Strongly Circularly Polarized Crystalline and β-Phase Emission from Poly(9,9-dioctylfluorene)-Based Deep-Blue Light-Emitting Diodes

A key challenge in the realization of circularly polarized polymer light-emitting diodes (CP-PLEDs) is the generation of highly circularly polarized deep-blue electroluminescence (EL). Here, by blending the achiral luminescent polymer poly(9,9-dioctylfluorene) (PFO) with a helically chiral molecule 1-aza[6]helicene the authors present CP-PLEDs with state-of-the-art device performance for deep-blue CP emission: for an inverted device with a semicrystalline microstructure a current efficiency (CE) of 1.13 cd A(-1), a power efficiency (PE) of 0.81 lm W-1, and an EL dissymmetry (gEL) of -0.42 are achieved; for the planarized and extended "beta-phase" chain conformation a CE of 1.23 cd A(-1), a PE of 0.63 lm W-1, and a gEL of -0.44 are achieved. While these two phases achieve both high CE, as well as gEL, the latter affords the first demonstration of chiral beta-phase emission from solid-state PFO devices. Such strongly circularly polarized light is generated from a supramolecular assembly of interacting planar polymer backbones. The authors rationalize that the strong chiroptical effects observed within such chiral beta-phase PFO domains originate from coupled interchain aggregates. The findings not only demonstrate efficient deep-blue CP-PLEDs, but also provide insight into the mechanisms that underpin the strong CP emission from excitonically coupled polymer chains.