Investigation On Carrier Dynamics In Quantum-dot Light-emitting Diodes

Quantum-dot light emitting diodes（QLEDs）which are based on inorganic quantum dots（QDs）have attracted extensive attention as one of the most potential next-generation display and lighting technologies due to the wide color gamut,pure color and good physical-chemical stability of QDs.Up to date,many studies have reported that the external quantum efficiency（EQE）of QLEDs for red,green and blue have exceeded 20%by optimizing the QDs or devices’structure.However,as to organic-inorganic hybrid devices,due to the different carrier mobilities and unmatched energy levels of charge transport layers,it is inevitable that non-radiative Auger recombination induced by imbalance charge injection could limit the device performance.At present,the understanding of the device operation mechanism and photoelectrical properties of QDs is still not deep enough,which limits the further optimization and application of QLEDs.As a result,to explore the operation mechanism of QLED,we revealed the carrier dynamics in devices by combining theoretical simulation and experimental results.In this paper,the carrier dynamics of QLEDs based on different structures are studied by transient electroluminescence（Tr EL）.We explored the influence of carrier injection,transport,distribution and recombination on device performance by controlling device structures.In addition,the theoretical results are consisting with the experimenatal results by using a modified Langevin recombination model,which further revealed the influence of Auger processes on device.The main contents of our study are as follows:1.The inverted QLEDs devices were fabricated based on three different electron transport layers（ETL）:Zn O,Ti O₂ and poly（3,4-ethylenedioxythiophene）:poly（styrenesulfonate）（PEDOT:PSS）.The influence of different electron transport layers on device operation are investigated by comparing the photoelectrical properties of devices.According to the different injection rate of electrons and holes,i.e.,carrier sequential injection behavior,a modified Langevin recombination model has been proposed to described carrier rate equation.And the simulated Tr EL curves of devices are in great agreement with the experimental results,which confirms the feasibility of this model.It is demonstrated that the formation of an exciton on the QDs with one electron injected first and a hole injected successively is much more effective than the reverse sequence.And the efficiency of negative trion Auger recombination（eeh type）is much lower than that of positive trion Auger recombination（e-h-h type）.The performance of Ti O₂-based devices is much lower than that of Zn O-based devices.The performance of Ti O₂-based devices is enhanced obviously by inserting 1,3,5-tris（1-phenyl-1H-benzimidazol-2-yl）benzene（TPBi）hole-blocking layer between QDs and the hole transport layer（HTL）.Finally,it is proved that a balanced charge injection is beneficial to improve device performance by Tr EL measurement and capacitance-voltage properties.Furthermore,the QDs are completely negative-charged by electrons or nearly each of the QDs is populated by one electron,which are beneficial to enhance the device performance while positive-charged QDs are unconducive to devices.2.The charge distribution in Zn O-based inverted-hybrid QLEDs were controlled by inverting TPBi hole blocking layer in 4,4’,4’’-tris（N-carbazolyl）-triphenylamine（TCTA）layer.And the relationship between charge distribution and carrier dynamics was explored by Tr EL measurement.The Tr EL rising edges and falling edges are highly dependent on the charge distribution in devices while the overshoots in rising edges are originated from holes’accumulation in devices,and the overshoot in falling edges are attributed to the process that the leaked electrons transport back to QDs layer and recombine with holes under reverse bias.Inserting TPBi hole blocking layers at the interface of QDs/TCTA can suppress hole injection efficiently and reduce the quenching effect of holes on excitons in QDs which are beneficial to improve device performance However,the electron leakage process induced by TPBi layer is harmful to deivce performance.In addition,inserting TPBi layer in TCTA can reduce the hole’s injection process and electron leakage process in the meantime which can enhance the device performance obviously.This result proves that reducing the rate of ehh type Auger recombination and electron leakage process are both beneficial to enhance devices’peak-efficiency.3.By comparing the onset time t_d of inverted QLED based on different thicknesses HTLs,we found an abnormal tendency that the HTL-thickness was non-proportional to the onset time t_d.By analyzing the charge transport process in devices and the film morphology which is characterized by atomic force microscope（AFM）images,we proved this abnormal tendency is originated to the influence on hole injection which is caused by the roughness of film surface when the HTL thickness is thin enough.Specifically,when the thickness of HTL is below 100nm,the film morphology would become rough with the increasing of film thickness which could generate preferred hole-injection paths,thus holes injection is much faster in a thicker HTL（i.e.,the smaller onset time t_d）.In addition,by comparing the relationship between the Tr EL onset time t_d and device’s pixel area（the smaller the device’s pixel area,the shorter the onset time t_d）,we proved the influence of ITO electrode stepwise effect on hole injection.According to the operation mechanism discussed above,we calibrated the hole mobility calculation by Tr EL measurement by optimizing the charge transport time,transport distance and the calculation of electric field.Finally,the hole mobility of TCTA organic film was calculated more accuratly by this method.Our study provides an intuitive comprehension of charge transport in devices.