2.1.

OLED Fabrication

Figure 1 shows the top-emitting red OLEDs we developed for this study. In Fig. 1(c), we present the energy diagram of the OLEDs to show energy barriers at each interface and understand the role of each layer. The energy levels of various materials were obtained from literature values for the solid state.^{21 –25} The energy level of DNP was calculated from the reported oxidation and reduction potentials in the solution state²⁶ converted by the reported empirical correlation to the energy level in the solid state.²⁷ We fabricated the OLEDs by thermally evaporating materials through four different custom-made shadow masks (LiMaB GmbH, $100\mu \mathrm{m}$ thick) in a vacuum chamber at a base pressure of ${10}^{-7}\mathrm{mbar}$ (Angstrom Engineering Inc., Evo Vac, Cambridge, Canada). First, we evaporated 200-nm-thick silver as wiring electrodes through the "wiring" mask on precleaned highly resistive silicon substrates (upper resistivity $3000\mathrm{\Omega }\mathrm{cm}$) with a thickness of $325\mu \mathrm{m}$ coated with 300-nm silicon dioxide (${\mathrm{SiO}}_2$) layers on both sides (Inseto). Then, we vented the evaporator chamber to introduce materials. Next, we deposited a 210-nm-thick aluminum as anodes using the "anode" mask. Once the "organic mask" was in place, we evaporated a 3-nm-thick molybdenum trioxide (${\mathrm{MoO}}_3$), followed by an 80-nm layer of 2,2′,7,7′-tetrakis($N,N^{\prime }$-di-$p$-methyl phenylamino)-9,9′-spirobifluorene (S-TTB) p-doped 2,2′-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (${\mathrm{F}}_6$-TNAP) as the hole transport layer (HTL). We then added a 10-nm layer of $N,N^{\prime }$-di(naphthalene-1-yl)-$N,N^{\prime }$-diphenylbenzidine (NPB) as an electron-blocking layer (EBL), followed by a blend of DCJTB at 1.5% (volume fraction) and ${\mathrm{Alq}}_3$ doped at 42% (volume fraction) with DNP¹⁹ as the emission layer (EML). The hole-blocking layer (HBL) comprised ${\mathrm{Alq}}_3$ and the n-doped electron transport layer (ETL) consisted of caesium-doped 4,7-diphenyl-1,10-phenanthroline (BPhen). The thicknesses of the EML and ETL of various types of OLEDs (A, B, C, and D) are detailed in Table 1. We fabricated different sizes of OLEDs to investigate the effect of the size on the bandwidth and the signal-to-noise ratio. Small (S) OLEDs had an active area of $4.7<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-4}{\mathrm{cm}}^2$ and large (L) OLEDs had an active area of $1.24<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}{\mathrm{cm}}^2$. Finally, we evaporated a 30-nm silver layer as the cathode through the "cathode mask" and an 80-nm-thick layer of NPB as the capping layer (CPL) without using a mask. After completing the evaporation process, we encapsulated all OLEDs under a nitrogen atmosphere using 1.1-mm-thick custom-made cavity glass lids (Luminescence Technology Corp., New Taipei City, Taiwan), an epoxy glue (Norland Products Inc., Norland Optical Adhesive 68, Jamesburg, New Jersey, United States) pre-baked under a nitrogen condition, and a moisture getter (Dynic Corporation, HD-071210T-50S, Kyoto, Japan). The active areas were measured from electroluminescence (EL) images of the operating OLEDs under a microscope (ECLIPSE LV100ND, Nikon, Minato City, Japan). The OLEDs were mounted on a custom-made holder equipped with a heat sink and a fan to dissipate Joule heating at high driving conditions.

Table 1

Summary of the layer thicknesses and performance of top-emitting OLED types.

