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  1. Abstract

    Perovskites have emerged as a forerunner of electronics research due to their vast potential for optoelectronic applications. The numerous combinations of constituent ions and the potential for doping of perovskites lead to a high demand to track the underlying electronic properties. Solution‐based electrochemistry is particularly promising for detailed and facile assessment of perovskites. Here, electrochemical impedance spectroscopy (EIS) of methylammonium lead iodide (MAPbI3) thin films is performed and model them with an equivalent circuit that accounts for solvent, ionic, and thin film effects. A dielectric constant consistent with prior AC studies and a diffusion constant harmonious with cation motion in MAPbI3are extracted. An electrical double layer thickness in the perovskite film of 54 nm is obtained, consistent with lithium doping in perovskite films. Comparing the EIS and equivalent circuit model of perovskite films to control ITO‐only data enabled the assignment of the ions at each interface. This comparison implied a double layer of primarily lithium ions inside the perovskite film at the solution interface with significant recombination of ions on the solution side of the interface. This demonstrates EIS as a powerful tool for studying the fundamental charge accumulation and transport processes in perovskite thin films.

     
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  2. Abstract

    Perovskite light‐emitting diodes (PeLEDs) are advancing because of their superior external quantum efficiencies (EQEs) and color purity. Still, additional work is needed for blue PeLEDs to achieve the same benchmarks as the other visible colors. This study demonstrates an extremely efficient blue PeLED with a 488 nm peak emission, a maximum luminance of 8600 cd m−2, and a maximum EQE of 12.2% by incorporating the double‐sided ethane‐1,2‐diammonium bromide (EDBr2) ligand salt along with the long‐chain ligand methylphenylammonium chloride (MeCl). The EDBr2successfully improves the interaction between 2D perovskite layers by reducing the weak van der Waals interaction and creating a Dion–Jacobson (DJ) structure. Whereas the pristine sample (without EDBr2) is inhibited by small stacking number (n) 2D phases with nonradiative recombination regions that diminish the PeLED performance, adding EDBr2successfully enables better energy transfer from smallnphases to largernphases. As evidenced by photoluminescence (PL), scanning electron microscopy (SEM), and atomic force microscopy (AFM) characterization, EDBr2improves the morphology by reduction of pinholes and passivation of defects, subsequently improving the efficiencies and operational lifetimes of quasi‐2D blue PeLEDs.

     
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  3. Abstract

    Blue electroluminescence is highly desired for emerging light‐emitting devices for display applications and optoelectronics in general. However, saturated, efficient, and stable blue emission has been challenging to achieve, particularly in mixed‐halide perovskites, where intrinsic ion motion and halide segregation compromises spectral purity. Here, CsPbBr3−xClxperovskites, polyelectrolytes, and a salt additive are leveraged to demonstrate pure blue emission from single‐layer light‐emitting electrochemical cells (LECs). The electrolytes transport the ions from salt additives, enhancing charge injection and stabilizing the inherent perovskite emissive lattice for highly pure and sustained blue emission. Substituting Cl into CsPbBr3tunes the perovskite luminescence from green through blue. Sky blue and saturated blue devices produce International Commission on Illumination coordinates of (0.105, 0.129) and (0.136, 0.068), respectively, with the latter meeting the US National Television Committee standard for the blue primary. Likewise, maximum luminances of 2900 and 1000 cd m−2, external quantum efficiencies (EQEs) of 4.3% and 3.9%, and luminance half‐lives of 5.7 and 4.9 h are obtained for sky blue and saturated blue devices, respectively. Polymer and LiPF6inclusion increases photoluminescence efficiency, suppresses halide segregation, induces thin‐film smoothness and uniformity, and reduces crystallite size. Overall, these devices show superior performance among blue perovskite light‐emitting diodes (PeLEDs) and general LECs.

