Charge Generation Layer Research Articles

Toward ultra-high efficiency tandem OLEDs: Benzothiazole-based bipolar hosts and specific device architectures

Tandem organic light-emitting devices (TOLEDs) satisfy the requirements of various applications owing to their high luminance, high efficiency, and long lifetime. A crucial factor in achieving optimal electroluminescence (EL) performance is the host material used for the emitting layer. In this study, a series of benzothiazole (BTZ)-based materials were linked via a phenyl bridge to three different electron-donor moieties, i.e., phenothiazine (PTZ), dimethylacridine (DMAC), and carbazole (CZ), named BTZ-PTZ, BTZ-DMAC, and BTZ-CZ, as bipolar hosts for green phosphors to realize ultra-high efficiency organic light-emitting devices (OLEDs). A Ir(ppy)3-based device based on a BTZ-CZ host is shown to exhibit a satisfactory peak efficiency of 28.0 % (98.8 cd A−1 and 111.6 lm W−1). Additionally, highly efficient green TOLEDs based on a BTZ-CZ host are demonstrated, and they are constructed with a specific charge generation layer and a TiO2 nanoparticle-based internal light extraction structure. The proposed TOLEDs achieved a maximum efficiency of 64.4 % (223.6 cd A−1 and 138.1 lm W−1) and a high maximum luminance of 24.9 × 104 cd m−2, one of the highest levels of efficiency and luminance reported in the literature for Ir(ppy)3-based tandem devices. These results show that the ingenious BTZ-based bipolar hosts exhibit immense potential for the development of tandem OLEDs, thus facilitating the realization of various high-performance EL applications.

Efficient Tandem White OLED/LEC Hybrid Devices

AbstractOrganic light‐emitting diodes (OLEDs) attract much research attention owing to their superior device characteristics in display and lighting applications. However, multi‐layered organic thin films deposited by thermal evaporation in a high‐vacuum chamber complicate the fabrication processes and increase the production costs. Alternatively, solution‐processable single‐layered light‐emitting electrochemical cells (LECs) possess easy fabrication and good device performance due to in situ electrochemical doping. To take advantages of both types of devices, the study proposes an efficient tandem white OLED/LEC hybrid device by stacking a red OLED on top of a blue LEC with a sophisticatedly designed charge‐generating layer (CGL). The proposed tandem device exhibits a high external quantum efficiency (EQE) of 21.53%, which is almost the sum of the EQEs of the two stacked devices. When fabricated on the diffusive substrate to recycle the trapped light in the substrate, the EQE can be further enhanced to reach 37.88%. Besides connecting the two stacked devices, the CGL improves the carrier balance of the blue LEC due to enhanced electron injection. This study demonstrates that the white tandem OLED/LEC hybrid device has a simpler device structure than all‐vacuum‐processed tandem OLEDs but it still delivers high device efficiency, revealing its potential in lighting applications.

Highly Electronegative V2CTx/Silicone Nanocomposite‐Based Serpentine Triboelectric Nanogenerator for Wearable Self‐Powered Sensors and Sign Language Interpretation

AbstractThe functionality and use of wearable triboelectric nanogenerators (TENGs) are significantly impacted by the development of their tribomaterial properties. Herein, a novel charge‐generating layer (CGL) composed of a V2CTx/Silicone nanocomposite and serpentine TENG (S‐TENG) is introduced. The V2CTx/Silicone nanocomposite exhibits a highly electronegative surface, which significantly enhances the surface potential and charge density. The fabricated nanocomposite‐based TENG exhibits a peak power density of 19.75 W m−2. It also exhibits a high mechanical stretchability (400%), which is significantly better than those of previously reported TENGs under small external pressure. The optimized S‐TENG is demonstrated as a self‐powered human motion sensor with high sensitivity (4.93 V kPa−1) and a quick rise time (21 ms). Furthermore, a real‐life demonstration of sign language interpretation using finger motion detection and text conversion via smartphones is successfully performed. Additionally, the real‐time sensing ability of the S‐TENG is demonstrated in virtual reality car games controlled by finger touch. The S‐TENG is demonstrated to capture different types of human motion energy for continuously powering a stopwatch and wristwatch. Therefore, it is expected that the V2CTx/Silicone nanocomposite will continue with the contributions of novel CGL materials for various TENG applications including wearable self‐powered sensors, robotics, and healthcare monitoring in IoT platforms.

