We explore the relationship between the morphology and ionic conductivity of block copolymer electrolytes over a wide range of salt concentrations for the system polystyrene-blockpoly(ethylene oxide) (PS-b-PEO, SEO) mixed with lithium bis-(trifluoromethanesulfonyl)imide salt (LiTFSI). Two SEO polymers were studied, SEO(16−16) and SEO(4.9−5.5), over the salt concentration range r = 0.03−0.55. The numbers x and y in SEO(x−y) are the molecular weights of the blocks in kg mol −1 , and the r value is the molar ratio of salt to ethylene oxide moieties. Smallangle X-ray scattering was used to characterize morphology and grain size at 120°C, differential scanning calorimetry was used to study the crystallinity and the glass transition temperature of the PEO-rich microphase, and ac impedance spectroscopy was used to measure ionic conductivity as a function of temperature. The most surprising observation of our study is that ionic conductivity in the concentration regime 0.11 ≤ r ≤ 0.21 increases in SEO electrolytes but decreases in PEO electrolytes. The maximum in ionic conductivity with salt concentration occurs at about twice the salt concentration in SEO (r = 0.21) as in PEO (r = 0.11). We propose that these observations are due to the effect of salt concentration on the grain structure in SEO electrolytes.

Solution-processable organic metal halide Ruddlesden− Popper phases have shown promise in optoelectronics because of their efficiencies in solar cells along with increased material stability relative to their three-dimensional counterparts (CH 3 NH 3 PbI 3 ). Here, we study the layered material butylammonium methylammonium lead iodide (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) n−1 Pb n I 3n+1 for values of n ranging from 1 to 4. Thin films cast from solution show a gradual change in the crystalline texture of the two-dimensional lead iodide layers from being parallel to the substrate to perpendicular with increasing n. Contactless timeresolved microwave conductivity measurements show that the average recombination rate order increases with n and that the yield−mobility products and carrier lifetimes of these thin films are much lower than that of CH 3 NH 3 PbI 3 , along with increased higher-order recombination rate constants.

Single-ion conducting block copolymers, such as poly(ethylene oxide)-b-poly[(styrene-4-sulfonyltrifluoromethylsulfonyl)imide lithium] (PEO−P[(STFSI)Li]), represent an exciting new class of materials capable of improving the performance of solid-state batteries with metal anodes. In this work, we report on the synthesis and characterization of a matched set of lithiated (PEO−P[(STFSI)Li]) and magnesiated (PEO−P[(STFSI) 2 Mg]) single-ion conducting diblock copolymers. We measure the temperature dependence of ionic conductivity, and through analysis using the Vogel−Tamman− Fulcher (VTF) relation, demonstrate that ion dissociation is significantly lower for all PEO−P[(STFSI) 2 Mg] samples when compared to their PEO−P[(STFSI)Li] counterparts. The VTF parameter characterizing the activation barrier to ion hopping was similar for both cations, but the VTF prefactor that reflects effective charge carrier concentration was higher in the lithiated samples by an order of magnitude. We study the melt morphology of the single-ion conducting block copolymers using temperature-dependent X-ray scattering and use the mean-field theory of Leibler to extract the effective Flory−Huggins interaction parameter (χ) for PEO/P[(STFSI)Li] and PEO/P[(STFSI) 2 Mg] from the X-ray scattering data. We demonstrate a linear relationship between the charge-concentration-related VTF parameter and the parameter quantifying the enthalpic contribution to χ. It is evident that ion dissociation and block copolymer thermodynamics are intimately coupled; ion dissociation in these systems suppresses microphase separation.

Organic metal halide Ruddlesden–Popper layered perovskite phases combine the excellent optoelectronic properties of three-dimensional, bulk hybrid perovskites with superior material stability under ambient conditions. However, the thin film structure of these layered perovskites is still poorly understood, as phase purity is typically determined solely by specular X-ray diffraction. The thin film structure of these Ruddlesden–Popper phases was examined by increasingly local characterization techniques. From the comparison of grazing-incidence wide-angle X-ray scattering patterns of cast films to expected scattering from single-crystal structures, significant in-plane disorder was observed. Spatially localized photoluminescence measurements show that films do not phase separate on the micrometer scale. Selected area electron diffraction measurements show the intergrowth of different phases within the same thin film, consistent with previous observations seen in epitaxially grown Ruddlesden–Popper complex oxides. Despite the presence of phase impurities that would typically be detrimental for device performance, fits to photothermal deflection spectroscopy measurements show relatively low Urbach energies of 33 meV for (C4H9NH3)2(CH3NH3)2Pb3I10 and 32 meV for (C4H9NH3)2(CH3NH3)3Pb4I13, indicating that the electronic properties are insensitive to the phase impurities.

