Emerging perovskite color converter for next-generation wireless communications

Selection of excitation light sources

The excitation light sources for PQDs can be categorized into two main types: LEDs and LDs. In recent years, μLEDs have emerged as a miniaturized variant of conventional LEDs. LEDs, as mature illumination sources, are characterized by their broad spectral band emission and wide beam divergence, which render them suitable for indoor multi-device communications scenarios^[1]. However, the modulation bandwidth of LEDs typically remains constrained to several MHz, as reported in studies^[40,54], which is inadequate to meet the 6G communication standards that demand bandwidths exceeding GHz^[7,9]. LDs, based on the principle of stimulated emission, are distinguished by their narrow spectrum linewidth, high brightness, strong directionality, and high modulation bandwidth (> GHz), establishing them as viable solutions for high-power, high-quality illumination and long-distance, directional high-speed transmission, such as UWOC^[32-34]. Nevertheless, the high cost and intense power density associated with LDs restrict their applicability for indoor lighting applications due to economic and safety considerations.

μLEDs have recently garnered significant attention for their applications in full-color displays, flexible displays, and augmented/virtual reality (AR/VR) applications^[14,17]. When integrated into VLC systems, μLEDs achieve higher current densities, substantially reduced RC time constants, and nanosecond carrier lifetimes due to their miniaturized structure, resulting in considerable bandwidth improvements^[1,7,8]. By combining μLEDs with specially engineered high-bandwidth PQD layers, μLED-based WLEDs can achieve data transmission rates exceeding Gbps, rivaling the performance of white LDs^[31]. Additionally, μLEDs offer ease of integration and scalability, which facilitates the construction of lighting arrays and MIMO communication systems, positioning them as optimal candidates for indoor VLC applications and potential components of Light Fidelity (LiFi) hardware infrastructure^[6,7]. However, several challenges, such as the strong quantum-confined Stark effect (QCSE) in GaN-based μLEDs grown on c-plane substrates, can compromise the performance of μLEDs^[9,15]. The QCSE induces spatial separation of the electron-hole wave function, consequently diminishing radiative recombination rates and decelerating the recombination process. These effects manifest in reduced efficiency and decreased bandwidth. The epitaxial growth of semipolar GaN layers can mitigate the effects of polarization and built-in electric fields, thereby improving the overlap of wave functions and suppressing the occurrence of QCSE^[43,55]. Recent investigations have demonstrated the successful implementation of semipolar μLEDs as light sources in PQD-based VLC systems^[41,43,53]. In 2023, Sadhu et al. employed a blue semipolar (20-21) μLED with a -3 dB bandwidth of 1.23 GHz as the excitation source^[53]. The WLED system, utilizing a color-conversion film composed of CH₃NH₃PbBr₃ and two organic emitter materials, achieved a color rendering index (CRI) of 82.4 and a bandwidth of 952 MHz. Beyond semipolar μLEDs, various approaches have been employed to suppress the QCSE, including the implementation of ultra-thin quantum wells^[56], nonpolar substrates^[57], and nano-engineered InGaN active regions^[58]. With advancements in device design and fabrication processes, the modulation bandwidth of μLEDs has recently exceeded 3 GHz^[58,59], establishing them as exceptional light sources comparable to LDs.

Optimization of PQD structures

To improve the modulation bandwidth and stability of PQD-based color converters, a variety of strategies have been implemented to refine the structures and encapsulations of PQDs. These include the use of inorganic oxide coating^{[40,43,51,60]}, organic polymer encapsulation^[30,31,53], inorganic glass embedding^[33], nanostructured PQD paper fabrication^[41], and solution dispersion^{[32,34,38,46]}. Encapsulating PQDs within a transparent, rigid, and chemically inert inorganic oxide matrix, such as SiO₂, ZrO₂, and Al₂O₃, has proven to be an effective surface passivation method^{[40,43,51,60]}. In 2021, Mo et al. synthesized CsPbBr₃@ZrO₂ nanocrystals (NCs) at room temperature by injecting Zr(C₄H₉O)₄ into CsPbBr₃ NCs solution^[51]. Figure 2A illustrates the synthesis process of CsPbBr₃@ZrO₂ NCs. The PLQY increased from 37% to 80% after ZrO₂ coating. The average size of the NCs decreased from 16.0 nm to 10.1 nm, possibly due to the etching of CsPbBr₃ NCs induced by the n-butanol present in Zr(C₄H₉O)₄. Numerous studies have demonstrated that reducing the size of QDs intensifies the quantum confinement effect, which leads to decreased PL lifetime^[61-64]. Figure 2B shows the TRPL curves of CsPbBr₃@ZrO₂ NCs with varying amounts of Zr(C₄H₉O)₄. The average PL decay time was decreased from 33.2 ns to as low as 14.2 ns. The stability of CsPbBr₃@ZrO₂ NCs was further confirmed, maintaining 81% and 70% of initial PL intensity after 90 min of heating at 60 °C and 70 min of water immersion, respectively.