     
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  4. Abstract

    Light‐emitting electrochemical cells (LECs) are devices that utilize efficient ion redistribution to produce high‐efficiency electroluminescence in a simple device architecture. Prototypical polymer LECs utilize three components in the active layer: a luminescent conducting polymer, a salt, and an electrolyte. Similarly, many small‐molecule LECs also utilize an electrolyte to disperse salts. In these systems, the electrolyte is incorporated to efficiently conduct ions and to maintain phase compatibility between all components. However, certain LEC approaches and materials systems enable device operation without a dedicated electrolyte. This review describes the general methods and materials used to circumvent the use of a dedicated electrolyte in LECs. The techniques of synthetically coupling electrolytes, incorporating ionic liquids, and introducing inorganic salts are presented in view of research efforts to date. The use of these techniques in emerging classes of light‐emitting electrochemical cells is also discussed. These approaches have yielded some of the most efficient, long‐lasting, and commercially applicable LECs to date.

     
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  5. Abstract

    Hybrid organic–inorganic perovskite light‐emitting devices (LEDs) have recently shown the characteristic dynamical behavior of light‐emitting electrochemical cells (LECs), with intrinsic ionic migration creating an electric double layer and internal p‐i‐n structure and by accumulation of ions at interfaces. Therefore, the development of perovskite light‐emitting and photovoltaic devices based on the concepts of LEC operation attracts much attention and clarifies general physical processes in perovskites. Here, new directions that can further improve perovskite optoelectronic devices and extend their functionalities using additive mobile ions are overviewed: 1) enhancing single‐layer LECs with lithium additives for increased efficiency and longer lifetime; 2) facilitating ionic motion in three‐layer perovskite LECs to create dual‐functional devices, operating as both LEC and solar cells; and 3) creating truly ambipolar LEC devices with carbon nanotubes as stable electrodes that leverage ionic doping. Taken together, the use of these approaches provides a strategy to create efficient, stable, and bright LECs, which use advantages of both LED and LEC operation. It is discussed that how the LEC behavior in perovskite LEDs can be further improved to address the long‐term challenges in perovskite optoelectronics, such as stability, through approaches like ionically reconfigurable host/guest systems.

     
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  6. Abstract

    Hybrid perovskites are emerging as highly efficient materials for optoelectronic applications, however, their operational lifetime has remained a limiting factor for their continued progress. In this work, perovskite light emitting electrochemical cells utilizing an optimized fraction of lithium hexafluorophosphate (LiPF6) salt additive exhibit enhanced stability. At 0.5 wt% LiPF6, devices exhibit 100 h operation at high brightness in excess of 800 cd m−2under constant current driving, achieving a maximum luminance of 3260 cd m−2and power efficiency of 9.1 Lm W−1. This performance extrapolates to a 6700 h luminance half‐life from 100 cd m−2, a 5.6‐fold improvement over devices with no LiPF6. Analysis under constant voltage driving reveals three current regimes, with lithium addition strongly enhancing current in the second and third regimes. The third regime correlates lower rates of luminance with lowered current flow. These losses are mitigated by LiPF6addition, an effect postulated to arise from preservation of perovskite structure. Electrochemical impedance spectroscopy with equivalent circuit modeling reveals that electrical double layer widths are minimized at 0.5 wt% LiPF6and inversely correlated with efficient performance. These results demonstrate that an optimal LiPF6concentration improves stability and efficiency through improved double layer formation and retention of perovskite structure.

     
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  7. Hybrid perovskites have attracted great interest in solar energy conversion and optoelectronic applications. The interconnected ionic and electronic effects complicate assessing the underlying electrical processes while contributing greatly to the efficiency and stability of devices. Fortunately, these coupled processes manifest on distinct timescales that enable frequency-specific electrochemical analysis. However, hybrid perovskites dissolve in most of the common aqueous and organic solvents utilized for electrochemistry. Here, we utilize a hydrofluoroether (HFE) solvent toolkit to perform nondestructive electrochemical impedance spectroscopy of methylammonium lead iodide (MAPbI3) perovskite thin films. This enables the extraction of dielectric constants and double-layer formation in these perovskite films. 
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