Analyses and optimizations of interface layers for perovskite light-emitting diodes

Abstract

Perovskite nanocrystals (NCs) are crystalline materials featuring unique and attractive properties, such as excellent carrier mobilities, long carrier diffusion length, and solution processability. As a promising candidate for efficient light emission devices, NCs based perovskite light-emitting diodes (PeLEDs) have been widely investigated. To obtain efficient and stable PeLEDs, realizing sufficient and balance carrier injection is vital. However, due to issues at the interface layer, such as the energy barrier and serious carrier accumulation, carrier injection may be affected. Meanwhile, strategies that can describe the carrier behaviours at interface layer are lacked. To solve these issues and promote the efficient PeLEDs, we have conducted the following researches: 1. Establish a capacitance-voltage (C-V) model for PeLEDs to describe carrier behaviours Analyzing and optimizing carrier behaviours are essential to achieve high electroluminescence performances for PeLEDs. In this work, C-V model for PeLEDs is established to describe carrier behaviours. Four distinct regions in this typical C-V model, including neutrality region, barrier region, carrier diffusion region, and carrier recombination region, were respectively analyzed. Importantly, the C-V model can guide the electroluminescence performance improvements for PeLEDs. By analyzing their C-V characteristics, issues of high hole injection barrier and insufficient recombination can be revealed. The C-V model helps quantitatively understand the essential carrier behaviours, and serves an efficient method to improve the EL performance for PeLEDs. 2. Introduce electric dipole layer for interfacial optimizations to enhance the hole injection in PeLEDs Carrier balance is essential to achieve high performance in PeLEDs. In this work, an efficient strategy by introducing an electric dipole layer is proposed to enhance the minority carrier injection, and realize carrier balance. The hopping theory demonstrates electric dipoles between hole injection layer (HIL) and hole transport layer (HTL) will enhance the hole injection. Then, MoO3 is chosen to generate electric dipoles due to its deep conduction band level. C-V analyses further prove there is the efficient hole injection. The proposed PeLEDs achieve a high current efficiency of 72.7 cd A-1, indicating a feasible approach to achieve a high PeLEDs performance. 3. Propose the efficient and stable PeLED based on poly(maleic anhydride-alt -1-octadecene) (PMA)-NCs and interfacial optimizations The efficient and stable PeLED based on PMA-NCs is realized through interfacial optimizations. First, the stable CsPbI3 NCs with PMA ligand is prepared. Then, interfacial optimizations are implemented to enhance the hole injection, and realize better carrier balance. Firstly, MoO3 is introduced to reduce the hole injection barrier between HIL and HTL. Then, 2,2'-(Perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ) is implemented as the electric dipole layer between HTL and emission layer (EML). Based on theoretical analysis and simulations, such an electric dipole layer can further enhance the hole injection and reduce the interfacial energy barrier. Therefore, the sufficient hole injection in the device can be realized. Additionally, optical optimization by microcavity design is implemented to realize sufficient light extraction. As the result, the performance of the proposed PeLED is significantly boosted, showing a relatively high external quantum efficiency of 20.9% and superior operational stability with T50 of 320 hours.