Recombination Zone Research Articles

Color-Switchable Subwavelength Organic Light-Emitting Antennas

Future photonic devices require efficient, multifunctional, electrically driven light sources with directional emission properties and subwavelength dimensions. Electrically driven plasmonic nanoantennas have been demonstrated as enabling technology. Here, we present the concept of a nanoscale organic light-emitting antenna (OLEA) as a color- and directionality-switchable point source. The device consists of laterally arranged electrically contacted gold nanoantennas with their gap filled by the organic semiconductor zinc phthalocyanine (ZnPc). Since ZnPc shows preferred hole conduction in combination with gold, the recombination zone relocates depending on the polarity of the applied voltage and couples selectively to either of the two antennas. Thereby, the emission characteristics of the device also depend on polarity. Contrary to large-area OLEDs where recombination at metal contacts significantly contributes to losses, our ultracompact OLEA structures facilitate efficient radiation into the far-field rendering transparent electrodes obsolete. We envision OLEA structures to serve as wavelength-scale pixels with tunable color and directionality for advanced display applications.

The Effect of Recombination Zone Positon on the Color Temperature Performance in the Color-Tunable White Organic Light-Emitting Diode

Using a color-tunable organic light-emitting diode (CT-OLED) can accord with the circadian cycle of humans and realize healthy lighting. The variation range of the correlated color temperature (CCT) is an important parameter to measure the performance of CT-OLEDs. In this paper, the effect of changing the utilization of phosphorescent materials and the position of the recombination zone (RZ) in the device are investigated by changing the thickness of the emissive layer (EML) and the doping ratio of the host and guest materials. The results show that reducing the red phosphorescent material and improving the blue phosphorescent material can affect the change direction of CCT, but it is not enough to expand the span of CCT (ΔCCT). It is more conducive to improving ΔCCT by more reasonable regulation of the position of the main RZ in EML and the energy transfer from the blue sub-EML to the red sub-EML. Device D obtains the best electro-optic and spectral characteristics, in which the maximum ΔCCT is 5746 K (2661 - 8407 K) as the voltage changes from 3.75 V to 9.75 V, the maximum current efficiency and luminance reach 18.34 cd·A-1 and 12,100 cd·m-2, respectively.

A comprehensive analytical model for ambipolar transport in nano-dimensional VOPc/p-6P OHJFET for applications in organic electronics

This paper presents a compact analytical DC model for high mobility VOPc (vanadyl pthalocyanine)/p-6P (para-sexiphenyl) ambipolar organic heterojunction field-effect transistor (OHJFET). The proposed model accounts for both unipolar and ambipolar regimes of VOPc/p-6P ambipolar OHJFET by considering spatial charge carrier density in the channel. The model incorporates subthreshold conduction phenomenon in addition to describing beyond threshold transport. The model is extended to describe ambipolar regime occurring in subthreshold region at low drain to source voltage, V DS. Device characteristics and various parameters obtained are presented and are further used to model recombination zone and channel potential profile. Results obtained, are compared with available experimental data and a good match is observed.

Light-emitting field-effect transistors with EQE over 20% enabled by a dielectric-quantum dots-dielectric sandwich structure

Emerging quantum dots (QDs) based light-emitting field-effect transistors (QLEFETs) could generate light emission with high color purity and provide facile route to tune optoelectronic properties at a low fabrication cost. Considerable efforts have been devoted to designing device structure and to understanding the underlying physics, yet the overall performance of QLEFETs remains low due to the charge/exciton loss at the interface and the large band offset of a QD layer with respect to the adjacent carrier transport layers. Here, we report highly efficient QLEFETs with an external quantum efficiency (EQE) of over 20% by employing a dielectric-QDs-dielectric (DQD) sandwich structure. Such DQD structure is used to control the carrier behavior by modulating energy band alignment, thus shifting the exciton recombination zone into the emissive layer. Also, enhanced radiative recombination is achieved by preventing the exciton loss due to presence of surface traps and the luminescence quenching induced by interfacial charge transfer. The DQD sandwiched design presents a new concept to improve the electroluminescence performance of QLEFETs, which can be transferred to other material systems and hence can facilitate exploitation of QDs in a new type of optoelectronic devices.

