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

Area of Science:

• Materials Science
• Organic Electronics
• Device Physics

Background:

• Solution-processed organic light-emitting diodes (s-OLEDs) offer potential for low-cost, flexible displays.
• Current s-OLEDs suffer from lower external quantum efficiencies (EQEs) and operational lifetimes compared to vacuum-deposited devices.
• Key challenges include inefficient hole transport, interfacial intermixing, and uncontrolled dopant aggregation.

Purpose of the Study:

• To investigate the role of hole transport layer (HTL) molecular weight in optimizing s-OLED performance.
• To understand how HTL molecular weight influences dopant aggregation, interfacial mixing, and carrier transport.
• To establish design principles for high-efficiency and reliable solution-processed green OLEDs.

Main Methods:

• Synthesized three poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) HTLs with varying molecular weights (51k, 60k, 130k).
• Fabricated green phosphorescent s-OLEDs using Ir(mppy)3:CBP emissive layers.
• Characterized device performance using atomic force microscopy, UV-visible absorption, electroluminescence, and J-V-luminance measurements.

Main Results:

• HTL molecular weight critically controls dopant aggregation and interfacial characteristics.
• The TFB_51k HTL (moderate aggregation) achieved a maximum EQE of ~9.9% with negligible roll-off and a lifetime of ~36 hours.
• The TFB_130k HTL (suppressed aggregation) resulted in lower EQE (~4.1%), significant roll-off, and a short lifetime (~5 hours) due to sharp interfaces and interfacial recombination.

Conclusions:

• Dopant aggregation is a critical, optimizable parameter, not merely a processing artifact.
• Controlled interfacial intermixing, achieved through judicious HTL molecular weight selection, is essential for device performance.
• Engineering HTL molecular weight provides a pathway to high-efficiency and stable solution-processed green OLEDs.