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Chemical vapor deposition (CVD) offers a large-area, scalable, and conformal growth of perovskite thin films without the use of solvents. Low-dimensional organic–inorganic halide perovskites, with alternating layers of organic spacer groups and inorganic perovskite layers, are promising for enhancing the stability of optoelectronic devices. Moreover, their multiple quantum-well structures provide a powerful platform for tuning excitonic physics. In this work, we show that the CVD process is conducive to the growth of 2D hybrid halide perovskite films. Using butylammonium (BA) and phenylethylammonium (PEA) cations, the growth parameters of BA2PbI4and PEA2PbI4and mixed halide perovskite films were first optimized. These films are characterized by well-defined grain boundaries and display characteristic absorption and emission features of the 2D quantum wells. X-ray diffraction (XRD) and a noninteger dimensionality model of the absorption spectrum provide insights into the orientation of the crystalline planes. Unlike BA2PbI4, temperature-dependent photoluminescence measurements from PEA2PbI4show a single excitonic peak throughout the temperature range from 20 to 350 K, highlighting the lack of defect states. These results further corroborate the temperature-dependent synchrotron-based XRD results. Furthermore, the nonlinear optical properties of the CVD-grown perovskite films are investigated, and a high third harmonic generation efficiency is observed.

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While the layered hybrid Ruddlesden-Popper (RP) halide perovskites have already established themselves as the frontrunners among the candidates in optoelectronics, their all-inorganic counterparts remain least explored in the RP-type perovskite family. Herein, we study and compare the optoelectronic properties of all-inorganic CsPbBr3perovskite nanocrystals (PNCs) with and without RP planar faults. We find that the RP-CsPbBr3PNCs possess both higher exciton binding energy and longer exciton lifetimes. The former is ascribed to a quantum confinement effect in the PNCs induced by the RP faults. The latter is attributed to a spatial electron–hole separation across the RP faults. A striking difference is found in the up-conversion photoluminescence response in the two types of CsPbBr3PNCs. For the first time, all-inorganic RP-CsPbBr3PNCs are tested in light-emitting devices and shown to significantly outperform the non-RP CsPbBr3PNCs.

Abstract: Chemical vapor deposition (CVD), a low-cost and a scalable deposition technique, allows the growth of methylammonium lead iodide (MAPbI3) films without the use of solvents, substantially increasing air stability while also inducing the stable cubic phase at room temperature and at pressures as low as 0.25 GPa. MAPbI3thin films were grown by a facile two-step low-pressure vapor deposition process in a single reactor. This method results in films, which are usually in the tetragonal phase (space group: I4/mcm) and occasionally in the cubic phase under ambient conditions. High-pressure synchrotron-based X-ray diffraction studies from CVD-grown MAPbI3crystallites show that the sample remains in the cubic phase (space group: Im\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{3}$$\end{document}) between 0.25 and 3.0 GPa. Temperature-dependent transport measurements show sharp anomalies, correlating with the structural changes. The transport measurements from the CVD-grown cubic MAPI3film is further compared with a film in the tetragonal phase. Graphic Abstract:

The use of polymer ferroelectric dielectrics in organic field-effect transistors (FETs) for nonvolatile memory application was demonstrated more than 15 years ago. The ferroelectric dielectric polyvinylidene fluoride (PVDF) and its copolymers are most widely used for such applications. In addition to memory applications, polymer ferroelectrics as a dielectric layer in organic FETs yield insights into interfacial transport properties. Advantages of polymer ferroelectric dielectrics are their high dielectric constant compared to other polymer dielectrics and their tunable dielectric constant with temperature. Further, the polarization strength may also be tuned by externally poling the ferroelectric dielectric layer. Thus, PVDF and its copolymers provide a unique testbed not just for investigating polarization induced transport in organic FETs, but also enhancing device performance. This article discusses recent developments of PVDF-based ferroelectric organic FETs and capacitors with a focus on tuning transport properties. It is shown that FET carrier mobilities exhibit a weak temperature dependence as long as the dielectric is in the ferroelectric phase, which is attributed to a polarization fluctuation driven process. The low carrier mobilities in PVDF-based FETs can be enhanced by tuning the poling condition of the dielectric. In particular, by using solution-processed small molecule semiconductors and other donor–acceptor copolymers, it is shown that selective poling of the PVDF-based dielectric layer dramatically improves FET properties. Finally, the prospects of further improvement in organic ferroelectric FETs and their challenges are provided.