Figure 2. (A) Synthesis process of CsPbBr₃@ZrO₂ NCs at room temperature; (B) PL decay curves of CsPbBr₃@ZrO₂ NCs with different amounts of Zr(C₄H₉O)₄; (C and D) transmission electron microscopy (TEM) images of yellow and red PNCs; (E) frequency response of red and yellow PNC-PMMA, and their bandwidth. Inset is the schematic of the test setup for the bandwidth measurement; (F) emission spectra of red and yellow PNC-PMMA. Insets are the photos of red and yellow PQD samples dispersed in n-hexane under 365-nm UV light irradiation. Reproduced with permission: (A and B)^[51], Copyright 2021, Wiley-VCH GmbH; (C-F)^[31], Copyright 2021, American Chemical Society. NC: nanocrystal; PQD: perovskite quantum dot; UV: ultraviolet; PNC-PMMA: perovskite nanocrystal-polymethyl methacrylate.

While the inorganic oxide coating of PQDs significantly improves stability and optical properties, current investigations have not achieved bandwidths exceeding 100 MHz, which suggests that the impact of oxide coating on the bandwidth of PQDs requires further in-depth investigation. To markedly reduce the particle size and PL lifetime of perovskites, thereby increasing modulation bandwidth, the encapsulations of perovskite-polymer composites, particularly using polymethyl methacrylate (PMMA), have gained widespread attention^[23,65,66]. PMMA effectively wraps and disperses PQDs via physical interactions, preventing exposure to air while maintaining small particle sizes. In 2021, Wang et al. synthesized yellow/red-emitting CsPb(Br/I)₃ and CsPbI₃ perovskite nanocrystal-polymethyl methacrylate (PNC-PMMA) films^[31]. The average particle sizes of CsPb(Br/I)₃ and CsPbI₃ QDs were 9.30 ± 1.53 and 5.73 ± 0.95 nm, respectively, as shown in Figure 2C and D. Attributed to the accelerated carrier relaxation rates associated with these extremely small particle sizes, the yellow and red PNC-PMMA films achieved PL lifetimes of only 11.2 and 3.5 ns, resulting in high modulation bandwidths of 347 and 822 MHz, respectively. However, upon six-month storage under ambient conditions at room temperature, both PNC-PMMA samples exhibited degradation in frequency response curves and normalized emission spectra, as illustrated in Figure 2E and F. The bandwidths of yellow and red PNC-PMMA films decreased to 230 and 624 MHz, accompanied by a 10 nm blue shift and a 21 nm red shift, respectively. Although PNC-PMMA demonstrated remarkable bandwidth, the inherent high thermal resistance of PMMA can lead to the thermal damage of PNCs, and the self-aggregation of PNCs can increase particle size^[33]. These factors contribute to the progressive deterioration of the bandwidth and emission quality of PNC-PMMA films over extended periods.

To bolster the long-term stability of PQDs, researchers have developed structures that embed PNCs within the inorganic glass matrix^[67-69]. In 2021, Xia et al. demonstrated in situ precipitation of CsPbBr₃ NCs in inorganic amorphous glass through solid-state reactions^[33]. The process schematic and photograph of CsPbBr₃ NCs-glass are shown in Figure 3A and B. Embedding CsPbBr₃ NCs into an amorphous glass matrix slightly reduces the transmittance of the color converter; however, the water and thermal stability are significantly enhanced. During the water immersion test, the NCs-glass initially experienced appreciable PLQY deterioration (from 68.5% to 56.0% in the first 6 h), attributed to the erosion of NCs that were not completely embedded in the glass. Nevertheless, as the testing period extended from 10 h to 2 weeks, the PLQY of the NCs-glass exhibited minimal change from 53.6% to 53.3%, demonstrating that the glass matrix effectively protected the fully embedded NCs. During another thermal stability test, the NCs-glass was subjected to a temperature increasing from room temperature to 200 °C. Although the luminescence intensity of NCs decreased with rising temperature, it recovered to its initial value upon cooling to room temperature. Besides its exceptional stability, CsPbBr₃ NCs-glass also demonstrated excellent communication performance. The NCs exhibited an average size of only 5.2 nm. Furthermore, the authors emphasized that higher refractive indexes of the matrix can enhance radiative transition rates of CsPbBr₃ NCs, thereby reducing their PL decay times. With glass matrix typically having a higher refractive index (1.54) than other matrixes, CsPbBr₃ NCs exhibited a minimum decay time of 3.32 ns. Under high-intensity LD excitation, a -3 dB bandwidth of 180 MHz was measured.

