Research On Transport Layer Interface Regulation And Carrier Behavior Of Perovskite Light-emitting Diodes

Emerging as a research hotspot and an urgent breakthrough in fields of next-generation display and lighting,perovskite light-emitting diodes（PeLEDs）demonstrate great application prospects.However,PeLEDs still suffer from key bottlenecks such as low blue-lighting efficiency,unbalanced charge injection and transport,and low stability.The imbalance in number and time scale of electron and hole injection will restrict the number of effective excitons,resulting in charge accumulation and recirculation.This leads to severe exciton quenching and performance degradation,which would seriously hinder commercial applications of PeLEDs.Therefore,it is of great scientific significance and application value to analyze the interface physical information of charge transport layers（CTLs）in PeLEDs and explore their carrier behaviors to promote the rapid commercial application of PeLEDs.In this paper,we carry out a series of interfacial physics work on issues such as carrier injection and transport,charge accumulation,and system relaxation at electrode/CTL/emitter heterointerface in PeLEDs.The main researches are listed as follows:（1）Aiming at the anomalous stagnation of the current density of PeLEDs with increasing voltage,we propose a synergistic characterization strategy with conbining visual contact potential/polarization difference of KPFM and multi-time-scale capacitance/voltage response of impedance spectroscopy,revealing hole interlayer-dependent interfacial carrier dynamics in LEDs.It is found that Poly-TPD features strong/weak alternating ordered nematic stripes,this ordered polarization distribution would lead to alternating dominance of capacitive and resistive behaviors when Poly-TPD directly contacts ITO.The dynamic accumulation and dissipation of interfacial charges inherently cause stagnation of current density in PeLEDs and OELDs.Introducing a semi-metallic hole interlayer（PEDOT:PSS）with more disordered polarization direction and higher polarization strength may construct abundant carrier hopping channels.Therefore,electrode polarization can be effectively reduced,dynamic charge transfer between electrode and Poly-TPD may be promoted,further improving capacitive reactance contribution of OLEDs at lower voltages and inductive reactance ratio at higher voltages,thereby achieving high-efficiency electroluminescence（EL）at lower voltages.Moreover,the higher capacitance ratio of system with PEDOT:PSS leads to the serious hysteresis of high-speed response capability of OLEDs.The frequency and electric field-mediated capacitance evolution of each functional layer provide important references for designing LEDs-based high-speed optical communication systems.（2）Aiming at unclear carrier injection behaviors at the interface between electrode and hole transport layer（HTL）caused by pre-biasing,TFB is chosen as the HTL and TFB-based OLEDs are constructed.Resulted,when TFB directly contacts ITO,the current density of OLEDs will stagnate with increasing voltage,which is similar to that of Poly-TPD system.The introduction of hole interlayer（PEDOT:PSS）can eliminate this stagnation phenomenon,enhance the anti-interference ability of the system under the smaller pre-biased stimulation,thereby improving device efficiencies.It is found that electrical stimulation with a smaller cut-off voltage can effectively smooth the electrode/TFB interface contact and make charge accumulation and dissipation occur earlier,thereby reducing system loss caused by ineffective charges and reducing the stagnation behavior.Low-frequency voltage-dependent real-part capacitance can effectively assess polarization degree at the HTL/electrode interface under initial small voltage stimulation.The capacitive behavior decreases first under low voltage stimulation,and the induced polarization accumulation enables capacitive characteristics to dominate again when the voltage continues to increase,while resistive behaviors dominates at higher voltages,the corresponding energy dissipation behavior can be evaluated by the high-frequency imaginary capacitance.（3）Aiming at unclear effects of doping on contact type and energy band bending of the heterointerfaces,we construct Na Cl-doped PEDOT:PSS systems,reveal the capacitive hysteresis phenomenon and elucidate the evolution mechanism of interfacial carrier behavior caused by doping.It is found that the capacitive charge hysteresis network caused by the composition gradient of doped PEDOT:PSS can be continuously identified by the capacitive response of the relaxation scale matching AC perturbation of a specific frequency band in a broad frequency range.The undoped PEDOT:PSS forms an ohmic contact with the electrode,and capacitive hysteresis network of doped systems will lead to carrier accumulation,causing the vacuum energy level to move down,aligning the Fermi plane and naturally deepening the HOMO energy level,thereby forming Schottky contacts with the electrode.The more matched energy level alignment between the doped PEDOT:PSS and perovskite significantly improves the hole injection efficiency and carrier balance,and the EQE（～11%）of the doped HTL-based PeLEDs is 1.87 times higher than that of the undoped system（～5.87%）.Furthermore,the capacitive hysteresis could be regulated by varying the doping concentration,the higher doping level can broaden the voltage hysteresis range and lower the current upper limit,and its stronger interface dipole orientation can more effectively suppress and shield the photogenerated current of PeLEDs.This study provides an insight into unraveling carrier behaviors of PeLEDs heterointerface.（4）Aiming at the instability of EL spectra of deep-blue hybrid halogen-based PeLEDs,we propose an ultrafast thermodynamics-induced interface carrier injection-enhancement strategy,and systematically explore its influence on the crystal quality,optical and electrical properties of perovskite quantum dots.Resulted,ultrafast thermodynamic control（UFTDC）can significantly inhibit the crystal growth rate,reduce crystal defects and increases the fluorescence lifetime.UFTDC effectively improves the hole injection efficiency and enhances the radiative recombination ratio.The resultant PeLEDs have a maximum EQE of3.66%in deep-blue light band（460 nm）,a maximum brightness of 2100 cd/m² and a lifetime(T₅₀)of 288 seconds.Moreover,the devices generate lower Joule heating during operation and less charge accumulation,significantly improving EL stability.This work provides an insight into improving the efficiency and stability of PeLEDs.（5）During the vapor evaporation of Al electrode in PeLEDs,the dissociation degree of LiF varies with thicknesses.We systematically explore carrier relaxation behaviors of LiF thickness-dependent systems.LiF with a smaller thickness can construct gradient carrier hopping channels,further slowing down the interfacial relaxation and energy disorder caused by direct contact between Al and TPBi.In addition,the voltage-dependent conductance modulation behavior and anti-interference ability to continuous electrical stimulation may be enhanced.Also,it can strengthen the Schottky contact behavior,reduce the ineffective accumulation of system charges,improve electron injection efficiency and promote carrier balance,thereby effectively improving PeLED performances.LiF with a smaller thickness will be completely dissociated.With the thickness increasing,the LiF part in contact with Al may dissociate,and the undissociated part close to the TPBi side will cause the polarization accumulation and reduce interface carrier hopping channels.As a result,low-frequency response of the imaginary-part capacitance to lower voltages and high-frequency response sensitivity of the real-part capacitance to higher voltages are enhanced.