Charge Balance in Red QLEDs for High Efficiency and Stability via Ionic Liquid Doping

Abstract

Colloidal quantum dot light-emitting diodes (QLEDs) simultaneously exhibiting high external quantum efficiency (EQE) and operation stability are achieved via a simple solution processing. A silver bis(trifluoromethanesulfonyl)imide treatment that is used to chemically dope the poly(3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) hole injection layer (HIL) is proposed. The ionic liquid salt acts as an effective p-dopant for high charge concentrations; and it raises the work functions and surface potentials of the PEDOT:PSS HIL for an interface energy band alignment. Benefiting from a raised hole mobility and a better charge mobility balance, the processing-simple colloidal red QLEDs yielded the highest EQE of up to 17.4%. In particular, a very broad plateau of EQE values over 16.5% in the brightness of 400-10 000 cd m −2 is obtained. Moreover, the packaged QLEDs exhibit an excellent operation stability in ambient air. The T 95 (time for the device brightness to decrease to 95% of its initial brightness) at an initial brightness of 1000 cd m −2 is 4160 h. The work highlights a promising ionic liquid strategy to guide the development of high-performance and costeffective QLEDs.

Ultrastable and High-Efficiency Deep Red QLEDs through Giant Continuously Graded Colloidal Quantum Dots with Shell Engineering

“…Quantum dot based light-emitting diodes (QLEDs) have shown great advantages in display and lighting due to their chromatic purity, high brightness, and all-solution-based fabrication process. − In recent years, the performance of QLEDs, especially the external quantum efficiency (EQE), has been rapidly improved by material and device optimization. − Color-tunable QLEDs based on CdSe quantum dots (QDs) targeting displays with red, green, and blue emission have reached maximum EQEs of over 20%. − However, the stability of state-of-the-art QLEDs has yet to meet the requirements of the current applications. This is mainly due to carrier injection imbalance, which leads not only to low device efficiency but also to the accumulation of charges at the interface between the emitting QD layers and the carrier transport layer. − …”

“…In 2018, Cao et al fabricated CdSe/Cd 1– x Zn x Se/ZnSe QLEDs with a maximum EQE of 15.1% and a T 95 operation lifetime of more than 2300 h at 1000 cd m –2 . Recently, Li et al used CdSe/ZnS QDs for stable red QLEDs with a maximum EQE of 17.6% and a T 95 of 4160 h at 1000 cd m –2 .…”

“…QLEDs emitting photons with wavelengths beyond 630 nm are highly desirable in order to reach a wide color gamut. However, the emission wavelength of red QLEDs with record EQE and operation lifetime had been limited to the range of 600–630 nm due to the challenge of synthesizing high-quality CdSe QDs emitting beyond 630 nm. ,,,,− One reason is the difficulty in increasing their size while maintaining the high PLQY. This difficulty is caused by the decrease in radiative decay rate with increasing size, while the defect-related nonradiative decay rate remains nearly unchanged. − Additionally, the number of internal and surface defects acting as nonemitting centers is proportional to the QDs’ volume and surface area. − Another challenge is the growth of a high-quality, defect-free passivating shell around larger seeds.…”

Flexible Quantum Dot Light‐Emitting Device for Emerging Multifunctional and Smart Applications

“…2020), high brightness, tunable emission wavelength, low operating voltage and easy processability. [39][40][41] For example, their band gap can be precisely modified to cover the emission from UV to the entire visible range and the near/mid-infrared range by tuning QDs sizes and components; [42][43][44][45] in addition, a thermodynamically-stable single-crystalline lattice of inorganic material enables QDs with high purity and improved lifetime and durability of QLEDs compared to organic light-emitting diodes (OLEDs, with broad and asymmetric emission and efficiency droop). [10,46,47] Moreover, QLEDs enjoy flexibility because of their ultra-thin device architecture (less than 1 μm for all functional layers), compared to conventional cathode ray tubes, liquid crystal displays and GaN LEDs.…”

Enhancing Efficiency of Organic Solar Cells with Alkyl Diamines Doped PEDOT: PSS

“…The famous conductive polymer, poly­(3,4-ethylenedioxythiophene): poly­(styrene-sulfonate) (PEDOT: PSS), has been widely applied as a hole transport material (HTM) owing to its good transparency, solution processing, and thermostability. − One of the key factors in determining device performance is the conductivity of the conductive polymer . However, the rapid development of solar cells raises higher requirements for the properties of PEDOT: PSS. − For instance, it is necessary to enhance the WF of PEDOT: PSS to match the deepening HOMO level of emerging donors. , More importantly, PEDOT: PSS’s shortcomings including the relatively low conductivity as well as the electrical and structural inhomogeneities brought by the redundant sulfonic acid groups of PSS will affect the performance of devices. , Doping strategy is an efficient method to modulate these properties for the requirement of practical applications. − Various polar organic solvents, − ionic liquids, − and surfactants − have been added into PEDOT: PSS and successfully improved its conductivity. However, the traditional post-treatment modification method is not only relatively cumbersome in the process of solvent removal, but also at the expense of the smoothness and uniformity of the surface morphology. − In previous work, our group developed phenylethylamine derivatives as dopants to achieve the enhancement of work function and conductivity. , It is worth noting that the sulfonic acid groups of PSS can interact with phenylethylamine, which is beneficial to the more ordered packing of conjugated backbones, resulting in the improvement of device performances.…”