Annada Sankar
Sadhu
ab,
Yi-Hua
Pai
a,
Li-Yin
Chen
*a,
Chung-An
Hsieh
a,
Hao-Wu
Lin
c and
Hao-Chung
Kuo
*ad
aDepartment of Photonics, Institute of Electro-Optical Engineering, College of Electrical and Computer Engineering, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan. E-mail: lychen@nycu.edu.tw; hckuo@faculty.nctu.edu.tw
bInternational Ph.D. Program in Photonics (UST), College of Electrical and Computer Engineering, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan
cDepartment of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
dSemiconductor Research Center, Hon Hai Research Institute, Taipei 11492, Taiwan
First published on 30th March 2023
We demonstrate semipolar (20–21) micro-LED-based high-bandwidth WLEDs utilizing perovskite QDs and organic emitters in color-conversion films. The WLEDs exhibit a bandwidth in excess of 1 GHz and a CCT of 6141 K, making these devices suitable for visible light communication and lighting applications.
Commercially available WLEDs used for illumination are typically made by combining a blue LED chip with yellow phosphors.2 However, the bandwidth of these conventional WLEDs is often limited by the long photoluminescence (PL) lifetimes of the phosphors and the resistive–capacitive (RC) delay of the blue LEDs, which can hinder high-speed VLC.3 To address this issue, micro-LEDs (μLEDs) are emerging as the most promising candidates for VLC implementation due to their small size, high efficiency, and low resistive–capacitive delay.4,5 Lan et al. demonstrated a GaN-based blue μLED in different configurations of an array that provides a bandwidth of ∼600 MHz and a data rate of 1 Gbps using the non-return-to-zero on–off keying (NRZ-OOK) format.6 Subsequently, Wei et al. reported a single-pixel μLED exhibiting a bandwidth of 1 GHz and a data rate of 8.7 Gbps using the quadrature phase shift keying-orthogonal frequency division multiplexing (QPSK-OFDM) scheme.7 Despite their excellent VLC capacity, the emission spectra of μLED chips are currently limited to the ultraviolet to green light range, due to the poor yield and efficiency in the case of red emission.8,9 This limits their direct implementation as broadband white light sources for VLC. This limitation can be overcome by using color-converting materials such as quantum dots (QDs) and organic emitters with short photoluminescence (PL) lifetimes. QDs have several advantages, including wide absorbance, tunable emission wavelength, high photoluminescence quantum yield (PLQY), short PL lifetime, and cost-effective production,10–14 and have been considered as promising color-converting materials, especially in display applications. Similarly, organic emitters are also potentially useful as color-converting materials due to their broad absorbance spectrum, wide visible light emission, large stock shift, high PLQY, and fast PL lifetime.15,16 The use of a blue μLED and CdSe/ZnS quantum dots (QDs) to create a white light emitting diode (WLED) with a bandwidth of 0.63 GHz and a correlated color temperature (CCT) of ∼20000 K was demonstrated by Cao et al.17 Ma et al. proposed the use of CsPb(Br/I)3 perovskite quantum dots (PQDs) in μLED-based WLEDs, which resulted in a white-light bandwidth of 0.75 GHz with a tunable CCT.18 Similarly, a commercial organic yellow emitter (Super Yellow) combined with a μLED (bandwidth of 60 MHz) was shown to produce a white light bandwidth of 0.53 MHz and a data rate of 1.68 Gbps using DC-biased optical OFDM.19 One strategy to further increase the bandwidth of a visible light communication (VLC) system is to use semipolar plane (20–21) grown μLEDs as pumping sources. These μLEDs have a lower quantum-confined Stark effect (QCSE) and a higher overlap between electron and hole wave functions compared to c-plane LEDs, leading to a fast photoluminescence (PL) lifetime, high bandwidth, high luminous efficiency, and minimal shift in the emission spectra. These characteristics make semipolar plane (20–21) grown μLEDs suitable for use in VLC and lighting applications.9,20
In this work, we propose a high-bandwidth white light system that combines a single-pixel semipolar (20–21) blue μLED, a phenothiazine/dimesitylborane-based organic blue emitter (CC-MP3), green-emitting CH3NH3PbBr3 perovskite quantum dots (PQDs), and a commercially available red-emitting phosphorescent emitter bis(2-(9,9-diethyl-fluoren-2-yl)-1-phenyl-1H-benzo[d] imidazolato)(actylacetonate)iridium(III) (Ir(fbi)2(acac)). This combination has not been previously studied for its optical and frequency response performance in both VLC and lighting applications. By designing the color-conversion layers appropriately, the white light emitting diodes (WLEDs) achieved a bandwidth of approximately 1.0 GHz, a CCT ranging from 4000 K to 6000 K, and a color rendering index (CRI) ranging from 61.3 to 82.4. To the best of our knowledge, this bandwidth is the highest among recently reported μLED-based VLC systems. The high bandwidth is attributed to the high modulation bandwidth of the μLED and the fast PL response of the emitters, which reduce the attenuation of high-frequency signals. We believe that the high bandwidth and excellent illuminating characteristics of the proposed WLED system will be highly beneficial for both VLC and lighting applications in practical settings.
