Márton Kollár

Clean, cleaved surfaces of the photovoltaic perovskite

The surface of a material is not only a window into its bulk physical properties, but also hosts unique phenomena important for understanding the properties of a solid as a whole. Surface sensitive techniques, like ARPES (Angle-resolved photoemission spectroscopy), STM (Scanning tunneling microscopy), AFM (Atomic force microscopy), pump-probe optical measurements etc. require flat, clean surfaces. These can be obtained by cleaving, which is usually possible for layered materials. Such measurements have proven their worth by providing valuable information about cuprate superconductors, graphene, transition metal dichalcogenides, topological insulators and many other novel materials. Unfortunately, this was so far not the case for the cubic, organo-metallic photovoltaic perovskite which morsels during the cleavage. Here we show a method which results in flat, clean surfaces of CH3NH3PbBr3 which allows surface sensitive measurements, badly needed for the understanding and further engineering of this material family.

Mechanical response of CH3NH3PbI3 nanowires

We report a systematic study of the mechanical response of methylammonium lead triiodide CH3NH3PbI3 nanowires by employing bending measurements using atomic force microscope on suspended wires over photo-lithographically patterned channels. Force-deflection curves measured at room temperature give a Youngs modulus between 2 and 14u2009GPa. This broad range of values is attributed to the variations in the microcrystalline texture of halide perovskite nanowires. The mechanical response of a highly crystalline nanowire is linear with force and has a brittle character. The braking modulus of 48u2009±u200920u2009MPa corresponds to 100u2009μm of radius of curvature of the nanowires, rendering them much better structures for flexible devices than spin coated films. The measured moduli decrease rapidly if the NW is exposed to water vapor.

Influence of the organic cation disorder on photoconductivity in ethylenediammonium lead iodide, NH3CH2CH2NH3PbI4

We report the synthesis and crystal structure of an organic–inorganic compound, ethylenediammonium lead iodide, NH3CH2CH2NH3PbI4. Synchrotron-based single crystal X-ray diffraction experiments revealed that the pristine and thermally treated crystals differ in the organic cation behaviour, which is characterized by a partial disorder in the thermally treated crystal. Based on current–voltage measurements, increased disorder of the organic cation is associated with enhanced photoconductivity. This compound could be a potential candidate for interface engineering in lead halide perovskite-based optoelectronic devices.

High-pressure transformation of MAPbI3: role of the noble-gas medium

It is well known that noble gases are relatively inert which justifies their use as pressure-transmitting medium (PTM) in highpressure (HP) studies. Recent reports [1] indicate that various noble gases taken as PTM apparently influence the outcome of structural transformations. We reach the same conclusion by a systematic study of the structural changes of methyl-ammonium lead iodide, CH3NH3PbI3 (MAPbI3), a photovoltaic perovskite [2] with pressure by choosing Ne and Ar as two PTMs. The single crystal XRD experiments were performed using synchrotron radiation at room temperature up to 20 GPa. The crystal structure of MAPbI3 consists of a framework of corner-sharing PbI6 octahedra and methyl-ammonium cations, CH3NH3+ (MA), which is rotating in the cuboctahedral surrounding of twelve I-atoms. Weak hydrogen bonds statistically appeared between MA and I atoms give a high flexibility to the structure, which depends, in particular, on temperature and pressure. The large structural interstices offer the possibility to host other molecules which can alter the hydrogen bonds and destabilise the structure (see e.g., the case of water molecules [3]). We found the following structural consequences of the incorporation of Ne and Ar PTM into the interstices at HP. (i) The first-order phase transition from the tetragonal phase at ambient pressure (unit cell parameters a ≈ 8.8 Å, c ≈ 12.7 Å) to the (pseudo)cubic HP-phase (unit cell parameter a ≈ 12.3 Å) starts at pressures of about 1 and 0.1 GPa for Ar and Ne PTM, respectively. (ii) Above these pressures, a phase amorphization starts at about 1.5 GPa for Ar and 4 GPa for Ne. It is documented that a crystal of poor quality (high patchiness, inter-domain disorientation) is irreversibly amorphized, while a higher quality single crystal preserves partially a crystalline state up to 20 GPa for Ne-PTM. (iii) Ne-PTM series shows the pressure dependent incorporation of Ne and the loss of MA (see a graphic illustration). The transformation of the crystal composition can be expressed as: MAPbI3→Ne(x).MAPbI3→Ne(x).MA(1y).H(y)PbI3→Ne(x).HPbI3. The decompressed crystal shows NeMAPbI3 composition even after four weeks upon the decompression. Ar is assimilated by the structure to form Ar1.4MAPbI3 at 2.4 GPa and a complete, irreversible amorphisation follows at about 3.5 GPa. The difference between Ne and Ar used PTM is related to the difference in their atomic radii (0.38 and 0.71 Å for Ne and Ar, respectively) and to their different ability to enter in “chemical reaction” in the restricted space between the PbI6 octahedra. Acknowledgment. This work was performed in collaboration with Eleonora Polini, Laura Henry, Dmitry Chernyshov, Andrzej Sienkiewicz, Gaetan Giriat, Anastasiia Glushkova. It is supported by the ERC Advanced Grant Picoprop of L.F.