For the purpose of exploring stable and efficient OLED structures, we also fabricated a bottom-emitting OLED (BOE-OLED) using a similar process, substituting indium tin oxide (ITO) as the anode and aluminum as the cathode. We treated the precleaned 1.1-mm-thick glass substrates with oxygen plasma and applied a 117-nm-thick pre-patterned ITO anode (Xin Yan Technology Ltd., Kowloon, Hong Kong). The substrates were then spin-coated with PEDOT:PSS (Heraeus Clevios, Al4800), diluted with deionized water in a 1:2 volume ratio at 3000 r/min, and baked at 120°C for 10 min. The use of PEDOT:PSS instead of ${\mathrm{MoO}}_3$ was to prevent shorting. The organic layers consisted of p-doped S-TTB as HTL, 10 nm NPB as EBL, EML, and either ${\mathrm{Alq}}_3$ or bis(8-hydroxy-2-methylquinoline)-(4-phenylphenoxy) aluminum (BAlq) as HBL, along with caesium-doped BPhen as n-doped ETL. For the purpose of comparison of operational lifetime with our previous best VLC-OLED in Ref. 17, we also fabricated the OLED based on 4,4′-bis[4-(diphenylamino)styryl]-biphenyl (BDAVBi), and the EML consisted of BDAVBi doped at 3% (volume fraction) in 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) with a thickness of 20 nm. The structural details of the BOE-OLEDs are summarized in Table S1 in the Supplementary Material. The active area of each device is $2{\mathrm{mm}}^2$. Section S1 in the Supplementary Material describes the performance of BOE-OLEDs.

2.2.

OLED Characterization

We evaluated the current density–voltage–radiance (J-V-R) characteristics using a source measure unit (Keithley Instruments, Keithley 2400, Cleveland, Ohio, United States) and a calibrated custom-made silicon photodiode module, with its output voltage via a multimeter (Keithley Instruments, Keithley 2000). We limited the maximum current during the voltage scan to prevent degradation. The external quantum efficiency of the OLEDs was estimated assuming the emission pattern was Lambertian. Emission spectra were measured using a calibrated fiber-coupled spectrograph (MS125, Oriel) equipped with a charge-coupled device camera (DV420-BU, Andor). The emission spectra were recorded by placing the fiber normal to the OLED substrate surface, with the OLEDs driven at a constant current density of around $20\mathrm{A}/{\mathrm{cm}}^2$.

For the frequency response measurements, a high-speed photodiode (Thorlabs, Newton, New Jersey, United States, APD210, $-3\mathrm{dB}$ bandwidth of 5 to 1000 MHz) was used. A pair of lenses with a diameter of 50 mm and back focal lengths of 25 and 85 mm was employed to collect and focus the OLED emission onto the photodiode. The photodiode was directly connected to a vector network analyzer (Keysight, E5061B, Santa Rosa, California, United States), whereas the OLEDs were connected through a bias tee (Mini-Circuits, ZFBT-4R2G, Brooklyn, New York, United States) to a direct current (DC) power supply (Keithley Instruments, Keithley 2401). The RF port on the bias tee was connected to port 1, the DC power supply to the DC port, and the photodiodes to port 2. The signal transmission coefficients ($S_{21}$) were recorded for frequencies between 1 kHz and 1 GHz, using a fixed RF power of $-6\mathrm{dBm}$ and a constant DC offset. The DC power supply controlled the DC output. The measured channel gain was normalized at its peak value. Prior to the measurement, both ports, including the bias tee, were calibrated using an electronic calibration module (Keysight, N7550A).

2.3.