Figure 3. (A) Synthesis process of CsPbBr₃ NCs-glass through solid-state reactions; (B) photograph of CsPbBr₃ NCs-glass with two sizes; (C) the -3 dB bandwidth and LOP of CsPbBr₃ PQDs with different concentrations; (D) frequency response of CsPbBr₃ PQDs with different volumes of ethanol; (E) benchmark of the modulation bandwidth versus PL lifetime for PQDs under different encapsulations. Reproduced with permission: (A and B)^[33], Copyright 2021, Wiley-VCH GmbH; (C and D)^[38], Copyright 2023, Optica Publishing Group. NC: nanocrystal; PQD: perovskite quantum dot; LOP: light output power; PL: photoluminescence.

In comparison to their solid-phase counterparts, liquid-phase PQDs, which have been extensively investigated in recent years, present several inherent advantages. Firstly, PQDs grown and preserved in solution maintain uniform distribution and suppress self-aggregation, thereby ensuring excellent optical properties and fast response speeds^[70]. Secondly, the mobility of PQDs in solution provides convective heat dissipation pathways, demonstrating low thermal resistance and robust heat dissipation capabilities. Liquid PQDs can be encapsulated within dense, rigid cavities such as glass, which further enhances their stability. Moreover, liquid PQDs-glass cavities can be molded into various shapes for color converters, offering flexibility and adaptability^[32,38]. Thirdly, liquid PQDs exhibit broad emission angles, enabling wide-angle or even omnidirectional transmitters that minimize link alignment requirements^[34,46,71]. In 2022, Ali et al. demonstrated high-bandwidth liquid-phase CsPbBr₃ in glass cavities^[32]. The bandwidth only marginally decreased from 1020 MHz to 883 MHz when a blue LD was combined with liquid-phase CsPbBr₃ color converters. Compared to solid-phase CsPbBr₃, liquid-phase CsPbBr₃ showed less efficiency droop over time. In 2023, Shan et al. investigated the impact of CsPbBr₃ PQD concentration in hexane solution on optical performance and modulation bandwidth^[38]. As depicted in Figure 3C, both modulation bandwidth and light output power (LOP) increased initially and then decreased with rising PQD concentration, achieving a maximum -3 dB bandwidth of 168.03 MHz at 0.5 mg/mL concentration. Furthermore, when pumped by a 100 mA LD, the addition of ethanol further improved the frequency response and bandwidth of CsPbBr₃ PQDs, as illustrated in Figure 3D. After the addition of 55 µL ethanol to 1 ml CsPbBr₃ PQDs, possibly attributed to the energy transfer assisted solvent effect, the PQDs achieved a -3 dB bandwidth of 363.68 MHz and a data rate of 1.25 Gbps, despite approximately halved PL intensity. This study reveals the benefits of energy transfer channels for bandwidth improvement, indicating that establishing stable energy transfer pathways in PQD solutions is a promising optimization strategy.

The benchmark of the modulation bandwidth for PQDs under different encapsulations is summarized in Figure 3E. With ongoing advancements in nano-engineering aimed at minimizing PL lifetimes, the modulation bandwidth of perovskite color converters is anticipated to exceed 1 GHz. Meanwhile, the enhancements in optical properties and stability achieved via different encapsulation strategies demand further in-depth investigation^[72]. In addition to the widely adopted polymer-film-based and solution-phase PQDs, alternative architectures of perovskite color converters warrant attention. For instance, perovskite-on-polymer microspheres, distinguished by their unique light diffusion properties, demonstrate a uniform light intensity distribution across a broad range of viewing angles, offering promising potential for applications of SSL and liquid-crystal display backlighting^[66,73].

Modulation schemes and multiplexing

Researchers have optimized the frequency response of systems through improvements in semiconductor light sources and PQD-based color converters. Furthermore, by selecting appropriate modulation schemes and multiplexing techniques, data transmission rates can be substantially increased, occasionally exceeding ten times the -3 dB bandwidth. Non-return-to-zero on-off keying (NRZ-OOK), a fundamental modulation scheme in wireless communications, employs high and low levels to encode sequences of ones and zeros. This straightforward modulation scheme typically enables the transmission rate to achieve twice the modulation bandwidth when the bit-error-rate (BER) is below the typical forward error correction (FEC) threshold of 3.8 × 10^-3.

In pursuit of higher data rates, advanced modulation schemes and multiplexing techniques have been implemented in PQD-based OWC systems^[5]. In OFDM technique, which combines modulation and multiplexing, data streams are transmitted across multiple subcarriers that maintain orthogonality, allowing spectral overlap without interference and enhancing spectral utilization efficiency. Bit loading for each subcarrier can be dynamically adjusted according to channel SNR, enabling the selection of different modulation schemes^[74]. Typically, low-frequency subcarriers support higher bit loading and can accommodate advanced modulation schemes, such as higher-order QAM, while high-frequency subcarriers with lower bit loading may utilize simpler modulation schemes like low-order QAM and binary phase shift keying (BPSK)^[45]. This adaptive bit loading maximizes channel utilization, achieving data rates in certain cases that exceed ten times the bandwidth^[37,51]. Nevertheless, OFDM introduces increased complexity in both hardware and software processing. Additionally, the high peak-to-average power ratio of OFDM makes it susceptible to nonlinear distortion and demands high linearity from amplifiers in transmitters^[75].

