The fabrication of solution deposited OLEDs is fraught with difficulties, largely due to the interlayer mixing and surface erosion during sequential deposition of the layers. We demonstrate that these problems...
The
problems with fabrication of solution-processed organic light-emitting
diodes (OLEDs) stem largely from the defects associated with sequential
deposition of the layers, solvent-induced surface erosion, and layer
mixing. Herein, we demonstrate that a photopolymerizable bis-diazirine
molecule can easily convert soluble polymers into cross-linked insoluble
materials, alleviating the problems associated with inter-layer mixing.
Upon 5–20 min irradiation with long wavelength/low power UV
(1.8 mW/cm2) bis-diazirines results in the formation of
carbenes that can react via carbon–hydrogen bond insertion
with polymers or small molecules yielding cross-linked networks. The
effectiveness of the bisdiazirine-mediated photo-cross-linking has
been studied by investigating the surface morphology of the hole-transporting
material polyvinylcarbazole (PVK), which exhibited 45% decrease in
surface roughness after 15 min of the UV irradiation. The effect of
this cross-linking procedure on device performance was studied in
OLEDs with the configuration of indium tin oxide/PEDOT:PSS/PVK:(0–10%)
cross-linker/PFO:2% F8BT/TPBI/CsF/Al. After the photolysis of the
diazirines, the experimental devices exhibited a 42% enhancement in
the maximum external quantum efficiency (EQEmax), from
1.2 to 1.7%, and a maximum luminous efficiency improvement from 3.9
to 5.4 cd/A. Overall, this observation suggests that 3-trifluoromethyl(aryl)diazirine-based
cross-linking processes is a promising method for fabrication of polymer
LEDs.
Density functional studies are performed to understand the role of chelating bi-phosphine ligands [(Ph 2 P(CH 2) m PPh 2); m = 1-4] in modulating the regio-selectivity of benzoic acid addition to 1-hexyne, in presence of ruthenium(II) catalyst [(Ph 2 P(CH 2) m PPh 2)Ru(methallyl) 2 ]. The Markovnikov addition to 1-hexyne is observed when catalyst 1 a [(Ph 2 P(CH 2)PPh 2)Ru(methallyl) 2 ] is employed, whereas a reverse regio-selectivity is witnessed in presence of 1 d [(Ph 2 P(CH 2) 4 PPh 2)Ru(methallyl) 2 ]. Anti-Markovnikov addition occurs via the neutral vinylidene intermediates (5 a/d) formed after 1,2-hydrogen shift in hexyne coordinated ruthenium(II) complexes 3 a/d. The energy profile shows clear preference for Markovnikov addition by 15.0 kcal/mol (G S L) in case of catalyst system 1 a. In contrast, anti-Markovnikov pathway following neutral vinylidenes are more favourable by 9.1 kcal/mol (G S L) for catalyst system 1 d .T h eZ-enol ester formation is more predominant in the anti-Markovnikov pathway since the activation barrier for this step requires less energy (5.9 kcal/mol, G S L) than the one furnishing the E-product. The calculated results are in good agreement with the reported experimental findings.
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