# High-speed OLED bandwidth optimisation method based on Relative Intensity Noise measurements

Mohamed Nihal Munshi, Nixson Loganathan, Mahmoud Chakaroun, Benoit

Racine, Luc Maret, Alexis P. A. Fischer

► To cite this version:

Mohamed Nihal Munshi, Nixson Loganathan, Mahmoud Chakaroun, Benoit Racine, Luc Maret, et al. High-speed OLED bandwidth optimisation method based on Relative Intensity Noise measurements. Organic Electronics, 2023, 123, pp.106935. 10.1016/j.orgel.2023.106935. cea-04303337

HAL Id: cea-04303337

https://cea.hal.science/cea-04303337

Submitted on 23 Nov 2023

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# High-speed OLED bandwidth optimisation method based on Relative Intensity Noise measurements

Mohamed Nihal Munshi<sup>a,b,c</sup>, Nixson Loganathan<sup>b,c</sup>, Mahmoud Chakaroun<sup>b,c</sup>, Benoit Racine<sup>a</sup>, Luc Maret<sup>a</sup>,

Alexis P.A. Fischer<sup>b,c</sup>

**Abstract**—In the context of visible light communication, we present a heuristic approach to optimize the bandwidth of organic light-emitting diodes. Our approach utilizes a vector network analyzer and an optoelectronic loop to directly and rapidly visualize the effects of stack modifications on the optical bandwidth of high-speed OLEDs. This technique is applied for the first time to conduct systematic heuristic studies on OLED bandwidth and to identify relevant parameters such as OLED capacitance, serial resistance, emission layer thickness, and active area. More precisely, we utilize high-speed OLEDs based on coplanar wave guided electrodes, and demonstrate that reducing the active area of OLEDs from 500 x 500 µm² to 200 x 200 µm² and further to 100 x 100 µm² increases the bandwidth from 49 MHz to 91 MHz and above 200 MHz, respectively. Moreover, and counter intuitively, the bandwidth is increased when the hole blocking layer is removed from the organic heterostructure.

**Index Terms**—OLED, Organic optoelectronics, Visible light communications, RIN measurement

## I. INTRODUCTION

The visible light spectrum of light emitting diodes (LEDs) and organic light emitting diodes (OLEDs) could prove as an alternative to the congested radio-frequency cellular systems for improved high-speed communication with even larger bandwidth, superior privacy and minimal-to-zero electromagnetic interference. To reach such objectives, light sources with large bandwidth and high brightness are at the heart of intense research [1]. Paths for bandwidth improvement deal with decreasing the electrical time constant at the device level and with reducing the fluorescence lifetime at the material level.

Recent works have demonstrated the use of violet micro-LED (24-µm diameter) to offer large bandwidths up to 655 MHz and data rates up to 7.9 Gb/s [2] and then with blue micro-LED (10-µm diameter) with a bandwidth of 200 MHz and a data rate of 7.7 Gb/s at [3]. Large data rate of 11 Gb/s has been achieved, with a blue micro-LEDs based 3x3 matrix [4]. The most recent and largest bandwidth have been obtained in 2021 by Pezeshki et al. with RF-optimized GaN microLEDs showing bandwidth above 3.5 GHz [5]. Note that, RF-optimized GaN LEDs are fabricated by increasing the number of defects in the high-speed material, increasing trap-assisted (non-radiative) recombination at the cost of lower quantum efficiency and larger current density. The paradigm in the GaN LED bandwidth improvement is based on non-radiative recombinations that are increased either by adding traps in the semiconductor or by increasing Auger effect with higher current densities.

Another significant breakthrough has emerged from a different category of light sources, namely organic light-emitting diodes (OLED). Until recently, OLEDs were predominantly recognized for their relatively slower switching times of around 10ms, which made them suitable primarily for display applications. However, there has been a growing

interest in utilizing OLEDs, including polymer OLEDs, for light communication purposes, despite their limited bandwidth in the MHz range [6, 7]. In more recent developments, it has been demonstrated that High-Speed-µ-OLEDs (HS-µ-OLEDs) equipped with CoPlanar Wave-guided (CPW) electrodes can produce optical pulses as short as 2.5 ns (227 MHz). This breakthrough makes HS-µ-OLEDs potentially appealing for applications in Light Communication (LC) [8, 9]. In 2020, Yoshida et al. using OLEDs with a similar cut-off frequency of 245 MHz reported LC with a data rate above 1 Gb/s [10]. In 2022, 400ps optical pulses were generated from Alq3 based OLED electrically driven with sub-ns pulses [11].