Another multiplexing technique, WDM, has been successfully implemented in RGB-organic light emitting diode (OLED) devices^[76,77], multicolor μLED devices^[74,78,79], and CdSe QD-based full-color devices^[80]. The applications of PQD-based color converters with different wavelengths in SSL systems have also been widely adopted, highlighting the adaptability of WDM. WDM can be effectively combined with lightweight modulation schemes such as NRZ-OOK. Instead of significantly increasing processing complexity, such systems emphasize optical system architecture. By utilizing multiple sets of independently modulated light sources and separate PDs, multiple optical channels at different wavelengths can be established, thereby multiplying the transmission rates. Additional physical multiplexing, such as time division multiplexing and polarization division multiplexing, holds potential for future research and applications. The selection of appropriate excitation sources and PQD structures, along with suitable modulation schemes and multiplexing, enables the optimal SSL-OWC system designs for specific application scenarios.

Lead-free PQDs

The presence of toxic lead in APbX₃-type perovskites presents significant environmental hazards throughout their entire life cycle, from manufacture to end-of-life stages. Although improved manufacturing processes, encapsulation methods, and disposal procedures can mitigate lead leakage^[43], these solutions increase costs and do not fundamentally resolve the underlying issue. Therefore, the development of lead-free perovskites for lighting and optical communication represents an attractive alternative^[81-83]. In recent years, copper halide perovskites, especially zero-dimensional (0D) Cs₃Cu₂X₅ and one-dimensional (1D) CsCu₂X₃, have emerged for high-quality SSL applications due to their superior optical properties, such as high PLQY, broad spectrum, and low self-absorption^[20,83,84]. These materials include green-emitting Cs₃Cu₂Cl₅^[60], yellow-emitting CsCu₂l₃, blue-emitting Cs₃Cu₂l₅^[42], Cs₃Cu₂Br₅^[45], and violet-emitting K₂CuBr₃^[85], with their integration into VLC systems being actively explored.

In 2022, Wang et al. demonstrated UV-excited Cs₃Cu₂I₅ and CsCu₂I₃ single crystals (SCs) as blue and yellow color converters for WLED^[42]. As shown in Figure 4A, they employed a pressure-assisted cooling method to synthesize large copper halide perovskites SCs, reducing the specific surface area to enhance the stability of the resulting materials. Scanning electron microscopy (SEM) images of Cs₃Cu₂I₅ and CsCu₂I₃ SCs are shown in Figure 4B and C, which exhibit uniform orientation and smooth surfaces of the large SCs, indicating their potential for superior optical performance. Figure 4D displays the electroluminescence spectra of the constructed WLED under various driving voltages, with the broad spectral coverage enabling the WLED to achieve a CRI of 91. In the VLC application, the system demonstrated a -3 dB bandwidth of 10.1 MHz and achieved a data rate of 87.7 Mbps using OFDM modulation. Figure 4E illustrates the frequency response of the VLC system and the captured 10-MHz eye diagram. In 2023, the research group further developed blue-emitting 0D Cs₃Cu₂Br₅ SCs^[45]. The resulting white-light system achieved a -3 dB bandwidth of 6.7 MHz and a data rate of 45 Mbps using OFDM modulation. Although copper halide perovskite SCs exhibit broadband emission suitable for high-CRI lighting, their PL lifetimes, ranging from tens of nanoseconds to several microseconds, limit their bandwidths to below 100 MHz. Future research should focus on improving the bandwidth of copper halide perovskites through structure optimization. Additionally, alternative lead-free perovskites incorporating Ag⁺, Sb³⁺, or Bi³⁺ as Pb substitutes have been successively developed for WLEDs^[20,66,84]. These diverse lead-free perovskites provide more options for color-conversion materials in PQD-based OWC systems. It is anticipated that more non-toxic perovskites will demonstrate high-speed communication capabilities in future research, becoming qualified alternatives to the current high-bandwidth CsPbX₃.

Figure 4. (A) Grown process of Cs₃Cu₂I₅ and CsCu₂I₃ SCs with pressure-assisted cooling method; (B and C) SEM images of Cs₃Cu₂I₅ and CsCu₂I₃ SCs grown with pressure-assisted; (D) EL spectra of the WLEDs under various driving voltages; (E) frequency response of the WLEDs. Inset is the captured eye diagram. Reproduced with permission: under a Creative Commons Attribution License^[42]. SEM: scanning electron microscopy; SC: single crystal; EL: electroluminescence; WLED: white light-emitting diode.

