Improved Thermally Activated Delayed Fluorescence based Electroluminescent Devices using Vacuum Processed Carbazole based Self- Assembled Monolayer

Engineering of the interface between organic emissive layer thin films and charge transport layers has fuelled the development of Organic light emitting diodes (OLEDs) over the past decade. This paper shows a comparative study between conventional poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) vs carbazole functional groups based self-assembled monolayer (SAM) as hole-injection layer (HIL). SAM enables an improved interface to achieve the charge carrier balance and reduced interfacial recombination to enhance the external quantum efficiency in the <i>p-i-n</i> OLEDs. An industry compatible vacuum processed SAM-HILs ((4-(3,6-Dimethyl-9H-carbazol-9-yl) butyl) phosphonic acid (Me-4PACz)) as a hole transport layer in thermally activated delayed fluorescence (TADF) based OLEDS is reported for first time, which facilitate to overcome the issues of poor device stability. Deeper highest occupied molecular orbital (HOMO @ -5.3 eV) of Me-4PACz enables the efficient hole injection with reduction in leakage current by 1 to 2 orders of magnitude compared to PEDOT: PSS (HOMO @ -5.00 eV) HIL based OLEDs. Also, built in potential is 1.3 times reduced to 2.45 V for Me-4PACz based OLEDs. With the further optimized thickness of Me-4PACz (d= 6 nm), OLED maximum external quantum efficiency (EQE) gets enhanced from 18.30 % to 20.79 % with luminous efficiency 62.79 cd/A. Further, the Me-4PACz is also investigated on the basis of its deposition method. Vacuum processed SAM based OLEDs showed a reduced operating voltages and enhanced brightness levels, which correlated with an improved electronic coupling between SAM/α-NPD interfaces. We note that, evaporated SAM-HTLs to complete OLED demonstrates comparable efficiency to their solution-processed counterparts. Furthermore, vacuum deposition SAM as HIL based OLEDs is found to provide a five- fold enhancement in the operational stability 60% (LT₆₀) of the initial luminance L₀ of the OLED in comparison to PEDOT: PSS as HIL based OLED. This study is further extended to a blue TADF OLED, with almost similar finding.