Color Conversion Light-Emitting Diodes Based on Carbon Dots: A Review

Abstract

This paper reviews the state-of-the-art technologies, characterizations, materials (precursors and encapsulants), and challenges concerning multicolor and white light-emitting diodes (LEDs) based on carbon dots (CDs) as color converters. Herein, CDs are exploited to achieve emission in LEDs at wavelengths longer than the pump wavelength. White LEDs are typically obtained by pumping broad band visible-emitting CDs by an UV LED, or yellow-green-emitting CDs by a blue LED. The most important methods used to produce CDs, top-down and bottom-up, are described in detail, together with the process that allows one to embed the synthetized CDs on the surface of the pumping LEDs. Experimental results show that CDs are very promising ecofriendly candidates with the potential to replace phosphors in traditional color conversion LEDs. The future for these devices is bright, but several goals must still be achieved to reach full maturity.

Keywords: LEDs; carbon dots; carbon-dot-based light-emitting diodes; color conversion; multicolor light-emitting diodes; organic materials; phosphors; white light-emitting diodes.

Figure 1

Trend of published papers related to CDs since their discovery. (Obtained using Scopus search results for “Carbon Dot”).

Figure 2

Semiconductor band diagram with charge transitions. (Reprinted from [23]; with the permission of the author).

Figure 3

Idealized densities of state for one band semiconductor 3D, 2D, 1D, and 0D structures, indicated in the figure, respectively, as 3d, 2d, 1d, and 0d. In the 3d case (i.e., bulk material), the energy levels are continuous, while in the 0d or molecular limit, the levels are discrete. (Reprinted from [28]; with the permission of the American Chemical Society).

Figure 4

Fluorescence emission spectra (excited at 365 nm) of NCD 220 powder and NCD 220 /epoxy resin composite (solid lines), and time-dependent intensity (gray circles) of the composite continuously exposed to UV light (365 nm). Inset: the composite under daylight (left) and 365 nm of UV light (right). (Reprinted from [77]; with the permission of John Wiley and Sons).

Figure 5

Photograph of the conventional CDs (1,4), trisodium citrate dihydrate (2,5), and NCD11 powders (3,6) under daylight (upper row) and 365 nm UV lamp (bottom row). (Reprinted from [78]; with the permission of Elsevier).

Figure 6

Mechanism representation of the dual fluorescence emission of solid-state N-doped carbon dots. (Reprinted from [47]; with the permission of Elsevier).

Figure 7

Energy band level alignments of monodispersed and aggregated CDs. Schematic of energy transfer that occurs in the aggregation process of (a) CD1 and (b) CD2. (Reprinted from [101]; with the permission of the American Chemical Society).

Figure 8

Energy level structures to explain the PL behaviors of the three different emissions from multicolor emissive CDs (indicated here as AC-CDs). (Reprinted from [111]; with the permission of the American Chemical Society).

Figure 9

Schematic illustration showing the mechanism of multicolor emission from CDs. (a) The intermolecular interaction between solvent and CDs. (b) The change in energy levels of the CDs in different polarity solvents. (Reprinted from [112]; with the permission of John Wiley and Sons).

Figure 10

Sketch showing the fluorescence mechanisms of m-PD, o-PD, p-PD embedded in starch.

Figure 11

Illustration showing the energy states with different color emission by (a) m–CDs and (b) o-CDs. (Reprinted from [116]; with the permission of John Wiley and Sons).

Figure 12

Schematic diagram of the luminescent and controllable synthesis mechanism. (Reproduction from [117]; with the permission of the Royal Society of Chemistry).

Figure 13

(a–c) HR-TEM images of blue, green, and orange color emissive CDs and (d–f) their size distributions histogram. (Reprinted from [118]; with the permission of the American Chemical Society).

Figure 14

Two-color fluorescent layers of CDs in different immiscible solvents. (Reprinted from [122]; with the permission of the American Chemical Society).

Figure 15

(a) N-doped CDs (NCDs in the figure) are embedded into melting recrystallized urea and biuret matrices. (b) Schematic illustration of possible energy structures of C=N bonds and phosphorescent emission processes. (Adapted from [124]; with the permission of the American Chemical Society).

Figure 16

A schematic illustration of the preparation procedure of different structure carbon dots by hydrothermal carbonization of L-serine and L-tryptophan (Ser + Trp) at different pH values and temperatures. (Reprinted from [126]; with the permission of the American Chemical Society).

Figure 17

Classical Jablonski diagram for the free-space condition (a) and the modified form in the presence of metallic particles (b). E indicates excitation; E_m indicates metal-enhanced excitation rate; Γ indicates radiative rate; K_nr indicates non-radiative decay rates for excited state relaxation and Γ_m indicates radiative rate in the presence of metal. (Reprinted from [134]; with the permission of Elsevier).

Figure 18

Visualized surface states of N-doped CDs with excitation-dependent and -independent behaviors. (Reprinted from [140]; with the permission of Elsevier).