PQDs for indoor VLC transmitters

With the ongoing development of 6G technology, indoor VLC, as a crucial branch of OWC, has become a prominent research field. LiFi technology based on VLC systems is evolving and demonstrates the potential to significantly complement the indoor communication landscape currently dominated by WiFi^[3,6,86-88]. Nevertheless, the VLC hardware infrastructure necessitates further refinement to achieve widespread commercial adoption. While VLC transmitters utilizing monochromatic light sources have reached a level of technological maturity, the integration of VLC transmitters with SSL sources, such as WLEDs and white laser diodes (WLDs), presents a more expansive opportunity^[9,20]. The integration of color converter emissions with excitation sources constitutes a predominant strategy for generating high-quality white light. For PL white-light systems, excitation sources typically employ blue or UV band emission. Blue light is the most commonly used wavelength of pump source, as mature commercial WLEDs adopt the “partial conversion” principle, which combines blue GaN LED emissions with color-converting light components from phosphors^[10]. As technological upgrades to traditional WLEDs, blue GaN μLEDs/LDs serve as ideal excitation sources, demonstrating excellent luminous efficiency and modulation bandwidth (> GHz). Additionally, semiconductor light sources in the UV band are also utilized to excite color conversion materials^[42]. Despite facing higher costs, lower external quantum efficiency, and reduced color conversion efficiency, the combination of UV LEDs/LDs with color converters still offers certain advantages for specific applications. For white lighting requiring high CRI, when white light is entirely emitted by color conversion materials, this approach facilitates achieving broader spectral coverage. Furthermore, this strategy enables dual-band communication across both UV and visible light spectra^[89,90].

Recent years have witnessed significant advancements in VLC systems based on PL white-light sources^[31,39,41]. WLEDs or WLDs with color converters typically exhibit reduced modulation bandwidths compared to monochromatic light sources. However, PQDs maintain relatively high bandwidths due to their shorter fluorescence lifetimes, with PQD-based WLED/WLD systems achieving the highest modulation bandwidths among research on PL white-light systems^{[26,31,41,53]}. In 2018, Mei et al. developed a WLED-VLC system combining a blue GaN-based μLED with yellow-emitting CsPbBr_1.8I_1.2 QDs^[39], demonstrating bandwidths of 73 MHz and 85 MHz for the PQDs and white-light system, respectively, with minimal degradation to 70 and 83 MHz after six months. The WLED achieved a maximum data rate of 300 Mbps. However, single yellow-emitting PQDs are insufficient to cover both the red and green spectral regions, which are critical for a high CRI, a metric essential for high-quality white lighting that requires values close to 100^[19]. To achieve superior optical effects in white-light sources, the integration of at least two color converters with different peak wavelengths becomes imperative.

Color converters combining PQDs with other luminescent materials, including phosphors^{[29,30,45,51,60]}, II-VI semiconductor QDs^[32,40,41], and organic emitters^[53,54], have been reported. Although semiconductor QDs and organic emitters typically exhibit lower -3 dB bandwidths than PQDs, they all significantly surpass phosphors in terms of bandwidths. PQDs, when integrated as one of the color-converting components alongside more established luminescent materials, highlight their application potential in WLEDs. In 2021, Singh et al. demonstrated a flexible white-light system that utilized a semipolar blue InGaN/GaN μLED as the pump source, with CsPbBr₃ PQD paper and CdSe QD paper serving as green and red converters, respectively^[41]. The semipolar μLED mitigated the QCSE, accelerating the radiative recombination process, and achieving a -3 dB bandwidth of 817 MHz and a data rate of 1.5 Gbps. The TRPL curves of the semipolar μLED, CsPbBr₃ PQD paper, and CdSe QD paper are illustrated in Figure 5A. The PQD paper exhibited a shorter average PL lifetime of 5.92 ns compared to 12.88 ns of the CdSe QD paper, attributable to the quantum confinement effect. Figure 5B compares the frequency responses and bandwidths between the PQD paper and a typical PQD film, with the PQD paper showing superior performance with a bandwidth of 229 MHz and a transmission rate of 400 Mbps. The white-light system and VLC test setup are depicted in Figure 5C. The -3 dB bandwidth of the WLED was limited to 95.5 MHz, due to a low bandwidth (< 25 MHz) of the red-emitting CdSe QD paper, underscoring the need to optimize the red component in this design. In 2022, Ali et al. developed a WLD by combining a blue LD with liquid-phase color converter (LCC) glass cavities containing green-emitting CsPbBr₃ and red-emitting CdSe/ZnS QDs^[32]. The liquid-phase QDs exhibited enhanced thermal stability, which is attributed to free movement facilitating convective heat dissipation. Additionally, reduced self-aggregation in liquid-phase QDs contributed to improved optical performance and a faster radiative recombination process. This WLD achieved a CRI of 85 and a CCT of 5520 K, making it suitable for high-power, high-quality SSL. As shown in Figure 5D, under high-bandwidth blue LD excitation, the CsPbBr₃ QDs, CdSe/ZnS QDs, and WLD demonstrated -3 dB bandwidths of 883, 871, and 855 MHz respectively, with the WLD achieving a remarkable data rate of 2.1 Gbps.

