Study On The Energy Transfer From Phosphorescent Molecules To Quantum Dots In Quantum-dot Light-emitting Diodes

It is generally believed that the energy transfer(ET)processes can improve the performance of organic/inorganic quantum dot(QDs)hybrid structure devices.Moreover,it has been widely reported that highly efficient quantum dot light emitting diodes(QLEDs)have been obtained by using similar hybrid device designs.However,the physical mechanism of adding a phosphorescent small molecule layer to a device to improve device performance is not very clear.Now,it is generally believed that the improvement in device performance is due to the energy transfer of small phosphorescent molecules to QDs.However,this inference has not been directly proven.So far,in inverted devices,researchers generally believe that the exciton-forming region is at the QDs/hole transport layer(HTL)interface,which means that the interface greatly affects device performance.After adding a bis(4,6-difluorophenylpyridinato-N,C2)-picolinatoiridium(FIrpic)small molecule layer at the QDs/HTL interface,the FIrpic small molecule layer changes the contact interface of the QDs on the one hand,on the other hand,the addition of FIrpic may also change the carrier distribution inside the device.We analyze and study the role of FIrpic layer in the device through the following aspects:First of all,by testing and analyzing the photoelectric characteristics of the device,we know that the addition of the FIrpic layer will change the carrier distribution in the device,but it will not change the carrier transmission and injection process.The current density of the device has hardly changed,but the luminance of the device has improved significantly with the addition of FIrpic.At the same time,combined with the energy level structure of the device,we speculate that excitons may form in FIrpic,and the exciton recombination in FIrpic transfers energy to QDs,thereby improving the performance of the device.Secondly,in order to further explore the changes in the distribution of carriers within the device and the energy transfer process between FIrpic and QDs,we also performed transient EL(TrEL)and impedance tests on the device.The impedance test can analyze the change of the accumulated carriers in the device.We found that after FIrpic was added to the device,the amount of accumulated carriers in the device increased,which proved that the distribution of carriers in the device changed.Transient EL can investigate the state of carrier injection,transmission,and accumulation in the device.By analyzing TrEL,we directly confirmed the energy transfer process between FIrpic and QDs.