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

Abstract

High quality white light generation with high colour rendering index (CRI) was achieved by integrating a cross-linkable azide functionalized polyfluorene derivative, namely poly[(9,9-dihexylfluorene)-co-alt-(9,9-bis(6-azidohexyl) fluorene)] (PFA), as a down-converting fluorescent material on the inorganic n-UV InGaN/GaN LED platform. For comparison, two other polyfluorene based polymers, namely poly[(9,9-dihexylfluorene)-co-alt-(9,9-bis(6-bromohexyl) fluorene)] (PFB) and poly[9,9-dihexyl-9H-fluorene] (PF), were tested for white light generation. While PFA and PF both led to white light generation, PFB fell out of the white region on the chromaticity diagram. Compared to PFA, both of the control groups (PF and PFB) exhibited much lower CRI. To gain a better insight into the mechanisms playing a key role for the generation of such high quality white light in PFA, all of these polymers were further subjected to a series of experiments such as controlled exposure to heat at 220 °C for 2 h under Ar and in air. The polymers PFA and PFB, which include cross-linkable groups, produced broad emission spectra in the region of 430-650 nm upon annealing in the absence of oxygen under Ar atmosphere while almost no change was observed in the emission spectrum of PF without any cross-linkable groups. PFA undergoes cross-linking through the decomposition of azide leading to reactive nitrene species, whereas in PFB cross-linking probably occurs via debromination. This result clearly proved that the broadening can not be attributed only to photo or thermal oxidation, but it is also due to cross-linking. PFA was also exposed to n-UV light from the InGaN/GaN LED to investigate its photostability. In these experiments, the spectral changes in absorbance and emission properties and thermal transitions of these polymers were monitored by FT-IR, UV-Vis and fluorescent spectrometry, and differential scanning calorimetry (DSC). These experiments indicated that PFA provides high quality white light opportunely via cross-linking and remains stable once cross-linking is formed in a solid film. © 2008 The Royal Society of Chemistry

Quantum efficiency enhancement in film by making nanoparticles of polyfluorene

“…Fluorene-based polymers have been extensively investigated because of their superior optoelectronic properties to be utilized in various photonic device applications [1][2][3][4][5][6]. Of such π -conjugated systems, poly(9,9-dialkylfluorene)s are particularly attractive as blue emitters in organic light emitting diodes (OLEDs), thanks to their high quantum efficiency, ease of processability, and good thermal stability [2].…”

White-Emitting Conjugated Polymer Nanoparticles with Cross-Linked Shell for Mechanical Stability and Controllable Photometric Properties in Color-Conversion LED Applications

“…While the intensity of blue emission band decreases, a new broad band peaking at 530 nm and extending to 650 nm appears as observed in our previous study. 33 In both spectra of PF3A in air and under nitrogen (Figure 5a,b, respectively), the intensity of blue emission further decreases while the intensity of green emission increases with respect to blue emission and a gradual decrease in the overall emission intensity is observed with increasing exposure time.…”

“…PF3B was synthesized using the similar procedure as previously reported. 32,33 However, in this synthesis, 2,7-dibromo-9,9-bis(3-bromopropyl)-9H-fluorene was used: yield 57%; , CH 3 ); gel permeation chromatography (GPC) M n = 0.6 Â 10 4 g mol À1 , M w = 1.6 Â 10 4 g mol À1 (THF was used as a solvent and polystyrene as standard).…”

“…34,35 However, we have shown in our previous paper that cross-linking via azide decomposition in the absence of oxygen, which is required for the keto defect formation, could still cause the emergence of the green emission band. 33 Azide groups can be decomposed under UV light to form very reactive nitrene species that facilitate the inter-and intra-cross-linking of polymer chains as well as might insert to the fluorene ring to create defects on the polymer backbone. However, there are no significant changes observed in the UVÀvis spectra of nanoparticles upon UV exposure, while the emission spectra change; this suggest that the defects must be in minute quantity and have low molar absorptivity not to be seen in the UVÀvis spectrum.…”

Fluorene-based macromolecular nanostructures and nanomaterials for organic (opto)electronics

“…PF-03e emitters collectively generate a very broad down-converting PL at longer wavelengths across the entirety of the visible spectrum, yielding high colour-rendering indices up to 91. Demir and co-workers [227] investigated the origin of high-quality white light emission from a hybrid organic/inorganic light-emitting diode using azide-functionalized PFs. Foulger and colleagues [228] described the preparation of hybrid nanoparticles composed of poly(2,5-dioctyl-1,4-phenylenevinylene), a green emitter (λ = 505 nm), and PF-01b (PFO), a blue emitter (λ = 417 nm).…”