Study On Charge Carrier Injection And Recombination In Quasi-2D Perovskite Light-emitting Diodes

Quasi-2D lead halide perovskite,as a solution processable semiconductor material,has showed great potentials as light-emitting materials in light-emitting diodes due to their outstanding optical and electrical properties,such as high carrier mobility,high photoluminescence efficiency,narrow luminescence spectrum,and light-emitting color tenability.To achieve high efficiency in the quasi-2D perovskite light-emitting diodes,it’s critical to achieve efficient carrier injection from the electrodes into the function materials at the carriers’injection contacts and high proportion of radiative recombination.The device structure of Pe LEDs commonly stems from that of solution-process polymer light-emitting diodes（PLEDs）.However,the carrier-injection mechanism is quite different for Pe LEDs.It has been shown that Pe LEDs could operate at low voltage even with a single mismatch hole injection layer.So far,the injection of Pe LED has not been fully understood.Understanding the carrier-injection mechanism of Pe LEDs will help find a more suitable device architecture for Pe LED rather than following the PLED device architecture.In this thesis,the hole-injection mechanism of the devices with ITO/PVK/perovskite structure was studied.The model of metal-insulator-semiconductor（MIS）contact is proposed to reveal the hole-injection mechanism.For MIS contact of ITO/insulator/perovskite,the electric field is concentrated in the insulator layer,thereby improving the hole injection from ITO to perovskite.By studying the transient electrical properties,it is confirmed that the accumulation of both electrons and ions at the insulator/perovskite interface is responsible for the screening of electric field.The model predicts that the Pe LEDs can operate efficiently with any insulator as hole injection layer.Polymer insulators PMMA and polystyrene（PS）are employed as efficient hole injection layer in lead halide perovskites light-emitting diodes（Pe LEDs）for the first time.Both devices exhibit efficient charge injection,confirming that MIS contact is universally applicable for any insulating layers.The ITO/PMMA/perovskite device exhibits a maximum LE of 28.4 cd/A,a peak EQE of 6.8%,and a turn-on voltage of 3.0 V.By optimizing the thickness of PS layer,the performance FAPb Br₃-based device achieves a turn-on voltage of 2.8 V,a peak luminous efficiency of 44.7 cd/A,and an external quantum efficiency of 10.6%,exhibiting a much better device performance than that of Pe LEDs with regular hole transport layer.The second part of the thesis have studied the hole-injection at the PEDOT:PSS interface in Pe LEDs.The hole-injection energy barrier is lowered by the space charge induced by the ionic charge accumulated at the interface.It’s revealed that there are three decay processes in the transient current of the devices.Two faster processes are attributed to the relaxation of cations and anions,respectively.A third decay process with the longest time is attributed to the strong bonding between the negatively charged Br^-and the positively charged PEDOT chain.By integrating the relaxation current,the space charge density is obtained.The space charge saturates at the applied voltage of 4 V,indicating that all the ions are moved to the contacts.As a result,the concentration of ionic defects in PEA₂（FAPb Br₃）_n-1Pb Br₄ perovskite film is estimated to be around 3.0×10¹⁸ cm^-3.The performance of perovskite light-emitting devices based on multiple quantum wells is strongly dependent on the quantum well（QW）distribution in the perovskite films.The third part of the thesis establish a theoretic framework is established to calculate the radiative efficiency based on the distribution of QWs in Pe LEDs by combining the charge-carrier recombination and the energy loss in energy transfer process.A ratio parameter?is introduced into the dependence of the radiative efficiency on the charger-carrier dynamics for the first time,by taking into account the fact that while the radiative recombination mainly occurs in large-n QWs,the non-radiative recombination exists in both small-n and large-n QWs.The interplay among the energy transfer,the Auger recombination rate,and the radiative recombination rate to determine the dependence of the radiative efficiency on the QW distribution are elucidated.It is revealed that the Auger recombination and the energy loss in energy transfer significantly reduce the radiative efficiency with a large ratio of n=2 QWs to n≥5 QWs.As n≥5 QWs proportion increases,the radiative recombination rate become lower due to the low charge density in n≥5 QWs,and the efficiency roll-off becomes milder.The theoretical framework successfully predicts and explains the trends of the peak efficiency and the efficiency roll-off of the Pe LEDs,providing a guideline for Pe LED fabrication.