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Abstract
CsPbBr3 perovskite quantum dots (PQDs) as promising color conversion materials have been widely used in display and visible light communication (VLC), but most CsPbBr3 PQDs for VLC are randomly selected without optimization. Thereby the exploration of fundamental experimental parameters of QDs is essential to better employ their performance advantages. Herein, we investigated the concentration and solvent effects on photoluminescence (PL) properties and communication performance of CsPbBr3 PQDs. The PL, time-resolved PL characterization and communication measurements of CsPbBr3 PQDs all exhibit concentration dependence, suggesting that there exists an optimum concentration to take advantages of performance merits. CsPbBr3 PQDs with a concentration of 0.5 mg/ml show the shortest carrier lifetime and achieve the highest −3 dB bandwidth (168.03 MHz) as well as the highest data rate (660 Mbps) comparing to other concentrations. And in terms of the optimal concentration, we further explored the approach to improve the communication performance, investigating the effect of polarity solvent on the communication performance of CsPbBr3 PQDs. Original 0.5 mg/ml CsPbBr3 PQDs (1 ml) with 55 μL ethanol added in obtain a higher −3 dB bandwidth of 363.68 MHz improved by ∼116.4% and a record data rate of 1.25 Gbps improved by ∼89.4% but weaker PL emission due to energy transfer. The energy transfer assisted improvement may open up a promising avenue to improve the communication performance of QDs, but more feasible energy transfer path needs to be explored to ensure the stability of QDs.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, all inorganic CsPbX$_{3}$ (X = I, Br, Cl) perovskite quantum dots (PQDs) are emerging as significant materials and have attracted considerable attention due to their narrow full-width at half-maximum (FWHM), tunable wavelength, wide color gamut, high photoluminescence quantum yield (PLQY) and short photoluminescence (PL) lifetime [1–3]. Green light source as an important component for display application has been concerned, and CsPbBr$_{3}$ PQDs with narrow band are regarded as the promising green-emitting materials. The excellent properties also endow CsPbBr$_{3}$ PQDs as promising candidates in applications of photodetectors, light emitting diodes (LEDs), lasing, multicolor display and visible light communication (VLC) [4–11]. Especially for the application of VLC, CsPbBr$_{3}$ PQDs as significant color-conversion materials have advantages of shorter carrier lifetime and thus higher modulation bandwidth comparing to the conventional phosphors with limited bandwidth of $\sim$2.5 MHz [12]. Several researches demonstrated the great potential of CsPbBr$_{3}$ PQDs based converted light to realize VLC and solid-state lighting (SSL) simultaneously and a high data rate of 2 Gbps was achieved by a blue laser diode (LD) combined with green-emitted CsPbBr$_{3}$ nanocrystals and red phosphor [4]. Also, liquid phase CsPbBr$_{3}$ and CdSe/ZnS QDs combined with blue LD to form a white-light source were demonstrated in [5] and the proposed white-light source exhibited a modulation bandwidth of 855 MHz and a data rate of 2.1 Gbps over a transmission distance of 1.2 m. Our previous work proposed a high-bandwidth white-light system combining 73 MHz yellow-emitting CsPbBr$_{1.8}$I$_{1.2}$ PQDs with a blue micro-LED, and achieved a real-time data rate of 300 Mbps under free space [6]. We also employed green-emitting CsPbBr$_{3}$ nanocrystals encapsulated in all-inorganic amorphous glass with a bandwidth of 180 MHz and a blue LD for underwater wireless optical communication (UWOC), achieving a data rate of 185 Mbps [7].
However, most researches randomly selected CsPbBr$_{3}$ PQDs for VLC without optimization, limiting the data rate of CsPbBr$_{3}$ PQDs. It is generally known that the surrounding environmental conditions of QDs materials have generally played vital roles in their performances and thus influence the application of the QDs materials [13]. In particular, the fundamental experimental parameters of QDs (e.g., package, concentration, the polarity of the solution solvent) have been widely discussed for the critical impact on the optical properties of QDs, which may limit their applications such as QDs inks in optoelectronics devices, printed electronics and color conversion [14–21]. Wang et al. fabricated high-quality white LED by combining red liquid-type QD (LQD) and phosphor-in-glass (PiG), finding that PiG-LQD exhibited high luminous efficacy, excellent color rendering index and low surface temperature in comparison to the PiG-SQD (QD packaged in silicone) [21]. On one hand, the concentration-dependent PL behavior has been investigated in different kinds of QDs materials, revealing that high concentration may induce self-absorption quenching [14,15]. Moreover, Lunz et al. have investigated the influence of CdTe QDs concentration on the PL wavelength and PL lifetime of QD monolayers dominated by Förster resonance energy transfer (FRET) from smaller to larger sized QDs [16]. Among these different kinds of QDs materials, several researches have also demonstrated concentration and temperature dependent PL of CsPbBr$_{3}$ PQDs, which will influence the PL intensity, FWHM and peak wavelength [17–19]. On the other hand, Mei et al. have reported and analyzed the effects of different polarity solvents on PL properties of CsPbBr$_{3}$ PQDs, indicating that the QDs dispersed in solvent with high polarities would emerge new PL wavelength peaks besides intrinsic fluorescence and occur energy transfer between the two luminous centers [20]. Although previous studies have investigated the concentration and solvent effects on optical properties of CsPbBr$_{3}$ PQDs in detail, but have not dealt with the effects on their communication performances and previous studies to date have failed to consider the improvement of modulation bandwidth and data rate of CsPbX$_{3}$ PQDs for VLC. Meanwhile, in terms of optimizing the traditional QDs for VLC, Leitao et al. have discussed the effect of self-absorption on the CdSSe/ZnS colloidal QDs by adjusting the thickness of the QDs/PMMA composites, finding that a lower thickness of the QDs/PMMA composites would minimize the effect of self-absorption and thus decrease PL lifetime as well as increase modulation bandwidth [22]. Nevertheless, the PL lifetime of CdSSe/ZnS colloidal QDs is in the range of ten to several tens of ns and thus the highest achievable modulation bandwidth is only 22 MHz even by optimization [22]. As CsPbX$_{3}$ PQDs with fast PL dynamics is a promising color converter for high-speed VLC, it is greatly essential to investigate the concentration and solvent effects on the communication performance of CsPbX$_{3}$ PQDs and thus optimize the CsPbX$_{3}$ PQDs to obtain higher communication performance.