Intermediate nanofibrous charge trapping layer-based wearable triboelectric self-powered sensor for human activity recognition and user identification

Significant research has been conducted to find practical methods to increase the triboelectric charge density of friction surfaces and improve the TENG output performance. In this study, a double-layer nanofibrous-TENG is newly proposed, consisting of MXene/P(VDF-TrFE) as a charge-generating layer and Siloxene/cobalt-nanoporous carbon/P(VDF-TrFE) as a charge-trapping layer, fabricated via a facile electrospinning process. The charge-generating layer generates abundant surface charges owing to the high electronegativity and electron affinity of MXene. Similarly, Siloxene as a filler in the charge-trapping layer improves the dielectric property of the layer, whereas hierarchically porous structure with a large surface area of cobalt nanoporous carbon provides more active sites for charge storage. After utilizing the charge-trapping layer, the current density and surface potential of the double-layer nanofibrous TENG is two-fold higher than the single-layer nanofibrous TENG. Furthermore, the TENG with Nylon 6/6 nanofiber as a positive friction layer, delivers a power density of 19 W m-2, which shows superior output performance compared to the state-of-the-art works. Finally, the fabricated device is attached to the shoe insole, and the triboelectric sensor data is analyzed using cutting-edge deep learning technology, which exhibited an accuracy of 99% in user identification and user activity recognition. Thus, this study investigates the possibilities of using the electrospun double-layer nanofibrous mat to boost the TENG output performance and explores its applications in artificial intelligence and human activity recognition systems.

Zirconium metal-organic framework and hybridized Co-NPC@MXene nanocomposite-coated fabric for stretchable, humidity-resistant triboelectric nanogenerators and self-powered tactile sensors

High-performance triboelectric nanogenerators (TENGs) have become more attractive for energy harvesting and self-powered sensing applications. Herein, a stretchable, humidity-resistant, and high-performance multilayered TENG (M-TENG) is newly developed for self-powered biomotion and tactile sensing applications by using a zirconium metal-organic framework (MOF-525)@Ecoflex nanocomposite with a layer of Ecoflex@cobalt-nanoporous carbon (Co-NPC)@MXene. The MOF-525 improved TENG’s performance four times due to homogeneous porosity and high charge accumulation. The porous structure of the Co-NPC functions as charge traps, promoting high charge-trapping capability, while the MXene nanosheets operate as microcapacitors, improving transport-ions inside the Ecoflex@Co-NPC@MXene nanocomposite (charge-trapping and charge-transport). When the Ecoflex@Co-NPC@MXene nanocomposite was added as an intermediate layer, it captured triboelectric charges from the charge-generating layer and accumulated more negative charges, improving the M-TENG performance by 13 times. A 3D-printed microstructure is apprehended to the surface to increase contact area and humidity-resistant. Besides, knitted fabrics provide flexible electrodes due to their excellent stretchability. The as-prepared M-TENG showed excellent performance, including a power density of 25.7 W/m2, sensitivity (149 V/KPa), humidity-resistant, and stretchability (245%) which are superior to the previously reported TENGs. By exploiting the outstanding stretchability of the M-TENG and its ultra-high sensitivity to mechanical stimuli, practical applications were successfully demonstrated in low-frequency wearable biomotion monitoring, biomechanical energy harvesting, high-precision character recognition, and as a self-powered tactile sensor. Furthermore, the real-time sensing capability was demonstrated via the LabVIEW interface and virtual-reality car games control by tracing the finger. Therefore, this work is anticipated to open exciting opportunities for wearable energy harvesters, self-powered sensing systems, and human-machine interfaces.

Triboelectric Nanogenerators: Enhancing Performance by Increasing the Charge-Generating Layer Compressibility.