These highly recent results unveil the promising dynamic properties of HS-µ-OLEDs, positioning them as a viable alternative to GaN LEDs. It is noteworthy that HS-µ-OLEDs have achieved such remarkable performance despite being the product of a comparatively shorter and emerging technological maturation processes. These results highlight the potential of HS-µ-OLEDs as a competitive option for various applications, suggesting exciting prospects for their further development and integration into practical systems. It is noteworthy that this fast response capability of organic semiconductors (OSC) was already predicted in 1992 by the Nobel prize winner A. Heeger [12]. These new ultra-fast light emitting organic devices could be the future of organic optoelectronic with applications "beyond display" [13]. Very likely, organic heterostructures need to be redesigned with new paradigms dedicated to bandwidth improvement rather than the sole efficiency improvement in steady-state regime as it is the case with OLEDs for displays and lighting.

Actually, the dynamics of OLEDs are governed by different mechanisms than those of GaN LEDs: indeed, the mobility expressed in the Poole-Frenkel law is non-linear with the electrical field which makes possible a paradigm different from that of LEDs [11]. Exploring heuristically these novel paradigms require new experimental tools and in particular

<footnote><sup>a</sup> Université Grenoble Alpes, CEA, Leti, F-38000 Grenoble, France</footnote>

<footnote><sup>b</sup> Université Sorbonne Paris Nord, Centrale de Proximité en Nanotechnologies de Paris Nord, 99 Avenue JB Clément, 93430 Villetaneuse, France</footnote>

<footnote><sup>c</sup> Université Sorbonne Paris Nord, Laboratoire de Physique des Lasers, 99 Avenue JB Clément, 93430 Villetaneuse, France</footnote>
efficient and rapid bandwidth measurement. To measure the bandwidths of OLEDs, Haas et al. used a Vector Network Analyzer (VNA) [10]. However, not only the bandwidth but the full modulation spectrum based on both the bandwidth and the Signal to Noise Ratio (SNR) have a significant impact on the overall performance of a transmission system. Indeed, the data rate *D* is expressed as *D* = *B*log(1 + *SNR*) where *B* is the bandwidth, *SNR* is the signal to noise ratio both being accessible directly with the Relative Intensity Noise (RIN) technique [14]. Therefore, we propose to measure the complete modulation spectrum of OLEDs using a technique similar to the Relative Intensity Noise (RIN) method employed in the optical telecommunications field [15]. More precisely, the spirit of this study is to apply a heuristic approach based on RIN measurements to uncover new paradigms and design principles for organic heterostructure stacks that enhance the modulation spectrum of OLEDs. The ultimate goal is to make OLEDs suitable for wireless optical communication applications. In this work, we provide a detailed explanation of how the VNA and the RIN-like technique can be utilized to accurately measure the OLED bandwidth. This approach offers a wide dynamic range and improved signal-to-noise ratio, enabling precise characterization of OLED performance.

The main originality lies in the use of the VNA measurement to carry out systematic studies at the electrical level, at the material level and at the device level to optimize the OLED bandwidth. At the electrical level, we investigate the effect of the OLED size on the RC time constant, while at the material level we study two different emitting materials to observe the impact of the fluorescence lifetime parameter. Also, at the device level, we investigate, the effect of the charge blocking layer on the OLED bandwidth. Another originality lies in the identification of conditions for obtaining an optimized RIN measurement system adapted to OLEDs.

The paper is structured as follows: Section II provides an overview of the characterization setup. Subsections within this section outline the techniques employed to minimize non-linear optical and electrical distortions, ensuring accurate measurements. In Section III, we present the design guidelines and structures of various high-speed OLED types. This section offers insights into different approaches for optimizing OLED performances. In section IV, the bandwidth analysis is conducted experimentally identifying the relevant parameters for the OLED bandwidth optimization. We discuss the experimental results in section V, and conclude in section VI.

## II. DESCRIPTION OF THE SETUP

The RIN measurement setup shown in Fig. 1 is made of a looped transmission chain consisting of an electrical part and an optical part. The electrical part consists of a VNA (Rohde & Schwarz ZNL-14 with a 5 kHz-14 GHz bandwidth) whose port 1 delivers a sweep signal with an amplitude $\Delta V = 230mV$ to a flat-gain amplifier (Amplifier 1 is Minicircuits ZFL-1000VH+ 20 dB gain with a 10 MHz to 1 GHz bandwidth, or Amplifier 2 is Minicircuits ZFL-1000+ 19 dB gain with a bandwidth from 100 kHz to 1 GHz) followed by a bias-tee (Picosecond 5575A-104 with a 10 kHz to 12 GHz bandwidth) which combines the amplified signal with a DC bias voltage $V_0$ delivered from a source meter unit (Keithley 2601A). The AC-DC combined

<figure><img src="image_2.png" /><figcaption>Figure 1: Experimental RIN measurement setup for bandwidth analysis</figcaption></figure>

signal is applied to the OLED via a ground-signal-ground (GSG) probe (Form Factor Z20-B-GSG-2000 20 GHz bandwidth). The amplifier is to be removed under certain conditions explained below.