Herein, in order to take advantages of communication performance merits of CsPbBr$_{3}$ PQDs, we optimized the CsPbBr$_{3}$ PQDs in solution by adjusting the concentrations and further explored the approach to improve the communication performance. Compared to PQDs in the polymer film, PQDs in solution are easier to adjust flexibility and have higher luminescence efficiency and thermal stability. The synthesized CsPbBr$_{3}$ PQDs were diluted in ten concentrations ranging from 0.02 mg/ml to 40 mg/ml and systematic analysis of PL, time-resolved photoluminescence (TRPL) characterization and communication performance were further conducted. The PL behavior of CsPbBr$_{3}$ PQDs exhibits concentration dependence, indicating the appropriate concentration of CsPbBr$_{3}$ PQDs solution could obtain a better color purity and better communication performance. CsPbBr$_{3}$ PQDs with a concentration of 0.5 mg/ml show the fastest carrier lifetime and achieve the highest −3 dB bandwidth of 168.03 MHz as well as the highest data rate of 660 Mbps compared to other concentrations. Furthermore, in terms of optimal concentration of CsPbBr$_{3}$ PQDs, we investigated the effect of polarity solvent on the communication performance of CsPbBr$_{3}$ PQDs. Original 0.5 mg/ml CsPbBr$_{3}$ PQDs (1 ml) with 55 $\mu$L ethanol added in obtain a higher −3 dB bandwidth of 363.68 MHz improved by $\sim$116.4% and a higher data rate of 1.25 Gbps improved by $\sim$89.4% but weaker PL emission due to energy transfer, compared with the original 0.5 mg/ml CsPbBr$_{3}$ PQDs. The improved modulation bandwidth and data rate of CsPbBr$_{3}$ PQDs by virtue of energy transfer assisted solvent effect exhibit a record performance, which is the highest data rate of QDs-based converted light to the best of our knowledge, as shown in Table 1. Moreover, the energy transfer assisted solvent effect on the improvement of communication performance inspires that more feasible energy transfer path can be explored to improve the communication performance while ensuring the stability of QDs.
Table 1. Comparison of QDs-based converted light for free space optical wireless communication. Converted light from the QDs without light emission from excitation source is summarized to minimize the effect of excitation source.
2. Materials and characterization
2.1 Synthesis of colloidal CsPbBr$_{3}$ PQDs
The colloidal CsPbBr$_{3}$ PQDs were initially prepared based on the modified hot-injection synthesis [1]. First, 0.814 g Cs$_{2}$CO$_{3}$ was mixed with 40 ml 1-octadecene (ODE) and 2.5 ml oleic acid (OA) in a three-neck flask and dried at 120 $^{\circ }$C for 1 h, followed by heating to 150 $^{\circ }$C in a nitrogen atmosphere. Second, 0.069 g PbBr$_{2}$ with 5 ml ODE was loaded into another three-neck flask and dried under vacuum for 1 h at 120 $^{\circ }$C. Then, 0.5 ml dried oleylamine (OLA) and OA were injected into the mixture in a nitrogen atmosphere at 120 $^{\circ }$C. When the PbBr$_{2}$ was dissolved completely, 0.4 ml prepared Cs-oleate solution was injected rapidly, followed by cooling the flask in ice-bath. Finally, the CsPbBr$_{3}$ PQDs crude solution was centrifuged and redispersed in hexane solution for good dispersion. To investigate the communication performance of different CsPbBr$_{3}$ QDs concentrations, ten different concentrations of CsPbBr$_{3}$ PQDs solution ranging from 0.02 mg/ml to 40 mg/ml were diluted with hexane solvent and then sealed in cuvettes to keep the effect of the environment from them.
