Charge balance in red QLEDs for high efficiency and stability via ionic liquid doping

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(trifluoromethane-sulfonyl)imide treatment that is used to chemically dope the poly(3,4-ethylenedioxythiophene):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 T95 (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 cost-effective QLEDs.