Discerning the Impact of a Lithium Salt Additive in Thin-Film Light-Emitting Electrochemical Cells with Electrochemical Impedance Spectroscopy

Bastatas, Lyndon D.; Lin, Kuo Yao; Moore, Matthew D.; Suhr, Kristin J.; Bowler, Melanie H.; Shen, Yulong; Holliday, Bradley J.; Slinker, Jason D.

doi:10.1021/acs.langmuir.6b02415

Cited by 40 publications

(45 citation statements)

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“…In stark contrast, 2 shows a significant increase of the overall resistance, highlighting the irreversible formation of new species that are hampering charge transport. This is in line with the electrochemical stability above described and could result in a lower device stability …”

Section: Resultssupporting

confidence: 90%

White Light‐Emitting Electrochemical Cells Based on Deep‐Red Cu(I) Complexes

Fresta

Weber

Fernández‐Cestau

et al. 2019

Advanced Optical Materials

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significantly reduce fabrication time and costs. [16][17][18] For this reason, the use of soluble ionic transition metal complexes (iTMCs) as emitters has intensively been investigated throughout the last decade. [15,19,20] However, up to date, the most efficient devices are based on Ir(III)-iTMCs. [15,[21][22][23][24][25] While these emitters yield light that spans the whole visible range, [15,26,27] the high cost and low abundance of iridium in the Earth's crust [28] do not match the future needs for sustainable large-area lighting. In this context, ionic copper(I) complexes-, i.e., Cu(I)-iTMCs-represent an appealing alternative to Ir(III)-iTMCs for LECs. Indeed, copper resources are abundant and considered as low-cost, while Cu(I)-iTMCs chemistry is well-known in literature, [29,30] allowing to tailor the features of the emitter-, e.g., redox stability, photoluminescence quantum yields (PLQYs), and/or the shift of the emission and the device color. As a matter of fact, Cu(I)-iTMCs have led to interesting results in LECs, achieving moderate stable and efficient blue and yellow devices. [13,[31][32][33][34][35][36] As in early works on Ir(III)-iTMC-LECs, [27] one of the major challenge is to identify possible complexes whose luminescent response covers the whole visible range. This has to be complemented with the right balance between efficiency, brightness, and stability, in order to achieve well-performing white LECs in the near future. [37][38][39] In contrast to Ir(III)-iTMCs, the state-of-the-art of Cu(I)-iTMC-LECs just involves yellow-and blue-emitting complexes. In short, Costa and co-workers have explored the possibility to prepare heteroleptic blue-emitting compounds with N^N ligands with high energy lowest unoccupied molecular orbital (LUMO) levels, achieving yellowemitting devices due to the lack of a thermally activated delayed fluorescence (TADF) process. [40] However, the same authors proposed to use another family of Cu(I) complexes bearing different N-heterocyclic carbenes and dipyridylamine ligands-, i.e., di-iso-propylphenyl)imidazole-2-ylidene and 2,2ʹ-bis-(3methylpyridyl)amine, showing an effective TADF emission that led to the first blue-emitting Cu(I)-iTMC LECs. [34,41,42] Other studies by Zhang et al., [43] Bolink and co-workers, [31,33,44] and Costa and co-workers [36,45,46] showed green-and yellow-emitting Cu(I)-iTMC based devices by changing the pattern substitution of heteroleptic diamine and diphosphine ligands.In light of the current art, deep-red LECs based on Cu(I)-iTMCs are still missing, hampering the preparation of whiteemitting LECs. In this context, we report the synthesis and The synthesis and characterization, as well as photoluminescent and electrochemical features of a series of ionic copper(I) complexes-, i.e., [Cu(N^N)(P^P)] + , where N^N is 4,4′-diethylester-2,2′-biquinoline (dcbq) and P^P is bis-triphenylphosphine, bis[2-(diphenylphosphino)phenyl)ether] (POP), or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos)-are reported along with their application to ach...

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Section: Resultssupporting

confidence: 90%

White Light‐Emitting Electrochemical Cells Based on Deep‐Red Cu(I) Complexes

Fresta

Weber

Fernández‐Cestau

et al. 2019

Advanced Optical Materials

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“…The Bode plot of impedance spectra in Figure a shows the distinct trend of an impedance plateau (lower frequency regime), which reflects the overall dc resistance of the film. As expected, the complexes with larger counterions produced devices with higher resistance, consistent with the general trend of iTMCs . EIS spectra were fit with the model circuit of Figure b discussed below.…”

Section: Resultssupporting

confidence: 77%

“…Electronic charge injection at 0 V is limited owing to the built‐in potential difference from the electrode work function imbalance that effectively reverse biases the sample, so charge transport is dominated by ion movement. With electronic charge injection restricted, we deduced the conductivity of the ions and the width of the electric double layers (EDLs) by following the approach employed in previous reports …”

Section: Resultsmentioning

confidence: 99%

“…Bastatas et al. observed that an ideal concentration of Li[PF 6 ] in iridium LEECs led to improved response, luminance, efficiency, and extrapolated lifetime . Alternatively, the counterions of lithium salt additives were recently varied and investigated in iridium iTMC LEECs, and the turn‐on times of these devices were found to be inversely proportional to the conductivity of the lithium salt in an electrolyte .…”