The optical part consists of a light collection objective (Magnification $M_1 = \times 20$, numerical aperture $NA_1 = 0.75$, working distance $W_{D1} = 1$ mm), a light focusing objective (Magnification $M_2 = \times 10$, numerical aperture $NA_2 = 0.22$, working distance $W_{D2} = 11.8$ mm) and an avalanche photodetector. The collected light from OLED is then collimated through the focusing objective onto the active area $S_{APD}$ of the avalanche photodiode (APD) with a diameter $D_{APD} = 500\mu m$. Two types of avalanche photodiodes (APD) are used (Thorlabs APD 430A or APD 210) with a bandwidth of DC-400 MHz and 5-1000MHz respectively and a low noise equivalent power of 0.14 and 0.24 pW/√Hz respectively. The output $U_0 + \Delta U$ of the photodetector is connected to port 2 of the VNA, and the forward transmission coefficient (S21) parameter is calculated internally and plotted in decibels (dB). The amplitude variation at the output of the photodetector is $\Delta U = G_T \cdot \Delta P_{OPT}$ where $G_T$ is the transimpedance amplifier gain of photodetector and $\Delta P_{OPT}$ is the light intensity variation resulting from modulation amplitude $\Delta V_{MOD}$. The RIN-like calculated for the OLED by VNA is given by:

$$ (1) \quad \text{RIN-like}_{\text{OLED}} = 10 \times \log_{10} \left( \frac{[G_T \cdot \Delta P_{OPT}(\omega)]^2}{[\Delta V_{MOD}(\omega)]^2} \right) $$

This measurement data is then post-processed using a custom-made computer software. The entire optical setup, including the distance between the optical source and the photodetector, spans a total length of 25 cm. To ensure accurate measurements, the photodetector is eventually enclosed within a Faraday cage. The criteria for selecting the optimal set of objectives are detailed below.

The optimization process faces challenges due to the non-linear effects inherent in the electrical and optical responses of OLEDs employed in the digital transmission chain. These nonlinearities have the potential to introduce distortions in the modulated signal, thereby affecting the spectral profile. Overcoming these non-linear effects is crucial for achieving accurate and reliable transmission in OLED-based systems.

### Electrical non-linearity optimizations

To prevent distortion in the modulated signal, two conditions must be met at the electrical level. Firstly, the bias
voltage $V_0$ should be above the threshold voltage $V_{th}$ in order to fulfil the condition:

$$ (2) \quad V_0 + \Delta V \geq V_{th} $$

Secondly, the electrical power $P_m$ of the modulated signal is to be chosen such that after amplification, the resulting power does not exceed the saturation power $P_{sat}$ of the chosen amplifier. Here, $P_m$ is the AC component of the electrical power delivered to the OLED:

$$ (3) \quad P_m = \Delta V \Delta I(V_0) $$

where $\Delta I(V_0)$ is the AC current amplitude around the operation point at the bias voltage $V_0$. The AC current amplitude is related to the bias voltage by:

$$ (4) \quad \Delta I = \frac{1}{R_d(V_0)} \Delta V $$

where $R_d(V_0)$ the dynamical resistance at the operation point is defined by the slope of the I-V curve:

$$ (5) \quad \frac{1}{R_d(V_0)} = \frac{dI}{dV_{V_0}} $$

Figure 2.A shows the OLED current-voltage (I-V) characteristics (blue curve) measured under DC conditions, together with the dynamical resistance $R_d$ (red curve) calculated as a function of the bias voltage $V_0$ in accordance with Eq. 5 and plotted with a log scale (right axis). Figure 2.B shows $P_m$ the AC component of the electrical power (right axis) as a function of the bias voltage before amplification (black curve) and after amplification by Amplifier 1 ($P_{sat1}$ cyan curve) or after amplification by Amplifier 2 ($P_{sat2}$ magenta curve). The saturation power of each amplifier $P_{sat1}$ and $P_{sat2}$ are indicated with dotted cyan and dotted magenta lines.