Fig. 3 Bandwidth measurement system architecture for (a) a white light system and (b) color-conversion films. |
The details of materials analysis, including transmission electron microscopy (TEM) of PQDs and nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS) results of CC-MP3, have been previously reported in our studies.21,22 The UV-visible absorbance and PL emission spectra of CC-MP3, PQDs, and Ir(fbi)2(acac) are shown in Fig. 5. The wide absorption spectra with an absorption onset at 466 nm, 530 nm, and 525 nm for CC-MP3, PQDs, and Ir(fbi)2(acac), respectively, indicate that the blue μLED can efficiently excite all the color-conversion materials. The wide PL emission of CC-MP3 has an emission peak at 477 nm, a shoulder at 504 nm, and a FWHM of 66 nm. The PQDs showed a narrow emission spectrum with a peak of 527 nm (FWHM of 28 nm). The commercial emitter Ir(fbi)2(acac) also exhibits wide PL emission with two peaks at 568 nm and 618 nm, and a shoulder at 685 nm (FWHM of ∼115 nm). The PLQY of CC-MP3 and PQDs in the solid-state thin films was 63% and 100%, respectively, indicating that they are very promising materials for color conversion.
To further understand the photophysical properties of the color-conversion materials, we first measured their PL lifetimes. Fig. 6a and b show that the PL lifetimes (to 1/e of the initial value) of CC-MP3, CH3NH3PbBr3, and Ir(fbi)2(acac) are 3.96 ns, 10 ns, and 15 ns, respectively, based on their central emission of 477, 528, and 568 nm. These lifetimes are much faster than those of conventional phosphors and comparable to those of recently reported materials for VLC and solid-state lighting.24–26 The modulation bandwidths of CC-MP3, CH3NH3PbBr3, and Ir(fbi)2(acac) were 170 MHz, 325 MHz, and 110 MHz, respectively, which suggest much higher communication speeds and channel capacities for VLC compared to conventional phosphors and CdSe/ZnS QDs used in WLED systems. These materials typically only have bandwidths of several to tens of MHz.17,27 Previous research has shown that there is a relationship between the cut-off frequency (f3 dB) and the PL lifetime (τPL) of CdSe/ZnS QDs using the equation .28 However, a negative correlation between f3 dB and τPL has been observed by many researchers, indicating that this equation does not always provide accurate quantitative results for materials like PQDs, organic emitters and aggregation-induced emission (AIE) materials.24,26,29,30 In our study, we also observed that the modulation bandwidth of the color-conversion materials did not show the negative correlation seen in CdSe/ZnS QDs. It is worth noting that the concentrations of CC-MP3, PQDs, and Ir(fbi)2(acac) in the PMMA matrix vary significantly in the color-conversion layers. However, previous studies have demonstrated that the −3 dB bandwidth of a composite film remains relatively constant with changes in the emitter concentration.15 As a result, the −3 dB bandwidth of the emitters depicted in Fig. 6 can serve as an indicator of the modulation capability of the color-conversion layers.
