# Progress of Organic/Inorganic Luminescent Materials for Optical Wireless Communication Systems
## 2 Luminescent Materials for VLC Systems
### Halide Perovskite Nanocrystals
By exhibiting a PLQY of ~70% and a fast radiative recombination pathway for carrier relaxation into the nanocrystals (7.0 \(\pm\) 0.3 ns), the composite light converter provides a high frequency bandwidth of 491.4 MHz, covering a wider electromagnetic spectrum than YAG-based phosphors (3-12 MHz), nitride-based active layers (\(\sim\)12.4 MHz), and organic molecules-based light converters (40-200 MHz) (Figure 4d). On the other hand, through bit error rate (BERs) measurements, CsPbBr\({}_{3}\) PNCs-red phosphor-based light converters can transmit data at 2 Gbps, which is a record in VLC applications. Despite the authors reporting the intrinsic stability of the PNCs in ambient air, it is well known that O\({}_{2}\) and humidity can progressively damage the perovskite structure, altering their integrity and, therefore, quenching their photophysical features. By introducing the concept of anion-exchange into the PNCs (interchange in bromide and iodide domains into the nanoparticle structure), mixed halide PNCs were prepared [93], with a PLQY up to 45% and a carrier lifetime of 5 ns. PNCs were deposited as films on blue-emitting InGaN LEDs and coated with a UV glue, with the purpose being to protect the prepared device against any air-borne contamination and eventual degradation of the perovskite. By measuring the perovskite device into the VLC configuration, a maximum of bandwidth of 700 MHz was reached. A fast radiative recombination dynamic into the nanocrystals is the reason to favor the data travelling at this frequency value. On the other hand, [PERSON] et al. [9] introduced PNCs-PMMA composites, with the perovskites being based on yellow-emitting mixed halide CsPbBr\({}_{1.5}\)I\({}_{1.5}\) and red-emitting CsPbI\({}_{3}\) nanoparticles. After encapsulation with PMMA, the PL spectra of the nanocrystals were obtained, being stable for around half year under ambient conditions, thus avoiding material degradation (eventual \(\alpha\)-to-\(\delta\) phase transformation). Interestingly, these materials provided enhanced frequency bandwidths of 347 and 822 MHz, respectively, which were much larger than those of traditional phosphors. Thus, PNCs were suitable for high-speed white-light VLC systems, depositing them on blue GaN \(\mu\)-LEDs. The corresponding combinations between yellow- and red-emitting active layers with the blue-\(\mu\)-LEDs produced a warm white emission with a CRI of ~75.7%, a maximum frequency bandwidth of 1005 MHz, and a real-time data transmission \(\sim\)1.7 Gbps (Figure 4e,f). These values represent the highest performance reached for LED-based VLC systems thus far, and this response is higher than a previous contribution, where a yellow-emitting CsPbBr1,8I12 PNCs was analyzed as a potential light converter [13]. Here, the luminescent material deposited on blue-\(\upmu\)-LEDs provided a maximum frequency bandwidth and data rate of \(\sim\)85 MHz and 300 Mbps, respectively. Differences in (i) the amount of iodide into the nanoparticles requiredto cover longer wavelengths and (ii) the presence of PMMA required to provide stability to the material are possible reasons for obtaining an enhancement in the VLC process. To compare the potentiality of the above-mentioned materials in accelerating wireless data travelling processes, Table 1 summarizes the parameters obtained from the most efficient VLC systems (in terms of estimated frequencies and data rates) depending on the nature of the light converter. Attending to the comparison between the luminescent materials, the combination of organic molecules is a suitable alternative used to achieve a wider frequency bandwidth and, thereby, a higher data rate, due to a favored carrier mobility. Nevertheless, actual records of these parameters were achieved using inorganic systems, such as PNCs, in which a higher absorption coefficient and an easier modulation of their band gap are exhibited through the quantum confinement regime or composition engineering (in this case, the presence of iodide will facilitate the data transmission at longer wavelengths).