To avoid electrical distortion caused by the amplifier, it is important to carefully select an appropriate amplifier based on the modulation power $P_m$. For higher values of $P_m$, it may even be beneficial to remove the amplifier. In the current scenario, where no distortion of the modulated signal is observed, both amplifiers are suitable for the transmission chain. However, in a setup involving amplifiers, it should be noted that a bias voltage $V_0$ exceeding 14 V may lead to surpassing the saturation power, potentially introducing electrical distortion to the transmitted AC signal. Therefore, it is crucial to consider the specific requirements and limitations of the amplification system when choosing the bias voltage to maintain optimal signal integrity.

### Optical distortion optimizations

To optimize the measurement setup from an optical standpoint, three aspects need to be considered. Firstly, it is important to maximize the amount of light detected by the photodetector. Secondly, suitable operating conditions must be identified to prevent distortions in the transmitted light, such as saturation. Thirdly, measures should be taken to avoid detector saturation.

To maximize the detection of light, three sub-conditions are applied to the working distance $W_{D1}$, numerical aperture $NA_1$, and the magnifications $M_1$ and $M_2$ of the lens objectives used.

In the first sub-condition, the working distance $W_{D1}$ of the first collection objective should be greater than the thickness of the OLED glass substrate (0.7 mm) to consider the focal point of the light source.

The second sub-condition states that a larger numerical aperture ($NA_1$ for the first objective results in better light collection from the OLED. However, it is important to note that a long working distance may limit the numerical aperture.

<figure><img src="image_3.png" /><figcaption>Figure 2: IV curve and dynamical resistance of an OLED (A) and electrical power of the modulated signal (B)</figcaption></figure>

Furthermore, this second sub-condition is also tampered by the following third sub-condition. The magnifications $M_1$ and $M_2$ of the first and second (focusing) objectives produce an image size $D_{im} = D_0 M_1/M_2$ with $D_0$ the OLED diameter. The image is to fit onto the active area $D_{APD}$ of the APD in order to maximize the light flux and the SNR. The condition on $D_2$ reads:

$$ (6) \quad D_2 = D_0 \frac{M_1}{M_2} < D_{APD} $$

In the current setup, with $NA_1 = 0.75$, $M_1 = \times 20$, and $M_2 = \times 10$, the aperture is close to the optimum. However, OLEDs larger than 250µm will result in images that are larger than $D_{APD} = 500µm$. Here, we need to consider a trade-off where we maximize the numerical aperture to optimize light collection for both the 100 x 100µm² and 200 x 200µm² OLEDs.

The second aspect involves preventing distortion in optical signals caused by either OLED saturation or threshold voltage effects. To ensure distortion-free modulated light, the operating point of the OLED should be carefully established by selecting appropriate values for the bias voltage $V_0$ and AC signal amplitude.

Figure 3 depicts a typical OLED Radiance-Voltage (R-V) curve, illustrating the threshold voltage ($V_{th}$) and the saturation effect of OLED light at $V_{sat} = 11.5V$. The saturation effect appears as a more abrupt phenomenon compared to the threshold voltage, resulting in a noticeable decrease in the amplitude of the modulated light when the voltage exceeds $V_{sat}$. To prevent distortion of the modulated light, it is crucial to avoid the voltage range above $V_{sat}$, ensuring that $V_0 + \Delta V \le V_{sat}$.
The derivative of the R-V curve (red curve) shown in Fig. 3 reveals a maximum at $V_m = 11.2V$. Since the slope is maximum at $V_m$, the amplitude of the modulated light is also at its maximum. This indicates an optimal bias voltage for detecting the largest amplitude of modulation. In the case of the current OLED, the saturation voltage ($V_{sat}$) and the optimal operation voltage ($V_m$) are close to each other. However, in PiN-doped OLEDs, there can be a significant difference between these two voltages. OLEDs based on materials with higher mobility exhibit less pronounced Joule heating effects, resulting in light saturation being reached at higher voltages. Consequently, in such cases, higher amplitudes of the modulation voltage can be used without concerns about distortion.

<figure><img src="image_4.png" /><figcaption>Figure 3: OLED radiance-voltage (RV) curve (blue - left scale) and its slope (red - right scale).</figcaption></figure>

A third condition on the photodetector to prevent distortion also exists. This condition specifically addresses the saturation of the photodetector. Fortunately, this condition is easily met, as the output power of the OLED in the micro-Watt range is significantly lower than the saturation power of the photodetectors, which is 1mW.