Figure 5. (A) TRPL curves for semipolar μLED, PQD paper, and CdSe QD paper; (B) frequency response and bandwidth of PQD paper and PQD film. Inset is an eye diagram for the PQD paper; (C) schematic of the white-light system setup for the VLC test; (D) frequency response of blue LD (B-LD) and B-LD with different QD converters. Reproduced with permission: (A-C)^[41], Copyright 2021, Chinese Laser Press; (D)^[32], Copyright 2022, Optica Publishing Group. SQW: single quantum well.

Considerable attention has been devoted to synthesizing perovskite/organic hybrid luminescent materials^[53,54]. Organic emitters, compared to QDs, exhibit a broader spectrum, rendering them more suitable for high-CRI white-light illumination and complementing the spectral coverage limitations of PQD-based color converters. Studies have demonstrated that specific organic emitters can achieve high bandwidths exceeding 100 MHz^[53,91-93], suggesting the potential for high-bandwidth performance in perovskite/organic hybrid materials. In 2024, Yao et al. fabricated a CsPbBr₃/organic color converter via an in situ solid-state synthesis technique^[54]. The green-emitting CsPbBr₃ and red-emitting 2,3,5,6-tetrakis(3,6-diphenylcarbazol-9-yl)-1,4-dicyanobenzene (4CzTPN-Ph) formed multi-site contacts, establishing energy transfer channels that overcame the challenge of low light intensity from the red organic emitters. As illustrated by the energy transfer mechanism shown in Figure 6A, the green emission from CsPbBr₃ PQDs can indirectly excite 4CzTPN-Ph, thereby significantly enhancing the PL intensity of the organic red emission. When combined with a blue LED, the resulting WLED device achieved a CRI of 86 and a luminous efficacy (LE) of 100 lm/W. For VLC applications, the short PL lifetime (12 ns) of the color converter indicated the potential for high-speed transmission. The WLED demonstrated a data rate of 5 Mbps, limited by the substantial RC constant of the blue LED. Replacing the excitation source with a high-bandwidth alternative could further unlock the high-speed potential of this perovskite/organic hybrid color converter.

Figure 6. (A) Schematic of the enhanced organic red emission through energy transfer channels; (B) optical spectrum of the WLED. Insets are the photographs of red and yellow PNC-PMMA, and films cured in cuvettes; (C) modulation bandwidth of the system based on μLED and μLED with different PNC-PMMA films; (D) BER of μLED with and without PNC-PMMA films at different data rates. Reproduced with permission: (A)^[54], Copyright 2024, Wiley-VCH GmbH; (B-D)^[31], Copyright 2021, American Chemical Society. 4CzTPN-Ph: 2,3,5,6-tetrakis(3,6-diphenylcarbazol-9-yl)-1,4-dicyanobenzene; ET: energy transfer; CB: conduction band; VB: valence band; LUMO: lowest unoccupied molecular orbital; HOMO: highest occupied molecular orbital. PNC-PMMA: perovskite nanocrystal-polymethyl methacrylate; WLED: white light-emitting diode; μLED: micro light-emitting diode; BER: bit-error-rate.

In the aforementioned studies on hybrid luminescent materials, PQDs-based color converters typically exhibit shorter PL lifetimes and higher bandwidths. Consequently, developing color converters composed entirely of PQDs could further advance VLC applications. In 2021, Wang et al. employed yellow-emitting CsPb(Br/I)₃ and red-emitting CsPbI₃ PNC-PMMA films as color converters for blue μLEDs^[31]. For SSL applications, the yellow/red-emitting PNC-PMMA films demonstrated FWHMs of 30 and 35 nm, respectively, which are sufficiently broad to support high-quality illumination. The WLED spectrum, depicted in Figure 6B, represents a warm white-light source with a CCT of 5670 K and a CRI of 75.7. In the VLC application, the yellow and red PNC-PMMA exhibited impressive modulation bandwidths of up to 347 and 822 MHz, respectively, owing to their exceptionally short PL lifetimes of 11.2 and 3.5 ns. Figure 6C illustrates the bandwidths of the μLED and three PNC-PMMA combinations under different driving currents. Attributed to the high bandwidths of PNC-PMMA, the modulation bandwidth at high currents only decreased from 1.130 GHz for the blue μLED to 1.005 GHz for the WLED. Figure 6D presents the BER of the μLED and WLED at different data rates under the NRZ-OOK modulation scheme. A record-breaking data rate of 1.7 Gbps was achieved for the white LED system while maintaining a BER below the FEC standard of 3.8 × 10^-3.