Triboelectric nanogenerators (TENGs) have received significant attention for next-generation wearable electronics due to their simple device structure and low cost. Although the performance of TENGs is intimately tied to compressibility effects in the charge-generating layer, achieving high compressibility with conventional elastomers is challenging because molecular entanglements place a lower bound on the softness of cross-linked networks. Here, we demonstrate that bottlebrush elastomers are efficient charge-generating layers that improve the output performance of TENGs, including voltage, current, and surface potential, by minimizing entanglements and decreasing the compressive modulus (E). For example, a cross-linked bottlebrush with poly(dimethylsiloxane) side chains yielded TENGs with an output voltage (120 V) more than two times larger than a linear PDMS network (55 V). In conclusion, this study highlights the advantage of designing new charge-generating layers with improved compressibility to enhance TENG performance.

Highly efficient tandem OLED based on a novel charge generation layer of HAT-CN/CuPc heterojunction

A novel non-doped organic heterojunction (OHJ) composed of HAT-CN and CuPc is proposed to be charge generation layers (CGL) for constructing high efficiency tandem organic light-emitting diodes (OLEDs). The highest current efficiency and external quantum efficiency (EQE) of tandem device with this CGL are 46.3 cd/A and 15.1%, which are 125% and 107% higher than those of single emissive unit OLED, respectively. Furthermore, the tandem device provides better stability. The efficiency roll-off from the maximum to 1000 cd/m2 is 7.2% for the tandem device, which delivers a great improvement compared to that of single emissive unit OLED (21.9%). And CuPc thickness in CGLs merely slightly affects the device performance, because of the tradeoff between improved charge injection and declined transmittance with the increase of the CuPc thickness. The experimental results demonstrate that the organic heterojunction consisting of HAT-CN/CuPc is a feasible CGL for developing high performance tandem OLEDs.

A Pure Blue Phosphorescent Organic Light‐Emitting Diode with an External Quantum Efficiency of Over 30% by Using a Tandem Device Architecture

AbstractAchieving efficient deep blue phosphorescent organic light‐emitting diodes (PHOLEDs) is one of the most difficult and challenging targets nowadays, especially when the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of y values are smaller than 0.1. Herein, tandem device architectures are fabricated by using an iridium(III) carbene phosphor. In the tandem structures, two electroluminescent units are stacked and connected by a charge generation layer (CGL) which consists of dipyrazino[2,3‐f:2′,3′‐h]quinoxaline‐2,3,6,7,10,11‐hexacarbonitrile (HATCN) and 4,4′‐(cyclohexane‐1,1‐diyl)bis(N,N‐di‐p‐tolylaniline) (TAPC). Meanwhile, 4,6‐bis(3,5‐di(pyridine‐3‐yl)phenyl)‐2‐methylpyrimidine (B3PYMPM) and diphenyl‐4‐triphenylsilylphenyl phosphine oxide (TSPO1) are used as electron transport layers. The best device gives an external quantum efficiency of 32.3% with the CIE coordinates of (0.15, 0.10), which is among the most efficient blue PHOLEDs made by iridium(III) phosphors. The pure blue emission and high performance are mainly attributed to the efficient and balanced recombination of charge carriers in the two emissive layers, which successfully suppresses the exciplex formation and efficiently confines the triplet excitons.

High‐Performance Inverted Tandem OLEDs with the Charge Generation Layer based on MoO x and Ag Doped Planar Heterojunction (Advanced Optical Materials 16/2022)

A high-performance inverted tandem organic light-emitting diode (OLED) based on a charge generation layer integrated with MoOx− and Ag-doped planar heterojunction is reported by Wei Shi, Bin Wei, Wai-Yeung Wong and co-workers in article number 2200984. The charge injection layer enhances the concentration of the charge carrier and balances the charge injection. The work provides a simple strategy to fabricate inverted tandem OLEDs with high performance.

High‐Performance Inverted Tandem OLEDs with the Charge Generation Layer based on MoOx and Ag Doped Planar Heterojunction

AbstractThe morphological and electrical characteristics of a planar‐heterojunction structured charge generation layer (CGL) consisting of MoO3 doped N,N′‐bis‐(1‐naphthalenyl)‐N, N′‐bis‐phenyl‐(1,1′‐biphenyl)‐4,4′‐diamine (NPB) as the p‐type material and Ag‐doped 4,7‐diphenyl‐1,10‐phenanthroline (Bphen) as the n‐type counterpart (NPB:MoO3/Bphen:Ag). A significant charge generation effect of CGL is observed and its mechanism is analyzed. The introduction of CGL can enhance the concentration of the charge carrier and balance the injection of carriers. Inverted tandem organic light‐emitting diodes (OLEDs) with the CGL are fabricated. The inverted tandem OLEDs exhibit excellent optoelectronic performance due to the charge generation effect and the suppression of the high concentration quenching. The maximum current efficiency and external quantum efficiency of the inverted tandem OLED reached 27.91 cd A−1 and 11.18%, respectively, which are 110.95% and 95.79% higher than that of the inverted OLED.

