# Progress of Organic/Inorganic Luminescent Materials for Optical Wireless Communication Systems
## 2 Luminescent Materials for VLC Systems
### Halide Perovskite Nanocrystals  
Given the outstanding intrinsic properties of perovskite nanocrystals (PNCs), such as their high absorption coefficient [76], modulo band gap generated via quantum confinement effect or composition engineering [77; 78], labile surface chemistry [79], and facile synthesis [80], these materials attracted growing interest in hot-research fields, such as optoelectronic and photovoltaics. PNCs unlocked high efficiencies in multicolor LEDs, with EQE up to 28% [81], while competitive PNCs-based solar cells achieved values closer to the records for the perovskite technology [82]. Most importantly, the PLQY of PNCs can  
Figure 3: (**a**) Typical structures and PL lifetimes of (**b**) Y–BODIPY– and (**b**) T–BODIPY–based molecules under different n-repetitions of oligofluorene arms. (**c**) BERs measurements by varying data rate for Y–BOPIPY (n = 1, 3) system, compared with a commercial phosphor color converter CL–840. Reproduced under terms of CC–BY license [4]. Copyright 2015, Authors. John Wiley and Sons, Inc. (**d**) EL spectra of OLEDs based on BODIPY doping on PMMA matrix: (I, green) 5% doped, (II, yellow) 10% doped, and (III, red) 100% emissive organic layer. Black and red curves indicate use of BODIPY with naphthalene and pyrene substitutes in its main structure, respectively. Reproduced with permission [72]. Copyright 2017, Elsevier. achieve values up to 100%, indicating the high quality and color purity of these systems [83]. This performance is associated with a lowered defect crystalline structure covered by a suitable density of capping ligands for material stabilization, suppressing the non-radiative carrier traps and promoting the radiative PL dynamics. However, unlike the chalcogenide-based nanocrystals, PNCs display color instability caused by the emergence of surface defects due to the eventual loss of capping ligands through aging time [84], as well as halide exchange reactions when PNCs with different halide natures are combined [85; 86]. Therefore, even perovskite mixtures can offer a high PLQY and CRI values of >80% during the generation of white emission, while the PLQY of the final combination is <70% [87; 88]. Additionally, red-emitting PNCs based mainly on CsPbI\({}_{3}\) are reported to be vulnerable to the presence of oxygen, humidity, and illumination conditions, inducing fast quenching of their optical features [89]. To overcome these issues, [PERSON] et al. [90] showed that the combination of carbon quantum dots (CQDs) with CsPbI\({}_{3}\) PNCs (Red light-converter) prepared via a modified synthetic route can prolong their chemical stability for more than 1 year. Here, CQDs produce a broad PL, covering the blue/green range from the visible spectrum (green/blue-light converter). By fixing a constant concentration of CQDs (10 mgmL\({}^{-1}\)) and adding different concentrations of CsPbI\({}_{3}\) PNCs (described as 10-CQDs-X-PNCs), they were able to modulate the white light tonality from cold to warm white emission, as shown in Figure 4a,b. The maximum PLQY of the CQDs-PNCs combination was 75%, and this composite material was used as an active layer for WLEDs, providing a CRI ~95%, as shown in Figure 4c. This result indicates an improvement in the white color quality of the device. For this reason, PNCs are considered convenient light converters for VLC [91; 92]. The enhancement of the CRI for white light emission coming from CQDs and PNCs active layers is in line with the typical CRI value of ~89% established by [PERSON] et al. [8], who introduced CsPbBr\({}_{3}\) PNCs, combined with commercial red-emitting phosphors, to generate warm-white light tonality.