Optical Simulation to Optimize OLED Stack

To optimize the thicknesses of HTL, ETL, and CPL for both high out-coupling efficiency and high bandwidth, we conducted optical simulations using a model based on the damped harmonic oscillator embedded within a thin-film stack.²⁸ Figure S2(a) in the Supplementary Material provides a schematic of the calculated OLED stacks. The refractive indexes of each layer were determined by fabricating thin films of the layers on glass substrates and testing them with a variable-angle ellipsometer (J. A. Woollam, M-2000 Ellipsometer, Lincoln, Nebraska, United States). Examples of the refractive index spectra for the EML, HBL, and ETL are shown in Fig. S2(b) in the Supplementary Material. We used reported values of the ratio of horizontal dipoles of 0.86 and the photoluminescence emission efficiency (${\mathrm{\Phi }}_{\mathrm{PL}}$) of 73% for DCJTB.²⁹ From the simulation results, we estimated both the out-coupling efficiency of the OLED stack and the Purcell factor. For situations where excited state dynamics limit bandwidth, the bandwidth is inversely proportional to the emission lifetime.² The change in the bandwidth ($\mathrm{\Delta }f$) in OLED stacks due to the microcavity effect can then be calculated using the following equation:³⁰

2.4.

Photoluminescence (PL) Bandwidth Measurement

We used a 450-nm laser (OSRAM, PLT5 450B) modulated by the vector network analyzer at a DC of $\sim 60\mathrm{mA}$ for optical excitation of the samples. As shown in Fig. S2(b) in the Supplementary Material, this wavelength primarily excites DNP. The excitation intensity was $31\mathrm{mW}/{\mathrm{cm}}^2$, with a beam size of 2 mm in diameter. The samples shown in Fig. S3(a) in the Supplementary Material were fabricated using large shadow mask openings, greater than 4 mm, to ensure the samples were large enough for laser excitation. We collected the photoluminescence from the samples using a fast photodiode with nearly flat frequency response (Thorlabs, APD430A2/m, $-3\text{-}\mathrm{dB}$ bandwidth of DC to 400 MHz). A pair of lenses was used to focus the emission onto the photodiode, and a 495-nm long-pass filter was placed in the optical path to block the excitation light while allowing the PL to pass through. The signals were recorded by the network analyzer. We tested the frequency response of the laser itself, as shown in Fig. S3(b) in the Supplementary Material, and performed a deconvolution from the PL frequency response of the various samples measured by subtracting the laser response (in dB) [see Fig. S3(c) in the Supplementary Material]. Finally, we estimated the $-6\text{-}\mathrm{dB}$ PL bandwidth by identifying the frequency at which the channel gain dropped by 6 dB from the reference value measured at 1 kHz.

2.5.

VLC Link Setup

The performance of the OLEDs was evaluated in a VLC system over two different link distances of 2 and 10 m. In this work, the direct current–biased orthogonal frequency division multiplexing (DC-OFDM) signals were generated by MATLAB^® and forwarded to an arbitrary waveform generator (AWG, Keysight M8195A). DC-OFDM was chosen due to its spectral efficiency, which surpasses other modulation schemes such as on-off keying, and it allowed for a direct comparison of data rate with other relevant OLED studies^{10 ,17 ,18 ,31 ,32} and inorganic LEDs³³ for VLC transmitters. An adaptive bit-loading and power-loading algorithm based on the weighted Hughes–Hartogs approach was applied³⁴ to maximize the achievable data transmission rate. Detailed information on the used DC-OFDM format can be found in Ref. 1. Figure S4 in the Supplementary Material illustrates the bit-loading per frequency results for the OFDM signal of both B-S and B-L OLEDs. The modulated signals generated by the AWG were amplified using a broadband amplifier (Mini-Circuits, ZHL-4240w+). The amplified signals were combined with DC from a power supply (Keysight, 36312A) through a bias tee (Mini-Circuits, ZFBT-282-1.5A). The combined output signals from the bias tee were applied to the OLEDs. The power supply controlled the offset voltages to be the DC density of $160\mathrm{A}/{\mathrm{cm}}^2$ for the S-OLED and $180\mathrm{A}/{\mathrm{cm}}^2$ for the L-OLEDs. Table S2 in the Supplementary Material summarizes the peak-to-peak voltage ($V_{\mathrm{pp}}$) of the OFDM signals set onto the AWG $V_{\mathrm{pp}}$ for different OLEDs under varying measurement conditions. It should be noted that the $V_{\mathrm{pp}}$ value and the DC bias current were carefully adjusted to maximize the average signal-to-noise ratio (SNR) across the modulation bandwidth.

