| Light-emitting diodes(LEDs)are now widely used in consumer electronics,offering higher current conversion efficiency,higher contrast ratio and larger color space than traditional liquid crystal displays,and are considered a revolutionary innovation in lighting and display.Recently,quantum dot-based LEDs have attracted a lot of attention worldwide,however quantum dots containing heavy metals Cd and Pb have problems such as high toxicity,poor stability and high price.As a member of the new carbon material family,carbon dots(CDs)have many advantages such as high stability,non-toxicity,low cost,abundant raw materials and solution processed,and also have unique color-tunable photoluminescence/electroluminescence(PL/EL)due to their size effect,surface effect and edge effect,making carbon dots a luminescent nanomaterial with great development potential.In this paper,we designed and synthesized two carbon dots based on solution-processed technology and used them in electroluminescent devices to achieve high-performance carbon dot-based LED devices(CDs-LEDs)by optimizing the device structure,suppressing the solid-state burst of carbon dots,and improving the dispersion and carrier transport properties of carbon dots.The specific research content and results are as follows:(1)W-CDs that can be used for white LEDs were synthesized by ultrasound-assisted hydrothermal method using ethanol and ethylenediamine as raw materials.The microscopic morphology,structural composition,optical properties and film-forming ability of W-CDs were studied in detail,and a series of LED devices based on W-CDs were prepared.In the case of W-CDs as a single light-emitting layer,the maximum brightness of W-CDs-LED is2.89 cd/m~2,the luminous efficiency is 0.030 cd/A,the power efficiency is 0.009 lm/W,and the external quantum efficiency(EQE)is 0.024%,the luminous peak almost covers the entire visible light range,and the luminous color is close to white.The PMMA-doped W-CDs were used as the host-guest doped luminescent system for the luminescent layer,the brightness of the device has been improved to a certain extent,but the luminous efficiency,power efficiency and EQE have been slightly reduced,and the EL luminescence peak of the device is very close to PL luminescence peak of W-CDs.Further,by optimizing the device structure and introducing PVK as the hole transport layer,a stable white LED was obtained.The maximum brightness of the LED based on PVK/W-CDs reaches 217.90 cd/m~2,the luminous efficiency is 0.457 cd/A,the power efficiency is 0.164 lm/W,and the EQE is 0.362%,realizing a high-performance electroluminescent white LED device based on CDs.(2)Special CDs with fewer defects,high carbonization rate,good film morphology and excitation wavelength independent PL emission were synthesized by a one-step hydrothermal method.The CDs were characterized in detail by X-ray diffraction,transmission electron microscopy,X-ray photoelectron spectroscopy and atomic force microscopy,and LED devices emitting from CDs were prepared.In the case of CDs as single emissive layer,the maximum luminance of CDs-LED is 19.34 cd/m~2,the luminous efficiency is 0.037 cd/A,the power efficiency is 0.039 lm/W,and EQE is 0.031%.In the case of PVK/CDs as dual emissive layers,the maximum luminance of PVK/CD-LED is30.01 cd/m~2,the luminous efficiency is 0.089 cd/A,the power efficiency is 0.046 lm/W,and EQE is 0.064%.With increasing voltage,the EL emission of both single-layer CDs and double-layer PVK/CDs LED devices undergoes an evolution from the ultraviolet/blue emission or interfacial emission of adjacent organic layers to the visible emission dominated by CDs.The evolution of EL and PL spectra provides an important reference for in-depth understanding of the transport and recombination mechanisms of carriers in CDs. |
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