Fig. 6 (a) PL decay curve of neat CC-MP3, neat PQDs, and neat Ir(fbi)2(acac) films. (b) −3 dB bandwidth of emitters. |
Three white LED (WLED) systems, S1, S2, and S3, were created by overlaying the color-conversion films, B1, B2, and B3, respectively, on the μLED. The emission spectra and the color performance of the resulting WLED systems are shown in Fig. 6. As shown in Fig. 7a, S1 exhibits emission peaks at 443 nm and 530 nm, which are contributed by the μLED and PQDs, respectively. Similarly, the peaks at 568 nm, 602 nm and 670 nm are from Ir(fbi)2(acac). It was also observed that PQDs enhance green emission in the WLED. However, the low intensity in the region around 500 nm in the emission spectrum reduced its color rendering capability. As a result, S1 provides white light with a CCT of 6141 K, a chromaticity coordinate of (0.3196, 0.3277) on the CIE 1931 color space, and a low CRI of Ra = 61.3. To improve the performance of WLED systems at around 500 nm in the wavelength region, we added CC-MP3 in the composite film, resulting in a new film called B2. The white light spectra of S2 (Fig. 7b) had a CCT of 5561 K, CIE coordinates at (0.3311, 0.3117), and a high CRI of 82.4, while S3 (Fig. 7c) exhibited a CCT of 4089 K, CIE coordinates at (0.3787, 0.3828), and a moderate CRI of ∼72. The CCT and CRI of S3 are lower compared to those of S2 because S3 does not contain PQDs. However, the CRI of S3 is improved compared to that of S1 because it includes CC-MP3. The corresponding CIE coordinates of the three WLEDs are denoted in Fig. 7d. The three different applied currents shown in Fig. 7 were chosen to achieve the highest attainable CRI in each case. However, varying the applied current did not result in significant changes in the CCT and CRI. For instance, when an applied current of 196 mA was used, S1, S2, and S3 showed CCTs of 6016 K, 5760 K, and 4203 K, respectively, and CRIs of 60.7, 80.8, and 69.4, respectively, indicating excellent color stability. Therefore, these white light emitters are suitable for a variety of applications, such as traffic communication, indoor lighting, and vehicle headlights. The color performance of these devices can be further enhanced by optimizing and improving the fabrication of μLED devices and developing new color-conversion emitters with wide emission spectra in the future.
Fig. 7 White light emission spectra of (a) S1, (b) S2, and (c) S3. (d) CIE coordinates of the S1, S2, and S3 WLED systems. |
The −3 dB bandwidth of a VLC-LED depends on the radiative carrier lifetime (τr), the non-radiative carrier lifetime (τnr), and the RC constant.28 However, the −3 dB BW of a device with an active region area of 100 μm or less is confined by its τr.31 As shown in Fig. 8a, increasing the injected current density of the μLED reduces its τr, resulting in an increase in its bandwidth, which can be attributed to the internal electric field screening and fast carrier lifetime at higher currents in the active region.9 As a result, the semi-polar μLED at a driving current of 196 mA showed a −3 dB bandwidth of 1230 MHz. Additionally, an increase in the BW with the injected current density (inset of Fig. 8a) indicates that the bandwidth is dependent on the carrier recombination lifetime rather than being constrained by the RC delay. The bandwidths of S1, S2, and S3 (Fig. 8b) were 1008, 952, and 988 MHz, respectively, at an applied current density of 2.5 kA cm−2. These values are among the highest reported in the literature for VLC using μLEDs and demonstrate a superior communication channel capacity for VLC.17,18,24,26 However, a reduction in white-light bandwidths was observed compared to the blue μLED due to the presence of high-frequency noise when the blue light passed through the composite films at high applied currents. These noises reduced the gain in BW exhibited by the μLED at higher current. In particular, the addition of an extra optical component (CC-MP3) in S2 led to an increase in noise levels,26 which further reduced the white-light bandwidth compared to S1 and S3. Nonetheless, the white-light bandwidths were still higher due to the larger bandwidth and optical power of the μLEDs.
Fig. 8 (a) Variation in the bandwidths of the μLED under different driving currents and equivalent applied current densities (inset). (b) White light bandwidths at 196 mA. |
Organic emitters and PQDs are generally prone to instability when exposed to atmospheric conditions.32–34 Therefore, the stability of CC-MP3 and PQD color-conversion films was evaluated after 180 days of storage (Fig. 9) in a humidity control cabinet. Both samples showed minor degradation of approximately 9% in the PL intensity and 11% in the bandwidth for CC-MP3, and 14% in the PL intensity and 15% in the bandwidth for PQDs. These values are lower than those reported in previous studies.24,26 The PQDs also exhibited a 4 nm blue shift which may due to the phase segregation of a halide component, which is still acceptable in SSL and VLC.
Fig. 9 Stability of CC-MP3 and PQDs after 180 days (a) in the optical and (b) in the frequency response performance. |
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