We used an aspheric lens (Thorlabs, ACL7560U-A) to collimate the emitted light from the OLEDs. On the receiver side, two different aspheric condenser lenses of the ACL50832U-A (Thorlabs) for the 2-m and ACL7560U-A (Thorlabs) for the 10-m link were employed. We used a larger lens for the 10-m link because the light has spread out more at this distance. These lenses focused light onto high-speed photoreceivers of the HSPR-X-I-1G4-SI (FEMTO Messtechnik GmbH) for the 2-m and the APD210 (Thorlabs) for the 10-m link. Avalanche photodiodes (APDs) compared with the p-type, intrinsic, and n-type (PIN) photodiodes have the advantage of higher responsivity making them suitable for detecting weak light signals.^{10 ,17 ,18 ,31} We initially tested the data transmission rate with an avalanche photodiode (APD210) in the 2-m link. However, the SNR did not improve with the C-L OLEDs compared with the C-S OLEDs due to the higher light emission of the C-L OLEDs saturating the APD. To accurately assess the effects of the OLED stacks and sizes, we used a PIN photodiode (HSPR-X-I-1G4-SI) for the comparison in the 2-m link. For the 10-m data link, we used the APD210 due to the weaker received light intensity.

We utilized a modulation bandwidth of 1.6 GHz for data communication, whereas the maximum communication bandwidths of the OLEDs employed were 1.53 GHz. The output signals of the photodiode modules were captured by a high-speed oscilloscope (Keysight, UXR0104B) and then sent to a laptop for decoding and further processing. Furthermore, a Volterra filter-based nonlinear equalizer was implemented at the receiver side to reduce the distortions introduced by nonlinearities in the optical link and improve the performance of the VLC system. The highest order Volterra filter used was 5, with filter lengths of {20, 5, 1, 1} and delay differences of {5, 0, 0, 0} for the second to fifth orders, respectively. Equalization and peak-to-peak drive voltage were optimized for each type of device studied. Figure S4 in the Supplementary Material shows the examples of SNR spectra and the bit-loading per frequency results for the OFDM signal of both B-S and B-L OLEDs. The oscillations observed in the SNR curves are likely due to reflections and multipath propagation in our optical setup³⁵ and/or meters-long co-axial cables used for the measurements. The use of long coaxial cables introduces electrical signal distortions, such as attenuation, impedance mismatches, reflections, and phase dispersion, which can lead to frequency-selective signal degradation. These combined effects contribute to the observed fluctuations in the SNR curves. Figure S5 in the Supplementary Material presents the measured bit error ratios (BERs) as a function of data rate for different OLEDs. To determine the data rate at given BERs, linear interpolation was performed between the nearest higher and lower data points. The results for every device at different BERs are shown in Table S2 in the Supplementary Material, together with their average.

3.1.

High-Speed OLEDs Based on DNP

Figure 1 shows an overview of the OLEDs investigated in this work. We initially tested the DNP OLED with a pin structure in bottom-emitting configuration to optimize blocking layers for high efficiency and high operational stability (see Sec. S1 in the Supplementary Material for device performance of the optimized OLED and comparison with BDAVBi OLED). We then made top-emitting devices (type A) on a silicon substrate using the same layers as for the bottom-emitting device. Using an optical simulation,²⁸ we optimized the thicknesses of HTL, ETL, and CPL for both high out-coupling efficiency and high reduction in emission lifetime in the OLED by the microcavity effect [Fig. S2(c) in the Supplementary Material and Table 1]. We initially set the OLED active area to $4.7<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-4}{\mathrm{cm}}^2$ [$\sim 120\mu \mathrm{m}<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>400\mu \mathrm{m}$, S-size in Fig. 1(b)] and describe this OLED as "A-S OLED."

