Improving the Performance of Polyfluorene-Based Organic Light-Emitting Diodes via End-capping

Abstract

Efficient deep‐blue electroluminescence(EL) is obtained via end‐capping polyfluorene (PF) homopolymers with hole‐transporting moieties (HTMs). Strong evidence that the observed improvement in device performance is related to the end‐capper moieties comes from the comparison of two different end‐cappers. The HTM does not disturb the liquid‐crystal properties of the PF polymer. Using doped polyimide alignment layers, polarized light‐emitting diodes (LEDs) with a polarization ratio in excess of twenty at an efficiency of 0.25 cd/A could be realized.

Nonlinear refractive index and three-photon absorption coefficient of poly(9,9-dioctylfluorence)

“…6,7 Their efficient photoluminescence, electroluminescence, and high chargedcarrier mobility, coupled with thermotropic liquid crystallinity, have been utilized for various optoelectronic devices including light-emitting diodes, field-effect transistors, polymer-based lasers, and display applications. [8][9][10][11][12][13] Poly͑9,9-dioctylfluorene͒ ͑PFO͒ is a standard liquid-crystalline homopolymer, composed of planar biphenyl units linked by a carbon atom in position 9 ͑see the inset of Fig. 1͒.…”

Polymer Light‐emitting Diodes Based on End‐capped Poly[9,9‐di‐(2′‐ethylhexyl)fluorenyl‐2,7‐diyl]

“…For polymer light-emitting diode (LED) applications, end-capping has been directed toward improving device efficiency, [1][2][3][4] stabilizing blue emission, 1,[4][5][6] tuning the emission color, 2,3,7 as well as realizing white electroluminescence. 8 As a means for increasing polymer LED device performance, end-capping will not result in phase separation (a common phenomenon in polymer blend systems) with time and does not alter the electronic properties of the polymer backbone.…”

“…8 As a means for increasing polymer LED device performance, end-capping will not result in phase separation (a common phenomenon in polymer blend systems) with time and does not alter the electronic properties of the polymer backbone. 1 Furthermore, when end-capped with some hole transport moieties, polyfluorenes can become more resistant to oxidation. 9 A few mechanisms have been proposed to explain the increase in polymer LED device efficiency arising from end-capping.…”

“…Miteva et al supposed that for their devices based on end-capped polymer, most holes in the emissive layer might be pointed to the end-capper groups instead of sites with less efficient emission, which subsequently recombined with the electrons on the polyfluorene main chain. 1 However, Nakazawa et al believed that the position of the exciton recombination zone has an effect on light emission and device efficiency. 4 They suggested that end-capping gives rise to improved hole injection so that the recombination zone is moved away from the polymer/anode interface.…”

“…A possible reason for this improvement of polyfluorene blue emission is that the chain ends, rather than aggregates and excimer forming sites, preferentially become the places where the electron-hole recombinations occur. 1 In some cases, suppression of low energy emission is ascribed to the shift of the recombination zone away from the polymer/ anode interface, since the polyfluorene molecules only have strong interchain interactions near the anode interface but not in the bulk. 4 Kadashchuk et al found that polyfluorenes end capped with certain hole-transporting molecules, like triphenylamine derivatives, demonstrate high stability against oxidation, which leads to considerably decreased concentration of keto defects.…”