Effect of film morphology on circular dichroism of low-dimensional chiral hybrid perovskites.

Chiral hybrid lead halide perovskites show interesting chiral optoelectronic properties. The extent of chirality is often estimated by their circular dichroism (CD) response. Here, we show that the CD data depend strongly on film morphology. Four of the six chiral hybrid lead halide films prepared, 2D (R- and S-MBA)2PbI4 and 1D (R- and S-MBA)PbI3 (MBA: methylbenzylammonium), form homogenous non-textured films and show an isotropic CD signal. In contrast, the other two samples, 1D (R- and S-MBA)PbBr3, form textured films, showing uncorrelated CD signals from different parts of the film. Therefore, the role of film morphology needs to be verified before designing and comparing the chiroptic and chiral optoelectronic properties of hybrid perovskites.

Emissive Dark Excitons in Monoclinic Two-Dimensional Hybrid Lead Iodide Perovskites.

Typically, bright excitons (XB) emit light in two-dimensional (2D) layered hybrid perovskites. There are also dark excitons (XD), for which radiative recombination is spin-forbidden. Application of a magnetic field can somewhat relax the spin-rule, yielding XD emission. Can we obtain XD light emission in the absence of a magnetic field? Indeed, we observe unusually intense XD emission at ∼7 K for (Rac-MBA)2PbI4, (Rac-4-Br-MBA)2PbI4, and (R-4-Br-MBA)2PbI4 (Rac-MBA: racemic methylbenzylammonium), which crystallize in a lower symmetry monoclinic phase. For comparison, orthorhombic (R-MBA)2PbI4 does not exhibit XD emission. XD has a lower energy than XB, with energy difference ΔE. In monoclinic samples, ΔE ∼ 20 meV is large enough to suppress the thermal excitation of XD to XB, at temperatures <30 K. Consequently, XD recombines by emitting light with a long lifetime (∼205 ns). At higher temperatures, the emission switches to the spin-allowed XB (lifetime < 1 ns).

Rational Design of Non-Centrosymmetric Hybrid Halide Perovskites.

Structural non-centrosymmetry in semiconducting organic-inorganic hybrid halide perovskites can introduce functionalities like anomalous photovoltaics and nonlinear optical properties. Here we introduce a design principle to prepare Pb- and Bi-based two- and one-dimensional hybrid perovskites with polar non-centrosymmetric space groups. The design principle relies on creating dissimilar hydrogen and halogen bonding non-covalent interactions at the organic-inorganic interface. For example, in organic cations like I-(CH2)3-NH2(CH3)+ (MIPA), -CH3 is substituted by -CH2I at one end, and -NH3+ is substituted by -NH2(CH3)+ at the other end. These substitutions of two -H atoms by -I and -CH3 reduce the rotational symmetry of MIPA at both ends, compared to an unsubstituted cation, for example, H3C-(CH2)3-NH3+. Consequently, the dissimilar hydrogen-iodine and iodine-iodine interactions at the organic-inorganic interface of (MIPA)2PbI4 2D perovskites break the local inversion symmetries of Pb-I octahedra. Owing to this non-centrosymmetry, (MIPA)2PbI4 displays visible to infrared tunable nonlinear optical properties with second and third harmonic generation susceptibility values of 5.73 pm V-1 and 3.45 × 10-18 m2 V-2, respectively. Also, the single crystal shows photocurrent on shining visible light at no external bias, exhibiting anomalous photovoltaic effect arising from the structural asymmetry. The design strategy was extended to synthesize four new non-centrosymmetric hybrid perovskite compounds. Among them, one-dimensional (H3N-(CH2)3-NH(CH3)2)BiI5 shows a second harmonic generation susceptibility of 7.3 pm V-1 and a high anomalous photovoltaic open-circuit voltage of 22.6 V.

Effect of chirality on the optical properties of layered hybrid perovskite R- and S-α-methylbenzylammonium lead iodide.

The introduction of chirality in layered hybrid perovskites breaks the symmetry of their inorganic sub-lattices. Consequently, they show intriguing linear and non-linear optical properties. Here we explore the effect of chirality on the excitonic photoluminescence of chiral (R- and S-α-MBA)2PbI4 (MBA: methylbenzylammonium) at cryogenic temperatures. The induced chirality splits the excitonic emissions below 150 K. Additionally, (R- and S-α-MBA)2PbI4 show wavelength-tunable second harmonic generation (SHG) that depends strongly on the polarization angle of the incident light.

Colloidal BaZrS3 chalcogenide perovskite nanocrystals for thin film device fabrication.

The theoretical optoelectronic properties of chalcogenide perovskites (e.g., BaZrS3) are as good as those of halide perovskites (e.g., CH3NH3PbI3). But the fabrication of optoelectronic devices is rarely reported, mainly because researchers still do not know how to prepare good quality thin films of chalcogenide perovskites. Here, we report colloidal BaZrS3 nanocrystals (NCs, 40-60 nm) and their solution processed thin film transistors. BaZrS3 NCs are first prepared using a solid-state synthesis route, and the subsequent surface modifications lead to a colloidal dispersion of NCs in both polar N-methyl-2-pyrrolidinone and non-polar chloroform solvents. The NCs exhibit good thermal (15-673 K) and aqueous stability. Colloidal BaZrS3 NCs in chloroform are then used to make field effect transistors showing ambipolar properties with a hole mobility of 0.059 cm2 V-1 s-1 and an electron mobility of 0.017 cm2 V-1 s-1. This report of solution processed chalcogenide perovskite thin films with reasonable carrier mobility and optical absorption and emission is expected to pave the way for future optoelectronic devices of chalcogenide perovskites.