Figure 1(a) is the transmission electron microscopy (TEM) image of the pre-obtained CsPbBr$_{3}$ PQDs, exhibiting that the pre-obtained CsPbBr$_{3}$ PQDs are monodispersed cubic structures with an average diameter of $\sim$11 nm. The PL and absorption spectra of 0.5 mg/ml CsPbBr$_{3}$ PQDs solution can be seen in Fig. 1(b). The absorption and PL spectra of QDs were measured using a 759S ultraviolet and visible (UV-vis) spectrophotometer and F97XP fluorescence spectrophotometer, respectively. The emission peak wavelength of pre-obtained QDs is at 517 nm with a narrow FWHM of $\sim$20 nm indicating good color purity. From the absorption spectrum, it can be observed that there is an absorption edge at $\sim$500 nm, showing a strong absorption below 500 nm. Furthermore, the overlap between the absorption spectrum and PL spectrum shows a tiny Stokes shift suggesting there is some self-absorption in the material, which is similar to that reported in the previous works [17].
Fig. 1. (a) Typical TEM image of pre-obtained CsPbBr$_{3}$ PQDs dissolved in hexane. (b) PL and absorption spectra of 0.5 mg/ml CsPbBr$_{3}$ PQDs.
2.2 VLC system setup
Figure 2 illustrates the experimental setup of the CsPbBr$_{3}$ PQDs based VLC system. The 450 nm LD combined with CsPbBr$_{3}$ PQDs packaged in cuvette was employed as the light source in the VLC system based on NRZ-OOK. The LD was driven by a bias-tee (Mini-circuits ZFBT-6GW+, 6000 MHz) combing a DC signal (output by the Keithley 2614B) and the pseudo-random binary sequences (PRBS) with a pattern length of 2$^{7}$-1 produced by the pulse pattern generator (PPG, 0.1-14 Gb/s) module from the signal quality analyzer (SQA, Anritsu MP1800A). Then the light output from the LD was collimated to pump CsPbBr$_{3}$ PQDs, and PL signal from the CsPbBr$_{3}$ PQDs was focused by the receiver lens into a high sensitivity silicon avalanche photodiode (APD, C5658, 1 GHz). To accurately measure the communication performance of the converted green light from CsPbBr$_{3}$ PQDs, a 495 nm long-pass optical filter was placed before APD to filter out remnant blue light from LD. The electrical signal converted by APD was finally analyzed by an error module from the SQA to obtain the bit error ratio (BER).
Modulation bandwidth. The modulation bandwidth measurement system is similar to the VLC system, except that the SQA is not required. A collimated 450 nm LD, which was driven by a bias-tee coupling a DC signal and an AC signal (output by the vector network analyzer (VNA, PicoVNA 106, 6 GHz)), was used as an excitation source to pump different concentrations of CsPbBr$_{3}$ PQDs. The light output from the CsPbBr$_{3}$ PQDs was collimated and focused into APD by the receiver lens. Specifically, a 495 nm long-pass optical filter was placed before APD to filter out remnant blue light from LD for ensuring the accuracy of modulation bandwidth measurement of the CsPbBr$_{3}$ PQDs. Finally, the electrical signal converted by APD was then analyzed by the VNA.
Time-resolved photoluminescence measurement. The experimental setup of the TRPL measurement system is also similar to the VLC system. The ultra-high speed pulse signal of the TRPL system was generated by the PPG from the SQA. The pulse width was set as 1 ns and the pulse cycle was set as 2000 ns (0.05% duty cycle), which ensured the accuracy of TRPL within a single pulse cycle. To accurately characterize the PL decay of the CsPbBr$_{3}$ PQDs, we combined the pulse signal with a DC signal through a bias-tee to drive the excitation source (450 nm LD). And a high-speed oscilloscope (OSC, Keysight DSA90604A, 6 GHz) was used to capture the final signal.
3. Optimization of PQDs concentration to improve optical communication performance
3.1 Concentration-dependent PL properties
To investigate the concentration-dependent optical properties of CsPbBr$_{3}$ PQDs, normalized PL spectra of CsPbBr$_{3}$ PQDs in different concentrations are demonstrated in Fig. 3(a), and the peak wavelength and FWHM of CsPbBr$_{3}$ PQDs with different concentrations are extracted in Table 2. It can be seen that the peak wavelength of CsPbBr$_{3}$ PQDs exhibits a red-shift phenomenon from 516 nm to 525 nm with increasing concentration from 0.02 mg/ml to 40 mg/ml, which may be attributed to self-absorption and FRET as QDs concentration increases [15]. With the increase of QDs concentration, higher energy light emitted by the QD will be re-absorbed by another QD with a smaller band gap (self-absorption) and/or QD with higher emitted energy will non-radiatively transfer its energy to the QD with a smaller band gap (FRET), and thus result in such concentration triggered red-shifts [16]. Figure 3(b) shows the corresponding calculated CIE coordinates of CsPbBr$_{3}$ PQDs solution at different concentrations presented in CIE 1931 color space according to the PL spectra. The variation of CIE coordinates with different concentrations presents concentration dependence, indicating that the appropriate concentration of CsPbBr$_{3}$ PQDs solution could obtain a better color purity. Furthermore, three selected concentrations (0.02 mg/ml, 0.5 mg/ml and 40 mg/ml) of these ten concentrations were further combined with red and blue color coordinates of National Television Systems Committee (NTSC) standard to calculate the color gamut, as shown in Fig. 3(c). The color gamut of CsPbBr$_{3}$ PQDs solution with 0.02, 0.5 and 40 mg/ml are 117.9%, 120.0% and 115.2% of NTSC, respectively, suggesting that the appropriate concentration of CsPbBr$_{3}$ PQDs solution could achieve a wider color gamut. These concentration-dependent results reveal that optimizing the appropriate concentration of CsPbBr$_{3}$ PQDs solution provides a potential path to improve display performance for future applications of display.