Figure 2 shows the A-S OLED current J-V-R characteristics. We could operate the OLED even at $160\mathrm{A}/{\mathrm{cm}}^2$ without breakdown due to the thermal conductive silicon substrates by suppressing the device temperature rise.³⁶ At $160\mathrm{A}/{\mathrm{cm}}^2$, the A-S OLED reached a radiance of $5.6\mathrm{kW}/({\mathrm{m}}^2·\mathrm{sr})$ at a driving voltage of 12 V, with ${\text{LT}}_{80}$ of 11 min even at this high drive current, as shown in Fig. S6(c) in the Supplementary Material. Lifetime reports at such high current densities are very rare. Ahmad et al.³⁷ operated OLEDs made from the polymer SuperYellow at $120\mathrm{A}/{\mathrm{cm}}^2$ in 300 ns pulses for 1,000,000 pulses, giving a lifetime of 0.3 s. In our previous work with BDAVBi at a much lower current density of $10\mathrm{A}/{\mathrm{cm}}^2$, we obtained ${\text{LT}}_{80}$ of 6 min.¹⁰

Next, we measured the OLED modulation bandwidth by evaluating the frequency response of an optical link with a vector network analyzer (Keysight, E5061B) producing a fixed power of $-6\mathrm{dBm}$ at different frequencies. This was achieved by modulating the OLED with a DC offset and detecting the emission from the OLED using a high-speed photodiode (Thorlabs, APD210) with a $-3\text{-}\mathrm{dB}$ bandwidth ranging from 5 to 1000 MHz.³⁸ Figure 3(a) shows the frequency response of the A-S OLED at $160\mathrm{A}/{\mathrm{cm}}^2$ normalized at the peak channel gain. We normalized at the peak instead of at low frequency because the detector exhibits a high-pass behavior, and the peaks for our OLEDs appear at several tens of MHz. The channel gain drops gradually with increasing frequency, limited by the speed of the OLED. We defined the $-6\text{-}\mathrm{dB}$ electrical bandwidth at a frequency where the gain dropped by $-6\mathrm{dB}$ from the peak gain ($-6\text{-}\mathrm{dB}$ EL bandwidth). The A-S OLED achieved a $-6\text{-}\mathrm{dB}$ EL bandwidth of 340 MHz at $160\mathrm{A}/{\mathrm{cm}}^2$, which is higher than the BDAVBi OLEDs operated at $<100\mathrm{A}/{\mathrm{cm}}^2$, $<255\mathrm{MHz}$.^{10 ,17} Usually, the bandwidth increases with voltage and, hence, current density,^{10 ,17} but device degradation limits the available current density. Our results highlight the importance of stability for high-speed operation. By making more stable devices, we could apply stronger fields and achieve higher bandwidth.

We set up a free-space data link with the A-S OLED at the transmitter. A receiver photodiode (HSPR-X-I-1G4-SI) was placed 2 m away, and data were transmitted using DC-OFDM (see Sec. 2 for more details). Error correction coding techniques have significantly advanced in recent years, driven by the increasing demand for higher data transmission reliability and efficiency. Although conventional coding schemes required a BER threshold of $3.8<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}$ with 7% overhead (OH) to achieve error-free data transmission, recent studies show that staircase coding, with a 7.15% overhead, can maintain error-free performance even at a higher BER threshold of $5.54<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}$.³⁹ Thus, we consider a data rate at a BER threshold of $5.54<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}$ to show the performance of our OLEDs, as presented in Fig. 4(a). To compare our results with previous works, the data rate at a BER of $3.8<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}$ is also plotted in Fig. 4(b), and Table S2 in the Supplementary Material summarizes data at the two different BERs. The A-S OLED achieved a data rate of 2.7 Gbps at a BER of $5.54<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}$ when operating at $160\mathrm{A}/{\mathrm{cm}}^2$ (coded data rate of 2.5 Gbps after deduction of the 7.15% OH). Also, the A-S OLED reached a data rate of 2.6 Gbps at a BER of $3.8<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}$, which is more than double the previously reported data rate of 1.1 Gbps using BDAVBi OLEDs^{10 ,17} due to the improved bandwidth.

