Understanding the Role of Small Cations on the Electroluminescence Performance of Perovskite Light‐Emitting Diodes

Abstract

The delicate engineering of monovalent cations in perovskite material has led to continuous performance breakthroughs and stability improvement for the perovskite light‐emitting diodes (PeLEDs). However, the exact role of A‐site cations on the electroluminescence (EL) performance and degradation mechanism of PeLEDs has not been systematically answered yet. Herein, it is demonstrated that the most commonly used methylammonium cation (MA+) has an adverse effect on the electrochemical reaction at the interface between perovskite and metal‐oxide layer, leading to deteriorated EL performance as compared to that of the formamidinium cation (FA+)‐based perovskite. It reveals that the accelerated deprotonation process of MA+ under an electric field will aggravate the reaction between iodide and metal ion in oxide layer. The further substitution of a small portion of FA+ with inorganic cesium cation (Cs+) results in much enhanced crystallinity and enlarged crystal size, leading to an optimized peak external quantum efficiency of 21.3%. The ion migration process in the PeLEDs can be significantly suppressed with Cs+ incorporation, leading to a smaller roll‐off under large current density and an elongated half‐lifetime of 190.1 h under a current density of 20 mA cm‐2, representing one of the most stable PeLEDs based on 3D perovskite layer.

Ion Migration in Lead-Halide Perovskites: Cation Matters

“…FA + and Cs + are the two other most popular A-site cations in MHPs. The migration of the two cations has also been reported in the literature. , Nagi et al utilized ToF-SIMS depth analysis and observed that FA + accumulated at the Au/MoO 3 interface in degraded FA-based PeLEDs, which could be a major contributor to the short operational lifetime . Pavlovetc et al reported bias-induced migration of both FA + and Cs + in mixed-cation perovskite lateral devices.…”

“…Additionally, Nagi et al reported that the incorporation of Cs + cations (with a Cs/Pb molar ratio of 0.15) in the FA-PeLEDs significantly suppressed the ion migration process and enhanced the device stability. This led to a smaller efficiency roll-off and an elongated operational half-lifetime of 190.1 h under a current density of 20 mA/cm 2 , while the FAPbI 3 device exhibited an operational half-lifetime of 38.4 h under the same testing conditions . Adverse impacts on device stability would occur with an increased doping ratio of Cs + .…”

“…53,71 Nagi et al utilized ToF-SIMS depth analysis and observed that FA + accumulated at the Au/ MoO 3 interface in degraded FA-based PeLEDs, which could be a major contributor to the short operational lifetime. 73 Pavlovetc et al reported bias-induced migration of both FA + and Cs + in mixed-cation perovskite lateral devices. Infrared photothermal heterodyne imaging ratio maps of lateral devices based on FA 0.85 Cs 0.15 PbI 3 and FA 0.76 MA 0.15 Cs 0.09 PbI 3 after biasing indicated FA + accumulation at the cathodes.…”

“…This led to a smaller efficiency roll-off and an elongated operational half-lifetime of 190.1 h under a current density of 20 mA/cm 2 , while the FAPbI 3 device exhibited an operational half-lifetime of 38.4 h under the same testing conditions. 73 Adverse impacts on device stability would occur with an increased doping ratio of Cs + . When the doping ratio is above 15%, Cs-rich domains could form and cause spatial inhomogeneity, leading to material degradation.…”

Charge Injection and Auger Recombination Modulation for Efficient and Stable Quasi‐2D Perovskite Light‐Emitting Diodes

“…[12][13][14][15][16] However, the PeLEDs suffer from serious ion migration issue under electric field, resulting in electrochemical reaction between adjacent layers and perovskite decomposition. [17,18] Bulky organic cations with various geometries have been introduced into 3D perovskites to form quasi-two-dimensional (quasi-2D) perovskites, consisting of alternative inorganic and organic layers, which aims to increase the ion migration barrier and suppress the decomposition process. [19][20][21] The quasi-2D perovskite films typically consist of quasi-2D phases with a series of different inorganic layers and also some 3D phases.…”