The phase diagram elucidates structural changes and phase separation effects, induced by halide substitution in hybrid perovskite MAPb(I,Br)3 solid solution.
Excellent conversion efficiencies of over 20 % and facile cell production have placed hybrid perovskites at the forefront of novel solar cell materials, with CH3NH3PbI3 being an archetypal compound. The question why CH3NH3PbI3 has such extraordinary characteristics, particularly a very efficient power conversion from absorbed light to electrical power, is hotly debated, with ferroelectricity being a promising candidate. This does, however, require the crystal structure to be non‐centrosymmetric and we herein present crystallographic evidence as to how the symmetry breaking occurs on a crystallographic and, therefore, long‐range level. Although the molecular cation CH3NH3+ is intrinsically polar, it is heavily disordered and this cannot be the sole reason for the ferroelectricity. We show that it, nonetheless, plays an important role, as it distorts the neighboring iodide positions from their centrosymmetric positions.
This paper discusses the full structural solution of the hybrid perovskite formamidinium lead tribromide (FAPbBr3) and its temperature‐dependent phase transitions in the range from 3 K to 300 K using neutron powder diffraction and synchrotron X‐ray diffraction. Special emphasis is put on the influence of deuteration on formamidinium, its position in the unit cell and disordering in comparison to fully hydrogenated FAPbBr3. The temperature‐dependent measurements show that deuteration critically influences the crystal structures, i.e. results in partially‐ordered temperature‐dependent structural modifications in which two symmetry‐independent molecule positions with additional dislocation of the molecular centre atom and molecular angle inclinations are present.
The hybrid halide perovskites MAPbI 3 , MAPbI 2.94 Cl 0.06 , and MAPbCl 3 (MAmethylammonium) have been investigated using inelastic and quasielastic neutron scattering (QENS) with the aim of elucidating the impact of chloride substitution on the rotational dynamics of MA. In this context, we discuss the influence of the inelastic neutron scattering caused by low-energy phonons on the QENS resulting from the MA rotational dynamics in MAPbI 3-x Cl x . Through a comparative temperature-dependent QENS investigation with different energy resolutions, which allow a wide Fourier time window, we achieved a consistent description of the influence of chlorine substitution in MAPbI 3 on to the MA dynamics. Our results show that chlorine substitution in the low temperature orthorhombic phase leads to a weakening of the hydrogen bridge bonds since the characteristic relaxation times of C 3 rotation at 70 K in MAPbCl 3 (135 ps) and MAPbI 2.94 Cl 0.06 (485 ps) are much shorter than in MAPbI 3 (1635 ps). For the orthorhombic phase, we obtained the activation energies from the temperature-dependent characteristic relaxation times τ C3 by Arrhenius fits indicating lower values of E a for MAPbCl 3 and MAPbI 2.94 Cl 0.06 compared to MAPbI 3 . We also performed QENS analyses at 190 K for all three samples. Here we observed that MAPbCl 3 shows slower MA rotational dynamics than MAPbI 3 in the disordered structure. FIGURE 1Visualization of the orthorhombic crystal structures of CH 3 NH 3 PbI 3 (a, b, and c) at 100K 21 and CH 3 ND 3 PbCl 3 (d, e, f, and g) at 80 K 23 (note that for MAPbCl 3 only partially deuterated structural data is available) as well as the MA molecule C 3 jump rotation movement (c, e, and f). Due to the hydrogen (deuterium) bonds between NH 3 (ND 3 ) and the halide atoms in the orthorhombic crystal structures, the orientation of the C-N axes is fixed and leads to distortions of the crystal lattice. In a), b), d), and g) only the C-N atoms and their orientation are shown together with the respective 12-fold coordination polyhedra. Because of the doubled unit cell of CH 3 ND 3 PbCl 3 (compared to the iodide) two different MA molecules can be identified: MA molecule 1 (e; blue color) and MA molecule 2 (f; red color). In MAPbI 3 , however, there is only one MA molecule (c) that occurs in four different orientations (a, b). Due to the double number of MA molecules in CH 3 ND 3 PbCl 3 there are a total of 8 different MA orientations (d, and g). In CH 3 ND 3 PbCl 3 the short N-Cl bond lengths are different in the two MA molecules: MA 1 (Cl2…N1 = 3.273(7) Å and Cl1-N1 = 3.336(11) Å) and MA 2 (Cl3…N2 = 3.300(6) Å and Cl4-N2 = 3.346(10) Å). Also the polyhedral volumes are different, V MA1 = 155.8 Å 3 , V MA2 = 149.4 Å 3 . In MAPbI 3 we can identify the following short N-I bond lengths: I2-N = 3.6804 Å and I1-N = 3.6113 Å. The polyhedral volume in MAPbI 3 is V MA = 206.9 Å 3 . The figures c, e, and f also show the C 3 three-fold jump rotation movement (blue arrow) of the MA molecules around the C-N axis, which is discussed in ...
