Stabilized Blue Emission from Polyfluorene‐Based Light‐Emitting Diodes: Elimination of Fluorenone Defects

Abstract

Polyfluorene (PF)‐based light‐emitting diodes (LEDs) typically exhibit device degradation under operation with the emergence of a strong low‐energy emission band (at ∼ 2.2–2.4 eV). This longer wavelength band converts the desired blue emission to blue–green or even yellow. We have studied both the photoluminescence (PL) and electroluminescence (EL) of PFs with different molecular structures and found that the low‐energy emission band originates from fluorenone defects which are introduced by photo‐oxidization, thermal oxidation, or during device fabrication. X‐ray photo‐emission spectroscopy (XPS) results show that the oxidation of PF is strongly catalyzed by the presence of calcium. The fluorenone defects generate a stronger contribution to the EL than to the PL. By utilization of a novel electron‐transporting material as a buffer layer between the emissive PF and the Ca/Ag (Ba/Ag) cathode, the blue EL emission from the PF was stabilized.

Single-molecule electroluminescence and photoluminescence of polyfluorene unveils the photophysics behind the green emission band

“…Later, however, it has been shown that emission in the green spectral range originates from a keto defect formed as a result of photooxidation or thermal oxidation [16][17][18][19][20][21][22] . Copolymers and oligomers containing well-defined fractions of fluorenone as a model of the oxidized species showed emission from the fluorenone unit that is photophysically identical to the green band in polyfluorene 23 .…”

Luminescent properties of a novel fluorene organic material

“…It is generally accepted that keto defects are generated in PF-type polymers by the oxidation of monoalkylfluorenes (42) and are extremely difficult to eliminate from the monomer feed and, therefore, are incorporated into the polymer. Some research groups give evidence of the formation of 9-fluorenone defects upon chemical, photo-or thermal oxidation (37)(38)(39)43). Considering the UHV atmosphere, however, fluorenone caused by the oxidation process can not be excluded.…”

“…In many fluorene-based devices, the development of a structured emission band around 490-500 nm, i.e. at a slightly lower wavelength than the usual green band (500-600 nm), can be observed (43,(46)(47)(48)(49) This emission band has previously been assigned to excimer emission in fresh PFs synthesized by Ni-catalysed Yamamoto coupling (19) and it can be observed also in uncapped PFs prepared by Suzuki coupling (11). The formation of intramolecular excimers with the new C-C bonds formed (38), electric field-induced excimer (48) and fluorenone defects introduced by the cathode (43) also would produce a green emission around 485 nm.…”

“…Nevertheless, OLEDs are usually fabricated and operated in an inert or vacuum atmosphere. In many devices operated under those conditions, this is also assigned to fluorenone defects introduced in the deposition of the cathode, which is a low-work-function metal (Ca or Ba) (43).…”

“…The formation of intramolecular excimers with the new C-C bonds formed (38), electric field-induced excimer (48) and fluorenone defects introduced by the cathode (43) also would produce a green emission around 485 nm. In our experiments, the BFLBBFLYQ film was spin-coated in air, then it was baked at 100°C for 10 min in air, which would induce the formation of fluorenone defects upon thermal or photo-oxidation in the presence of oxygen (19,43,(50)(51)(52) Therefore, the peak at 452 originates from the emitter of the BFLBBFLYQ molecule, and the peaks of 490, 528 and 576 nm can be attributed to fluorenone defects.…”