### III. DESIGN AND FABRICATION OF HIGH-SPEED OLED

This section summarizes the design rules and the fabrication process of HS-μ-OLEDs. These sources are designed following three guidelines at the electrical and optical levels. At the electrical level, the reduction of the electrical time constant is obtained firstly from the equivalent electrical circuit optimization, and secondly by the implementation of microwave techniques in OLED with CPW electrodes [8, 9].

The reduction of the electrical time constant is guided by the expression on the equivalent electrical circuit as [8]:

$$ (7) \quad \tau = \frac{C}{\frac{1}{R_d} + \frac{1}{R_m + R_s}} $$

where $C = SC_M$ is the OLED junction parasitic capacitance proportional to S the OLED active area by a factor $C_M = 25 nF/cm^2$, $R_d$ is the dynamical resistance defined by Eq. 5, $R_m$ is the measurement resistance if any, and $R_s$ is the serial resistance. It appears clearly that the electrical constant of the OLED can be decreased by reducing its active area A. If the

geometry is one way to change the electrical time constant, changing the condition of operation is another way. Indeed, when increasing the bias voltage, the dynamical resistance $R_d$ will decrease and the electrical time constant will follow provided $R_d < R_m + R_s$ [8].

The reduction of the time constant resulting from the use of CPW electrodes on top of which the organic heterostructure is deposited, comes from the characteristic impedance continuity with the pulse or modulation driver. CPW electrodes consist of a conductor strip surrounded by two ground planes as presented in earlier works [5, 16]. The three strips lie on the same plane and the overall characteristic impedance depends on the geometry. In the current study, the electrodes are patterned in a 340nm thin layer of Indium Tin Oxide (ITO) coating a 700μm thick glass substrate with an ITO resistivity of 15Ω. The ITO layer is partially metallized in order to reduce its series resistance.

At the optical level, the high-speed properties are enhanced by choosing organic compounds with shorter fluorescence lifetime. For example, Tris-(8-hydroxyquinoline) aluminium (Alq3) exhibits a fluorescence lifetime of 12 ns, whereas 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) exhibits a 1.2 ns fluorescence lifetime [17, 18]. The design rules guide the fabrication process that is divided into four phases: etching, metallization, planarization and organic layers deposition.

As a first step, the glass substrates covering the ITO layer are cleaned with organic solvents followed by deposition of a positive photo-resist (AZ1518). The photoresist is then exposed to UV light (I-line) through a photolithography mask (Mask 1) followed by immersion in a developer solution (AZ351B) and water at 1:4 ratio. Then, the ITO layer is etched in an inductively coupled plasma chamber (Coriol ILS200) to free the ITO layer from the photo-resist. The resulting ITO pattern is made of two lateral planes aside two centrally aligned ribbons, one for the cathode and one for the anode. The CPW anodes are ended with three ribbons with different widths of 100μm, 200μm and 500μm anticipating active areas such as 100 x 100 μm², 200 x 200 μm², and 500 x 500 μm².

As a second step, the metallization is realized through a lift-off process consisting of similar photolithography to the etching phase but with an inverted photolithography mask (Mask 2) followed by metal depositions (with Chromium-Gold) to minimize the resistivity effects of the CPW ITO electrodes. Only the active area is not metallized.

The third step is the insulation: it consists of spin coating an insulating resist and then in exposing it to UV light (I-line) through another photolithography mask (Mask 3). The insulating resist is used to planarize the active area and its vicinity to avoid electrical discontinuities induced by ITO sharp flanks at the anode ribbon level. The samples are then baked in an oven to soften the insulation resists and smooth the edges.

The fourth step is the deposition of organic materials and cathode. It is performed in a thermal deposition chamber under vacuum ($10^{-7}$ torr) where materials are heated to a gaseous state and deposited as a stack on the ITO layer through hollow masks.

Figure 4 presents four different types of OLED to be compared in terms of bandwidth:
The type 1 OLED heterostructure is shown in Fig. 4.a. It is made of a hole injection layer (HIL) of 35nm of m-MTDATA, a hole transporting layer (HTL) of 15nm thick NPD, and 55nm thick Alq3 layer that is both an emitting layer (EL) and an electron transporting layer (ETL). The stack is deposited on non-metallized CPW electrodes made of ITO, and the cathode is made of LiF (1.3nm) and 100nm of Aluminium.

<figure><img src="image_5.png" /><figcaption>Figure 4: Different types of organic heterostructures for OLED</figcaption></figure>

The OLED type 2 consists of the same heterostructure as in first type shown in Fig. 4.a except that the CPW electrodes are partially metallized with chromium and gold except for the active area where the ITO cathode is left uncovered.