Fig. 3. (a) Normalized PL spectra of CsPbBr$_{3}$ PQDs in different concentrations. (b) CIE coordinates of CsPbBr$_{3}$ PQDs solution at different concentrations. (c) The color gamut of CsPbBr$_{3}$ PQDs solution for 0.02, 0.5 and 40 mg/ml combined with red and blue color coordinates of NTSC.
Table 2. The extracted peak wavelength and FWHM of CsPbBr$_{3}$ PQDs solution with different concentrations.
To further gain insight into the performance of CsPbBr$_{3}$ PQDs solution with different concentrations, TRPL measurement was employed to investigate the carrier dynamics. The time constant $\tau _{ave}$ obtained from TRPL measurement is an average exciton lifetime for a population of QDs, which is inversely proportional to the modulation bandwidth of QDs [28]. Four selected concentration (0.1 mg/ml, 0.5 mg/ml, 2 mg/ml and 20 mg/ml) of these ten concentrations were further carried out the TRPL test, and the results are shown in Fig. 4. We used the following bi-exponential decay model Eq. (1) to fit the TRPL curves for estimating the carrier lifetimes of CsPbBr$_{3}$ PQDs solution with different concentrations [29]:
where $\tau _{1}$, $\tau _{2}$ are the fast and slow decay time to fit the PL intensity decay. $A_{1}$ and $A_{2}$ are the corresponding relative amplitude. And the average lifetime ($\tau _{ave}$) can be estimated using the following formula [30]: where $\tau _{1}$, $\tau _{2}$, $A_{1}$ and $A_{2}$ can be calculated by Eq. (1) mentioned above. From Fig. 4, the calculated $\tau _{ave}$ for 0.1 mg/ml, 0.5 mg/ml, 2 mg/ml and 20 mg/ml are 10.47 ns, 4.29 ns, 6.24 ns and 12 ns, respectively. Notably, the PL decays become faster with the decrease of CsPbBr$_{3}$ PQDs concentration, but become slower with the decrease of concentration below 0.5 mg/ml, which may be ascribed to FRET and self-absorption effects [15,16,31], and detailed mechanism of FRET as well as self-absorption effects associated with PL decay needs to be further investigated.Fig. 4. Normalized TRPL decay spectra of CsPbBr$_{3}$ PQDs in different concentrations. The solid lines are the corresponding fitted TRPL decay spectra by bi-exponential decay model (LD excitation current of 20 mA).
3.2 Concentration-dependent modulation bandwidth
As well known, modulation bandwidth is the key parameter for VLC performance. Therefore, we further investigated the modulation bandwidth performance of CsPbBr$_{3}$ PQDs with different concentrations. Figure 5(a) presents the frequency responses of CsPbBr$_{3}$ PQDs with different concentrations under the LD excitation current of 100 mA. The ripples in the frequency response curve may originate from the influence of electrical as well as optical noise, which is unavoidable for the actual test circuit, but has almost no effect on the −3 dB bandwidth measurement. And as shown in Fig. 5(b), the extracted −3 dB bandwidth of CsPbBr$_{3}$ PQDs with ten different concentrations and the corresponding emitted light output power (LOP) from CsPbBr$_{3}$ PQDs at the receiver side under the LD excitation current of 100 mA are plotted. It can be seen that −3 dB bandwidth of CsPbBr$_{3}$ PQDs first increases and then decreases as the concentration increases from 0.02 mg/ml, which is consistent with their TRPL behaviors. The carrier lifetime of CsPbBr$_{3}$ PQDs is first fast and then slow with the increasing concentration, while the modulation bandwidth is inversely proportional to PL lifetime, which corroborates the variation trend of modulation bandwidth. And CsPbBr$_{3}$ PQDs achieve the highest bandwidth of 168.03 MHz at the concentration of 0.5 mg/ml. Obviously, the LOP characteristics of CsPbBr$_{3}$ PQDs with different concentrations present the similar concentration-dependent trend. With the increase of CsPbBr$_{3}$ PQDs concentration from 0.02 mg/ml, more and more QDs are pumped, thus the QDs emission becomes stronger until the QDs concentration reaches 2 mg/ml and then drops due to concentration quenching. The concentration quenching may be attributed to the FRET and self-absorption effects at high QDs concentration [19]. There is therefore an optimum concentration of CsPbBr$_{3}$ PQDs to achieve maximum modulation bandwidth and color conversion. Based on the modulation bandwidth and LOP characteristics of CsPbBr$_{3}$ PQDs with different concentrations mentioned above, to better investigate the concentration-dependent communication performance, we selected four representative concentrations of CsPbBr$_{3}$ PQDs for further measurements in detail: 0.02 mg/ml represents very low concentration, 0.5 mg/ml represents the optimum concentration to achieve the highest −3 dB bandwidth, 2 mg/ml represents the optimum concentration to achieve the highest LOP, and 20 mg/ml represents the high concentration.
