Thermal Annealing Of Emitting Layer And Interface Modification In CdSe/ZnS Quantum Dot Light-emitting Diodes

Quantum dot light-emitting diodes（QLEDs）are considered to be a prominent candidate as the next generation optoelectronic display,because of the excellent properties such as wide tunability of luminescence spectrum,narrow full width at half peak and solution processability.However,QLEDs still have the following problems:（1）Organic ligands modifiying on quantum dots（QDs）can passivate surface defects of QDs and enhance the dispersion and solubility of QDs,but the insulation of organic ligands is not conducive to the charge transfer of QDs and thus affects the luminous efficiency of QLEDs;（2）the relatively high electron mobility of ZnO as a common electron transport material in QLEDs usually leads to imbalanced carrier injection,and the electron self‐transfer between QDs and ZnO also results in exciton quenching.Therefore,thermal annealing of light-emitting layer and interface modification of ZnO layer with PF‐ffBTz polymer are used to improve the performance of QLED in this work,and the contents are descriebed as follows:Firstly,thermal annealing of CdSe/ZnS red QDs was used to improve the performance of QLEDs.The RQDs light-emitting layer by thermal annealing with different temperatures（60,90,120,150℃,12 min）was used to fabricate the inverted QLEDs.The QLEDs without thermal annealing showed external quantum efficiency（EQE）of 10.02%and turn-on voltage(V_on)of 2.8 V.The luminous efficiency of QLEDs with thermal anealing first increased and then declined with the increase of annealing temperature,and moreover the QLEDs with thermal annealing at 120℃showed the highest EQE of 12.07%and the lowest Von of 2.2 V.The measurements of the carrier‐only devices showed that the carrier mobility of RQDs light-emitting layer gradually increased with the increase of anealing temperature.It was found that the energy level of RQDs light-emitting layer should not be affected by thernmal annealing through UPS and UV‐vis measurements.The charge transfer characteristic of QLEDs based on RQDs with different thermal annealing was studied by impedance method.According to the analysis of Nyquist spectrum,the bulk resistance of QLEDs gradually decreased with the increase of annealing temperature,indicating gradually enhanced charge transfer.However,the tests of capacitance versus voltage suggested that the increase of annealing temperature would significantly aggravate the electron self-transfer between ZnO/QDs.Therefore,appropriate annealing temperature is particularly important for improving the luminescence performance of QLEDs.Second,Interface modification of ZnO with polymer PF‐ffBTz was used to improve the performance of QLEDs.PF‐ffBTz was an alcohol soluble and wide band-gap interfacial polymer.PF‐ffBTz was used as the interface modification layer between ZnO and QDs to construct inverted QLEDs.The blue QLEDs without PF‐ffBTz modification layer showed EQE of 2.42%,while the EQE of B-QLEDs with PF‐ffBTz modified layer（thickness of 12 nm）was increased to 4.25%（increase range of 75%）.The measurements of UPS and UV‐vis showed that compared with ZnO without PF‐ffBTz modified layer,the electron injection barrier of ZnO with PF‐ffBTz modified layer was reduced.The measurements of electron-only device showed that compared with ZnO without PF‐ffBTz modifation layer,the electron transport efficiency of ZnO with PF‐ffBTz modified layer was reduced,which might be due to stronger charge tunneling barrier of PF‐ffBTz polymer.It was found that the surface defects of ZnO could be effectively passivated by PF‐ffBTz modified layer through the fluorescence emission measurements.The measurements of TRPL showed that the interfacial charge transfer rate and efficiency of ZnO/QDs were 1.28×10⁸ S^-1 and 31.5%,respectively,and moreover the interfacial charge transfer rate and efficiency of ZnO/PF‐ffBTz/QDs were decreased to 0.53×10⁸ S^-1 and 15.8%,respectively,which indicated that PF‐ffBTz interface modification layer could effectively reduce the electron self‐transfer between ZnO and QDs and inhibit the exciton quenching of QDs,and thus improve the luminescence performance of QLEDs.Further,PF‐ffBTz interface modification layer was applied to red and green QLEDs,respectively.The current efficiency of R-QLEDs and G-QLEDs based on PF‐ffBTz modification layer is 20.72 and 22.80 cd/A,respectively,which were both higher than QLEDs without PF‐ffBTz modified layer.