Bis(N-naphthyl-N-phenylamino)benzophenones as exciton-modulating materials for white TADF OLEDs with separated charge and exciton recombination zones

Organic semiconductors were employed as exciton modulators, blue emitters, hole-transporting materials and hosts with resonant-appropriate singlet and triplet energies for efficient and stable white organic light emitting diodes (OLEDs). Two 4,4'-bis(N-naphthyl-N-phenylamino)benzophenones were synthesized using isomeric N-naphthyl-N-phenylamines as the donors and benzophenone as the acceptor moiety. Molecular design of new compounds allowed to obtain required combination of properties, i.e. blue prompt fluorescence in solid state with singlet energies close to those of the selected blue emitter exhibiting thermally activated delayed fluorescence (TADF), low triplet energies of 2.32 and 2.45 eV which are close to those of orange TADF emitter, good charge injecting properties (ionization potentials of 5.68 and 5.79 eV), good charge transporting properties with hole mobilities exceeding 10−4 cm2 (V s)−1 and high thermal stability with five percent weight loss temperatures up to 428 °C. The blue-emitting compounds were used as exciton modulators between the known blue and orange TADF emitters for fabrication of white OLEDs exploiting spatial exciton allocation strategy. In the frame of this strategy, resonant energy transfers: NPABP emitters → blue TADF and NPABP emitters → orange TADF emitters were investigated using different device structures towards efficient white electroluminescence. About twice higher external quantum efficiency was obtained for devices with two resonant energy transfers in comparison to that of the reference devices with one resonant energy transfer proving efficiency of spatial exciton allocation strategy for white TADF OLEDs. The best quality of white electroluminescence is characterized by CIE coordinates of (0.32, 0.31), colour temperature of 4490 K and colour rendering index of 80. Similar stability of blue and orange emission bands in white electroluminescence spectra was achieved due to the separation of charge and exciton recombination zones in the device structure.

Co-solvented solution filling and interfacial phenomena of sublimation transferred emitting layer for high-resolution OLED fabrication

We investigated co-solvented solution properties and relations in the sublimation transfer process and optimized intense pulse light (IPL) using time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis. The capillary force induced co-solvented solution into the microchannel, with the co-solvented solution properties influencing the thickness and morphology of the sublimation transferred patterns of the emitting layer. The sublimation transferred pattern was optimized by tuning the ratio of solvents that were mixtures of chlorobenzene and ortho-dichlorobenzene. In addition, an analysis of the interface between pre-deposited layers and the evaporated layer fabricated by variation of IPL energy was conducted by means of ToF-SIMS depth profiling. Host emissions or the occurrence of the intermixing of layers was detected due to insufficient or excessive IPL energy exposure. With the optimized IPL energy, the current efficiency and external quantum efficiency were maximized without a recombination zone shift or intermixing during the IPL evaporation process.

White PeLED with the composite emission layer of organic small molecule and perovskite materials preparedby thermal evaporation

