Underwater wireless optical communication (UWOC) has attracted increasing interest in various underwater activities because of its order-of-magnitude higher bandwidth compared to acoustic and radio-frequency technologies. Testbeds and pre-aligned UWOC links were constructed for physical layer evaluation, which verified that UWOC systems can operate at tens of gigabits per second or close to a hundred meters of distance. This holds promise for realizing a globally connected Internet of Underwater Things (IoUT). However, due to the fundamental complexity of the ocean water environment, there are considerable practical challenges in establishing reliable UWOC links. Thus, in addition to providing an exhaustive overview of recent advances in UWOC, this paper addresses various underwater challenges and offers insights into the solutions for these challenges. In particular, oceanic turbulence, which induces scintillation and misalignment in underwater links, is one key factor in degrading UWOC performance. Novel solutions are proposed to ease the requirements on pointing, acquisition, and tracking (PAT) for establishing robustness in UWOC links. The solutions include light-scattering-based non-line-of-sight (NLOS) communication modality as well as PAT-relieving scintillating-fiber-based photoreceiver and large-photovoltaic cells as the optical signal detectors. Naturally, the dual-function photovoltaic-photodetector device readily offers a means of energy harvesting for powering up future IoUT sensors.
Enhancing robustness and energy efficiency is critical in visible light communication (VLC) to support large-scale data traffic and connectivity of smart devices in the era of fifthgeneration networks. To this end, we demonstrate that amorphous silicon (a-Si) thin-film solar cells with a high light absorption coefficient are particularly useful for simultaneous robust signal detection and efficient energy harvesting under the condition of weak light in this study. Moreover, a first-generation prototype called AquaE-lite is developed that consists of an a-Si thin-film solar panel and receiver circuits, which can detect weak light as low as 1 µW/cm 2. Using AquaE-lite and a white-light laser, orthogonal frequency-division multiplexing signals with data rates of 1 Mb/s and 908.2 kb/s are achieved over a 20-m long-distance air channel and 2.4-m turbid outdoor pool water, respectively, under the condition of strong background light. The reliable VLC system based on energy-efficient a-Si thin-film solar cells opens a new pathway for future satellite-air-ground-ocean optical wireless communication to realize connectivity among millions of Internet of Things devices.
In this work, a simple carbazole-based hole transport material with triphenylamine moieties, termed LD22, has been used in perovskite solar cells (PSCs). It is noted that LD22 exhibits a proper HOMO level of −5.27 eV, high hole mobility of 1.65 × 10–5 cm2 V–1 s–1, and relatively high glass-transition temperature of 132 °C. When LD22 was used in PSCs, pristine LD22-based PSCs showed a power conversion efficiency (PCE) of 13.04%. When LD22 is doped, the PCE improves to a promising 17.18%. More importantly, the concentration of LD22 has little influence on the PSC performance regardless of the existence of dopants, which shows good repeatability. As a reference, the device with doped 2,2′,7,7′-tetrakis(N,N-bis(4-methoxyphenyl)amino)-9,9′-spirobifluorene (spiro-OMeTAD) shows a PCE of 17.73%. On the other hand, the laboratory synthesis cost of LD22 is much lower than that of spiro-OMeTAD. Therefore, the results indicate that the simple carbazole-triphenylamine compounds own the potential to be doped-free HTM and LD22 could be a promising HTM candidate for high-performance PSCs due to its simple structure.
The use of autonomous underwater vehicles (AUVs) is highly desirable for collecting data from seafloor sensor platforms within a close range. With the recent innovations in underwater wireless optical communication (UWOC) for deep-sea exploration, UWOC could be used in conjunction with AUVs for high-speed data uploads near the surface. In addition to absorption and scattering effects, UWOC undergoes scintillation induced by temperature-and salinity-related turbulence. However, studies on scintillation have been limited to emulating channels with uniform temperature and salinity gradients, rather than incorporating the effects of turbulent motion. Such turbulent flow results in an ocean mixing process that degrades optical communication. This study presents a turbulent model for investigating the impact of vehicle-motion-induced turbulence via the turbulent kinetic energy dissipation rate. This scintillationrelated parameter offers a representation of the change in the refractive index (RI) due to the turbulent flow and ocean mixing. Monte Carlo simulations are carried out to validate the impact of turbulent flow on optical scintillation. In experimental measurements, the scintillation index (SI) and signal-to-noise ratio (SNR) are similar with (SI = 0.4824, SNR = 5.56) and without (SI = 0.4823, SNR = 5.87) water mixing under uniform temperature channels. By introducing a temperature gradient of 4 °C, SI (SNR) with and without turbulent flow changed to 0.5417 (5.06) and 0.8790 (3.40), respectively. The experimental results show a similar trend with the simulation results. Thus, turbulent flow was shown to significantly impact underwater optical communications.
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