Fig. 5. (a) The frequency response of CsPbBr$_{3}$ PQDs with different concentrations under LD excitation current of 100 mA. (b) The extracted −3 dB bandwidth of CsPbBr$_{3}$ PQDs with different concentrations and the corresponding LOP from CsPbBr$_{3}$ PQDs at the receiver side under the LD excitation current of 100 mA.
Interestingly, we also found that the modulation bandwidth of CsPbBr$_{3}$ PQDs increases with the increase of LD excitation. This phenomenon is due to comprehensive factors. One possible reason is that the overall recombination is faster due to the changes in the radiative recombination as well as the non-radiative recombination mechanism within QDs with the increase of excitation light intensity [26]. As shown in Fig. 6, the frequency response of four representative CsPbBr$_{3}$ PQDs concentrations (0.02 mg/ml, 0.5 mg/ml, 2 mg/ml and 20 mg/ml) at different excitation currents are further plotted. It can be observed that the modulation bandwidths of all the concentrations exhibit the similar excitation-dependent trend. In the case of 0.5 mg/ml CsPbBr$_{3}$ PQDs, we measured its TRPL decays under different LD excitation currents, as shown in Fig. 7. With the increased LD excitation current from 20 mA to 100 mA, the average PL lifetime decreases from 4.29 ns to 3 ns and thus increases the modulation bandwidth [26].
Fig. 6. The frequency response of (a) 0.02 mg/ml, (b) 0.5 mg/ml, (c) 2 mg/ml, and (d) 20 mg/ml CsPbBr$_{3}$ PQDs concentrations at different LD excitation currents.
Fig. 7. Normalized TRPL decay spectra of 0.5 mg/ml CsPbBr$_{3}$ PQDs under different LD excitation currents. The solid lines are the corresponding fitted TRPL decay spectra by bi-exponential decay model.
3.3 Concentration-dependent optical communication performance
To evaluate the concentration effect on communication performance, VLC data transmission experiment of CsPbBr$_{3}$ PQDs with different concentrations was conducted. To maximum the data transmission capacity, the LD excitation current and modulation depth which could influence the signal to noise ratio (SNR) of the communication system need to be optimized [32]. Adjusting peak-to-peak voltage (V$_{pp}$) and driving current can make full use of the linear operating area of the light source, thereby avoiding problems such as nonlinear distortion or zero-level clipping [33]. As shown in Fig. 8(a), the BER as a function of different excitation currents under a fixed V$_{pp}$ of 1.4 V and a transmission data rate of 500 Mbps is presented. Figure 8(b) shows the BER as a function of different V$_{pp}$ under a fixed LD excitation current of 35 mA and a transmission data rate of 500 Mbps. The optimum LD excitation current and V$_{pp}$ to achieve the minimum BER is 35 mA and 2 V, respectively. Based on the optimum parameters, the BERs versus the data rates for four representative concentrations (0.02 mg/ml, 0.5 mg/ml, 2 mg/ml and 20 mg/ml) of CsPbBr$_{3}$ PQDs with NRZ-OOK modulation format are shown in Fig. 8(c).
Fig. 8. (a) BER versus different excitation currents. (b) BER versus different peak-to-peak voltages. (c) BERs versus data rates with four representative concentrations of CsPbBr$_{3}$ PQDs.
And Table 3 extracted and summarized the communication performances of CsPbBr$_{3}$ PQDs with different concentrations. It can be seen that 0.5 mg/ml CsPbBr$_{3}$ PQDs achieves the highest data rate of 660 Mbps with a BER of 2.1$\times$10$^{-3}$, below the forward error correction (FEC) threshold of 3.8$\times$10$^{-3}$. Besides, 0.5 mg/ml CsPbBr$_{3}$ PQDs is also an appropriate concentration for display according to the analysis of Fig. 3. In contrast, the data rate for very low concentration (0.02 mg/ml) and high concentration (20 mg/ml) CsPbBr$_{3}$ PQDs are both much lower due to the limited modulation bandwidth and LOP. Therefore, there is an optimum concentration of CsPbBr$_{3}$ PQDs to achieve the best performance of VLC.
Table 3. Performances of CsPbBr$_{3}$ PQDs with different concentrations.