Recently, unconventional bright magnetic dipole (MD) radiation was observed from two-dimensional (2D) hybrid organic–inorganic perovskites (HOIPs). According to commonly accepted HOIP band structure calculations, such MD light emission from the ground-state exciton should be strictly symmetry forbidden. These results suggest that MD emission arises in conjunction with an as-yet unidentified symmetry-breaking mechanism. In this paper, we show that MD light emission originates from a self-trapped p-like exciton stabilized at energies below the primary electric dipole (ED)-emitting 1s exciton. Using suitable combinations of sample and collection geometries, we isolate the distinct temperature-dependent properties of the ED and MD photoluminescence (PL). We show that the ED emission wavelength is nearly constant with temperature, whereas the MD emission wavelength exhibits substantial red shifts with heating. To explain these results, we derive a microscopic model comprising two distinct parity exciton states coupled to lattice distortions. The model explains many experimental observations, including the thermal red shift, the difference in emission wavelengths, and the relative intensities of the ED and MD emission. Thermodynamic analysis of temperature-dependent PL reveals that the MD emission originates from a locally distorted structure. Finally, we demonstrate unusual hysteresis effects of the MD-emitting state near structural phase transitions. We hypothesize that this is another manifestation of the local distortions, indicating that they are insensitive to phase changes in the equilibrium lattice structure.

Light-matter interactions in semiconductor systems are uniformly treated within the electric dipole (ED) approximation, as multipolar interactions are considered "forbidden". Here, we demonstrate that this approximation inadequately describes light emission in novel two-dimensional hybrid organic-inorganic perovskite materials (2D HOIPs) -a class of solution processable layered semiconductor with promising optoelectronic properties. Consequently, photoluminescence (PL) spectra become strongly dependent on the experimental geometry, a fact that is often overlooked, though critical for correct optical characterization of materials. Using energy-momentum and time-resolved spectroscopies, we experimentally demonstrate that low-energy sideband emission in 2D HOIPs exhibits a highly unusual, multipolar polarization and angle dependence. Using combined electromagnetic and quantum-mechanical analyses, we attribute this radiation pattern to an out-of-plane oriented magnetic dipole transition arising from the 2D character of the excited and ground state orbitals. Symmetry arguments point toward the presence of significant inversion symmetry-breaking mechanisms that are currently under great debate. These results provide a new perspective on the origins of unexpected sideband emission in HOIPs, clarify discrepancies in previous literature, and generally challenge the paradigm of ED-dominated light-matter interactions in novel optoelectronic materials. 1 arXiv:1901.05136v2 [cond-mat.mtrl-sci]

Time-resolved microwave conductivity reveals good carrier mobility and long carrier lifetimes in the layered organic metal halide compound (CH3NH3)2Pb(SCN)2I2.

Layered hybrid organic–inorganic perovskites such as the lead halide Ruddlesden–Popper (RP) series are solution-processable two-dimensional (2D) materials with tunable optoelectronic properties. Dynamic interactions between the ionic perovskite substructure and organic spacer cations impact optoelectronic properties relevant for device applications. Here, the static and dynamic structures of linear alkylammonium and aromatic spacers in lead iodide RP phases (n = 1) are characterized at ambient temperatures using solid-state NMR (ssNMR) spectroscopy and compared with previously reported crystal structures derived from X-ray diffraction. Rigid and flexible sites of spacers are distinguished by examining 13C­{1H} and 15N­{1H} cross-polarization magic-angle spinning (CP-MAS) signal intensity build-up. Different trends in site-specific rigidity are observed for short and long alkylammonium spacers. Short spacers (e.g., butylammonium) are attached by strong affinity interactions to lead iodide octahedra, whereas longer spacers (e.g., dodecylammonium) are more rigid within the RP interlayer than near the octahedral surface. Phenethylammonium and butylammonium spacers are similarly rigid, and we estimate that the local reorientation time scale of phenyl rings is 10–100 μs by 2D 13C CP-variable contact (CP-VC) experiments. These ssNMR results indicate that the interplay between spacer interactions with lead iodide octahedra (Coulombic and hydrogen-bonding) and van der Waals forces between spacers is responsible for a variety of site-specific dynamics and local structural distortions at intermediate time scales (microsecond to millisecond). This study demonstrates a general method to characterize nanoscale structures and site-specific dynamics that contribute to structural and electronic disorder in functional optoelectronic RP phases.