3.2.

OLED Stack for High Speed and High Brightness

To further improve both speed and light output, we next altered the OLED stack. Munshi et al.⁴⁰ observed bandwidth enhancement by omitting their HBL and explained this by a reduction in the junction capacitance formed at the organic interface. We also note that the electron mobility of our HBL, ${\mathrm{Alq}}_3$, is many orders of magnitude lower than that of the EBL and NPB.⁴¹ Thus, removing ${\mathrm{Alq}}_3$ may increase the bandwidth by reducing the charge transit time. Based on the Shannon–Hartley theory, data transmission speed depends on bandwidth and SNR. The latter is increased by high light output. Doped transport layers are essential to realize ohmic injection from contacts, and their high conductivity is crucial to give design freedom to optimize the optical cavity for high light output while not affecting driving voltage.⁴² However, metals in the doped ETL can cause quenching.^{43 ,44} Blocking layers are used to separate emission layers from the doped transport layers;⁴² hence, removing the HBL may reduce light output. Therefore, to investigate the trade-off between the bandwidth and light output for a high data transmission rate, we fabricated OLEDs with different HBL thicknesses, 5 nm as "type B" and 0 nm as "type C" (Table 1). Jarikov⁴⁵ reported that in an OLED based on dibenzo [b,k] perylene (DBP) (which is a similar aggregation-prone perylene derivatives to DNP), and with NPB and ${\mathrm{Alq}}_3$ as blocking layers, the recombination zone expands from EBL toward the HBL to cover the entire 15-nm EML due to the good hole injection and hole transport of DBP. We expect that the recombination zone of our DNP OLEDs similarly expands from the EBL toward the HBL and fills the entire 15-nm EML with excitons. In the device without the HBL, there is additional quenching of excitons by the ETL. By increasing the EML thickness, we expect charge recombination at the EML/HBL interface to be reduced, leading to a reduction in quenching. Thus, we fabricated the OLEDs without HBL and increased the EML thickness to 45 nm (type D) to avoid charge recombination at the EML/ETL interface and prevent quenching at the ETL. This is also expected to broaden the charge recombination zone, leading to an enhancement in operational stability.²⁶ We adjusted the ETL thickness for the different OLED types to be the same total thickness as type A (see Table 1) to minimize the differences in the microcavity effects.

Figure 2 presents the J-V-R characteristics of the B-S, C-S, and D-S OLEDs. At $160\mathrm{A}/{\mathrm{cm}}^2$, the radiance and the driving voltage are $4.8\mathrm{kW}/({\mathrm{m}}^2·\mathrm{sr})$ and 12 V for the B-S OLED, $4\mathrm{kW}/({\mathrm{m}}^2·\mathrm{sr})$ and 11 V for the C-S OLED, and $6.8\mathrm{kW}/({\mathrm{m}}^2·\mathrm{sr})$ at 14 V for the D-S OLED. Comparing the different OLED types, the C-S OLED requires the lowest voltage, whereas the D-S OLED requires the highest voltage. These differences are attributed to the doped ETL being more conductive than the HBL and EML. The radiance increases in the order C-S then B-S then A-S and D-S. The C-S OLED shows the lowest radiance because it has more quenching caused by the ETL compared with the A-S and B-S OLEDs. Although the peak efficiency of the D-S OLED is slightly lower than that of the A-S OLED, the reduction in emission efficiency of the D-S OLED with current density is less than that of the A-S OLED [see Fig. S6(b) in the Supplementary Material]. It becomes higher than the A-S OLED at $160\mathrm{A}/{\mathrm{cm}}^2$. Again, we compare with the reported material, DBP, due to the smaller oxidation potential of DNP than DBP and structural similarities of them, the charge recombination zone can spread more than 15 nm in the D-S OLED, similar to the reported OLEDs based on DBP,⁴⁵ and thus reducing the densities of charge and excitons⁴⁶ compared with the A-S OLED. Thus, it suppresses quenching at the EML/ETL and emission efficiency roll-off caused by charges and triplet excitons.⁴⁷ The operational stability at $160\mathrm{A}/{\mathrm{cm}}^2$ of the B-S and C-S OLEDs was similar to that of the A-S OLEDs. However, the D-S OLED showed a 50% increase in operational stability at a current density of $160\mathrm{A}/{\mathrm{cm}}^2$, as shown in Fig. S6(c) in the Supplementary Material. This improvement in stability arises from the large recombination zone of the D-S OLED, which reduces the formation rate of degradation products by lowering excited state densities and spreads the products.²⁶ As the degradation products act as quenchers, the large recombination zone improves the stability of the D-S OLED, showing the frequency responses of these OLEDs. The $-6\text{-}\mathrm{dB}$ EL bandwidths of the B-S and C-S OLEDs are 360 MHz and 390 MHz, respectively, both higher than those of the A-S OLEDs. The D-S OLEDs show a bandwidth of 280 MHz, lower than the A-S OLEDs (Table 1).

