The Role of Triphenylamine in the Stabilization of Highly Efficient Polyfluorene‐Based OLEDs: A Model Oligomers Study

Abstract

In order to understand the factors responsible for the improved efficiency and stability of organic light-emitting diodes (OLEDs) based on poly(9,9-dioctylfluorene) (PFO) when triphenylamine (TPA) is introduced as lateral fluorene substituent, we synthetize mono-disperse fluorene-thiophene oligomers as model compounds. Their blends with different concentrations of the fluorenone containing oligomer are studied in order to verify if only a reduction of ketonic defect sites or also an impeded energy transfer (ET) towards such sites are responsible for the suppression of the green emission band. We show that the introduction of TPA groups leads specifically both to an antioxidant action and a reduced ET towards residual defect sites, thanks to the environmental micro-encapsulation role played by TPA units surrounding the polymer backbone. As a result, the performances and colour stability of a device fabricated with TPA-substituted PFO (PFTPA) are strongly improved with respect to standard PFO device prepared in the same conditions.

Deep Blue Light Amplification from a Novel Triphenylamine Functionalized Fluorene Thin Film

“…The PL spectrum results to be stable in air with no appearance of the typical green large emission mainly due to the keto defect. This result is consistent with previous reports [18,22] evidencing that the introduction of TPA group as substituent on 9-position of a fluorene unit reduces the number of fluorenone defects as a result of its relevant antioxidant action. In fact, it is generally accepted that keto-defects are generated in PFs by the oxidation of residual monoalkylfluorenes [37] already present in the polymer (monoalkylated fluorene residues present as impurities incorporated into the polymer during polymerization), or generated during operation by thermal-, photo-, or electro-oxidative degradation processes catalyzed by the presence of residual metal impurities [38] of some PFs.…”

“…Among the large class of semiconducting organic conjugated materials, fluorene derivatives are promising compounds for organic lasers because of their efficient blue emission and lasing characteristics with low threshold [14][15][16][17][18]. However, most of the fluorene-based materials suffer from chemical instability due to easy oxidation with formation of fluorenone moieties responsible for the appearance of a low energy green emission band [19].…”

“…Triphenylamine (TPA) moiety has been proposed as a versatile building block able to suppress both keto defects and aggregation during time [26,27]. Hole-transporting molecules such as TPA derivatives are reported to act as antioxidants for conjugated materials and to inhibit the oxidation of fluorene derivatives, hence considerably reducing the concentration of keto defects [18,28]. The bulky and propeller-shaped TPA structure incorporated into the polymer backbone tends to be packing-disruptive, most of the TPA substituted materials are amorphous, with high thermal stability, good film-forming ability, and good solubility [29][30][31].…”

π-Stacked polybenzofulvene derivatives as hosts for yellow and red emitting OLEDs

“…Devices fabricated with P1 are quite unstable (see Supplementary material), showing a weak red-shifted EL probably related to the formation of fluorenone defects [15]. On the other hand, the devices based on the P2 polymer are stable and show EL spectra corresponding to the PL (see Fig.…”

Bis(pyridylpyrazolate)platinum(ii): a mechanochromic complex useful as a dopant for colour-tunable polymer OLEDs

“…48 As it is well known, keto defect sites in polyfluorenes constitute an impediment for the fabrication of devices with a pure blue light emission. 49,50 However, they can play a key role in the design of white polymer OLEDs by doping with a red emitter. 51,52 In this context, we consider the strategic introduction of fluorenone defects on the active layer during device fabrication.…”