Conjugated Polyelectrolyte Hybridized ZnO Nanoparticles as a Cathode Interfacial Layer for Efficient Polymer Light‐Emitting Diodes

Abstract

IntroductionPolymer light-emitting diodes (PLEDs) have attracted a great attention as a possibility for next-generation large-area display and lighting devices due to solution processability and Herein, we present CPE-hybridized ZnO NPs (CPE:ZnO hybrid NPs), so-called organic/inorganic hybrid material, as a solution-processable CIL for high-effi ciency PLEDs. Alkoxy side-chain tethered polyfl uorene CPE, poly [(9,9-bis((8-(3-methyl-1-imidazolium)octyl)-2,7-fluorene)-alt-(9,9-bis(2-(2-methoxyethoxy)ethyl)-fl uorene)] dibromide (F8imFO 4 , Figure 1 a), was synthesized in order to offer not only good miscibility with ZnO NPs for preventing phase separation but also make use of the charged nature of CPE for better electron injection at EML/electrode interface. X-ray photoelectron spectroscopy (XPS) analysis results reveal that the surface defects of ZnO NPs are passivated by forming two different kinds of coordination bond with functional groups, alkoxy side-chains and bromide anions, from F8imFO 4 . We fi nd that a specifi c CPE concentration (4.5 wt% to the weight of ZnO NPs) shows the most effi cient passivation effect of ZnO NPs and reduces photoluminescence (PL) quenching of yellow-emitting PPV layer (commercial name "Super Yellow," SY). Consequently, the device effi ciencies of SYbased PLEDs are greatly increased with the CPE:ZnO hybrid NPs (11.7 cd A −1 at 5.2 V and 8.6 lm W −1 at 3.8 V) compared to the PLEDs with ZnO NPs only (4.8 cd A −1 at 7 V and 2.2 lm W −1 at 6.6 V) or F8imFO 4 CPE only (7.3 cd A −1 at 5.2 V and 4.9 lm W −1 at 4.2 V). Results and DiscussionF8imFO 4 CPE was synthesized through Suzuki cross-coupling polymerization, followed by substitution reaction using n -methylimidazole to functionalize imidazolium bromide ionic pendent group on alkoxy side-chain tethered polyfl uorene. [ 25,26 ] The resulting polymer, F8imFO 4 , is soluble in typical polar organic solvents such as 2-methoxyethanol. F8im CPE was similarly prepared. For the preparation of colloidal ZnO solutions, ZnO NPs were synthesized using zinc acetate dihydrate and lithium hydroxide monohydrate as precursors in ethanol. [ 14 ] The X-ray diffraction (XRD) result identifi es that the crystal structure of the ZnO NPs is corresponding to wurtzite structure of ZnO ( Figure S1, Supporting Information). F8imFO 4 at desired concentration to the weight of ZnO NPs was added to the colloidal solution in 2-methoxyethanol to obtain the CPE:ZnO hybrid NPs. (See the details of the synthetic procedures in the Supporting Information.) Figure 1 b,c shows the transmission electron microscopy (TEM) images of CPE:ZnO hybrid NPs using two different CPEs; F8imFO 4 and F8im, respectively. It is observed that ZnO NPs are uniformly dispersed in the case of the hybrid NPs with F8imFO 4 , whereas ZnO NPs blended with F8im form agglomerated clusters. Note that the concentration of CPEs was fi xed at 9 wt% for both hybrid NPs. The particle size distribution results in Figure 1 show a better dispersion of the particles (13.4 nm) than that of CPE:ZnO hybrid NP...

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“…Each binding energy was obtained from previous studies. [1], [8][9] As seen in Fig. 3 (b) and (c), the O L peak increased in ZRS 3% and decreased in ZRS 7%.…”

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