White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene as the Blue Host and a 2,1,3‐Benzothiadiazole Derivative as the Orange Dopant

Abstract

New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue (λmax = 421 nm/445 nm) and orange emission (λmax = 564 nm) from the corresponding emitting species. The influence of the photoluminescence (PL) efficiencies of both the blue and orange species on the electroluminescence (EL) efficiencies of white polymer light‐emitting diodes (PLEDs) based on the single‐polymer systems has been investigated. The introduction of the highly efficient 4,7‐bis(4‐(N‐phenyl‐N‐(4‐methylphenyl)amino)phenyl)‐2,1,3‐benzothiadiazole unit to the main chain of polyfluorene provides significant improvement in EL efficiency. For a single‐layer device fabricated in air (indium tin oxide/poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonic acid/polymer/Ca/Al), pure‐white electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.35,0.32), maximum brightness of 12 300 cd m–2, luminance efficiency of 7.30 cd A–1, and power efficiency of 3.34 lm W–1 can be obtained. This device is approximately two times more efficient than that utilizing a single polyfluorene containing 1,8‐naphthalimide moieties, and shows remarkable improvement over the corresponding blend systems in terms of efficiency and color stability. Thermal treatment of the single‐layer device before cathode deposition leads to the further improvement of the device performance, with CIE coordinates of (0.35,0.34), turn‐on voltage of 3.5 V, luminance efficiency of 8.99 cd A–1, power efficiency of 5.75 lm W–1, external quantum efficiency of 3.8 %, and maximum brightness of 12 680 cd m–2. This performance is roughly comparable to that of white organic light‐emitting diodes (WOLEDs) with multilayer device structures and complicated fabrication processes.

Organic emitters for solid state lighting

“…11a and b, respectively. In both cases, the molar ratio of monomers plays a key role in control of polymer emission [30,31].…”

Improving the Performance of Phosphorescent Based White Polymer Light Emitting Devices Using Iridium Complexes

“…Various approaches towards realizing WOLEDs have been reported, including multilayer structures capable of sequential energy transfer, multiple component emissive layers containing an appropriate of RG phosphorescent or fluorescent dopants [11][12][13][14][15]. Polymer blends containing RGB emitting species, charge transfer exciplexes or excimers braoad emission and single component layers that utilize a polymer with broad emission [16][17][18][19][20][21][22].…”

An effective method to prepare polymer/nanocrystal composites with tunable emission over the whole visible light range

“…Normally, blue, green, and red are the basic colors required to obtain white-light emission [20,21]. Two complementary color molecules are also good choices [22][23][24]. For example, Wang et al prepared a single-polymer white-light system involving two-colored chromophore emission [24].…”