Section: Introductionmentioning

confidence: 99%

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The Effect of the Dielectric Constant and Ion Mobility in Light‐Emitting Electrochemical Cells

2017

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Light‐emitting electrochemical cells (LEECs) are a promising low‐cost option for display and solid‐state lighting. In these devices, the interplay of mobile ions, electrons, and holes makes for rich physics that can be leveraged for high performance. One example of this interplay is in the formation and radiative decay of excitons–bound electron and hole pairs. Considerations from exciton binding and Langevin recombination suggest that a low dielectric constant (ϵ) would enhance emission. However, emission is also enhanced by the product of the bulk hole and electron concentrations, which in LEECs are enhanced by the motion of small mobile ions yielding high dielectric constants. These competing effects make it difficult to predict whether active layers with low or high dielectric constants will optimize device performance. Here, the effect of varying the dielectric constant on the performance of LEEC devices from ionic transition‐metal complexes was studied by systematically exchanging the negative counterions paired with an iridium complex emitter. Electrochemical impedance spectroscopy, constant voltage and constant current device studies, and drift‐diffusion simulations were performed. The results clarify the competing effects of Langevin bimolecular recombination and ion‐assisted injection processes occurring in LEECs.

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“…To satisfy the other requirements, we introduce LiPF6 salt, a salt that we have previously used to attain high performance in LECs utilizing ionic transition metal complexes. [16][17][18] We prepared films with an optimal concentration of the polymer electrolyte poly(ethylene oxide) (PEO) and systematically studied the effect of LiPF6 salt addition.…”

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Bright and Effectual Perovskite Light-Emitting Electrochemical Cells Leveraging Ionic Additives

et al. 2019

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Perovskite light emitting diodes (PeLEDs) have drawn considerable attention for their favorable optoelectronic properties. Perovskite light emitting electrochemical cells (PeLECs)-devices that utilize mobile ions -have recently been reported but have yet to reach the performance of the best PeLEDs. We leveraged a poly(ethylene oxide) electrolyte and lithium dopant in CsPbBr3 thin films to produce PeLECs of improved brightness and efficiency. In particular, we found that a single layer PeLEC from CsPbBr3:PEO:LiPF6 with 0.5% wt. LiPF6 produced highly efficient (22 cd/A) and bright (~15000 cd/m 2 ) electroluminescence. To understand this improved performance among PeLECs, we characterized these perovskite thin films with photoluminescence (PL) spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD).These studies revealed that this optimal LiPF6 concentration improves electrical double layer formation, reduces the occurrence of voids, charge traps, and pinholes, and increases grain size and packing density. TOC GRAPHICSPerovskite light-emitting diodes (PeLEDs) based on inorgano−organometallic halide perovskites, such as CH3NH3PbX3 and CsPbX3 (X = Cl, Br, or I), have attracted much attention due to their low-temperature solution processability, high color purity with narrow spectral width (FWHM of 20 nm), band gap tunability and large charge carrier mobility. [1][2][3][4] To date, devices based on these perovskites have achieved high luminance in excess of 10000 cd/m 2 with high efficiencies (EQE ~10%), comparable to organic LEDs and quantum dot (QD) LEDs. [1][2][3][4][5][6][7] Interestingly, effects such as hysteresis and high capacitance in perovskite semiconductor devices suggest that ion motion can largely influence device operation. In this vein, researchers have recently been investigating perovskite materials in light-emitting electrochemical cell (LEC) architectures instead of traditional LEDs. [8][9][10][11] These LEC devices (PeLEC leverage ion redistribution to achieve balanced and high carrier injection, resulting in high electroluminescence efficiency. Due to this mechanism, LEC devices can be prepared from a simple architecture consisting of a single semiconducting composite layer sandwiched between two electrodes. In addition, they can operate at low voltages below the bandgap, yielding highly efficient devices.Recently, perovskite LECs (PeLECs) have been reported and show promise as electroluminescent devices. [8][9][10][11] However, these PeLECs are generally limited to luminance maxima of 1000 cd/m 2 or lower, below what has been typically observed in PeLEDs. This disparity suggests that further understanding and refinement of PeLEC materials and devices could produce significant improvements of brightness, efficiency, and other performance metrics. To this end, we fabricated a highly efficient (22 cd/A) and bright (~15000 cd/m 2 ) single layer LEC based on a cesium lead halide perovskite, CsPbBr3. To achieve...

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Discerning the Impact of a Lithium Salt Additive in Thin-Film Light-Emitting Electrochemical Cells with Electrochemical Impedance Spectroscopy

Cited by 40 publications

References 30 publications

White Light‐Emitting Electrochemical Cells Based on Deep‐Red Cu(I) Complexes

White Light‐Emitting Electrochemical Cells Based on Deep‐Red Cu(I) Complexes

The Effect of the Dielectric Constant and Ion Mobility in Light‐Emitting Electrochemical Cells

Bright and Effectual Perovskite Light-Emitting Electrochemical Cells Leveraging Ionic Additives

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