# Progress of Organic/Inorganic Luminescent Materials for Optical Wireless Communication Systems
## 1 Introduction  
With the purpose of ensuring the increase in the speed of both data transmission and consumption, advanced strategies were previously studied to overcome the limitations imposed by using radio-frequency technology, establishing efficient optical communication approaches, such as wireless network systems [1, 2]. Among the most prominent alternatives used to accelerate information transference, visible light communication (VLC) is highlighted due to its use of solid-state light (SSL) converters, enabling access to a wide bandwidth in the scale of THz [3, 4]. This fact allows users to carry out wireless communication across the electromagnetic spectrum from UV to NIR [4]. One of the main light sources for VLC is based on light-emitting diodes (LEDs), where a phosphor-based active layer is incorporated into a device to absorb the blue light coming from a commercial component, usually InGaN, and re-emits illumination at larger wavelengths [5, 6, 7]. This configuration produces luminescent white color LEDs (WLEDs) with high color purity and an improved color rendering index (CRI) [8, 9, 10], facilitating the development and subsequent commercialization of LEDs-based technologies, such as lighting systems, LCD displays, organic-LEDs (OLEDs), etc. As seen Figure 1, VLC systems are generally based on light link between a blue-emitting LED, a luminescent active layer, and a photodiode. The composed LED device, which produces the illumination, is named the emitter subsystem, while the photodiode, which receives the signal coming from the LED, is known as the receiver subsystem [11]. Next, the emitter obtains the signal from a signal generator, which imposes a bias current to drive the LED. Here, the signal that reaches the LED (known as the original binary response) is previously transformed through encoding, modulation, and pre-equalization processes, before being converted into a digital signal to modulate the light intensity of the emitting device [12]. The LED into the VLC system must work continuously while the data are transmitted, and, simultaneously, the power stage ensures that the emitted light serves as a continuous illumination source. At this point, the electrical signals are transformed into an optical response, which contains all the information through its frequency [13]. Clearly, the frequency bandwidth will depend on the intrinsic properties of the luminescent material employed as the active layer, and commonly, a long-pass optical filter is introduced to filter out the remanent blue-light emitted by the LED. After the light transmission step, the optical signal is collected through the optical lens (transmitter and receiver ones, Tx and Rx, respectively) with the purpose of enhancing the signal strength to reach the photodiode and extend the transmission distance [13]. Interestingly, the modulated light can travel through open spaces, such as air or water. Lastly, the photodiode perceives the optical signal (also called the original transmission response), transforming it into an electrical response, and treats it under the post-equalization, demodulation, and decoding processes to obtain the desired digital signal [11]. One of the most useful SSL converters is based on yellow phosphors, such as Y\({}_{3}\)Al\({}_{5}\)O\({}_{12}\):Ce\({}^{3+}\) (YAG:Ce), providing high performance and brightness to the WLEDs; however, this luminescent species shows a long excited state lifetime (in the microscale range) [14; 15; 16]. This intrinsic characteristic only favors the accessibility to a bandwidth of a few MHz, restricting the frequency modulation and, thereby, limiting the speed of the wireless communication [17]. To overcome these drawbacks, several strategies were applied, such as (i) the optimization of the LED structure, where the resistance and capacitance of the device components are reduced to accelerate the carrier recombination dynamics [18; 19]; and (ii) the blue-filtering process [20; 21], which is pivotal to suppressing the parasite signals produced by phosphor emitters.