Physical multiplexing techniques, particularly WDM, can further exploit the data transmission potential of PQD-based VLC transmitters. In 2024, Fu et al. constructed a WDM-VLC system utilizing green/red-emitting liquid CsPbBr₃/CsPbI₃ QDs^[46]. Benefiting from the wide-angle characteristics of liquid PQDs, the system achieved a field of view of ± 40° at a distance of 60 cm, with each green/red channel operating at 50 Mbps, yielding a total data rate of 100 Mbps. At the software level, the authors implemented a computational temporal ghost imaging (CTGI) algorithm to reduce BER, extending the maximum transmission angle to ± 60°. The feasibility of WDM-VLC schemes utilizing PQD-based color converters with different emission wavelengths has been demonstrated. Further research should prioritize the enhancement of communication parameters of WDM-VLC systems, such as modulation bandwidth, data rate, transmission distance and angle. Table 2 summarizes the configurations and SSL-VLC parameters of recently developed PQD-based white-light systems. Future research should aim to advance WLED or WLD systems with variable CCT, higher CRI, increased transmission rates, and improved stability to enhance the practicality of SSL-VLC solutions.

PQDs for UWOC transmitters

With the advancement of underwater engineering such as marine exploration and environmental monitoring, there is an increasing demand for network connectivity among underwater devices. Consequently, UWOC has emerged as a significant research focus in recent years. UWOC, in conjunction with RF communication and underwater acoustic communication (UAC), is poised to meet the diverse requirements of the Internet of Underwater Things (IoUT) and underwater optical cellular networks. While UAC is distinguished by its capability in kilometer-range long-distance communication, UWOC has garnered attention for its advantages of low multipath loss, high transmission rate, low latency, and compact equipment size, rendering it highly suitable for last-meter solutions in underwater networking^[94-97].

In pure water conditions, blue light undergoes minimal optical absorption loss, which is one of the reasons why blue LDs are extensively utilized in UWOC systems. Nevertheless, in actual aquatic environments, the wavelength that corresponds to the minimum loss shifts toward longer wavelengths as water turbidity increases^[95,97,98]. PQDs are particularly advantageous as color conversion layers when paired with blue LDs to achieve green emission for UWOC applications, yielding two primary advantages. Firstly, while the “green gap” limits the electroluminescence efficiency of green diodes^[98,99], the combination of blue LD and inorganic perovskite has demonstrated high efficiency in generating green light through a color conversion approach. Secondly, PQDs offer wavelength tunability, allowing for matching with the minimum-loss wavelength specific to different water environments. This flexibility enables UWOC systems to maintain minimal link loss^[33,34].

As the first application of PQDs in UWOC, in 2021, Xia et al. integrated CsPbBr₃ NCs with a blue LD to construct a UWOC system operating over a 15 cm distance^[33]. To enhance the stability of CsPbBr₃ NCs against water, heat, and intense laser irradiation of 3.6 kW/cm², they synthesized CsPbBr₃ NCs-glass through in situ precipitation in inorganic glass via solid-state reaction. This color converter achieved green emission with a narrow FWHM (~ 20 nm). Additionally, the high refractive index (1.54) of the glass matrix enhanced the radiative transition rates of CsPbBr₃ NCs, reducing the decay time to 3.22 ns. The experimental setup of the UWOC system is schematically depicted in Figure 7A. The study demonstrated a UWOC system with a bandwidth of 180 MHz and a data rate of 185 Mbps. As shown in Figure 7B, the eye diagram remained open after 60 minutes of operation, indicating the reliability of this UWOC system. Furthermore, Li et al. advanced PQD-based UWOC systems with improved transmission distance and networking capabilities. In 2022, the group developed a quasi-omnidirectional UWOC transmitter through the integration of liquid CsPbBr₃ QDs sealed in a cylindrical bottle with a blue LD, mitigating link alignment challenges^[34]. Figure 7C and D presents the structure of this transmitter, and its uniform optical power distribution across the spherical surface. The UWOC system was validated in a standard swimming pool. Using on-off-keying (OOK) modulation, transmission rates of 60 Mbps and 40 Mbps were achieved at distances of 10 m and 20 m, respectively. With code division multiple access (CDMA) networking of four clients, each client reached 10 Mbps and 7.5 Mbps at 10 m and 20 m, respectively. Figure 7E demonstrates that in different communication modes, BER slightly increased with yaw angle changes, but the variations remained within acceptable ranges. The authors noted that their design, incorporating a blue-emitting LD and a green-emitting CsPbBr₃ converter, could operate in dual modes: long-distance directional communication and short-distance omnidirectional communication, which could enhance the functionality of a single UWOC transmitter. The application of PQDs in UWOC transmitters has demonstrated initial promise for short-range underwater networking, and further research is needed to approach commercialization.

Figure 7. (A) Schematic of the experimental setup for the UWOC system based on CsPbBr₃ NCs-glass; (B) the eye diagrams for the UWOC system at different operating times; (C) structure of the quasi-omnidirectional UWOC transmitter; (D) measured optical power distribution of the UWOC transmitter; (E) BER for two communication modes at different yaw angles. Reproduced with permission: (A, B)^[33], Copyright 2021, Wiley-VCH GmbH; (C-E)^[34], Copyright 2022, Optica Publishing Group. SQA: signal quality analyzer; OSC: oscilloscope; APD: avalanche photodiode; UWOC: underwater wireless optical communication; NC: nanocrystal; BER: bit-error-rate.