The type 3 shown in Fig. 4.b corresponds to red-emitting OLEDs with an HBL. The heterostructure is similar to that of type 2 except for the emitting layer made of Alq3 as a host with DCM at 2% as a guest. The stack is m-MTDATA (35nm)/ NPD(15nm)/ Alq3:DCM(30nm)/ TPBi (5nm) / Alq3 (25nm) and LiF with aluminium electrodes. The CPW electrodes are partially metallized as in type 2.

The type 4 is shown in Fig. 4.c. It consists of a red-emitting OLEDs without HBL; The stack consists of m-MTDATA (35nm)/ NPD(15nm)/ Alq3:DCM (30nm)/ Alq3 (25nm)/ LiF (1.3nm) with LiF and aluminium electrodes and partially metallized CPW electrodes.

After the deposition of organic materials, encapsulation by Atomic Layer Deposition with 35nm of Al₂O₃ is carried out to prevent the OLED degradation from oxygen and moisture once removed from the glovebox. Each sample type contains 3 OLED dimensions: 100 x 100 µm², 200 x 200 µm², and 500 x 500 µm².

## IV. RIN MEASUREMENTS AND RESULTS

The different types of HS µ-OLEDs are then characterized successively using the RIN measurement setup shown in Fig.1 To avoid systematic errors and perform accurate measurements with the above mentioned dynamics, both ports of VNA are connected with rigid co-axial cables and a calibration step is performed before measurements. In our case, a manual calibration kit (ZN-Z135) was used and the type of calibration performed was Through, Open, Short, Matched (TOSM).

<figure><img src="image_6.png" /><figcaption>Figure 5: RIN-like measurements of the bandwidth of 500 µm x 500 µm OLEDs with a 12V bias for four different OLED types; non-metallized electrodes with Alq3 as emitting layer (dash green), Cr-Au metallized CPW electrodes with Alq3 as emitting layer (green solid line), Red-emitting OLED with HBL (magenta solid line), red OLED without HBL (red solid line). Background noise signal (grey). The arrows indicate the -6dB OLED bandwidths of 14MHz, 17MHz, 35MHz and 49MHz respectively.</figcaption></figure>

Figure 5 compares the RIN profiles for the four different types of 500 µm x 500 µm OLED biased with 12V. As a preliminary step before measurement, the light-path is blocked, to observe the idle state response (noise floor) of the photodetector (grey solid line) at about -100 dB. The RIN of the first type of non-metallized OLED with Alq3 as emitting layer exhibits a bandwidth at -6 dB of 14 MHz and a maximum at -47 dB (green-dashed curve) whereas OLED with metallized electrodes (second type) exhibits a bandwidth increased to 17 MHz (solid green curve) corresponding to an increase of 21% and a maximum at -30 dB. Obviously, the metallization of the CPW electrodes helps to reduce the electrical time constant and thus to enhance the OLEDs bandwidth. The third type of 500 µm x 500 µm² OLEDs with DCM as a red emitter and with HBL exhibits a bandwidth of 35 MHz and a maximum at -21.6 dB when biased with 12V. The fourth type of OLED with an emitting layer made of Alq3 doped with DCM (Alq3:DCM) but without HBL shows red light emission and a bandwidth increased to 49 MHz and a maximum at -19 dB (red). Clearly, guest-host based OLEDs exhibit larger bandwidth and better signal to noise ratio that are further increased with the removal of the hole blocking layer.
<figure><img src="image_7.png" /><figcaption>Figure 6: RIN-like measurements of the bandwidth of 500 x 500 µm² (red solid line), 200 x 200 µm² (green solid line) and 100 x 100 µm² (blue solid line) OLEDs biased with 12V. Inset: Bandwidth as a function of OLEDs sizes for red-emitting OLED with (black circles) and without HBL (magenta squares). Uncertainties are induced by external noise from the photodiode power supply.</figcaption></figure>

Figure 6 shows how the RIN fluctuations evolve as a function of the size of the OLEDs for type 3 and 4. The -6 dB bandwidths of the red-emitting OLEDs with HBL are 18 MHz 59 MHz, and 88 MHz for the 500 x 500 µm², 200 x 200 µm², and 100 x 100 µm² respectively. Red-emitting OLEDs without HBL exhibit bandwidths of 49 MHz and 91 MHz for OLED active area of 500 x 500 µm² and 200 x 200 µm² respectively which is 2.7 and 1.5 times larger than that with HBL. The bandwidth of the 100 x 100 µm² red-emitting OLED without HBL is estimated in the range of 104 MHz to more than 200 MHz. In the inset of Fig. 6, despite the uncertainties the trends for the different OLED size with and without HBL are clear. For this size, the bandwidth enhancement resulting from the HBL removal is estimated between 120% and 230%. Note that the uncertainties in the RIN measurement of small OLEDs result from external parasitic resonances and low light-emitted level. Note that the slopes in the spectral profiles of the different OLEDs show low progressive decrease, inversely proportional to the OLED size.