<p indent="0mm">Metal halide perovskite material has received considerable attention in recent years because of its unique properties, such as high photoluminescence quantum yield, adjustable bandgap, high carrier mobility, and long diffusion length, which provide promising research and application potential in the following optoelectronic fields: Photovoltaic, light-emitting diode, and photodetector. By improving emission layer morphology and structure quality as well as controlling emission materials’ dimensional size, the external quantum efficiency (EQE) of the sole color light-emitting PeLED has already been over 20%, which is comparable to the performance of the optimal organic light-emitting diode. However, the development of PeLED still faces serious challenges, such as poor temperature, long-term stability, and hard reproducibility of the fabrication. However, metal halide PeLED demonstrates superior stability compared with routine hybrid PeLED owing to the replacement of organic cations like MA⁺ and FA⁺ at the A site of the ABX₃ molecular structure by monovalent metal cations such as Cs⁺, Ag⁺, and Na⁺, which provides a promising substitute to the routine hybrid PeLED. There are few reports regarding PeLED fabrication using physical vapor deposition methods such as thermal evaporation, which can be attributed to its typically higher cost and lower light-emitting efficiency. However, the thermal evaporation procedure has distinct advantages, such as precise control of the element content and layer thickness, which is especially important for achieving a superthin layer. In this study, the fabrication and EL performance of the white PeLED with the structure of ITO/MoO₃/TAPC/TCTA/CsPbBr₃/mCP/TmPyPb/LiF/Al have been investigated, in which the CsPbBr₃/mCP is employed as emission layer. Consequently, all functional layers are fabricated through thermal evaporation, while the CsPbBr₃ layer is formed through the dual-source coevaporation of CsBr and PbBr₂. The passivation effects of the organic small molecule material mCP on the perovskite emitter are observed with considerably reduced trap densities in the emission layer. Meanwhile, as the mCP thickness increases, the trap density decreases. Furthermore, the thickness of the mCP strongly affects the ratio of the different emitting colors in white light and the shifting of the exciton recombination zone. Therefore, the white PeLED can be realized while the thicknesses of CsPbBr₃ and mCP are kept at <sc>20 nm</sc> and within <sc>~10–30 nm,</sc> respectively, in which the 20-nm mCP will yield the highest quality white light with the highest color rending index (CRI) of 89 and optimal CIE coordinates of (0.33, 0.34) due to the balanced emission of blue, green, and red light. The optimal EL performance can be obtained at the mCP thickness of <sc>30 nm,</sc> with the maximum current efficiency, brightness, EQE, and CRI of <sc>0.35 cd/A,</sc> <sc>341 cd/m²,</sc> 0.42%, and 82, respectively. Therefore, the improvement of the EL performance with the increasing mCP layers thickness <sc>(~10–30 nm)</sc> can be demonstrated. The blue-purple and red light at 380 and 620-nm wavelengths are attributed to the mCP, whereas the green light at <sc>520 nm</sc> is attributed to CsPbBr₃. The research demonstrates a practical method for developing white PeLEDs with high-quality white light emission and a simple structure.

Spatial Control of Dynamic p-i-n Junctions in Transition Metal Dichalcogenide Light-Emitting Devices.

Emerging transition metal dichalcogenides (TMDCs) offer an attractive platform for investigating functional light-emitting devices, such as flexible devices, quantum and chiral devices, high-performance optical modulators, and ultralow threshold lasers. In these devices, the key operation is to control the light-emitting position, that is, the spatial position of the recombination zone to generate electroluminescence, which permits precise light guides/passes/confinement to ensure favorable device performance. Although various structures of TMDC light-emitting devices have been demonstrated, including the transistor configuration and heterostructured diodes, it is still difficult to tune the light-emitting position precisely owing to the structural device complexity. In this study, we fabricated two-terminal light-emitting devices with chemically synthesized WSe2, MoSe2, and WS2 monolayers, and performed direct observations of their electroluminescence, from which we discovered a divergence in their light-emitting positions. Subsequently, we propose a method to associate spatial electroluminescence imaging with transport properties among different samples; consequently, a common rule for determining the locations of recombination zones is revealed. Owing to dynamic carrier accumulations and p-i-n junction formations, the light-emitting positions in electrolyte-based devices can be tuned continuously. The proposed method will expand the device applicability for designing functional optoelectronic applications based on TMDCs.

Color-Tunable All-Fluorescent White Organic Light-Emitting Diodes with a High External Quantum Efficiency Over 30% and Extended Device Lifetime.

