Although formamidinium lead iodide (FAPbI3) perovskite has shown great promise in the field of perovskite-based optoelectronic devices, it suffers the complications of a structural phase transition from a black perovskite phase (α-FAPbI3) to a yellow non-perovskite phase (δ-FAPbI3). Generally, it is pivotal to avoid δ-FAPbI3 since only α-FAPbI3 is desirable for photoelectric conversion and near-infrared (NIR) emission. However, herein, we firstly exploited the undesirable δ-FAPbI3 to enable structurally stable, pure FAPbI3 films with a controllable α/δ phase junction at low annealing temperature (60 °C) through stoichiometrically modified precursors (FAI/PbI2 = 1.1–1.5). The α/δ phase junction contributes to a striking stabilization of the perovskite phase of FAPbI3 at low temperature and significantly enhanced NIR emission at 780 nm, which is markedly different from pure α-FAPbI3 (815 nm). In particular, the optimal α/δ phase junction with FAI/PbI2 = 1.2 exhibited preferable long-term stability against humidity and high PLQY of 6.9%, nearly 10-fold higher than that of pure α-FAPbI3 (0.7%). The present study opens a new approach to realize highly stable and efficient emitting perovskite materials by utilizing the phase junctions.

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Stable α/δ phase junction of formamidinium lead iodide perovskites for enhanced near-infrared emission

This study presents the electrical properties, complex impedance analysis and dielectrical behavior of La05Ca05−xAgxMnO3 manganites with compositions below the concentration limit of silver solubility in perovskites (0 ≤ x ≤ 02) Transport measurements indicate that all the samples have a semiconductor-like behavior The metal-semiconductor transition is not observed across the whole temperature range explored [80 K–700 K] At a specific temperature, a saturation region was marked in the σ (T) curves We obtained a maximum σdc value at ambient temperature with the introduction of 20% Ag content Two hopping models were applied to study the conduction mechanism We found that activation energy (Ea) related to ac-conductivity is lower than the Ea implicated in dc-conductivity Complex impedance analysis confirms the contribution of grain boundary to conductivity and permits the attribution of grain boundary capacitance evolution to the temperature dependence of the barrier layer width From the temperature dependence of the average normalized change (ANC), we deduce the temperature at which the available density of trapped charge states vanishes Such a temperature is close to the temperature at which the saturation region appears in σ(T) curves Moreover, complex impedance analysis (CIA) indicates the presence of electrical relaxation in materials It is noteworthy that relaxation species such as defects may be responsible for electrical conduction The dielectric behavior of La05Ca05−xAgxMnO3 manganites has a Debye-like relaxation with a sharp decrease in the real part of permittivity at a frequency where the imaginary part of permittivity (e′′) and tg δ plots versus frequency demonstrate a relaxation peak The Debye-like relaxation is explained by Maxwell–Wagner (MW) polarization Experimental results are found to be in good agreement with the Smit and Wijn theory

Conduction mechanism, impedance spectroscopic investigation and dielectric behavior of La0.5Ca0.5-xAgxMnO3 manganites with compositions below the concentration limit of silver solubility in perovskites (0 ≤ x ≤ 0.2).

CaCu 3 Ti 4 O 12 (CCTO) ceramics with high dielectric constant (2-4 x 10 4 ) and low loss (0.04) were prepared by the sol-gel process and sintered at 1050°C for different times. The sintering time has a sensitive influence on the values of the dielectric constant and nonlinear coefficient. Tailored dielectric constant and nonlinear coefficient can be obtained by selecting a suitable sintering time according to different desired device application. The result of current-voltage characteristics and Cole-Cole plots in a broad temperature range (60-400 K) provide more effective evidence of the high dielectric constant supported by the grain boundary barrier layer (GBBL) capacitors model. Below 150 K, the GBBL capacitors effect weakens and gradually disappears with further decrease of temperature, thus leading the dielectric constant to decrease rapidly. Two values of grain activation energy acting at different temperature for each sample were obtained.

Structure, Properties, and Impedance Spectroscopy of CaCu3Ti4O12 Ceramics Prepared by Sol–Gel Process

In this work a comparative study of undoped CaCu3Ti4O12 (CCTO) and doped with Fe3+(CCTOF) and Nb5+(CCTON) ceramics, was aimed to modify the electronic transport. XRD patterns, FE-SEM microstructural analysis, impedance spectroscopy and I–V response curves were afforded to correlate the microstructure with the nonlinear I–V behaviour. The appearance of nonlinear behaviour in doped CCTO samples has been correlated with the ceramic microstructure that consists in n-type semiconductor grains, surrounded by a grain boundary phase based on CuO. The presence of this secondary grain boundary phase is the responsible of the assisted liquid phase sintering in CCTO ceramics. Doped samples showed cleaner grain boundaries than CCTO and nonlinearity in the I–V response.

Nonlinear I–V electrical behaviour of doped CaCu3Ti4O12 ceramics

A combined powder X-ray lattice parameter and ceramic impedance spectroscopy study is presented on materials within the CaO–CuO–TiO2 ternary phase diagram. Several compositions containing CaCu3Ti4O12 (CCTO) and small amounts of secondary phases such as TiO2, CaTiO3 and CuO are analysed and two different defect mechanisms are identified as the cause of the non-stoichiometry in CCTO. The first mechanism involves a variation in the Cu content, which explains the large differences in the intrinsic bulk and extrinsic grain boundary (GB) resistance, and the formation of the ceramic internal barrier layer capacitor (IBLC) structure. The second mechanism is associated with Ca–Cu anti-site disorder causing an unusually high intrinsic bulk permittivity above that predicted from Clausius–Mossotti calculations.

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Non-stoichiometry in “CaCu3Ti4O12” (CCTO) ceramics

The CaCu3Ti4O12 (CCTO) compound shows an unusually high and almost temperature independent dielectric constant at low frequencies. CCTO powders have been synthesized by an organic gel-assisted citrate process. The ceramic microstructure was optimized for a given sintering process. Both the grain size and density are shown to be maximum when PVA is introduced in powder before the complete formation of CCTO. A correlated increase of the dielectric constant is evidenced by impedance spectroscopy measurements. Results support the IBLC model proposed to explain the high dielectric constant of CCTO.

G. Gruener

Papers

Grain size effects on the dielectric constant of CaCu3Ti4O12 ceramics