## V. DISCUSSION

The RIN measurement techniques presented in the current study offer several advantages over alternative techniques like pulse analysis in the time domain performed with a scope [5]. Firstly, a Bode diagram is directly and explicitly displayed contrary to pulse responses analysed in the temporal domain. Secondly, the VNA power is set to 0 dBm with a noise floor of about -100 dBm resulting in dynamics much larger than that of a scope offering a noise level as large as 1mV (-47 dBm). The simplicity to access the modulation spectrum of different OLEDs is a major advantage for comparing them and for identifying the parameters relevant for bandwidth enhancement.

The drawbacks are related to the presence of different electrical band-pass filters, several non-linearities induced by

components in the measurement loop. They induce distortion and noise, and require careful design of both the electrical and optical part.

The first drawback deals with the measured RIN spectrum that is not that of the sole OLED, but it is convoluted with the band-pass of amplifiers (10 MHz to 1 GHz for amplifier 1 and 100 kHz to 1 GHz for amplifier 2) and of the bias-tee (10 kHz to 12 GHz bandwidth) and that of the VNA. This is the reason why the measured spectral profiles show a drop below 1 MHz as in Fig. 5 and 6 whereas OLED are fully operating in DC regime. But as the filter profiles of the surrounding components are directly accessible through VNA measurements, it can be de-convoluted to access the filter profile of the sole OLED.

A second drawback deals with non-linearities presents in the devices like threshold voltage, and like OLED saturation, or present in the measurement schemes. But, as previously explained, careful conditions can be applied to circumvent or limit these issues. For example, the smaller the amplitudes of modulation, the more limited the effect of non-linearities.

The third drawback deals with the need to make a careful choice of the optical elements to increase the collected light and hence the SNR.

Nevertheless, despite the uncertainties, these experiments highlights the potential of the RIN measurement technique for direct access to the OLED bandwidth. The RIN measurement unveils experimentally that µ-OLEDs are able to exhibit a large modulation spectrum extending much beyond a 200 MHz bandwidth with a very gradually decreasing magnitude. The latter could prove to be very useful for high data rate light transmission. The main results of the application of RIN-type techniques to the measurement of OLED bandwidth are that four paths for the design of organic heterostructure have been identified that allow to increase the bandwidth of OLEDs: Firstly, the use of metallized CPW electrodes, secondly, the reduction of the OLED active area, thirdly, the removal of the HBL, fourthly, the use of organic emitter with short fluorescence lifetime in the ns range.

Three of these parameters have an impact at the electrical level namely the metallization, the reduction of the active area and the removal of the HBL. The metallization of CPW electrodes have reduced the serial resistance of the devices from $R_s$ 100Ω to 48Ω and in turn reduces the electrical time constant as predicted from the OLED dynamical model equation (7). With $R_d = 10^4$Ω and $R_m = 0$, the reduction of $R_s$ from 100 Ω to 48 Ω leads to a decrease of the time constant by 48% from (6) which is more than the 21% increase of the cut-off frequency from 14 MHz to 17 MHz.

The reduction of the device size $S$ by 6.25-fold and 25-fold from 500 x 500 µm², to 200 x 200 µm², and to 100 x 100 µm² respectively, is expected to reduce the parasitic capacitances $C$ by as much and in turn the bandwidth is expected to increase inversely. Instead, we observe a bandwidth enhancement of 1.8 and 4-times only. A more systematic and theoretical study is necessary to identify other limiting factors.

The effect of the HBL removal is clearly visible in the inset of Fig. 6. It is to induce an enhancement of the bandwidth by 2.7, 1.5 and 1.2-fold depending on the OLED size. This can be understood when considering the spatial charge distribution among the different layer interfaces inside the organic
heterostructure. Indeed, as reported by Ruhstaller and al. charges accumulate at organic layer interfaces [19]. These accumulations are similar to what happens with junction capacitors; therefore, the removal of HBL has an effect similar to a junction capacitance removal. Removing HBL to improve the bandwidth of OLED under AC excitation is an approach opposite to add HBL in order to improve the OLED efficiencies under DC regime. This novel and counter intuitive first paradigm for bandwidth enhancement of high-speed OLEDs is therefore here clearly unveiled thanks to the RIN-like measurement technique. Again, and more clearly, our main but counter-intuitive conclusion is that high-speed OLEDs driven under AC regime have to be tailored differently than OLED driven in DC or quasi DC regime.