To understand these bandwidth differences and the limiting processes on OLED speed, we compared the bandwidths of the photoluminescence processes of the emitters (PL bandwidth) in the different OLED stacks with and without electrodes, as well as films of the emission layer only [see Fig. S3(a) in the Supplementary Material]. Regardless of the presence or absence of electrodes, the PL bandwidths increased in the order of types A, B, and C OLED due to increased quenching (likely due to the ETL for the thinner EBLs). Type D OLEDs have three times thicker EML than the type C OLEDs, so the contribution from the quenching region to the observed PL will be reduced. OLED stacks with electrodes show a higher PL bandwidth than those without electrodes because of the microcavity effect. On the other hand, the EL bandwidths are nearly double the PL bandwidths; this can be attributed to another mechanism, such as quenching that occurs during OLED operation. At $160\mathrm{A}/{\mathrm{cm}}^2$, the EQE drops to below 20% of the peak values [Fig. S6(b) in the Supplementary Material], indicating significant quenching. Assuming that the EQE reduction is entirely due to additional quenching in the operating device, the EL bandwidth of the OLED stack may be increased by up to five times compared with the PL bandwidth. Thus, the higher measured EL bandwidths of the OLEDs may reasonably be attributed to quenching. In most devices, the emission process limits the speed, but the lower bandwidth of the D-S OLEDs suggests that charge transport is also a limitation in these thicker devices.

We tested the VLC performance of these OLEDs in the 2-m data link. The data rates at BER of $5.54<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}$ and the coded data transmission rates with 7.15% OH were 2.8 and 2.6 Gbps for the B-S OLED, 2.5 and 2.3 Gbps for the C-S OLED, 2.8 and 2.6 Gbps for the D-S OLEDs (Table 1). Although the C-S OLED shows the highest EL bandwidth and the D-S OLED the lowest, the D-S device shows the highest equal data rate. We attribute this to its higher light output.

3.3.

Improving Signal-to-Noise Ratio with Larger OLEDs

Increasing the size of OLEDs can enhance their optical power, improving SNR and data rates. Typically, enlarging OLED size can also increase capacitance and lead to higher electrical time constants. However, if the bandwidth of the OLED is limited by factors such as charge transport and emission processes rather than the electrical time constant, increasing the OLED size will not reduce its bandwidth. Instead, it will enhance their optical power. We therefore fabricated larger versions of the type B and C OLEDs with 2.6 times the active area of the small devices, i.e., $1.24<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}{\mathrm{cm}}^2$ [$310\mu \mathrm{m}<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>400\mu \mathrm{m}$, B-L and C-L OLEDs, see Fig. 1(b)], showing the J-V-R characteristics of the B-L and C-L OLEDs. We found that the J-V-R characteristics are similar for different OLED sizes, which means that total light output is increased with the larger devices at the same current density. On the other hand, the emission efficiency of the L-size OLED is 14% to 20% lower than the S-OLED [see Fig. S6(b) in the Supplementary Material]. This lower emission efficiency could be due to the light being trapped inside the active area of the L-OLEDs, but extracted from the edges of the narrower aluminum electrodes in the S-OLEDs, which are $\sim 120\mu \mathrm{m}$ wide. At a current density of $180\mathrm{A}/{\mathrm{cm}}^2$, the $-6\text{-}\mathrm{dB}$ electrical bandwidth for the B-L OLED is 340 and 390 MHz for the C-L OLED (Fig. 3 and Table 1), similar to the corresponding S-size OLEDs. This indicates that the electrical time constants of the L-size OLEDs are still not limiting their speed.

