Interface Modification Of Quantum Dots And Organic Light-emitting Diodes

Quantum dots（QDs）are semiconductor nanocrystals with a size smaller than the exciton Bohr radius,which induces quantum confinement effect.The development of quantum dots light-emitting diodes（QLEDs）is rapid due to QDs’narrow emission spectra,adjustable emission peaks,high color purity,and excellent solution processability.QLEDs are multi-layer thin film devices,in which each layer has distinct functions.Many key electrodynamic processes,such as carrier injection and extraction,exciton recombination,energy transfer and dissociation,take place at the interface.Therefore,the interface is critical to the QLEDs’efficiency and stability.In order to drive large-size and high-resolution displays,active matrix（AM）driving scheme is adopted,in which each pixel is driven by a few thin film transistors（TFT）.Metal oxide TFTs（MOTFT）offer high electron mobility and low manufacturing cost,making them the ultimate choice in AM display.The inverted QLEDs device structure better fit the active matrix panel with n-type MOTFT than the regular device structure,because the cathode of the inverted QLEDs can be directly connected to the drain of MOTFT,thereby reducing the pixels’driving voltage.However,most inverted QLEDs are fabricated by combining solution-processed QDs layer with thermally-evaporated hole transporting layer（HTL）which increases the manufacturing cost.All-solution processed QLEDs are therefore preferred.Currently,the overall efficiency and the lifetime of all-solution processed inverted RGB-QLEDs device are much lower than those of the regular devices.To enhance the performance of the all-solution processed inverted QLEDs,we first dually doped poly-ethylene dioxythiophene:polystyrene sulfonate（PEDOT:PSS）to form a smooth and uniform film on the hydrophobic HTL.To reduce the hole-injection barrier between HTL and QDs layer,polyethylenimine（PEI）was introduced.Not only does PEI form a dipole layer to lower down the vacuum level,but also the amine group in PEI as electron donor passivates the electron traps,suppressing fluorescence quenching.As the result,the peak CE reaches 28.1 cd/A,43.1 cd/A,and 1.26 cd/A,the maximum external quantum efficiency（EQE）increases to 20.6%,10.4%,and 2.95%,and the maximum luminance reaches 5.06×10⁴ cd/m²,1.21×10⁵ cd/m²,and 2.96×10³ cd/m² for RGB QLEDs,respectively.To the best of our knowledge,the red device’s EQE is one of the highest among all inverted red QLEDs.Moreover,the red QLED’s extrapolated lifetime extends to8253 hours at an initial brightness of 100 cd/m²,which achieved the longest life at that time.The effects of temperature and Zn O interface contact on carrier dynamics have been investigated by the radiative recombination rate K_r,non-radiative recombination rate K_nr,and the interface charge transfer rate K_et and interface charge transfer efficiencyη_et.Theτ_ave of the quantum dot films without annealing,and with thermal annealing at 60°C,120°C,180°C,240°C,are 11.4 ns,10.3 ns,10.7 ns,10.5 ns and 10.1 ns,respectively.With the increase of annealing temperature,τ_ave gradually decreases.Fluorescence quantum yield（PLQY）decreases from 44.5%to 28.6%,and K_r decreases from 3.90×10⁷ s^-1 to 2.83×10⁷ s^-1,while K_nr increases from 4.86×10⁷ s^-1 to 7.07×10⁷ s^-1.The decrease of PLQY is due to the decrease of K_r and the increase of K_nr.The results show that thermal annealing increases the number of defects on the surface,which lowers the PLQY.When QDs are in contact with zinc oxide（Zn O）,τ_ave becomes shorter under different thermal annealing conditions.As the temperature increases to 240°C,τ_ave decreases from 8.5 ns to 7.2 ns due to the decrease of radiative recombination lifetime and the increase of non-radiative recombination lifetime.Concurrently,the PLQY of the quartz/QDs/Zn O film decreases from 32.5%to 18.9%,K_et increases from 3.06×10⁷ s^-1 to 3.99 spontaneous charge transfer.Thermal annealing causes more non-radiative recombination or trapped fluorescence emission,which reduces the utilization rate of excitons.As a result,the efficiency of QLEDs is reduced.The efficiency and the lifetime of RG-QLEDs have met the commercialization requirements,while B-QLEDs is still large behind.We introduced an interfacial dipole layer at the organic/metal interface by solvent vapor annealing（SVA）.By treating the blue light-emitting polymer poly[（dibenzo-thiophene-S,S-dioxide）-co-（dioctyl-2,7-fluorene）]（PFSO）with methanol vapor for 90 minutes,the current efficiency increases from 3.8 cd/A to 6.0 cd/A,while the turn-on voltage is reduced from 3.4 V to 2.8 V.It’s discovered that the interface dipole between the cathode and the emission layer is responsible for the performance enhancement.The interface dipole improves the electron injection by lowering the electron injection barrier.