Optical And Electrical Properties Of Light-emitting Diodes Based On Low-dimensional Perovskites And Organic Materials

Light-emitting diode or light-emitting device（LED）has been widely used in the field of display and lighting due to its high brightness,wide gamut,low power consumption,long life,and being friendly to the environment.It has become one of the most competitive products in the optoelectronic industry.With the development of the LED industry and higher requirement of LED display in the new era,the researchers have gradually made more exploration of the novle electroluminescent materials.Among them,the organic molecules and new low-dimensional halide perovskites have attracted great attention due to their many virtues.The performance of the LED depends on the type of the luminescent material and the device structure.Understanding of the carrier transport and recombination mechanisms inside the devices is also very important.Therefore,the study of the carrier transport and recombination mechanisms of the LEDs employing different luminescent materials is scientifically important,and it can also provide reliable theoretical basis for further improvement of their performances.We fabricated different structures of organic light-emitting diodes（OLEDs）by employing the organic material poly-TPD as the light-emitting layer and hole transport layer.Their electroluminescence properties were studied.It was found that the photoluminescence and electroluminescence properties of poly-TPD are quite different.The PL is dominated by the intrinsic luminescence,while the electroluminescence was dominated by the exciplexes.After addition of the electron transport layer TPBi to optimize the device structure,the luminescence color of the OLED changes from blue to white and red with increasing bias.This is attributed to the existence of the two competitive luminescence mechanisms（intrinsic and exciplex luminescence）.We studied the LEDs employing the CsPbBr₃ nanocrystals（NCs）as the luminescent materials and their operation mechanisms at working voltages as high as40 V.These LEDs exhibit anomalous hump-shaped luminescence intensity versus bias curves.A theoretical model based on the Auger recombination was proposed to explain this phenomenon.The imbalanced carrier injections lead to enhancement of the Auger recombination,which causes the efficiency droop of the CsPbBr₃ NC-LEDs.In addition,we found the simultaneous fluctuation of the linewidth and peak position of the electroluminescence spectrum of the CsPbBr₃ NC-LED,which is ascribed to the inner self-heating effect in the emission layer of the LED.We studied the synthesis and growth mechanism of the CsPbBr₃ nanowires（NWs）.We fabricated the LEDs by employing these NWs and the emission layers.Some of the CsPbBr₃ nanorods can grow into bulk single crystals.The combination of the NWs and the nanorods leads to growth of the nanochains and curved nanorings.The driving force for the alignment of the CsPbBr₃ NCs is the dipole–dipole interaction.We fabricated two types of CsPbBr₃ NWs-based LEDs which employ different concentrations of the NWs as the emissive layers.One LED emitted pure green light,whereas the emission color of the other LED changed from green to quasi-white with increasing bias.We investigated the electroluminescence properties and the mechanisms of carrier transport and recombination.The result shows that the carrier transport shifts from quantum tunneling to direct injection with increasing bias.