Figure 4 illustrates the data rates of the B-L and C-L OLEDs in the 2-m data link. Both OLED types achieved higher data rates than the S-size OLEDs, with B-L OLEDs reaching a maximum data rate of 4.0 Gbps at a BER of $5.54<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}$ (coded data transmission rate of 3.7 Gbps after deducting the 7.15% OH). The higher light output of the larger OLEDs resulted in a 3-dB higher SNR across the overall frequency range [Fig. S4(a) in the Supplementary Material]. To assess the performance of this OLED-based VLC system over a much longer distance than ever used before for OLED data transmission, we extended the link to 10 m. Using the B-L OLED, we achieved a data rate of 2.9 Gbps at a BER of $5.54<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}$ (coded data transmission rate of 2.7 Gbps after deducting the 7.15% OH). The data rate result over 10 m is a very significant step forward for OLED-based VLC communication, as previous studies^{9 ,10 ,17 ,18 ,31 ,48 –53} were conducted over much shorter distances. Of the 11 previous reports, nine were over distances up to 0.5 m,^{9 ,10 ,18 ,48 –53} and two were for distances up to 2 m.^{17 ,31} Not only have we achieved a data rate over a much longer distance, but it is also very fast—almost the same as the previous record yet over 40 times the distance. Figure 4(b) compares the data transmission rate at a BER of $3.8<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}$ as a function of the link distance with previous reports.^{10 ,17 ,18 ,31 ,48 –53} It shows that we achieve the highest data rates for OLED transmitters, and also the longest link distance. Squares are drawn from the measured distance and the data rate toward zero to help compare different results measured at different distances. For a given communication system, the data rate will decrease as the link distance increases, but the amount by which this occurs depends on many factors such as the optics used and the properties of the source. Nevertheless, any system that achieves a higher data rate at a longer distance performs better than one achieving a lower data rate at a shorter or equal distance. Hence, any result covered within a square has inferior performance.

We achieved a record data transmission rate of 3.8 Gbps at BER of $3.8<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-3}$, which is 1.3 times higher and a link distance eight times longer than the previous record for OLEDs, which was 2.85 Gbps in a 25-cm data link.¹⁸ This improvement in data rate is due to the larger bandwidth and higher light output of our OLEDs, which are almost 100 times larger in area than the OLEDs in Ref. 18, which had a bandwidth of 275 MHz under driving conditions optimized for data communication and a size of $1.6<span\; class="naked\_sign">\times </span><span\; class="naked\_aural">ばつ</span>{10}^{-5}{\mathrm{cm}}^2$. Moreover, our data rate at a 10-m distance demonstrates that B-L OLEDs are promising light sources for high-speed VLC systems targeting longer transmission distances. Figure S4(b) in the Supplementary Material shows the comparison of the SNR spectra of B-L OLEDs at different link distances. We found that the SNR of B-L OLEDs at 10 m is lower than the SNR of B-L OLEDs at 2 m by 3 dB across almost the entire frequency range. Thus, the reduction in data rate is due to the reduction in SNR at longer distances and due to the reduction in optical power.³³ The finite OLED size limits the degree of collimation that can be achieved. As the link distance increases, this imperfect collimation causes the spreading of the beam, preventing all the light from being collected by the lens on the receiver side.

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