4. Solvent effect on improving optical communication performance
After a series of concentration correlative experiment analysis mentioned above, we determined the optimum concentration of CsPbBr$_{3}$ PQDs with 0.5 mg/ml and further explored the possible methods to improve the communication performance. As well known, the environmental conditions of QDs especially the polarity of the solvent can influence the optical properties of the QDs [20]. Therefore, we further investigated the solvent effect on the communication performance of CsPbBr$_{3}$ PQDs by adding a small amount of ethanol in the CsPbBr$_{3}$ PQDs dispersed in hexane. We selected CsPbBr$_{3}$ PQDs dispersed in hexane with the optimum concentration of 0.5 mg/ml as original QDs. We then added 15 $\mu$L and 55 $\mu$L ethanol in two original QDs samples (1 ml) as sample 2 and sample 3, respectively. Two representative ethanol addition amounts were selected in this experiment. Adding excess ethanol can easily lead to fluorescence quenching [34], while too little ethanol can hardly provide significant improvement in PQDs bandwidth. Afterwards, VLC data transmission experiment was conducted on these three CsPbBr$_{3}$ PQDs samples for detailed investigation. Figure 9(a) presents the normalized emitted LOP from these three CsPbBr$_{3}$ PQDs samples at the receiver side of the VLC system under the different LD excitation currents. As the amount of ethanol added increases, the LOP of CsPbBr$_{3}$ PQDs reduces, which is consistent with the observation in the previous study [20]. The reduced LOP of CsPbBr$_{3}$ PQDs may be attributed to the formation of hydrogen bonding with amino groups (-NH$_{2}$) in solution served as energy acceptor and thus occurs energy transfer as well as weaker the intrinsic emission of CsPbBr$_{3}$ PQDs [20]. Then the frequency responses of these three CsPbBr$_{3}$ PQDs samples under LD excitation of 100 mA are shown in Fig. 9(b). Interestingly, original QDs with 55 $\mu$L ethanol added achieves the highest −3 dB bandwidth of 363.68 MHz, which is much higher than that of original QDs with −3 dB bandwidth of 168.03 MHz. The much-enhanced modulation bandwidth of CsPbBr$_{3}$ PQDs may also be abscised to that QDs as the energy donor transfer its energy to the hydrogen bonding system of -NH$_{2}$ and the polar solvents, and thus shorter lifetime [20] as well as improves the −3 dB bandwidth. Finally, we compared the VLC transmission data rates of original QDs and original QDs with 55 $\mu$L ethanol added, as shown in Fig. 9(c). Compared to the original QDs with a data rate of 660 Mbps, the original QDs with 55 $\mu$L ethanol added obtain a record data rate of 1.25 Gbps and the corresponding BER is 3.58$\times$10$^{-3}$, below the FEC threshold of 3.8$\times$10$^{-3}$. Although adding polar solvent has an impact on the optical properties of CsPbBr$_{3}$ PQDs to a certain extent, the energy transfer assisted solvent effect improves the data rate of CsPbBr$_{3}$ PQDs by $\sim$89.4%, which is the highest data rate of QDs-based converted light to the best of our knowledge. These results suggest that using energy transfer effect to improve the communication performance of CsPbBr$_{3}$ PQDs may be a promising way. And this provides some inspiration for our future work that we may explore more feasible energy transfer path to improve the communication performance while ensuring the stability of QDs.
Fig. 9. (a) The normalized LOP from three CsPbBr$_{3}$ PQDs samples at the receiver side under different LD excitation currents. (b) The frequency responses of these three CsPbBr$_{3}$ PQDs samples under LD excitation of 100 mA. (c) BERs versus data rates with two CsPbBr$_{3}$ PQDs samples.
5. Conclusion
In this work, we on one hand investigated the concentration effect on PL properties and communication performance of CsPbBr$_{3}$ PQDs. CsPbBr$_{3}$ PQDs with appropriate concentration (0.5 mg/ml-2 mg/ml) show better color purity, higher LOP and better communication performance due to the reduction of concentration induced self-absorption effect. Among these different concentrations, CsPbBr$_{3}$ PQDs with the concentration of 0.5 mg/ml obtain the highest −3 dB bandwidth of 168.03 MHz due to the shorter carrier lifetime, and the highest data rate of 660 Mbps is also achieved. These concentration dependent results indicate that optimizing the appropriate concentration of CsPbBr$_{3}$ PQDs provides a potential path to improve performances in display and VLC for future application of color conversion. On the other hand, after the determination of the optimal concentration, we further explore the possible approach to improve the communication performance, investigating the solvent effect on the communication performance of CsPbBr$_{3}$ PQDs by adding a small amount of ethanol in the CsPbBr$_{3}$ PQDs dispersed in hexane (with a concentration of 0.5 mg/ml). The original CsPbBr$_{3}$ PQDs (1 ml) with 55 $\mu$L ethanol added obtain an improved −3 dB bandwidth of 363.68 MHz and a record data rate of 1.25 Gbps but weaker PL emission, which may be attributed to energy transfer. The energy transfer assisted nearly 100% improvements of −3 dB bandwidth and data rate may open up a promising avenue to improve the communication performance of QDs, but more feasible energy transfer path needs to be explored to ensure the stability of QDs. Thereby these findings lay down guidance for employing CsPbBr$_{3}$ PQDs to realize better performance towards versatile applications such as display and VLC.
Funding
National Key Research and Development Program of China (No. 2021YFB3601000, No. 2021YFB3601003, No. 2021YFE0105300); National Natural Science Foundation of China (No. 61974031); Science and Technology Commission of Shanghai Municipality (No. 21511101303).
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.