Applications of nitrogen-vacancy (NV) centers in diamond in quantum technology have attracted considerable attention in recent years. Deterministic generation of ensembles of NV centers can advance the research on quantum sensing, many-body quantum systems, multipartite entanglement and so on. Here we report the complete process of controlled generation of NV centers in diamond as well as their characterisation: growing diamond films through chemical vapor deposition (CVD), ion implantation and spectroscopic characterization of the defect centers using a confocal microscope. A microwave-assisted CVD set-up is presented which we constructed for the preparation of single-crystalline homoepitaxial diamond films. The films were prepared with minimized nitrogen concentration, which is confirmed through photoluminescence measurements. We demonstrate an in situ ultra high vacuum (UHV) implantation and heating process for creation of NV centers using a novel experimental set-up. For the first time hot implantation has been shown which prevents surface charging effects. We do not observe graphitization due to UHV heating. By optimizing the implantation parameters it has been possible to implant NV centers in a precise way. We present large area mapping of the samples to determine the distribution of the centers and describe the characterization of the centers by spectroscopic techniques. Reducing the decoherence caused by environmental noise is of primary importance for many applications in quantum technology. We demonstrate improvement on coherence time T2 of the NV spins by suppression of their interaction with the surrounding spin-bath using robust dynamical decoupling sequences.
In recent years, inorganic cesium-lead-halide perovskites, CsPbX3 (X=I, Br, Cl), have attracted 2 interest for optoelectronic applications such as highly efficient thin-film light-emitting diodes or wide-gap absorber materials for photovoltaics. However, phase segregation and secondary phases in as-deposited thin films are still considered to be limiting factors for devices based on CsPbX3. Here, we report a correlative electron microscopy and spectroscopy approach for the identification of secondary phases and their distributions in Cs-Pb-Br thin films, deposited by solution-based and coevaporation methods on various substrates. We identified phases by their compositional, structural, and optoelectronic properties, using X-ray diffraction, spectroscopy and a variety of microscopy techniques. We found that the Cs-Pb-Br films contain ternary Cs4PbBr6 and CsPb2Br5 phases in addition to CsPbBr3, a finding consistent with calculations of formation enthalpies by means of density functional theory showing that these values are very similar for the three ternary phases. We find that these phases can exhibit different spatial distributions inside the film and discuss the influence of the deposition method and synthesis parameters on the resulting phase composition of the Cs-Pb-Br layers.
The halide perovskite CsPbBr3 belongs to the Cs‐Pb‐Br material system, which features two additional thermodynamically stable ternary phases, Cs4PbBr6 and CsPb2Br5. The coexistence of these phases and their reportedly similar photoluminescence (PL) have resulted in a debate on the nature of the emission in these systems. Herein, optical and microscopic characterizations are combined with an effective mass, correlated electron–hole model of excitons in confined systems, to investigate the emission properties of the ternary phases in the Cs‐Pb‐Br system. It is found that all Cs‐Pb‐Br phases exhibit green emission and the non‐perovskite phases exhibit PL quantum yields orders of magnitude larger than CsPbBr3. In particular, blue‐ and red‐shifted emission for the Cs‐ and Pb‐rich phases, respectively, are measured, stemming from embedded CsPbBr3 nanocrystals (NCs). This model reveals that the difference in emission shift is caused by the combined effects of NC size and different band mismatch. Furthermore, the importance of including the dielectric mismatch in the calculation of the emission energy for Cs‐Pb‐Br composites is demonstrated. The results explain the reportedly limited blue shift in CsPbBr3@Cs4PbBr6 composites and rationalize some of its differences with CsPb2Br5.
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