Star-shaped cross-linkable hole transport materials with high triplet energy and deep HOMO energy enable efficient solution-processed deep-blue TADF OLEDs

Abstract

Cross-linkable hole transport materials (x-HTMs) play a crucial role in solving the issue of interlayer mixing of solution-processed organic light-emitting diodes (OLEDs). However, issues such as energy level mismatch and low hole mobility hinder the application of x-HTMs in deep-blue OLEDs. In particular, thermally activated delayed fluorescent (TADF) emitters require HTMs with high triplet energies (E_T) to ensure high exciton utilization efficiency. Here, two star-shaped cross-linkable HTMs 5-(9H-carbazol-9-yl)-N¹,N³-di(p-tolyl)-N¹,N³-bis(4-vinylphenyl)benzene-1,3-diamine (m-V-CzDPA) and N¹,N¹-diphenyl-N³,N⁵-di(p-tolyl)-N³,N⁵-bis(4-vinylphenyl) benzene-1,3,5-triamine (m-V-DPADPA) were designed and synthesized. Owing to their aromatic torsion structures, m-V-CzDPA and m-V-DPADPA possessed high E_Ts of 2.89 and 2.87 eV, respectively, which can effectively confine triplet excitons in the emitting layer (EML). The carrier diffusion coefficients of their x-HTMs, x-m-CzDPA and x-m-DPADPA, which were obtained via carrier diffusion imaging characterization were 0.54 and 0.44 cm² s⁻¹, respectively, thus indicating outstanding intrinsic hole transport capacity, with hole mobilities of 4.30×10⁻⁴ and 1.39×10⁻⁴ cm² V⁻¹ s⁻¹, respectively. Solution-processed deep-blue TADF-OLEDs employing x-m-CzDPA as the HTM achieved a maximum current efficiency/maximum external quantum efficiency of 5.25 cd A⁻¹/18.06%, with CIE coordinates of (0.162, 0.042). This is the first time that x-HTMs have served as efficient deep-blue TADF-OLEDs via a solution process, which also meets the latest BT. 2020 standard (CIE_y ⩽ 0.046).