Enhancing Interfacial Exciton Conversion in Blue Upconversion OLEDs through Oligomer-Controlled Molecular Orientation

Controlling molecular orientation is central to high-performance organic optoelectronics. Anthracene oligomers provide an orientation-tunable platform for elucidating orientation effects in upconversion organic light-emitting diodes (UC-OLEDs), which generate blue emission at exceptionally low driving voltages through interfacial charge-transfer (CT) states, CT-to-triplet (CT-to-T1) conversion, and triplet–triplet annihilation. Across anthracene derivatives, increasing horizontal orientation enhances external quantum efficiency (EQE) beyond that expected from light outcoupling alone. To identify the underlying processes, the randomly oriented monomer 1,2-ADN and the preferentially horizontally oriented dimer p-ADN-1 serve as representatives. Both interface-exchanged devices, in which interfacial and emitting-layer anthracene species are varied independently, and transient absorption measurements consistently show that horizontal orientation enhances interfacial CT-to-T1 conversion. Overall, a more horizontal orientation of the host molecules improves light outcoupling by only ~4% but enhances interfacial CT-to-T1 conversion efficiency by 1.3–1.5-fold. Beyond its ultralow turn-on voltage of 1.5 V, the optimized p-ADN-1 device achieves an EQE of 2.61%, a 1.5-fold improvement over the 1,2-ADN device, and a LT50 (time to 50% of the initial luminance) exceeding 1750 h at 1000 cd m−2. To our knowledge, these values represent the highest EQE and longest operational lifetime reported for blue OLEDs with turn-on voltages below 2 V. These findings establish molecular orientation as an active structural parameter governing both light outcoupling and interfacial exciton conversion.