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Abstract
In recent years, cesium lead bromide (CsPbBr3) and cadmium selenide/zinc sulfide (CdSe/ZnS) quantum dots have been widely investigated to enhance the capacity of visible light communication (VLC) and solid-state lighting (SSL). Herein, liquid-phase color converter (LCC) glass cavities and solid-phase color converter (SCC) films with green-emitting CsPbBr3 and red-emitting CdSe/ZnS are fabricated to investigate and compare their performance. A facile high-quality LCC-based white laser diode (WLD) is fabricated by combining blue LD with LCC CsPbBr3 and CdSe/ZnS glass cavities as color conversion layers. The LCC-based WLD achieves bright white light with a color rendering index of 85, a correlated color temperature of 5520 K, and a Commission Internationale de L'Eclairage (CIE) coordinates at (0.32, 0.34). Moreover, the VLC system exhibits a modulation bandwidth of 855 MHz and the capability to transmit a real-time data rate of up to 2.1 Gbps over a transmission distance of 1.2 meters. These results indicate that the fabricated WLD is a promising lighting device for simultaneous high-speed VLC and high-efficiency SSL.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Corrections
29 September 2022: A correction was made to the funding section.
1. Introduction
Visible light communication (VLC) is one of the next generation's most promising wireless communication technologies. VLC offers many advantages over current wireless communication systems, including global availability, broad bandwidth, network security, unlicensed bands, and stable transmission with electromagnetic immunity [1]. One of the primary advantages of VLC over other wireless communication technologies is the ability to utilize readily available solid-state lighting (SSL) for illumination and communication simultaneously. Together with the function of lighting, VLC using white light-emitting diodes (WLED) has become more popular to fulfil the growing demand for wireless data communication [2]. In 2019, Tian et al. demonstrated WLED VLC system exhibits a modulation bandwidth of 75 MHz with a CRI of 89 [3]. The modulation bandwidth of gallium nitride-based LED is tens of megahertz (MHz), limited by carrier lifetime and resistance-capacitance (RC) time constant [4]. According to the Shannon limit, the low modulation bandwidth transmitter cannot support high-speed data transmission as channel capacity and bandwidth are related [5].
Micro LEDs (µ-LEDs) have been proposed to overcome the bandwidth limitation due to their high internal quantum efficiency (IQE) and bandwidth. Despite the outstanding performance of µ-LEDs, they have a few limitations. Because of its ultra-small light-emitting area, a single µ-LED has relatively low emission power, [6–8] which limits its application in high color rendering index (CRI) lighting and high-speed VLC. The light source with high emission power and modulation bandwidth, such as laser diode (LD), is suitable for high CRI illumination and high-speed communication systems. LDs have a higher pumping efficiency and a larger modulation bandwidth (>5 GHz) than LEDs [9]. LDs have several advantages in communication applications, e.g., fast response speed, minimum light-emitting surface, and high coupling efficiency. Polychromatic light is usually used as the light source for human-eye-friendly indoor applications. LDs alone are not suitable for most lighting applications due to their nearly monochromatic emission. White laser diodes (WLDs) are commonly fabricated by combining blue or ultraviolet LD beams with color-converter materials (CCMs), including organic and inorganic compounds. The bandwidth and wide color gamut of WLDs depend on the pumping light source and the optical properties of CCMs [10–14].
Commercially available inorganic CCMs have a relatively long PL lifetime to the order of microseconds to milliseconds, limiting the modulation bandwidth to a few (3-12) MHz [15–17]. Organic color converters such as BODIPY, MEH-PPV, and BBEHP-PPV were proposed to address the bandwidth limitation for white light VLC. However, they still suffer from their long PL lifetime, limiting the modulation bandwidth to the range of 40 to 200 MHz [18,19]. The effect of CCMs with different PL lifetimes on the bandwidth of generated white light is reported in [10,20]. Equation (1) shows the relationship between the frequency response and PL lifetime of CCMs [14].
Where H(f) is the degree of modulation and $\tau $ is a PL lifetime of CCMs. When H(f) is $\frac{1}{2}$ the corresponding 3-dB bandwidth is shown in Eq. (2).It can be seen in Eq. (2) that the 3-dB bandwidth is inversely proportional to the PL lifetime of CCMs [14].