References
1. L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R. Caputo, C. H. Hendon, R. X. Yang, A. Walsh, and M. V. Kovalenko, “Nanocrystals of cesium lead halide perovskites (CsPbX3, X= Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut,” Nano Lett. 15(6), 3692–3696 (2015). [CrossRef]
2. A. Swarnkar, R. Chulliyil, V. K. Ravi, M. Irfanullah, A. Chowdhury, and A. Nag, “Colloidal CsPbBr3 perovskite nanocrystals: luminescence beyond traditional quantum dots,” Angew. Chem. 127(51), 15644–15648 (2015). [CrossRef]
3. W. Chen, X. Xin, Z. Zang, X. Tang, C. Li, W. Hu, M. Zhou, and J. Du, “Tunable photoluminescence of CsPbBr3 perovskite quantum dots for light emitting diodes application,” J. Solid State Chem. 255, 115–120 (2017). [CrossRef]
4. I. Dursun, C. Shen, M. R. Parida, J. Pan, S. P. Sarmah, D. Priante, N. Alyami, J. Liu, M. I. Saidaminov, M. S. Alias, A. L. Abdelhady, N. T. Khee, O. F. Mohammed, B. S. Ooi, and O. M. Bakr, “Perovskite nanocrystals as a color converter for visible light communication,” ACS Photonics 3(7), 1150–1156 (2016). [CrossRef]
5. A. Ali, Z. A. Qasem, Y. Li, Q. Li, and H. Fu, “All-inorganic liquid phase quantum dots and blue laser diode-based white-light source for simultaneous high-speed visible light communication and high-efficiency solid-state lighting,” Opt. Express 30(20), 35112–35124 (2022). [CrossRef]
6. S. Mei, X. Liu, W. Zhang, R. Liu, L. Zheng, R. Guo, and P. Tian, “High-bandwidth white-light system combining a micro-LED with perovskite quantum dots for visible light communication,” ACS Appl. Mater. Interfaces 10(6), 5641–5648 (2018). [CrossRef]
7. M. Xia, S. Zhu, J. Luo, Y. Xu, P. Tian, G. Niu, and J. Tang, “Ultrastable perovskite nanocrystals in all-inorganic transparent matrix for high-speed underwater wireless optical communication,” Adv. Opt. Mater. 9(12), 2002239 (2021). [CrossRef]
8. K. Shen, H. Xu, X. Li, J. Guo, S. Sathasivam, M. Wang, A. Ren, K. L. Choy, I. P. Parkin, Z. Guo, and J. Wu, “Flexible and self-powered photodetector arrays based on all-inorganic CsPbBr3 quantum dots,” Adv. Mater. 32(22), 2000004 (2020). [CrossRef]
9. Y. Yin, Z. Hu, M. U. Ali, M. Duan, L. Gao, M. Liu, W. Peng, J. Geng, S. Pan, Y. Wu, J. Hou, J. Fan, D. Li, X. Zhang, and H. Meng, “Full-color micro-LED display with CsPbBr3 perovskite and CdSe quantum dots as color conversion layers,” Adv. Mater. Technol. 5(8), 2000251 (2020). [CrossRef]
10. C.-Y. Huang, C. Zou, C. Mao, K. L. Corp, Y.-C. Yao, Y.-J. Lee, C. W. Schlenker, A. K. Jen, and L. Y. Lin, “CsPbBr3 perovskite quantum dot vertical cavity lasers with low threshold and high stability,” ACS Photonics 4(9), 2281–2289 (2017). [CrossRef]
11. G. Li, J. Huang, Y. Li, J. Tang, and Y. Jiang, “Highly bright and low turn-on voltage CsPbBr3 quantum dot LEDs via conjugation molecular ligand exchange,” Nano Res. 12(1), 109–114 (2019). [CrossRef]
12. H. Le Minh, D. O’Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung, Y. Oh, and E. T. Won, “100-Mb/s NRZ visible light communications using a postequalized white LED,” IEEE Photonics Technol. Lett. 21(15), 1063–1065 (2009). [CrossRef]
13. Z. Shangguan, X. Zheng, J. Zhang, W. Lin, W. Guo, C. Li, T. Wu, Y. Lin, and Z. Chen, “The stability of metal halide perovskite nanocrystals—a key issue for the application on quantum-dot-based micro light-emitting diodes display,” Nanomaterials 10(7), 1375 (2020). [CrossRef]
14. Y. Song, S. Zhu, S. Xiang, X. Zhao, J. Zhang, H. Zhang, Y. Fu, and B. Yang, “Investigation into the fluorescence quenching behaviors and applications of carbon dots,” Nanoscale 6(9), 4676–4682 (2014). [CrossRef]
15. X. Hu, Y. Xie, C. Geng, S. Xu, and W. Bi, “Study on the color compensation effect of composite orange-red quantum dots in WLED application,” Nanoscale Res. Lett. 15(1), 118 (2020). [CrossRef]
16. M. Lunz, A. L. Bradley, W.-Y. Chen, V. A. Gerard, S. J. Byrne, Y. K. Gun’ko, V. Lesnyak, and N. Gaponik, “Influence of quantum dot concentration on Förster resonant energy transfer in monodispersed nanocrystal quantum dot monolayers,” Phys. Rev. B 81(20), 205316 (2010). [CrossRef]
17. Y. Wang, Y. Yang, P. Wang, and X. Bai, “Concentration-and temperature-dependent photoluminescence of CsPbBr3 perovskite quantum dots,” Optik 139, 56–60 (2017). [CrossRef]
