No 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 (PeLECs) utilizing an optimized fraction of LiPF 6 salt additive exhibit enhanced stability. At 0.5 wt% LiPF 6 , devices exhibit 100 h operation at high brightness in excess of 800 cd m −2 under constant current driving, achieving a maximum luminance of 3260 cd m −2 and 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 LiPF 6 . 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 LiPF 6 addition, an effect postulated to arise from preservation of perovskite structure. Electrochemical impedance spectroscopy with equivalent circuit modeling revealed that electrical double layer widths are minimized at 0.5 wt% LiPF 6 and inversely correlate with efficient performance. These results demonstrate that an optimal LiPF 6 concentration improves stability and efficiency through improved double layer formation and retention of perovskite structure.

Light-emitting electrochemical cells (LEECs) offer the potential for high efficiency operation from an inexpensive device. However, long turn-on times and low luminance under steady-state operation are longstanding LEEC issues. Here, we present a single-layer LEEC with a custom-designed iridium(III) complex and a lithium salt additive for enhanced device performance. These devices display reduced response times, modest lifetimes, and peak luminances as high as 5500 cd/m(2), 80% higher than a comparable device from an unoptimized complex and 50% higher than the salt-free device. Improved device efficiency suggests that salt addition balances space charge effects at the interfaces. Extrapolation suggests favorable half-lives of 120 ± 10 h at 1000 cd/m(2) and 3800 ± 400 h at 100 cd/m(2). Overall, complex design and device engineering produce competitive LEECs from simple, single-layer architectures.

Light-emitting electrochemical cells (LEECs) from small molecules, such as iridium complexes, have great potential as low-cost emissive devices. In these devices, ions rearrange during operation to facilitate carrier injection, bringing about efficient operation from simple, single-layer devices. Prior work has shown that the luminance, efficiency, and responsiveness of iridium LEECs is greatly enhanced by the inclusion of small fractions of lithium salts, but much remains to be understood about the origin of this enhancement. Recent work with planar devices demonstrates that lithium additives in iridium LEECs enhance double-layer formation. However, the quantitative influence of lithium salts on the underlying physics of conventional thin-film, sandwich structure LEECs, which beneficially operate at low voltages and generate higher luminance, has yet to be clarified. Here, we use electrochemical impedance spectroscopy to discern the impact of the lithium salt concentration on double-layer formation within the device and draw correlations with performance metrics, such as current, luminance, and external quantum efficiency.

Light-emitting electrochemical cells (LEECs) utilizing small molecule emitters such as iridium complexes have great potential as low-cost emissive devices. In these devices, ions rearrange during operation to facilitate carrier injection, bringing about efficient operation from simple, single layer devices. Recent work has shown that the luminance, efficiency, and responsiveness of iridium-based LEECs are greatly enhanced by the inclusion of small amounts of lithium salts (≤0.5%/wt) into the active layer. However, the origin of this enhancement has yet to be demonstrated experimentally. Furthermore, although iridium-based devices have been the longstanding leader among small molecule LEECs, fundamental understanding of the ionic distribution in these devices under operation is lacking. Herein, we use scanning Kelvin probe microscopy to measure the in situ potential profiles and electric field distributions of planar iridium-based LEECs and clarify the role of ionic lithium additives. In pristine devices, it is found that ions do not pack densely at the cathode, and ionic redistribution is slow. Inclusion of small amounts of Li[PF6] greatly increases ionic space charge near the cathode that doubles the peak electric fields and enhances electronic injection relative to pristine devices. This study confirms and clarifies a number of longstanding hypotheses regarding iridium LEECs and recent postulates concerning optimization of their operation.