Recently, QDs materials have been widely investigated to enhance the capacity of SSL and VLC based on LEDs/LDs because of their short PL lifetime, narrow full width at half maximum (FWHM), and desirable optical properties. QDs CCMs exhibit a modulation bandwidth of hundreds of (200–800) MHz, as reported in [9]. Promising classes of such materials are metal-halide perovskites $\textrm{Cspb}{X_3}$ (X = Cl, Br, or I) and cadmium selenide quantum dots ($\textrm{Cdse} - \textrm{QDs}$). Cesium lead bromide ($\textrm{CsPb}B{r_3}$) and cadmium selenide ${\; }({CdSe} )$ QDs have recently emerged as potential CCMs in VLC and SSL due to tunable and stable emission wavelength, high photoluminescence quantum yield (PLQY), short PL lifetime, broad absorption, narrowband emission, and size-engineering bandgap. In 2018, Mei et al. demonstrated a white light VLC system with a modulation bandwidth of 85 MHz combining blue µLED with yellow cesium lead halide perovskite quantum dots [21]. A white light VLC system based on red and green perovskite QDs with a modulation bandwidth of 162 MHz was experimentally demonstrated for both VLC and SSL [22]. The properties of $CsPbB{r_3}$ and $CdSe/ZnS$ are given in Table S1 (Supplement 1).
The low resistance of QDs to oxygen, moisture, temperature, and water limits their practical application. Recently, numerous efforts have been made to resolve the stability problem of QDs through surface engineering and surface coating techniques. Ibrahim et al. used Polydimethylsiloxane (PDMA) doped $CsPbB{r_3}{\; }$ and red nitride phosphor film to demonstrate a real-time data rate of 2.0 Gbps with a CRI of 89 [23]. In 2021, Ali et al. demonstrated a VLC system with a CRI of 93 and a data rate of 1.6 Gbps by using polymethyl-methacrylate-doped phosphor film based on $CsPbB{r_3}$ and potassium fluorosilicate [24]. A VLC system with a CRI of 63 and a data rate of 9.6 Gbps using PDMA doped phosphor film based on dual-sized $CdSe/ZnS$ core-shell-QDs with two luminescent wavelengths centered at 515 and 630 nm is reported in [25]. Most reported CCMs are dispersed in organic, inorganic or silicone resin to form a film. However, resins’ bulky size, poor dispersion stability, low heat conductivity and resistance can lead to severe discoloration and luminescent degradation in prolonged continuous operation [26,27]. Moreover, heat and water can damage these components, so they are unsuitable for outdoor applications. CCMs in the glass cavity has been proposed to address this limitation [28–30]. In 2018, Wu et al. demonstrated a VLC system with a data rate of 1.6 Gbps over 1 m using fluorescent glass as a color converter phosphor [31]. The authors of [32] experimentally demonstrated extremely stable and long-term water resistant $CsPbB{r_3}$ in glass for underwater wireless optical communication (UWOC) and illumination with a data rate of 185 Mbps.
Packing of QDs color converter can be made in solid and liquid phases to fabricate white LEDs/LDs. The constraints of QDs, i.e. nonuniform QDs dispersion and limited thermal conductivity of the solid matrix, hamper white LEDs/LDs fabrication in the solid phase [33,34]. Another major concern in the QDs is the reduction in quantum efficiency when the solvent is dried up. This impediment can be overcome by packing QDs in the liquid phase to fabricate white LEDs/LDs. QDs in the liquid phase exhibit a much better light emission capability than in the solid phase due to self-aggregation. Additionally, QDs can be more easily disseminated in a liquid than in a solid matrix. The radiative recombination process of liquid-phase QDs is faster than that of solid-phase; therefore, the color gamut and bandwidth of liquid-phase QDs are higher than that of solid-phase [35,36]. Additionally, the liquid phase QDs may increase heat dissipation efficiency in the color converter cell and avoid thermal quenching of QDs, hence increasing their emission stability. Liang et al. synthesized liquid perovskite QDs $CsPbB{r_3}$ for LD-based VLC and SSL [36]. Li et al. demonstrated a quasi-omnidirectional UWOC system with a data rate of 60 Mbps over a transmission distance of 10-m by using liquid-phase $CsPbB{r_3}$ [37]. To the best of the author's knowledge, this paper is the first to study the combination of blue LD, liquid phase $CsPbB{r_3}{\; }$ and $CdSe/ZnS$ for simultaneous illumination and communication.
