Abstract
Semiconducting polymers that feature thermally activated delayed fluorescence (TADF) can deliver a much desired combination of high-efficiency and metal-free electroluminescence and cost-efficient solution-based fabrication. A TADF polymer is thus a very good fit for the emitting compound in light-emitting electrochemical cells (LECs) because the commonly employed air-stabile and few-layer LEC architecture is well suited for such solution-based fabrication. Herein we report on the first LEC device based on a TADF polymer as the emitting species, which delivers a luminance of 96 cd m–2 at 4 V and a current efficacy of 1.4 cd A–1 and >600 cd m–2 at 6 V, which is competitive with the performance of multilayer organic light-emitting diodes based on the same TADF polymer. We further utilize the established sensitivity of the emission of the TADF polymer to its environment to draw conclusions on the exciton populations in host-guest and host-free TADF LEC devices.
Highlights
Semiconducting polymers that feature thermally activated delayed fluorescence (TADF) can deliver a much desired combination of high-efficiency and metal-free electroluminescence and cost-efficient solution-based fabrication
The singlet excitons in phosphorescent emitters can be efficiently transferred to the triplet state by intersystem crossing (ISC), so that, in principle, all electrically generated excitons can be harvested for light emission.[27]
A recently invented class of heavy-metal free organic compounds addresses the above issue through the emission process of thermally activated delayed fluorescence (TADF).[28−32] Efficient TADF emitters are designed so that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are spatially separated, which render the lowest excited singlet state (S1) and the lowest excited triplet state (T1) positioned close in energy; with such a design, a thermally promoted reverse intersystem crossing (RISC) from the nonemissive T1 to the emissive S1 can take place.[33]
Summary
The blend and active-material inks were prepared by blending the master inks in a desired mass ratio, followed by stirring on a magnetic hot plate at 323 K for. The ink-under-study was spin-coated (2000 rpm, 1000 rpm s−1, 60 s) on a carefully cleaned substrate and thereafter dried at 343 K for >2 h. The optical transmission of 100 nm thick spin-coated thin films on quartz substrates (thickness = 1 mm, Ted Pella) was measured with a spectrometer The active-material ink was spin-coated either on top of the ITO or on top of the ITO/PEDOT:PSS and dried at 343 K for >3 h. The thickness of the active material was controlled by the spin-coating parameters (800 rpm, 800 rpm s−1, 60 s).
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