White Electroluminescence from a Single Polymer System: Improved Performance by Means of Enhanced Efficiency and Red‐Shifted Luminescence of the Blue‐Light‐Emitting Species

Abstract

Recently, white polymer light-emitting diodes (WPLEDs) have received considerable attention because of their potential utilization in display and lighting applications via low-cost solution processing. One common approach to obtaining white emission from polymer LEDs is to use blends as emissive layers, such as polymer/polymer blends [1][2][3][4][5][6] or polymer/ small-molecule blends. [7][8][9][10][11][12][13][14][15][16] Although high electroluminescence (EL) efficiencies have been achieved, WPLEDs based on blend systems suffer from bias-dependent EL spectra and intrinsic phase separation during long-term device operation. Another approach to WPLEDs is the use of a single polymer as the emissive layer. [17][18][19] Together, blue emission from the polymer itself and orange emission from an aggregate/excimer/exciplex give white emission. Although phase separation is completely prevented, WPLEDs based on this strategy suffer from low EL efficiencies. Our group has proposed a novel strategy for a white electroluminescent single polymer, which involves incorporating an orange-light-emitting unit into a blue-emitting polymer host. [20][21][22][23][24][25] WPLEDs of this design have the great advantages of high EL efficiencies, bias-independent EL spectra, and no phase separation. We also found that the EL efficiencies of white polymers could be improved by either enhancing the PL quantum efficiency of the orange-emitting species [22] or attaching an orange-emitting species to the side chain of a blueemitting polymer backbone.[23]In all of our white single polymers, the blue-emitting species is polyfluorene (PF). Moreover, the aforementioned efficient WPLEDs based on blends also employ PF as the blue-emitting material; however, as a blue electroluminescent polymer PF suffers from two drawbacks: i) the PL quantum efficiency of PF in solid films is low (U PL = 0.55) [27,28] because of the interaction of polymer chains, and ii) the emission spectrum of PF lies in the deep-blue region, to which human eyes are insensitive, and limits the luminous efficiency of the white emission. Hence, we believe that the blue-emitting species is the bottleneck for the EL efficiency of white electroluminescent polymers and devices. Our group has reported a highly efficient blue-emitting polymer of a dopant/host system, in which a light-blue-emitting dimethylamino naphtalimide (DMAN) unit (model compound 4-dimethylamino-9-decyl-1,8-naphthalimide (MC-B1), see Supporting Information) is attached to the side chain of deep-blue-emitting PF (PFB5, see Scheme 1).[29] This polymer exhibits a much higher EL efficiency than PF because its emission dominantly originates from the highly fluorescent, dispersed DMAN unit (U PL = 0.84), and because its EL spectrum is partially red-shifted compared with that of PF (ca. 30 nm). In this Communication, we report how we have replaced PF by a DMAN-containing PF as the blue-emitting species in a white-emitting single polymer, with the aim of improving the EL efficiencies of white-emitting single polymers by...

On the origin of high quality white light emission from a hybrid organic/inorganic light emitting diode using azide functionalized polyfluorene

“…[14][15][16][17][18][19] Due to the complexity of multi-chromophore polymers or blend systems there arises some difficulty in the formation of uniform multiple polymer films which can be attributed to the intrinsic phase separations among different components in the system. 2,3 It is also difficult to synthesize such multi-chromophore polymers in high yields.…”

“…2,5 For these reasons, light emitting conjugated polymers have been explored for their potential use, for instance, in full colour displays. [3][4][5][12][13][14][15][16][17][18][19] Recently, various strategies have been developed for the efficient use of those materials also as downconverting fluorescent materials on inorganic light emitting diodes or in electrically driven polymer based LEDs to generate white light.…”

Studying the Effect of Nicl2 Addition on the Optical Properties of Synthesized Chitosan

“…The optical properties of these materials can be easily tuned by controlling contents of the different concentrations. Though a great deal of excellent work has been reported on such materials [7][8][9] , it is still meaningful to extend the research of these polymers.…”

White emitting polyfluorene functionalized with azide hybridized on near-UV light emitting diode for high color rendering index

“…Similar approaches have also been successfully shown to make electrically driven polymer-based LEDs. Another method also shown in such polymer LEDs combines the emission of multiple primary colors, necessary for the full white light generation, on a single but rather complex polymer, with multiple chromophores each of which emits a primary color simultaneously [11][12][13][14][15][16]. However, in these approaches, it has been difficult to obtain very high color rendering index (CRI), which is required to be >80 for the future solid-state lighting [17].…”

“…First, polymers exhibit high absorption coefficients and feature high solid-state photoluminescence quantum efficiencies; second, they are easily processed, molecularly engineered, and coated using very low-cost techniques [2,8]. To this end, light emitting conjugated polymers have been extensively studied to explore their potential use, mainly in full color displays till date [6][7][8][9][10][11][12][13][14][15][16].…”