Lead halide perovskites have a rich landscape of structural and optical properties, which can be explored and possibly controlled by applying high pressure. Despite several reports on high-pressure studies of CsPbBr3nanocrystals (NCs), there have so far been no studies under pressure that incorporate planar defects. CsPbBr3NCs with Ruddlesden–Popper (RP) faults, formed via post-synthetic fusion growth, are significantly larger in size than as-synthesized NCs and display exceptional emission stability. Here, we compare synchrotron-based high-pressure X-ray diffraction and photoluminescence (PL) properties of CsPbBr3(without RP) and RP-CsPbBr3(with RP) and resolve their crystal structure under pressure for the first time. CsPbBr3undergoes a phase transition from the orthorhombic Pnmaphase at ambient pressure to the cubic Pm3̅mphase at 1.7 GPa, and RP-CsPbBr3transforms from Pnmato the monoclinic P21/mphase at 0.74 GPa in addition to several isostructural transitions. Density-functional calculations predict a narrowing of the band gap with pressure, concomitant with the PL energies. The RP-CsPbBr3NCs exhibit enhanced PL intensity at 1 GPa and show band gap opening at high pressures. This study opens new strategies for not only tuning just the structural properties but also tuning planar defects in alkali halide lead crystals for improved optical properties.

Copolymers based on diketopyrrolopyrrole (DPP) cores have attracted a lot of attention because of their high p-type as well as n-type carrier mobilities in organic field-effect transistors (FETs) and high power conversion efficiencies in solar cell structures. We report the structural and charge transport properties of n-dialkyl side-chain-substituted thiophene DPP end-capped with a phenyl group (Ph-TDPP-Ph) monomer in FETs which were fabricated by vacuum deposition and solvent coating. Grazing-incidence X-ray diffraction (GIXRD) from bottom-gate, bottom-contact FET architectures was measured with and without biasing. Ph-TDPP-Ph reveals a polymorphic structure with π-conjugated stacking direction oriented in-plane. The unit cell comprises either one monomer with a= 20.89 Å, b= 13.02 Å, c= 5.85 Å, α = 101.4°, β = 90.6°, and γ = 94.7° for one phase (TR1) or two monomers with a= 24.92 Å, b= 25.59 Å, c= 5.42 Å, α = 80.3°, β = 83.5°, and γ = 111.8° for the second phase (TR2). The TR2 phase thus signals a shift from a coplanar to herringbone orientation of the molecules. The device performance is sensitive to the ratio of the two triclinic phases found in the film. Some of the best FET performances with p-type carrier mobilities of 0.1 cm2/V s and an on/off ratio of 106are for films that comprise mainly the TR1 phase. GIXRD from in operando FETs demonstrates the crystalline stability of Ph-TDPP-Ph.

The semiconducting bithiophene systems are largely unexplored structurally at high-pressures. Here, we characterize the high-pressure structure and phase behavior of 5,5′-bis(naphth-2-yl)-2,2′-bithiophene (NaT2) by single crystal X-ray diffraction, photoluminescence and Raman compression measurements in the diamond anvil cell to 7.6 GPa. Experimental results are supported by empirical interaction energy calculations. NaT2 maintains its ambient pressure structure (P21/c) on compression but undergoes a subtle second-order phase transition at ca. 3.5 GPa. The newly formed phase at 4.0 GPa is characterized by the formation of a new S···H interaction between thiophene moieties in adjacent molecules, as well as by the emergence of a more favorable compression pathway in its long-chain direction. NaT2 also undergoes a band gap closure manifested as a distinct yellow to red color change on compression that is fully reversible on decompression.