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

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. [Display omitted] •Achieving high performance device by an ultrathin nondoped emissive layer strategy.•Extremely small efficiency roll-off of 1.5% from peak to the value at 5000 cd m−2.•Peak luminance of the nondoped device was far higher than that of doped devices.•Color shift of (0.114, 0.076) by altering the emissive layer from 0.02 to 1.0 nm.•Realizing color shift of (0.056, 0.023) with the voltage for 1.0-nm-thick device.