3PTZ and 3PXZ small molecular hole-transporting materials in polymer light-emitting diodes

In the field of organic display devices, the solution process of organic materials still requires a lot of research in order to realize advantages such as a simple manufacturing process and flexible characteristics. Here, we propose novel small-molecule hole-transporting materials of 1,3,5-tri(10H-phenothiazin-10-yl)benzene (3PTZ) and 1,3,5-tri(10H-phenoxazin-10-yl)benzene (3PXZ) applicable to spin-coating techniques for solution-processed polymer light-emitting diodes (PLEDs). With optical analysis, electrical analysis, and density functional theory (DFT) modeling, accurate energy band diagrams of 3PTZ and 3PXZ as hole transport layers (HTLs) in the PLED structure were revealed. The current-voltage (I-V) and electroluminescence (EL) results of PLEDs with 3PTZ (3PXZ) HTLs showed enhanced luminance and quantum efficiency. The maximum luminance and quantum efficiencies of PLEDs with the 3PTZ HTL increased 1.4 and 1.76 times compared to those of the reference PLED without a HTL. The 3PTZ (3PXZ) HTLs with the optimal energy band structure exhibited similar performances regardless of the deposition processes like spin-coating and thermal evaporation. Devices based on 3PTZ (3PXZ) materials show 1.2 (1.11) times increased quantum efficiencies compared with that of devices based on poly-TPD, a representative polymer HTL material. Comparing two small molecular materials as a HTL, the PLED with 3PTZ showed 1.08 times increased quantum efficiency compared with the PED with 3PXZ, attributed to the higher electron barrier height of 3PTZ than that of 3PXZ. To investigate parameters for quantum efficiency improvement of PLEDs with small molecular 3PTZ HTL, time-resolved photoluminescence (TRPL) and I-V curve fittings were additionally analyzed.