Studies On The Phase Segregation And LED Optical Properties Of Perovskite Nanocrystals

Synthesized by the chemical approach,semiconductor nanocrystals?NC?are composed of 100-10000 atoms with a typical size ranging from 1-20nm.Since the size of a semiconductor NC is smaller than or close to the Bohr diameter,it belongs to the quantum confined system with a size-dependent energy bandgap.By adjusting the size continuously,the emission wavelength of a semiconductor NC could vary to cover the entire visible region from 400 to 800nm.Meanwhile,the quantum confinement effect leads to discrete energy levels in a semiconductor NC,and this atomic-like feature can be utilized to generate high-purity single photons.Moreover,semiconductor NCs are also associated with other excellent properties,such as high fluorescence quantum yield,narrow photoluminescence?PL?spectrum,as well as low-cost and simple synthesis procedure.These excellent physical,chemical,material and optical properties make it possible for semiconductor NCs to be investigated in both fundamental researches and device applications,including quantum information processing,lasers,light-emitting diodes?LEDs?,biomarkers,and solar cells.In this thesis,we focus on a novel type of semiconductor NCs with the perovskite structures,showing how their phase segregation process occurs due to ion migration and exploring how their fundamental optoelectronic properties evolve in the normal operation of LED devices.In the second chapter,we mainly study the optical and electrical properties of mixed-halide Cs Pb Br_1.2I_1.8NCs.At room temperature,the PL spectrum of Cs Pb Br_1.2I_1.8NCs show a blue shift under continuous laser illumination,which can recover back to the original position in the dark.In order to reveal the underlying mechanism for the above phenomenon,we apply electric field to the Cs Pb Br_1.2I_1.8NCs and find out that the PL spectrum still has a blue shift even without the light illumination.Based on the above observations,we conclude that the phase segregation process occurs in Cs Pb Br_1.2I_1.8NCs,which proceeds through the migration of iodine ions out of and back to the laser illumination area.In the third chapter,we focus on the fabrications of perovskite NC LEDs and the characterizations of their fundamental optoelectronic properties.The LED fabrications and characterizations of perovskite NCs have been studied for more than five years,however,there still exist a bunch of critical problems to be solved,such as realizing long-term stable operation,improving the external quantum efficiency and inhibiting the efficiency droop.By choosing suitable perovskite NCs as well as the electron and hole transport materials,we have successfully fabricated green-emitting LEDs after gradually optimizing the overall device structures.When driven under high voltage,the as-fabricated LEDs show an efficiency droop,which we have thoroughly studied by combining the PL and electroluminescence techniques.Through the combination of photoluminescence and electroluminescence measurement,we have made a preliminary exploration of the reasons for the roll-off of the efficiency,which provides a new way to solve the stability problem of perovskite nanocrystal LED.We have provided preliminary explanations for the possible reasons of this efficiency droop,which will surely help to solve the instability issue of perovskite NC LEDs.