Molecular weight-dependent control of interfacial intermixing and dopant aggregation: a design principle for...

Solution processed organic light emitting diodes (s-OLEDs) are promising for low cost, large area, and flexible displays, yet their external quantum efficiencies (EQEs) and operational lifetimes still lag far behind those of vacuum deposited devices owing to inefficient hole transport, interfacial intermixing, and poorly controlled dopant aggregation in the emissive layer. Here, we identify the molecular weight of the polymeric hole transport layer (HTL) as a powerful design parameter for simultaneously tuning dopant aggregation, interfacial intermixing, and carrier transport in green phosphorescent s-OLEDs. Three poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) HTLs with number-average molecular weights of 51k, 60k, and 130k were incorporated into Ir(mppy)₃:CBP green emissive-layer devices and systematically investigated using atomic force microscopy, UV-visible absorption, electroluminescence, and J-V-luminance measurements. We show that the dopant aggregation state is not an undesirable byproduct of solution processing but a critical variable that must be optimized: complete suppression of aggregation by the highly solvent-resistant TFB_130k leads to a low maximum EQE (∼4.1%), severe EQE roll-off, strong parasitic TFB emission, and a shortened lifetime (∼5 h), whereas a moderate level of dopant-dopant interaction and aggregation realized with TFB_51k yields a maximum EQE of ∼9.9% with negligible roll-off at 1000 cd m^-2 and an extended lifetime of ∼36 h. These trends arise from molecular weight dependent interfacial intermixing at the TFB/CBP boundary: the relatively low solvent resistance of TFB_51k forms a graded intermixed junction that smooths hole injection and confines the recombination zone within the Ir(mppy)₃:CBP bulk, while the sharp interface of TFB_130k drives hole accumulation and interfacial recombination on TFB. Our results establish that engineering an optimal dopant aggregation window and controlled interfacial intermixing via HTL molecular weight design is essential for achieving high efficiency and reliable solution processed green OLEDs.