Efficient tandem organic light-emitting diode with fluorinated hexaazatrinaphthylene charge generation layer

ABSTRACT We present the fluorinated hexaazatrinaphthylene derivative, 1,2,3,4,7,8,9,10,13,14,15,16-dodecafluoro-5,6,11,12,17,18-hexaazatrinaphthylene (HATNA-F12), as a new material to replace another hexaazatrinaphthylene molecule, 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN), which is widely used as an intermediate connector and charge generation layer (CGL) in tandem organic light-emitting diodes (OLEDs). Upon comparison with the reference single emitting layer unit OLED, the tandem device with HATNA-F12 CGL had improved current efficiency and external quantum efficiency of 42.1% and 41.7%, respectively. Our results show that HATNA-F12 is an excellent substitute for HAT-CN and has more promising characteristics as a CGL material for applications in tandem OLEDs. When compared to devices with HAT-CN CGL, the green tandem OLEDs with CGL consisting of HATNA-F12 demonstrated improved current efficiency, power efficacy, and external quantum efficiency values, while the change of the CGL material had almost no effect on the operating voltage, current density, and color coordinates of the OLEDs. We confirm that using the newly suggested HATNA-F12 molecule as a CGL in these OLEDs can enhance device efficiency.

Deep blue exciplex tandem OLEDs using n- and p-doped planar heterojunction as a charge generation layer

The tandem organic light-emitting diodes (OLEDs) have the advantages of small current density, high current efficiency (CE), and long lifetime. We have developed the conventional and inverted tandem OLEDs using n- and p-doped planar heterojunction as a charge generation layer (CGL). The CGL consists of the bathophenanthroline:Cs2CO3 and N,N’-di-[(1-naphthalenyl)-N,N’-diphenyl]-1,1′-biphenyl)-4,4′-diamine:MoO3 bilayer structure to connect the deep blue- and deep red-emitting units. The turn-on voltage, luminescence, CE, and external quantum efficiencies of the conventional tandem OLED are 7.2 V, 5083 cd m−2, 8.45 cd A−1, and 13.94%, respectively, and the color rendering ability remains stable at a high current density of 60 mA cm−2. Moreover, the efficiency roll-off of the inverted tandem OLED is optimized to 5.5% at a luminance of 1000 cd m−2. Furthermore, a large-area (50 × 50 mm2) parallel OLEDs with a tunable red-emitting area are fabricated. The development of the OLEDs provides a new direction for the application of OLEDs in plant growth.

Efficient flexible quantum-dot light-emitting diodes with unipolar charge injection.

The exfoliation between the electrode film and the adjacent functional layer is still a big challenge for the flexible light emitting diodes, especially for the devices dependent on the direct charge injection from the electrodes. To address this issue, we design a flexible quantum-dot light-emitting diodes (QLEDs) with a charge-generation layer (CGL) on the bottom electrode as the electron supplier. The CGL consisting of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ZnO can provide sufficient electron injection into the QDs, enabling a balanced charge injection. As a result, the CGL-based QLED exhibits a peak external quantum efficiency 18.6%, over 25% enhancement in comparison with the device with ZnO as the electron transport layer. Moreover, the residual electrons in the ZnO can be pulled back to the PEDOT:PSS/ZnO interface by the storage holes in the CGL, which are released and accelerates the electron injection during the next driving voltage pulse, hence improving the electroluminescence response speed of the QLEDs.

