The achievement of high-performance and allochroic organic light-emitting diodes via nondoped 4CzPN ultrathin emissive layer

Abstract

The device fabrication process can be simplified by the employment of ultrathin nondoped emission layer (UT-EML). Based on 1,2,3,4-tetrakis(carbazol-9-yl)-5,6-dicyanobenzene (4CzPN) emitter, the influence of the thickness of ultrathin emissive layer on the device properties was systemically investigated. The high-performance nondoped device was fulfilled by utilizing a 0.02 nm ultrathin emissive layer. The nondoped device delivered the maximum efficiencies of 9.0%, 30.0 cd A−1, and 15.0 lm W−1, accompanied by the peak luminance of 30 597 cd m−2, whilst the corresponding properties of the doped device with a similar configuration were 7.4%/23.7 cd A−1/14.9 lm W−1, and 10 072 cd m−2. Moreover, the efficiency roll-off (just 1.5% from peak to the value at 5 000 cd m−2) of nondoped device was much superior to that of doped devices (46%). Besides, the color can be tuned by controlling the thickness of ultrathin emissive layer and applied voltage, exhibiting potential applications in the field of smart lighting and anti-counterfeiting. When the emission layer thickness was increased from 0.02 to 1.0 nm, the CIE coordinates of devices under 6 V shifted from (0.322, 0.603) to (0.436, 0.537). And the device with a 1.0-nm-thick emissive layer showed a ΔCIE of (0.056, 0.023) with the increase of bias voltage from 5 to 12 V. These results shed light on that nondoped ultrathin emission layer holds great potential for high performance and color-tunable TADF OLEDs.