Unicolored phosphor-sensitized fluorescence for efficient and stable blue OLEDs

Abstract

Improving lifetimes and efficiencies of blue organic light-emitting diodes is clearly a scientific challenge. Towards solving this challenge, we propose a unicolored phosphor-sensitized fluorescence approach, with phosphorescent and fluorescent emitters tailored to preserve the initial color of phosphorescence. Using this approach, we design an efficient sky-blue light-emitting diode with radiative decay times in the submicrosecond regime. By changing the concentration of fluorescent emitter, we show that the lifetime is proportional to the reduction of the radiative decay time and tune the operational stability to lifetimes of up to 320 h (80% decay, initial luminance of 1000 cd/m²). Unicolored phosphor-sensitized fluorescence provides a clear path towards efficient and stable blue light-emitting diodes, helping to overcome the limitations of thermally activated delayed fluorescence.

Fig. 1

Working principle of unicolored phosphor-sensitized fluorescence. a Radiative decay paths of phosphor-sensitized fluorescence. Long-range Förster resonance energy transfer from the triplet state of the phosphorescent donor to the energy-matched singlet state of a fluorescent acceptor reduces excited state decay time while preserving the emission color. Unwanted Dexter transfer to the acceptor triplet state can lead to a reduced quantum efficiency. b Spectral properties: absorption (dashed lines) and emission (solid lines) of donor (blue) and acceptor (red) in a unicolored phosphor-sensitized fluorescence system. The shaded area indicates the spectral overlap J of donor emission and acceptor absorption

Fig. 2

Molecular structures and photophysical properties of the unicolored phosphor-sensitized fluorescence system. a Molecular structures of the matrix, acceptor, and donor molecules. b UV–visible spectra of neat matrix, donor and acceptor layers on glass and normalized photoluminescence spectra of donor and acceptor in matrix. The red line shows the excitation wavelength of 375 nm used for the photoluminescent experiments

Fig. 3

Time-resolved photoluminescent experiments. All samples were excited at 375 nm to suppress direct acceptor excitation. a Sketch and TRPL-data of bilayer samples. The thickness of the matrix spacer layer, which separates the donor from the acceptor layer, was varied from 0 to 6 nm. Förster-radius and Dexter-radius are depicted (not to scale) as black and red circles around the donor molecules. The inset shows a zoom-in to illustrate the increase of counts. b Sketch and TRPL-spectra of mixed layers. The A concentration was gradually increased from 0% to 1.5%. The decays were fitted with a multi-exponential fit (red lines) to derive the mean (intensity-weighted) radiative decay time

Fig. 4

Characteristics of the light-emitting diodes (OLEDs) using mixed emission layers. The acceptor concentration was varied from 0 to 1.5%. a J-V-L characteristics. b External quantum efficiency vs luminance. c Normalized electroluminescence spectra and d normalized photoluminescence spectra. Donor-only and acceptor-only photoluminescence spectra were fitted with three Gaussian peaks. The mixed samples were fitted with linear combinations of the donor-only and acceptor-only fits: e and d show the donor and acceptor coefficients with respect to the acceptor concentration

Fig. 5

Correlation between lifetimes and radiative decay rates. Comparison of the relative radiative decay rates (black) and relative lifetime values (70% decay, LT₇₀) as a function of acceptor concentration. The device lifetimes were measured at constant current with initial current density of 25 mA/cm² (blue) and initial luminance of 4000 cd/m² (red)