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

Abstract 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 (g_EL) of −0.42 are achieved; for the planarized and extended “β‐phase” chain conformation a CE of 1.23 cd A⁻¹, a PE of 0.63 lm W⁻¹, and a g_EL of −0.44 are achieved. While these two phases achieve both high CE, as well as g_EL, the latter affords the first demonstration of chiral β‐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 β‐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.