Another parameter is assumed to have an impact at the optical level namely the fluorescence lifetime of the emitter; The green OLEDs use an Alq3-based emitting layer with 12 ns fluorescence lifetime, while red OLEDs use DCM-based emitting layer with 1.2 ns fluorescence lifetime which corresponds to a ten-fold reduction. While the bandwidth is expected to increase by as much, the bandwidth enhancement from OLED type 2 to type 4 is observed to be only 2.9. We believe OLED type 2 and 4 suffer from other limiting factors at the electrical level preventing the full enhancement to be observed such as trapping effect from the DCM dopant [20]. A more systematic small-signal analysis is necessary to distinguish the relevant limiting factors. Surprisingly, while the bandwidth increase from 14 MHz to 17 MHz, 35 MHz and to 49 MHz, the SNR is shown to increase from -47 dB to -30 dB, -21.6 dB and to -19 dB indicating that the bandwidth enhancement is not the result of an increase of non radiative recombinations contrary to what happens with GaN µ-LED. Rather it indicates that another paradigm specific to OLEDs possibly related to the Poole Frenkel mobility of organic semiconductors is at stake in the modulation spectrum enhancement.

## VI. CONCLUSION

We conducted a systematic heuristic study to extend OLED modulation bandwidth in the context of wireless optical communication links. The systematic and direct measurement of the light intensity modulation spectrum of high-speed OLEDs relies on the RIN technique and uses a VNA-based experimental setup. The electrical and optical design rules for optimizing the RIN measurements setup have been presented. By analyzing the resulting light intensity modulation spectra we measure the bandwidth and the SNR of four different OLED types with different electrodes, different emitting layers, and the presence or the absence of hole blocking layers in the organic heterostructures. This leads to identify four paths to optimize the organic devices for improvements of their bandwidth; firstly, the reduction of serial resistance via electrodes partial metallization, secondly the use of short fluorescence lifetime emitting compound like DCM (1.2 ns) rather than Alq3 with 12 ns fluorescence lifetime. A third path deals with the removal of the HBL layer. The latter is a counter intuitive design rule and a novel paradigm highlighting the fact that OLEDs for light communication have to be optimized in different and specific manner than OLEDs for lighting or

displays. The fourth path deals with the OLED size reduction and we reported that three OLEDs of different sizes 500 x 500 µm², 200 x 200 µm² and 100 x 100 µm² exhibiting bandwidths of 49 MHz, 91 MHz and above 200 MHz respectively. These results confirm that contrary to common knowledge, OLEDs are able to exhibit bandwidth in the 100-200 MHz range making them a promising technology for light communications.

Surprisingly, while the bandwidth increase from 14 MHz to 17 MHz, 35 MHz and to 49 MHz, the SNR is shown to increase from -47 dB to -30 dB, -21.6 dB and to -19 dB indicating that the bandwidth enhancement is not the result of an increase of non-radiative recombinations contrary to what happens with GaN µ-LED. Another important conclusion and second novel paradigm is therefore that the physics behind OLED bandwidth enhancement is different from that for LEDs.

Perspectives deals with improvement of the RIN measurement setup by noise removal and SNR enhancement. We believe OLEDs bandwidth can be enhanced further by optimizing OLED heterostructures with layer thickness optimization, shorter fluorescence lifetime of the organic emitter and PIN doping.

## ACKNOWLEDGMENT

This work was supported by the Agence Nationale de la Recherche through the 453 Investissement d'Avenir Programs under Grant ANR-11-IDEX-0005-02, by the Labex SEAM: Science Engineering, 454 Advanced Materials, This study was also supported by the IdEx Université de Paris, ANR-18-IDEX-0001. The authors would like to thank especially the Agence Nationale de la Recherche (ANR) for the funding of the DEµS CRP project (ANR-21-CE24-0016-02). The authors also thank D. Kocic and Christelle Laugier for their technical support. Relative Intensity Noise measurement of High-speed µ-OLEDs for bandwidth enhancements © 2022 by Mohamed Nihal Munshi, Nixson Loganathan, Alexis P.A. Fischer, Luc Maret, Mahmoud Chakaroun, Benoit Racine is licensed under CC BY-ND 4.0. To see a copy of this license, visit http://creativecommons.org/licenses/by-nd/4.0/

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