 8.8589(6)), (CH3(CH2)3NH3)2(CH3NH3)2Pb3I10 (n = 3, C2cb; a = 8.9275(6), b = 51.959(4) Å, c = 8.8777(6)), and
(CH3(CH2)3NH3)2(CH3NH3)3Pb4I13 (n = 4, Cc2m; a = 8.9274(4), b = 64.383(4) Å, c = 8.8816(4)) have been solved by single-crystal X-ray diffraction
and are reported here for the first time. The compounds are noncentrosymmetric,
as supported by measurements of the nonlinear optical properties of
the compounds and density functional theory (DFT) calculations. The
band gaps of the series change progressively between 2.43 eV for the n = 1 member to 1.50 eV for the n = ∞
adopting intermediate values of 2.17 eV (n = 2),
2.03 eV (n = 3), and 1.91 eV (n =
4) for those between the two compositional extrema. DFT calculations
confirm this experimental trend and predict a direct band gap for
all the members of the Ruddlesden–Popper series. The estimated
effective masses have values of mh =
0.14 m0 and me = 0.08 m0 for holes and electrons, respectively, and are found to be
nearly composition independent. The band gaps of higher n members indicate that these compounds can be used as efficient light
absorbers in solar cells, which offer better solution processability
and good environmental stability. The compounds exhibit intense room-temperature
photoluminescence with emission wavelengths consistent with their
energy gaps, 2.35 eV (n = 1), 2.12 eV (n = 2), 2.01 eV (n = 3), and 1.90 eV (n = 4) and point to their potential use in light-emitting diodes.
In addition, owing to the low dimensionality and the difference in
dielectric properties between the organic spacers and the inorganic
perovskite layers, these compounds are naturally occurring multiple
quantum well structures, which give rise to stable excitons at room
temperature.