PQDs for photodetectors

In OWC systems, receivers play a pivotal role, and the link transmission rate is fundamentally constrained by the response characteristics of the receiver^[1,9]. While color-converting PQDs are widely employed in transmitters, there is an emerging body of research that explores the placement of PQDs in proximity to PDs. This approach aims to convert the received light into specific wavelengths with a narrow FWHM to match the optimal spectral response of the target PDs. This strategy has the potential to significantly enhance the sensitivity and efficiency of PDs in OWC systems.

PQDs have been recognized for their potential in PDs for UV-based communication systems. UV light communication has emerged as a competitor to VLC due to its superior non-line-of-sight (NLOS) capability and reduced susceptibility to solar background radiation^{[89,90,100-102]}. However, the development of efficient UV communication receivers presents several challenges. Traditional photomultiplier tubes (PMTs) are limited by high power consumption and large sizes^[103], while III-nitride-based PDs face issues with dislocation and lattice mismatch^[104]. Meanwhile, although Si-based PDs for visible light have reached technological maturity, they exhibit poor response to UV light^[105]. Therefore, integrating PQD-based color converters with Si-based PDs offers an effective approach to UV detection. In 2019, Kang et al. developed a hybrid CsPbBr₃-silicon PD by integrating a CsPbBr₃ NCs layer with a Si-based PD^[35]. CsPbBr₃ NCs can convert UVC (100-280 nm) light into green light, matching the high-response band of the Si-based PD. After installing the NCs layer in front of the bare Si-based PD, the responsivity to 270-nm UVC light increased from 29 to 84 mA/W. The PL decay time of CsPbBr₃ NCs was only 4.5 ns, enabling a small-signal modulation bandwidth of 70.92 MHz. Using a 278 nm UVC-LED as the transmitter with the OOK modulation scheme, the receiver achieved a data rate of 34 Mbps, compared to 25 Mbps without CsPbBr₃ NCs. In 2023, the same group incorporated CsPbBr₃ NCs in front of the optical concentrator of a Si-based receiver, expanding the FoV half-angle to 60°^[36]. This improvement was attributed to CsPbBr₃ NCs acting as a secondary light source, emitting green light uniformly in 360°. Additionally, the data rates were improved to 100 Mbps and 71.6 Mbps under NRZ-OOK and OFDM modulation schemes, respectively. Figure 8A illustrates the experimental setup for UV light communication using the NRZ-OOK modulation scheme.

Figure 8. (A) Schematic of the experimental setup for UV light communication based on CsPbBr₃ NCs, using the NRZ-OOK modulation scheme; (B) image of the CsPbBr₃ NCs-polymer fiber under UV illumination; (C) image of the NCs-polymer fibers detection arrays. Reproduced with permission: (A)^[36], Copyright 2023, Optica Publishing Group; (B, C)^[44], Copyright 2022, Optica Publishing Group. AWG: arbitrary waveform generator; TEC: thermoelectric controller; APD: avalanche photodiode. NRZ-OOK: non-return-to-zero on-off keying; UV: ultraviolet; NC: nanocrystal.

For the expansion of OWC receivers, perovskite fibers exhibit multifunctional characteristics. These fibers can facilitate wavelength conversion to better couple with PDs. Additionally, compared to planar-based PDs, perovskite fibers can form near-omnidirectional array reception platforms similar to radio-frequency antennas, owing to their inherent flexibility and scalability. This feature significantly reduces the difficulty of signal reception. In 2022, Kang et al. developed a near-omnidirectional OWC receiver based on perovskite-polymer-based scintillating fibers^[44]. The design incorporated CsPbBr₃ NCs embedded within polymer fibers, which were cascaded and expanded to form a fiber PD array, coupled with either silica fibers or Si-based avalanche photodiodes (APDs). Figure 8B and C illustrates the CsPbBr₃ NCs-polymer fiber and its corresponding fiber detection arrays. Isobornyl acetate (IBOA) monomer was utilized to combine with CsPbBr₃ NCs to form a polymer composite. Although IBOA effectively enhanced the photostability of CsPbBr₃, it absorbed light at wavelengths below 400 nm, consequently limiting the communication band to the short-wavelength visible spectrum (400-475 nm). When transmitting data through the channel using 405 nm light, the perovskite-polymer-fibers re-emitted green light at approximately 514 nm. The system achieved data rates of 23 Mbps and 152.5 Mbps using NRZ-OOK modulation for VLC and OFDM modulation for UWOC, respectively. Both planar-based and fiber-based perovskite color-converting PDs have demonstrated their distinct advantages and application scenarios, offering promising strategies for advancing OWC receivers. Moreover, rapidly evolving perovskite PDs that directly perform photoelectric conversion are attracting increased attention. Compared to PQD-based color-converting PDs, photoelectric conversion perovskite PDs show promise as more versatile detection solutions due to their higher energy conversion efficiency and integration density^[106-108].