# Progress of Organic/Inorganic Luminescent Materials for Optical Wireless Communication Systems
## 2 Luminescent Materials for VLC Systems
### Organic Molecules-Organic-Light Emitting Diodes (OLEDs) in VLC  
Recently, by addressing advantages in material science and fabrication processes, solid-state devices, such as OLEDs, were gradually implemented in various small and medium displays. However, the fastest VLC systems reported so far make use of optical transmitters, which consist of LEDs [22; 33] and laser diodes (LDs) [34; 35; 33] and feature inorganic semiconductors (typically gallium nitride) as the emissive medium. This characteristic affords high optical output power and broad bandwidth, deducing that inorganic LEDs [36; 37] and LDs [38; 39] are suitable for integration into a dual system capable of simultaneously providing white lighting and fast data transmission. However, the use of  
Figure 2: (**a**) Schematic illustration depicting synthetic conditions required to obtain typical molecular structures of Zr–MOF (NU–1000) and PIM–1. (**b**) Frequency response measurements for NU–1000/PIM–1 MMMs (where D is associated with MOF as donor, A is ascribed to PIM–1 as acceptor, and D–A0.9 is donor–acceptor assembly with a 90 wt% A). Reproduced with permission [29]. Copyright 2022, American Chemical Society. (**c**) Frequency response measurements and (**d**) data rate (7% forward error correction limit) of diverse OLEDs by varying transporting layer (diverse generation, G, and sizes) between OLEDs in function of operational voltage. Reproduced under terms of CC-BY license [32]. Copyright 2020, Authors, Springer Nature. OLEDs represents a valid alternative, which gained considerable attention for VLC in the last few years [40; 41; 42; 43; 44; 45]. In the literature, different types of OLEDs were proposed for VLC (named as OLED-based VLC, OVLC), with studies concluding that the performance of the OLEDs is defined by the properties of the luminescent organic molecules for this application. However, the organic devices are hampered by scenarios in which there is a low level of visible light penetration in non-transparent media. The suitable solution found by research groups is related to extending the communication system operations to the \"almost (in)visible\" region of the near-infrared (NIR, 700-1000 nm) using a new OLED type based on Pt(II) and Ir(III) complexes, modified porphyrins, donor-acceptor copolymers, small molecules-polymer blends [46; 47], etc. However, OLEDs have a low modulation bandwidth (hundreds of kHz) compared to their analogous inorganic LEDs (typically several MHz). This fact is mainly attributed to the restrained charge carrier mobility in the organic semiconductors, limiting the applicability of the organic devices [46; 47; 48; 49]. In this context, equalization processes (supply a higher charging voltage to the device for improving the carrier transport) [50] were applied to OLEDs, with one study obtaining a frequency bandwidth up to 10 MHz and a datalink supporting up to 100 Mbps [51]. This performance could be the initial step in the process of fabricating efficient OVLC systems. Recently, small organic molecules were employed as optoelectronic components in VLC [52]. It is known that organic devices present several advantages compared to the inorganic ones, since these devices offer lower heat dissipation, reduced production cost, and wide photoactive areas. However, it is quite difficult to replace inorganic devices with organic devices because the former types dominate the current market [53]. The first electroluminescent device based on OLEDs containing small organic molecules was reported by [PERSON], which was known as SMOLEDs [54]; later, an important number of improved OVLCs were developed using organic photonic devices as the device's transmitter, the receiver, or both components [55; 56]. In addition, organic photodiodes (OPDs) could be seen as future devices for image sensor-based applications and optical communications because of their elevated sensitivity and superior responsivities [57].