Quantum-dot Light-emitting Diodes Based on Inorganic Charge-generation Layer

利用WO3/ZnO作为电荷产生层（CGLï¼‰åˆ¶å¤‡äº†å ·æœ‰å€’ç½®ç»“æž„çš„é‡å­ç‚¹ç”µè‡´å‘å ‰å™¨ä»¶ï¼ˆQLEDï¼‰ï¼Œç›¸æ¯”äºŽä¼ ç»Ÿçš„åŸºäºŽå•å±‚ZnOä½œä¸ºç”µå­ä¼ è¾“å±‚çš„QLED，利用CGL‐QLED的电流效率提高了近30%ã€‚è¿™ä¸»è¦å½’å› äºŽCGLçš„ç”µå­æ³¨å ¥å ·æœ‰ç”µåœºä¾èµ–ç‰¹æ€§ï¼Œä»Žè€Œä½¿å¾—å™¨ä»¶ä¸­çš„ç”µè·æ³¨å ¥æ›´åŠ å¹³è¡¡ï¼Œæé«˜äº†æ¿€å­çš„å½¢æˆæ•ˆçŽ‡ï¼ŒæŠ‘åˆ¶äº†è½½æµå­å¯¼è‡´çš„çŒç­è¿‡ç¨‹ã€‚æ­¤å¤–ï¼Œæˆ‘ä»¬é€šè¿‡çž¬æ€ç”µè‡´å‘å ‰å ‰è°±æŠ€æœ¯åŠç”µå®¹ç‰¹æ€§æµ‹è¯•ï¼Œåˆ†æžäº†åŸºäºŽCGL的QLED的器件工作机制，发现CGLä¸­å¯ä»¥å­˜å‚¨å¤§é‡çš„è½½æµå­ï¼Œä»Žè€Œä½¿å¾—å™¨ä»¶åœ¨è„‰å†²ç”µåŽ‹é©±åŠ¨æ—¶å‡ºçŽ°å‘å ‰è¿‡å†²çŽ°è±¡ã€‚å ¶çŽ¯å¢ƒç¨³å®šæ€§ä¹Ÿä¸Žå¸¸è§„çš„åŸºäºŽZnO的器件一致。而由于CGLç‹¬ç‰¹çš„ç”µè·äº§ç”Ÿæœºåˆ¶ï¼Œä½¿å¾—å ¶ä¸ä¾èµ–äºŽç”µæžåŠŸå‡½æ•°ç‰¹æ€§ã€‚æˆ‘ä»¬ç›¸ä¿¡ï¼Œè¿™ç§å™¨ä»¶ç»“æž„åœ¨æ”¹å–„å™¨ä»¶ç¨³å®šæ€§åŠè‰¯çŽ‡æ–¹é¢æœ‰ç€å·¨å¤§æ½œåŠ›ã€‚

Color-Tunable Alternating-Current Quantum Dot Light-Emitting Devices.

To date, it remains a central challenge to achieve electroluminescence in both positive and negative half cycles of alternating-current (AC) voltage for a light-emitting device. Herein, we successfully demonstrated a novel structure to construct a real AC quantum dot light-emitting device (QLED) with two charge generation layers (CGLs) consisting of the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/ZnO nanoparticle bilayer structure. Besides the conventional driving way with power input from a pair of opposite electrodes, this AC QLED can also work in the manner of in-planar-electrode driving mode, achieving simultaneous electroluminescence of each pixel. By employing a bilayer emissive layer composed of red and green quantum dots, the emission color of the AC QLED can be tuned by both the polarity and amplitude of the driving voltage. Leveraging the excellent electron injection and negligible voltage consumption from the CGLs, this QLED can be turned on at a record low voltage of 5.6 V. We believe that this AC QLED can provide a platform for the realization of simple and smart plug-and-play QLED-based display and lighting systems.

Light extraction in tandem organic light emitting diodes

Since the invention of organic light emitting diodes (OLEDs), great research efforts have been dedicated to improving their efficiency and lifetime. For high-brightness applications, tandem OLED structures have advantages because of the lower current densities required to achieve high brightness. With the successful development of highly efficient charge generation layers, high brightness tandem OLEDs are used in displays and lighting. However, the major challenge for tandem OLEDs is the low light extraction efficiency, because about 50% of the light is trapped inside the device as waveguide modes. In this Perspective, we first review the recent works done on light extraction, analyze different waveguide mode extraction structures, and then identify the key factors determining the extraction efficiencies in tandem OLEDs.

