color qualityand spectral efficiency. These LEDs cannot simultaneously accomplish a good colorrendition, a good spectral match with the spectral sensitivity of the human eye, and a warm white shade,although individual high performances are possible. This basically emanatesfrom the difficulties in the spectral tuning of phosphors. In addition,concerns regarding the supply of and current commercial monopoly on phosphors have increased the demand for alternative colorconverters 5. At this point,quantum dots are rising as a promising candidate since they exhibit finespectral tuning, achieved by their size control and narrow-band emission 6.Therefore, with optimized spectral designs,the real colorsof objects can berendered properly whileachieving a warmwhite shade and a good spectraloverlap with the human eye sensitiv-ity function, which in turn increases the efficiency of thelight source; all of these improvements can be achieved at the same time with QDs employed in white LEDs 7, 8.
Furthermore, their high photoluminescence quantumeffi- ciencies can contribute to realizing the high electrical effi- ciency of the device 9, 10. Considering thesefeatures of QDs, theyoffer great potential for white LEDsby possess- ing high color qualityalong with photometric and electri- cal efficiency. In addition to general lighting applications, QD-based LEDs can easily respondto the demands of the backlights used in liquidcrystal displays (LCD).In partic- ular, the narrow emissionband of QDs enables the repro-duction of high purity colors. Moreover, a much larger number of colors can be generated using these materials; in other words, the color gamut of the LCDs can be broad- ened beyond the industrial standards.At this point it is useful to distinguish two typesof LEDs using QDs,which rely on twodifferent means of exci-tation. One is based on electrically exciting the QDs, which makes LEDs based on the directelectroluminescence of QDs, and the other is throughoptically exciting them, which makes color-conversion LEDs using QDs asthe nanophosphors. As the name implies, in the electrically excited QD basedLEDs, electrons and holes are directlyinjected into the quantum dots, and the white light emis- sion is thus obtained through the radiative recombination of these injected carriers within QDs having different sizes and emitting different colors.
Over the years to date,the overall efficiencies of these LEDs typically remainedlower compared to those of color converting QD-WLEDs mainly because of the charge injection problem. The organicligands surrounding the QDs, whose main functionis the passivation of the QD surface, are poor conductive mate- rials and generate a large barrier that makes the charge injection difficult. Therefore, the injection of carriers into theQDs is not an easy task which in the end decreasesthe