Our study presents a novel thermally stable LCC-based WLD composed of blue LD; liquid phased green-emitting $CsPbB{r_3}$ and red-emitting $CdSe/ZnS$. We demonstrate LCC-based WLD with a CRI of 85 and modulation bandwidth of 855 MHz, which is higher than one that is achieved with the combination of QDs with a conventional red phosphor [24]. Thus, our proposed LCC-based WLD could offer opportunities for both high-speed VLC and high-performance SSL simultaneously. In our case, 50 mA bias current was considered as optimal DC bias for illumination and communication. Furthermore, the electrical and optical properties of monochromatic light (green/red) from $CsPbB{r_3}$ and $CdSe/ZnS$ in solid and liquid phases are reported based on measurement data. We believe these findings could help readers better understand the fabrication of QDs-based white LD for VLC and SSL applications.
2. Experimental Methods
2.1 Preparation of LCC ${CsPbB}{{r}_3}$ glass cavity
$CsPbB{r_3}$ QDs solution was made by dissolving $CsPbB{r_3}$ powder in toluene at a 3 mg/1 ml concentration ratio. A syringe injected the solution into a glass cavity with a 45∗12.5∗1 mm (0.35 ml) volume. The glass cavity was then sealed using paraffin, preventing solvent volatilization and limiting exposure to the environment.
2.2 Preparation of the LCC ${CdSe}/{ZnS}$ glass cavity
$CdSe/ZnS$ solution was prepared by dissolving $CdSe/ZnS$ powder in toluene with the concentration ratio of 3 mg/1 ml. LCC $CdSe/ZnS$ QD glass cavity was prepared under the same conditions as the $CsPbB{r_3}$ LCC glass cavity, except that the volume of the glass cavity was 45∗12.5∗2 mm (0.7 ml).
2.3 Fabrication of LCC-Based White-LD
LCC-based white LD was fabricated by combining blue LD and LCC $CsPbB{r_3}{\; }$ glass cavity with LCC $CdSe/ZnS$ glass cavity. We used transparent glass tape to stick two glass cavities with each other, as it is colorless and will not affect the color and efficiency of generated light. These glass cavities have adhered to the top of the blue laser diode. Aiming to evaluate the LCC-based white LD, SCC-based LD was fabricated using blue LD and $CsPbB{r_3}$ green QD film with $CdSe/ZnS$ QD film. The fabrication method of the LCC- and SCC-based WLD is shown in Fig. S1 (Supplement 1).
2.4 Measurement of the SSL system
OHSP spectral and illuminance analyzer was used to characterize the illuminance performance of generated white light, including spectrum, intensity, RGB ratio, correlated color temperature (CCT), and de L'Eclairage (CIE) color coordinates, and color rendering index (CRI, Ra-R15) values. An Ocean Optics spectrometer (USB2000, Ocean Optics) was used to measure the spectra of blue, green, red and white light.
2.5 Measurement of the communication system
The schematic diagram of the experimental setup is shown in Fig. 1(a). LCC-based white LD was applied as a light source for a 1.2 m long indoor VLC system. The LCC-based LD was driven by real-time pseudorandom binary sequences (PRBS), with a pattern length of ${2^{15}} - 1$ OOK-NRZ data stream with a peak-to-peak voltage of 0.8 V was generated repeatedly by a bit error rate tester (BERT) (MP2100B, Anritsu). Two aspherical lenses were used to collimate and focus the optical signal into a high-speed Si Avalanche photodiode (APD, APD210). The BERT captured the generated electrical signal to measure the bit error rate (BER) at different data rates. A real-time oscilloscope (DPO75902SX, Tektronix) was used to measure the eye diagrams of received signals at different data rates. A programmable network analyzer (Keysight, N5227A) measured the frequency response of blue LD and LCC-based WLD.
Fig. 1. (a) Schematic diagrams of the white LD-based VLC system. (b) Voltage/light output power versus driving current curves. (c) EL spectra at different bias currents. (d) Peak wavelength extracted from the EL spectra.