Degradation analysis of doped organic p-n heterojunction charge generation layers by impedance and optical spectroscopy

Investigation of the long-term stability of charge generation layers (CGLs) provides a fundamental and an essential approach in achieving highly efficient tandem organic electronic devices. Thus, in this foremost study, the degradation mechanism of electrically aged organic p-n heterojunction CGLs has been investigated by impedance and optical spectroscopy. Rubidium carbonate (Rb2CO3)–doped 2,2,2-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi) and molybdenum trioxide (MoO3)–doped 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′-diamine (NPB) are used as the n-type and p-type organic semiconductors, respectively. A detailed analysis from capacitance-frequency (C–F) and capacitance-voltage (C–V) characteristics reveals reduced charge generation and 19.6% reduction in the geometric capacitance of the CGL after electrical aging. Reduced peak intensity from UV–Vis–NIR spectra of the aged CGL points to 21.4% charge transfer complex decomposition of the Rb2CO3-doped TPBi. We propose that the rate-limiting step of charge generation in the CGL is caused by the electron transport in the TPBi:Rb2CO3 layer and not the charge generation itself at the TPBi:Rb2CO3/NPB:MoO3 heterojunction. This simple, comprehensive, and non-destructive technique facilitates a crucial analysis that underpins the mechanism of device degradation and further provides a fundamental approach in developing highly stable CGLs for efficient organic electronic devices.

Highly efficient tandem PHOLEDs with lithium-doped BPhen/NDP-9-doped TAPC as a charge generation layer

The development of large-area organic light-emitting diode (OLED) displays requires a highly efficient tandem device architecture and an easily processable charge generation layer (CGL) with a low voltage drop and high optical transparency. In this study, we investigated and applied a doped organic n-CGL/p-CGL using thermal vacuum deposition in tandem OLED devices. A doping concentration of 1.0 wt.% for Li in 4, 7-Diphenyl-1, 10-phenanthroline (BPhen) was optimal for the n-CGL with 8 wt.% for 2-(7-dicyanomethylene-1,3,4,5,6,8,9,10-octafluoro-7H-pyrene-2-ylidene)-malononitrile (NDP-9)-doped N,N-bis(4-methylphenyl)benzenamine (TAPC) as a p-CGL. Maximum luminous efficiencies of 42.5 and 63.4 cd/A and a 4,000 cd/m2 current density for the target luminance values of 11.2 and 6.5 mA/cm2 were demonstrated for double-stack and triple-stack tandem blue phosphorescent OLED devices, respectively. Implementing these highly efficient tandem device structures will improve the overall lifetime of OLED displays by lowering their operating current density at the target luminance.

38.1: Invited Paper: High Performance Deep Red Three‐Unit Tandem OLEDs for Automotive Lighting Applications

We report high performance deep red tandem OLEDs suitable for automotive lighting applications. We developed a deep red phosphorescent emitter that has an intrinsic photoluminescent (PL) peak wavelength at 639 nm and a photoluminescent quantum yield PLQY of 0.80. A low‐voltage red host (LVRH) material was developed to match this emitter, which enables a single‐unit, bottom emission device to reach 2,000 cd/m2 with an external quantum efficiency (EQE) of 25.8%, bias voltage of 3.7 V, and LT95 of 6,000 hrs. A 3‐unit device incorporating this emissive system along with a novel charge‐generation layer (CGL) was fabricated and its device structure was optimized by tuning the hole transport layer (HTL) thickness. The device achieved a current efficiency of 39 cd/A, overall EQE of 77%, LT95 of 46,000 hrs, and bias voltage of 9.9 V at 2,000 cd/m2. This tandem device also reaches a deep red chromaticity with a peak wavelength of 641 nm, and CIE (X, Y) = (0.706, 0.293). The LT95 of the same device at 85 °C was 9,700 hr at 2,000 cd/m2. Such a deep red OLED is desirable for automotive lighting applications.

52.1: Invited Paper: Novel dual‐branch SPICE model for tandem structure OLED

Tandem‐structure OLED devices have been developed using charge generation layers to implement two or more OLED units in a single stack. They can achieve higher brightness at a given current density than the conventional OLEDs. In order to incorporate the novel tandem structure OLED within the computer aided design (CAD) flow of microdisplays, we have developed an equivalent circuit model that accurately describes the tandem OLED electrical characteristics.