Efficient Thermally Evaporated Near-Infrared Perovskite Light-Emitting Diodes via Phase Regulation

Abstract

α-phase formamidinium lead triiodide (FAPbI3) has demonstrated extraordinary properties for near-infrared perovskite light-emitting diodes (NIR-PeLEDs). The vacuum processing technique has recently received increasing attention from industry and academia due to its solvent-free feature and compatibility with large-scale production. Nevertheless, vacuum-deposited NIR-PeLEDs have been less studied, and their efficiencies lag far behind those of solution-based PeLEDs as it is still challenging to prepare pure α-FAPbI3 by the thermal evaporation. Herein, we report a Cs-containing triple-source co-evaporation approach to develop the perovskite films. The addition of thermally stable Cs cation fills in the perovskite crystal lattice and eliminates the formation of metallic Pb caused by the degradation of FA cation during the evaporation process. The tri-source co-evaporation strategy significantly promotes the phase transition from yellow δ-phase FAPbI3 to black α-phase FACsPbI3, fostering smooth, uniform, and pinhole-free perovskite films with higher crystallinity and fewer defects. On this basis, the resulting NIR-PeLED based on FACsPbI3 yields a maximum EQE of 10.25%, which is around sixfold higher than that of FAPbI3-based PeLEDs. Our work demonstrates a reliable and effective strategy to achieve α-FAPbI3 via thermal evaporation and paves the pathway toward highly efficient perovskite optoelectronic devices for future commercialization.

1. Introduction

Metal halide perovskites have rapidly emerged as a revolutionary frontier in display and lighting owing to their fascinating properties including high color purity, excellent charge transport properties, tunable chromaticity, and cost-effectiveness [1–5]. Specially, near-infrared perovskite light-emitting diodes (NIR-PeLEDs) hold great potential for a variety of applications such as medical treatment, data storage, optical communications, hyperspectral imaging, night vision for surveillance, and automotive safety [6, 7]. With tremendous efforts of researchers, the external quantum efficiencies (EQEs) of NIR-PeLEDs have exceeded 23% within the last few years [2, 4], enabling them to be a strong competitor of the III-V NIR-LEDs.

Up to date, the fabrication of formamidinium-based emitters such as FAPbI3 was regarded as the most successful approach to develop NIR-PeLEDs [2, 4, 7–9]. Nevertheless, most of the reported state-of-the-art NIR-PeLEDs were prepared via solution-based spin-coating, limiting their application in large-scale production. Inspired by the deposition of commercialized organic light-emitting diodes (OLEDs), thermal evaporation has been proposed as an alternative and effective technique to develop scalable PeLEDs [10]. Unlike solution-processing techniques, thermal evaporation can effectively avoid the problem of low solubility of some perovskite precursors. Besides, it can prevent the utilization of environmentally unfriendly organic solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Furthermore, the process of thermal evaporation can be uniformly fabricated onto various substrates, paving a novel pathway for the heterogeneous integration of NIR-PeLEDs. Most importantly, thermal evaporation is compatible with the currently available OLED industry, thus lowering the initial investment and speeding up the commercialization of PeLEDs [3, 10–13].