Facilely synthesized carbon dots and configurated light-emitting diodes with efficient electroluminescence

Carbon dots (CDs) are recently emerging photoluminescence (PL) and electroluminescence (EL) emitters that are highly spotlighted in solid-state lighting. In this work, exceptional CDs with few defects, high carbonation, good film morphology and excitation wavelength-independent PL emission have been facilely synthesized and characterized with X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and atomic force microscopy. Superior EL emission from CDs has been demonstrated. With CDs as a single emissive layer, the light-emitting diode (LED) shows a maximum luminance of 19.34 cd m^-2, luminous efficiency of 0.037 cd A^-1, power efficiency of 0.039 lm W^-1, and external quantum efficiency (EQE) of 0.031%. With poly(9-vinylcarbazole)/CDs as dual emissive layers, the LED shows a maximum luminance of 30.01 cd m^-2, luminous efficiency of 0.089 cd A^-1, power efficiency of 0.046 lm W^-1, and EQE of 0.064%. The latter is better than the former because of the improved carrier balance and extended emission zone. The EL emission of both CD-based LEDs moves from the ultraviolet/blue emission of organics and/or interface emission to predominantly visible CD emission with increasing voltage. The evolution of EL and PL allows for a deeper understanding of the mechanism of CD emission and the advancement of CD-based LEDs.