Electroluminescent and photoluminescent light-emitting diodes from carbon dots and device architecture optimization

Light emission from carbon dots (CDs) is of great interest in both electroluminescence and photoluminescence. Herein, we construct electroluminescent and photoluminescent light-emitting diodes (LEDs) from emitters of CDs. The electroluminescent LED with host-guest-doped dual emissive layers (EMLs) of [poly(9-vinylcarbazole) (PVK)-CDs] × 2 gives satisfactory electro-optical properties, with maximum luminance of 560 cd m^-2 at 16 V, luminous efficiency of 0.183 cd A^-1, power efficiency of 0.082 lm W^-1, and external quantum efficiency of 0.25%, which are superior to the counterparts with single-EML of CDs, single-EML of [PVK-CDs], and triple-EMLs of [PVK-CDs] × 3. These enhanced properties are rationally ascribed to optimization of the device architecture, carrier balance improvement, and reduction in concentration-induced quenching. The electroluminescent LEDs also show color evolution from PVK to CDs, and/or to the PVK/CDs interface, with increasing driving voltages, owing to incomplete energy transfer from PVK to CDs. Highly efficient photoluminescent LEDs with 365- and 395-nm UV excitation are demonstrated. With a CDs : polyvinyl pyrrolidone ratio of 1 : 3, the 365-nm excited photoluminescent LED gives a maximum luminance of 512 347 cd m^-2 with a power efficiency of 25.2 lm W^-1, while the 395-nm excited photoluminescent device gives a maximum luminance of 670 954 cd m^-2 with a power efficiency of 22.0 lm W^-1, with both showing yellow emission. Our experiments provide some new ideas for broadening CD applications and advancing LEDs.