3. Results and discussion
The optical and electrical properties of the blue LD are summarized in Fig. 1. The operation voltage/ light output power versus driving current curves of the LD is shown in Fig. 1(b). The normalized EL spectra of blue LD with increasing bias currents are shown in Fig. 1(c), which illustrates the slight redshift of the peak wavelength. The redshift of peak wavelength was on a small scale, which will not affect the generated white light. Figure 1(d) shows the redshift peak wavelength from 451 nm to 453.5 nm as an increasing bias current, extracted from Fig. 1(c). The redshift of the emission wavelength observed is attributed to high temperatures at high currents. The bandgap energy decreases, and the emitted wavelength increases by increasing the temperature.
The transmission electron microscopy (TEM) image of $CsPbB{r_3}$ is shown in Fig. 2(a). The EL emission is mainly due to the electrons in semiconductor nanoparticles being excited by pump light and subsequently decaying to a lower energy level, emitting a stream of photons. The wavelength conversion occurs during this spontaneous emission process [38,39]. The absorption (red) and EL emission spectrum of LCC (blue) and SCC (black) of $CsPbB{r_3}$ QDs are shown in Fig. 2(b). LCC $CsPbB{r_3}$ has a wide absorption spectrum and EL emission peak at 519 nm with a narrow FWHM of 21 nm. SCC $CsPbB{r_3}$ exhibited an EL emission peak at 524 nm with a narrow FWHM of 24 nm. A slight redshift and widening of the FWHM in the SCC spectrum were observed compared to the spectrum of LCC. In previous studies, it had been demonstrated that a higher density of QDs weakens the quantum confinement and shrinks the bandgap, which causes redshift and wide FWHM [40,41].
Fig. 2. (a) TEM image of $CsPbB{r_3}$. (b) Normalized absorption and EL spectra of LCC $CsPbB{r_3}$; for reference, SCC $CsPbB{r_3}$ EL spectrum is also overlaid on the LCC EL spectrum. The inset shows the photography of LCC and SCC $CsPbB{r_3}$.
The peak wavelength shifts of the LCC $CsPbB{r_3}$ and SCC $CsPbB{r_3}$ in response to injection current from 30 mA to 80 mA are shown in Fig. 3(a). When the current increases, the peak wavelength shifts towards higher wavelengths. The peak wavelength shifts for the LCC $CsPbB{r_3}$ and SCC $CsPbB{r_3}$ were 2 and 4 nm, respectively. The intensity decay of LCC $CsPbB{r_3}$ and SCC $CsPbB{r_3}$ under various operation times extracted from Fig. S2 (Supplement 1) is shown in Fig. 3(b). It can be depicted from the experiment results that the efficiency reductions of the LCC and SCC $CsPbB{r_3}$ are 10% and 18% after 30 minutes of testing, respectively. The emission spectra intensity of both LCC and SCC $CsPbB{r_3}$ decreased over the operating time, but that of SCC $CsPbB{r_3}$ declined faster, mainly caused by increased surface temperature. Liquid phases serve as excellent heat conductors and protect QDs from photo-oxidation and thermal quench. This may maintain the intensity and keep the color properties of the system during a long period of time and under high-power operation conditions.
Fig. 3. (a) Peak wavelengths of LCC $CsPbB{r_3}$ and SCC $CsPbB{r_3}{\; }$ for 30-80 mA. (b) Intensity decay of LCC and SCC $CsPbB{r_3}$ under various operation times.
