Analysis Of Long-lived Traps In Organic Materials And Their Effects On Quantum-dot Light-emitting Diodes

Quantum dots（QDs）,due to their high photoluminescence efficiency,wide tunable emission wavelength range,narrow emission spectrum,and low-cost solution processing,have attracted much attention.Thus,quantum-dot light-emitting diodes（QLEDs）provide great application prospects and commercial value in the fields of high-quality displays and solid-state lighting,and becoming one of the strong candidates for next-generation lighting and display applications.However,the working mechanism of QLEDs has not been fully explored so far,such as electroluminescence（EL）turn-on behavior,the positive aging effect,origin of degradation,etc.Therefore,multidisciplinary efforts are needed to reveal the underlying physical mechanisms.For QLEDs,the carrier dynamics process is the most essential and fundamental physical process during the operation process.And there is no doubt that the defect states in every functional layer of the QLEDs have an important impact on the carrier dynamic processes of the devices.However,there is rare report on this problem.Defects widely exist in the semiconductor materials,which deserve great effort due to their various significant influences on the properties of semiconductors and corresponding photo-electronic devices.In some cases,the defects play a positive role in the photo-electronic properties of semiconductor materials.For example,the defects could function as the main pathway for carrier transport,or as a luminescent center in light-emitting materials.However,in most cases,defects always deteriorate the properties of semiconductor films.The carrier trapping process induced by defects decreases the free carrier concentration,reducing the carrier mobility.Meanwhile,these defect states could also result in exciton quenching,thus reducing the efficiency of photo-electronic devices.Therefore,it is urgent to characterize the defects and to uncover their influence on the EL devices.In this work,we develop an easy-to-operate and low-cost technology to characterize the long-lifetime traps in organic semiconductor films,which is achieved through recording the Current by a Periodic Stepwise-Increasing Voltage（CPSIV）.This method is based on the principle that the captured carriers can induce scatter effect and localized space electric fields due to the trap capturing,which decreases the current through the film.This means the carrier trapping process can be obtained by measuring the current difference through the film at a periodic step voltage.Specifically,the defect states in the materials can be evaluated by calculating the current difference,ΔI,between the first and second cycles of the film.In this paper,the detailed principle,the measurement circuit,the measurement procedure and the related data processing progress of the CPSIV are described in detail.The effect of each parameter on the CPSIV measurement was analyzed by characterizing the QLEDs with the structure of ITO/Zn O/QDs/4,4′,4″-tris（N-carbazolyl）-triphenylamine（TCTA）/Mo O₃/Al.First,the CPSIV was performed for two hole-only devices consisiting of two commonly used hole transport layers（HTL）,TCTA and 2,2′-bis（4-（carbazol-9-yl）phenyl）biphenyl（BCBP）.To validate the CPSIV,the following measurements were carried out.The effect of device aging was excluded by the recoverability of current and the current differenceΔI between the first and second cycles.Then,by the temperature measurement of the device before and after the CPSIV measurement,we excluded the possibility that the temperature change causes the current difference.Finally,the injection barrier between TCTA and Mo O₃ was eliminated by inserting a 5-nm 4,4’-bis（9-carbazolyl）-2,2’-biphenyl（CBP）between TCTA and Mo O₃ to form Ohmic contact.However,the current difference of this ohmic contact device could still be observed,which excludes the injection barrier as the origin of current difference.Thus,we verified the effectiveness of CPSIV for the characterization of the defect states.In addition,one of the key parameters in QLEDs,the hole injection voltage V_h-inj,could be obtained by the CPSIV.Then we demonstrated that the EL turn-on process of QLED was determined by the hole injection.Meanwhile,by correlating the trap capturing process in QLEDs with the luminance overshoot of QLEDs under a fixed current source,we found that they are highly similar in intensity and time scale.Therefore,we suggested that the luminance overshoot phenomenon of QLEDs under a constant current source is also caused by these long-lifetime defects in HTL.