Since the pioneering work of Burroughes et al. in 1990, [ 1 ] polymer light emitting diodes (PLEDs) based on conjugated polymers have attracted widely research interest for their potential applications in full color fl at-panel displays and illuminations. Compared to inorganic and small molecular organic light-emitting diodes, PLEDs can be conveniently manufactured by solution processing techniques such as spin coating or ink-jet printing, which is very important for low cost, fl exible and largearea displays. For full color displays, three basic color-red, green and blue are required. Compared with green and blue emissive polymers, the development of red EL conjugated polymers is far behind in terms of both color purity and EL effi ciency. Most red-emitting conjugated polymers are realized by incorporating red fl uorophores, such as 2,1,3-benzothiadiazole derivatives, [ 7–9 ] 2,1,3-benzoselenadiazole derivatives, [ 9–11 ] 2,1,3-naphthaselenadiazole, [ 12 ] cyanovinyl-containing units, [ 13 , 14 ] 3,4-diphenylmaleimide derivatives [ 15 ] and phenothiazine derivatives, [ 16 ] perylene-containing dyes, [ 17 ] etc., into backbones, side chains or end groups of conjugated polymers. Due to the nature of red emissive materials, the red fl uorophores emitting longer wavelength are more extensively ð -conjugated or stronger polar compounds, which usually have lower fl uorescence quantum yields ( Ö PL ). Meanwhile, they are highly susceptible to concentration quenching in solid state because of the intermolecular dipole–dipole interactions or ð – ð stacking. [ 18 ] So, the devices of red polymers with red enough emission wavelengths to achieve excellent CIE coordinates usually show inferior EL effi - ciencies. Until now, all of the fl uorescent pure and saturated red-emitting polymers show EL efficiencies no more than 2.0 cd A − 1 . [ 12 , 16 , 19,20 ] Recently, our group has reported a new approach to design high effi ciency EL polymers of dopant/host system with molecular dispersion feature by covalently attaching small amounts of dopant units to the side chain of polyfl uorene host. [ 21–25 ] By this means, the problems of phase separation in physical blend material systems [ 26–28 ] and voltage dependence of the EL spectrum existing in multilayer devices [ 29–32 ] will be avoided due to the homogeneous distribution of the dopant units in the polymer host. Meanwhile, the concentration quenching effect aforementioned will be partly suppressed because of the low content of dopant units, so highly effi cient blue, [ 21 ] green, [ 22 ] red [ 23 ] and white [ 24 , 25 ] PLEDs have been realized by using dopant/host strategy. However, for red-emitting polymers, [ 23 ] the key problem in our previous report is that we haven’t realized the EL effi ciency/color purity trade-offs. We fi nd that highly effi cient PLED (5.10 cd A −1 ) is not red enough (orange-red emission at 615 nm, CIE of (0.59, 0.39)), and deep-red PLED at 645 nm is not so effi cient (1.70 cd A −1 ), which is clearly corresponding to the PL efficiencies and emission wavelengths of the red dopant units. Thus, optimization of red dopant units is very important for realizing red PLEDs EL effi ciency/color purity trade-offs. In this article, we report a series of red EL polymers based on polyfl uorene as blue host and 2,1,3-benzothiadiazole derivatives with fi ne-tuned PL efficiencies and emission wavelengths as red dopant units on the side chain, aiming at tuning the EL efficiencies and color purities of the resulting polyfl uorene copolymers of dopant/host system and realizing the optimization of red PLEDs EL effi ciency/color purity trade-offs. The difference between this article and previous study is on achieving excellent PLEDs EL effi ciency/color purity trade-offs for red fl uorescent polymers based on polyfl uorene as blue host and 2,1,3-benzothiadiazole derivatives as red dopant. As a result, the single-layer devices of these polymers (ITO/PEDOT:PSS/ Polymer/Ca/Al) show pure red emission at 624 nm with a luminous effi ciency of 3.83 cd A −1 and CIE of (0.63, 0.35) for PFR1, and saturated red emission at 636 nm with a luminous effi ciency of 2.29 cd A −1 and CIE of (0.64, 0.33) for PFR2 respectively. Furthermore, by using ethanol soluble phosphonate- functionalized polyfl uorene (PF-EP) as the electron injection layer (EIL) reported in our previous study, [ 33 ] high performance two-layer devices (ITO/PEDOT:PSS/Polymer/PF-EP/ LiF/Al) are obtained with maximum luminous efficiencies of 5.50 cd A −1 and CIE of (0.62, 0.35) for pure red emission (PFR1), and 3.10 cd A −1 and CIE of (0.63, 0.33) for saturated red emission (PFR2), respectively. To the best of our knowledge, these are the highest efficiencies of pure and saturated red fl uorescent EL polymers reported so far.
Result and Discussion
We designed a series of saturated red emission model compounds (MCs), as shown in Scheme 1, by introducing different alkoxy groups to increasing the electron donating ability of 4-(5-(4-( N -phenyl- N -(4-methoxy)phenylamino)phenyl)thienyl-2)- 7-(4-(diphenylamino)phenyl)-2,1,3-benzothiadiazole (MC-R2) reported in our previous work, which show high PL effi ciency (59%), but orange-red emission at 620 nm. Firstly, we designed MCR1 by incorporating two methoxy groups to the triphenylamine segment of MC-R2, which will lead to about 10 nm redshift according to our experience. [ 24 , 34 ] Secondly, we designed MCR2 by changing the thiophene unit of MC-R2 to 3,4-ethylenedioxythiophene (EDOT) unit, which will lead to more than 10 nm redshift due to the stronger electron-donating effect of the ethylenedioxy bridge compared to dialkoxy groups. [ 35 ] Thirdly, for comparison, we designed MCR3 by adopting above-mentioned two modifi cations at the same time. Thus, we hope to obtain saturated red dopants with fi ne-tuned PL efficiencies and emission wavelengths. Then we obtained red polymers through covalently attaching these red dopant units to the side chain of polyfl uorene using alkyl spacers. The contents of the red dopant units have been controlled to be 0.3 mol% in order to achieve almost complete energy transfer from polyfl uorene host to red dopant units and suppress the concentration quenching effect. Thus, high EL efficiencies of these polymers with superior CIE coordinates will be expected.