The transmission electron microscopy (TEM) image of $CdSe/ZnS$ is shown in Fig. 4(a). The absorption (red) and EL emission spectrum of LCC (blue) and SCC (black) of $CdSe/ZnS$ are shown in Fig. 4(b). The LCC $CdSe/ZnS$ have a wide absorption spectrum and EL emission peak at 630 nm with a narrow FWHM of 24 nm. SCC $CdSe/ZnS$ exhibited an EL emission peak at 634 nm with a narrow FWHM of 26 nm. Figure 5(a) shows Peak wavelength shifts of LCC $CdSe/ZnS$ and SCC $CdSe/ZnS$ as a function of injection current from 30 mA to 80 mA. The peak wavelength shift for the LCC $CdSe/ZnS$ and SCC $CdSe/ZnS$ was 2 nm, and 5 nm. The intensity decay of LCC- and SCC- $CdSe/ZnS$ under various operation times extracted from Fig. S3 (Supplement 1) is shown in Fig. 5(b). It can be depicted from the experimental results that the efficiency reductions of the LCC and SCC $CdSe/ZnS$ are 2.86% and 6.15% after 30 minutes of testing, respectively. The intensity of LCC $CdSe/ZnS$ slowly decreases while that of SCC $CdSe/ZnS$ decreases sharply. The results indicate that the LCC $CdSe/ZnS$ achieves stable performance over a long operating time.
Fig. 4. (a) TEM image of $CdSe/ZnS$. (b) Normalized absorption and EL spectra of LCC $CdSe/ZnS$; for reference, SCC $CdSe/ZnS$ EL spectrum is also overlaid on the LCC EL spectrum. The inset shows the photography of LCC and SCC $CdSe/ZnS$.
Fig. 5. (a) Peak wavelengths of LCC $CdSe/ZnS$ and SCC $CdSe/ZnS$ for 30-80 mA $CdSe/ZnS$. (b) Intensity decay of LCC and SCC $CdSe/ZnS$ under various operation times.
The EL spectra of the LCC- and SCC-based WLD under different bias currents from 40 mA to 80 mA is shown in Fig. 6(a), (b). The current dependent integrated intensity of blue, red, and green components is extracted from Fig. 6(a) and (b), as shown in Fig. S4 (Supplement 1). The data show that the fraction of blue light linearly increased in SCC-based WLD compared to LCC-based WLD with increasing current. CRIs of LCC- and SCC-based WLD at different bias currents are shown in Fig. 6(c). LCC-based WLD exhibited a CRI of 85 at 50 mA. Likewise, LCC-based WLD exhibited a CRI of 74 at 70 mA. Marginal variations in CRI are observed with varying driving currents from 30 mA to 70 mA. The CRI of SCC-based WLD is 71.4 at 50 mA. Further increasing the bias current at 70 mA, the CRI of SCC-based WLD decreases to 55 due to the excessive blue light component. The relationship between surface temperature and applied currents was examined for both LCC- and SCC-based WLD, as shown in Fig. 6(d). At 120 mA for 20 min, the surface temperature of the SCC-based WLD grew to 37 °C, while the LCC-based WLD only reached 31°C. The QDs in LCC-based WLD can freely move, which would facilitate convective heat dissipation.
Fig. 6. EL spectra of (a) LCC-based WLD and (b) SCC-based WLD for various currents from 40 mA to 80 mA. (c) Current-dependent CRI of both LCC- and SCC-based WLD. (d) The relationship between surface temperature and applied current for LCC- and SCC-based WLD.
The white light performance of LCC-based WLD consisting of blue LD, $CsPbB{r_3}$ and $CdSe/ZnS$ is characterized at the receiver side. Figure 7(a) shows the EL spectrum of LCC-based WLD in which blue, red, and green fluorescent components are observed. The wavelength and FWHM are 451 nm and 2.85 nm for blue LD, 511 nm and 24 nm for green $CsPbB{r_3}$ And 630 nm and 26 nm for red $CdSe/ZnS$, respectively. Due to its narrow emission wavelength, a wide color gamut and high CRI white light was generated. The insets show the photographs of the white light spot on white paper. The CCT and CIE chromaticity coordinate of LCC-based WLD are 5520 K and (0.332, 0.342), as shown in Fig. 7(b). The chromaticity coordinates are in proximity on the Planckian locus, indicating the presence of neutral white light. The illuminance of white light versus driving currents is shown in Fig. 7(c). The illuminance of white light linearly increases with the forward current. Figure 7(d) illustrates the CRI and R1-R15 of LCC-based WLD. The generated white light exhibited a high CRI of 85 at the optimized current, which is essential for indoor solid-state lighting [42] and optical display application [43].