18. Y. Li, J. Tao, Q. Wang, Y. Zhao, Y. Sun, P. Li, J. Lv, Y. Qin, W. Wang, Q. Zeng, and J. Liang, “Microfluidics-based quantum dot color conversion layers for full-color micro-LED display,” Appl. Phys. Lett. 118(17), 173501 (2021). [CrossRef]
19. H. Lee, S. Lee, and H. Lee, “Energy/Charge transfer modulation with spacer ligands for highly emissive quantum dot–polymer blend,” ACS Appl. Mater. Interfaces 13(18), 21534–21543 (2021). [CrossRef]
20. J. Mei, F. Wang, Y. Wang, C. Tian, H. Liu, and D. Zhao, “Energy transfer assisted solvent effects on CsPbBr3 quantum dots,” J. Mater. Chem. C 5(42), 11076–11082 (2017). [CrossRef]
21. H. Wang, Y. Mou, Y. Peng, J. Liu, R. Liang, M. Chen, J. Dai, and C. Chen, “White light-emitting diodes with high color quality fabricated using phosphor-in-glass integrated with liquid-type quantum dot,” IEEE Electron Device Lett. 40(4), 601–604 (2019). [CrossRef]
22. M. F. Leitao, J. M. Santos, B. Guilhabert, S. Watson, A. E. Kelly, M. S. Islim, H. Haas, M. D. Dawson, and N. Laurand, “Gb/s visible light communications with colloidal quantum dot color converters,” IEEE J. Sel. Top. Quantum Electron. 23(5), 1–10 (2017). [CrossRef]
23. Z. Zhou, P. Tian, X. Liu, S. Mei, D. Zhou, D. Li, P. Jing, W. Zhang, R. Guo, S. Qu, and A. L. Rogach, “Hydrogen peroxide-treated carbon dot phosphor with a bathochromic-shifted, aggregation-enhanced emission for light-emitting devices and visible light communication,” Adv. Sci. 5(8), 1800369 (2018). [CrossRef]
24. H. Cao, S. Lin, Z. Ma, X. Li, J. Li, and L. Zhao, “Color converted white light-emitting diodes with 637.6 MHz modulation bandwidth,” IEEE Electron Device Lett. 40(2), 267–270 (2019). [CrossRef]
25. C. H. Kang, I. Dursun, G. Liu, L. Sinatra, X. Sun, M. Kong, J. Pan, P. Maity, E.-N. Ooi, T. K. Ng, O. F. Mohammed, O. M. Bakr, and B. S. Ooi, “High-speed colour-converting photodetector with all-inorganic CsPbBr3 perovskite nanocrystals for ultraviolet light communication,” Light: Sci. Appl. 8(1), 94 (2019). [CrossRef]
26. M. F. Leit ao, M. S. Islim, L. Yin, S. Viola, S. Watson, A. Kelly, Y. Dong, X. Li, H. Zeng, S. Videv, H. Haas, E. Gu, I. M. Watson, N. Laurand, and M. D. Dawson, “Pump-power-dependence of a CsPbBr3-in-Cs4PbBr6 quantum dot color converter,” Opt. Mater. Express 9(8), 3504–3518 (2019). [CrossRef]
27. K. J. Singh, X. Fan, A. S. Sadhu, C.-H. Lin, F.-J. Liou, T. Wu, Y.-J. Lu, J.-H. He, Z. Chen, T. Wu, and H.-C. Kuo, “CsPbBr3 perovskite quantum-dot paper exhibiting a highest 3 dB bandwidth and realizing a flexible white-light system for visible-light communication,” Photonics Res. 9(12), 2341–2350 (2021). [CrossRef]
28. N. Laurand, B. Guilhabert, J. Mckendry, A. Kelly, B. Rae, D. Massoubre, Z. Gong, E. Gu, R. Henderson, and M. Dawson, “Colloidal quantum dot nanocomposites for visible wavelength conversion of modulated optical signals,” Opt. Mater. Express 2(3), 250–260 (2012). [CrossRef]
29. Y. Li, L. Duan, Z. Zhang, H. Wang, T. Chen, and J. Luo, “Healing the defects in CsPbI3 solar cells by CsPbBr3 quantum dots,” Nano Res. 2, 1–7 (2021). [CrossRef]
30. D. Wang, W. U. Khan, and Y. Wang, “Solid-state carbon dots with efficient cyan emission towards white light-emitting diodes,” Chem. - Asian J. 14(2), 286–292 (2018). [CrossRef]
31. Z. Gan, H. Xu, and Y. Fu, “Photon reabsorption and nonradiative energy-transfer-induced quenching of blue photoluminescence from aggregated graphene quantum dots,” J. Phys. Chem. C 120(51), 29432–29438 (2016). [CrossRef]
32. C.-Y. Su, H.-Y. Wang, C.-H. Cheng, and G.-R. Lin, “Compact high-power visible laser diode wavelength division multiplexing for white-light communication,” Adv. Photonics Res. 2(8), 2100075 (2021). [CrossRef]
33. S. Zhu, P. Qiu, X. Shan, R. Lin, Z. Wang, Z. Jin, X. Cui, G. Zhang, and P. Tian, “High-speed long-distance visible light communication based on multicolor series connection micro-LEDs and wavelength division multiplexing,” Photonics Res. 10(8), 1892–1899 (2022). [CrossRef]
34. X. Li, W. Cai, H. Guan, S. Zhao, S. Cao, C. Chen, M. Liu, and Z. Zang, “Highly stable CsPbBr3 quantum dots by silica-coating and ligand modification for white light-emitting diodes and visible light communication,” Chem. Eng. J. 419, 129551 (2021). [CrossRef]