Fig. 7. The white light performance of LCC-based WLD. (a) EL spectrum. Inset: photograph of a white light spot on white paper. (b) CIE chromaticity coordinates. (c) The illuminance versus driving currents. (d) CRI (Ra and R1-R15)
Time-resolved photoluminescence (TRPL) was employed to investigate the carrier lifetimes of both LCC $CsPbB{r_3}$ and $CdSe/ZnS$, as shown in Fig. 8(a). The average PL lifetimes calculated for $CsPbB{r_3}$ and $CdSe/ZnS$ QDs solutions were 16 ns and 22 ns, which are much shorter than traditional color converter phosphors, indicating the potential of $CsPbB{r_3}$ and $CdSe/ZnS$ as a fast color converter in VLC and SSL.
Fig. 8. (a) TRPL curves for LCC $CsPbB{r_3}$ and $CdSe/ZnS$. (b) Frequency response of B-LD and B-LD with different LCC QDs under a driving current of 50 mA.
The frequency response of B-LD and B-LD with different LCC QDs is shown in Fig. 8(b). The measured maximum 3-dB bandwidth of B-LD, B-LD + $CsPbB{r_3}$, B-LD+ $CdSe/ZnS$ and B-LD + $CsPbB{r_3}{\; }$& $CdSe/ZnS$ stabilize at 1020, 883, 871, and 855 MHz, respectively. After adhering to the LCC QDs glass cavity, we can observe that the throughput response intensity was reduced by approximately 11 dB compared to the pure B-LD link. It is mainly due to the relevant absorption, scattering and reflection by the LCC QDs glass cavities.
Figure 9(a) shows the schematic diagram of LCC-based WLD for simultaneous illumination and communication. The communication performance of blue LD and LCC-based WLD are tested in Fig. 9(b), illustrating the BER performance under various data rates. A real-time PRBS with the pattern length of ${2^{15}} - 1{\; }$ data stream with a peak-to-peak voltage of 0.8 V modulates the emitted light. A data rate of 2.4 Gbps with a BER of $2.1{\ast }{10^{ - 3}}$ was achieved for a 1.2 m free space real-time VLC based on blue LD. Using LCC-based WLD, a data rate of 2.1 Gbps with BER of $1.9{\ast }{10^{ - 3}}$ was achieved for the same transmission distance. The obtained BERs measurement adheres to the standard FEC threshold of $3.8{\ast }{10^{ - 3}}$. The LCC-based WLD system's data rate is reduced compared to the blue LD system due to scattering, reflection, relevant absorption, and beam divergence. The clear open eye diagrams are observed at 1.5 Gbps, 2 Gbps of LCC-based WLD system and 1.8 Gbps, 2.4 Gbps of blue LD-based system are presented in Fig. 9(c), (d), respectively. The open-eye diagram suggests that the LCC-based WLD can transmit a high data rate of up to 2.1 Gbps. Recent progress in VLC and SSL systems based on blue/violet LD and quantum dots are summarized in Table 1.
Fig. 9. (a) Schematic diagram of LCC-based WLD for illumination and communication (b)BER at different data rates with blue LD and LCC-based WLD. Eye diagrams of (c) LCC-based WLD link at 1.5 Gbps (left) and 1.9 Gbps (right), and (d) Blue LD-based link at 1.8 Gbps (left) and 2.4 Gbps (right)
Table 1. Recent progress in VLC and SSL systems based on blue/violet LD and quantum dots
4. Conclusion
This study investigated the thermal and optical stability of $CsPbB{r_3}$ and $CdSe/ZnS$ in the liquid and solid phases. We fabricated novel thermally stable LCC-based WLD by combining blue LD with liquid phase green-emitting $CsPbB{r_3}$ and red-emitting $CdSe/ZnS$ for high-speed visible light communication and high-quality solid-state lighting. A data rate of 2.1 Gbps over a transmission distance of 1.2 m is achieved by employing the NRZ OOK modulation scheme. Moreover, generated white light exhibits CRI of 85, CCT of 5520, and CIE 1931 chromaticity coordinate at (0.332, 0.3420), which is close to the ideal CIE value of white light (0.333, 0.333). These results will support the development of LCC-based WLD for high-speed VLC applications and high-efficiency solid-state lighting systems.
Funding
Shenzhen Technology and Innovation Council (WDZC20200820160650001-20200827130534001).
Disclosures
The authors declare no conflicts of interest
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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