White organic light-emitting diodes (WOLEDs) with high efficiencies and tunable colors attracts considerable interest from the industry and academia. Thermally activated delayed-fluorescence (TADF) emitters can revolutionize such WOLED devices; however, they still suffer from poor performances. In this study, an advanced double-emissive-layer device architecture capable of hole-trapping TADF-sensitized emissions is proposed to not only achieve a recombination zone shift for the tunable colors but also accelerate exciton emission dynamics for high efficiency and alleviated roll-off. The proof-of-concept WOLEDs exhibit significant shifts in their Commission Internationale de l'Eclairage (CIE) coordinates and correlated color temperatures from (0.40, 0.47) and 4088 K at 100cd m-2 to (0.27, 0.33) and 9269 K at 5000cd m-2 . Additionally, the maximum external quantum efficiency (EQE) reaches 30.7% and remains >25%over a wide luminance range of 500-5000cd m-2 , along with an extended LT80 of over 20000 h at an initial luminance of 100cd m-2 . This is the first time that all-fluorescent WOLEDs have been used to realize an EQE exceeding 30%, thereby establishing a new benchmark in this field.

Combining complementary emissions of hole- and electron-transport layers for ultra-simple white organic light-emitting diodes achieving high device performance

In this work, we proposed a novel and ultra-simple all fluorescent di-chromatic white organic light-emitting diodes (WOLEDs) containing only three non-doped organic layers, which are yellow emitting hole-transport layer, blue emitting electron-transport layer, and charge adjusting layer inserted between hole- and electron-transport layers. White light emission can be realized by simultaneously combining complementary emissions of hole- and electron-transport layers. Based on this structure, a series of all fluorescent di-chromatic WOLEDs were developed, and it was demonstrated the emission spectra could be easily improved by simply changing the thickness of charge adjusting layer. As a consequence, the optimized di-chromatic WOLED achieves a high color rendering index (CRI) of 86, to the best of our knowledge, which is among the highest CRI for such simple structures reported in the literatures. And the resulting device also exhibits high color stability with a slight change of (0.00, −0.01) for CIE coordinates at a wide luminance range from 1011 cd/m2 to 9310 cd/m2. Moreover, efficiency roll-off is rarely observed for target device before luminance reaching 10,000 cd/m2. Such high device performance can be attributed to i) absolutely complementary emissions for hole- and electron-transport layers; ii) a balanced excitons distribution on two transport layers; and iii) a wide carriers recombination zone extending throughout both transport layers. We believe this work will provide a guide to develop simplified but high performance WOLEDs.

Luminance behaviour of PLEDs with integration of nanomaterials

Incorporation of nanomaterials in device structure is the key to enhance performance of polymer light emitting diodes (PLEDs). The major challenges that impede competence of PLEDs, for application in display technology, are (i) non-availability of stable low work function metals to act as cathode, (ii) presence of charge trapping centers in the polymer chains and (iii) total internal reflection of light at ITO/glass and glass/air interfaces. The foremost problem leads to increase in turn ON voltage of the device and reduction in electron injection from cathode. Low injection and high trapping probability of electrons lead to charge imbalance in the emissive layer and shifting of recombination zone towards cathode. This immensely constrains the formation and radiative decay of excitons in the emissive layer and declines the luminosity of the device. In this review, experimental studies on the integration of nanomaterials in PLED structures to enhance device luminance are presented. The diverse impact of their geometric features, ionization potential, electrical conductivity and refractive index on the carrier transport and light extraction in PLEDs is discussed and a perspective on this evolving research path is provided.

Degradation Mechanism of Solution-Processed Organic Light-Emitting Diodes: Sputter Depth-Profile Study

We have studied the origin of degradation in solution-processed organic light-emitting diodes (s-OLEDs) by using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) depth profiling. Successful mechanical exfoliation of top cathode layers from degraded s-OLEDs and Ar gas cluster ion beam sputtering of remaining organic layer stack enabled us to probe chemical and positional changes at the relevant buried interfaces in the degraded device. Our XPS depth profile data showed that device operation induced migration of PEDOT in PEDOT:PSS layer toward the interface between hole transport layer (HTL) and emissive layer (EML), which lowers the hole injection efficiency and shifts the carrier recombination zone toward the interface. TOF-SIMS depth profile data showed that molecular decomposition occurred only in the EML after the device operation. These results depict a scenario in which migration of PEDOT lowers the hole injection efficiency which in turn shifts the recombination zone toward the mixed interface between HTL and EML, where an accumulation of excitons and hole polarons induces strong exciton-polaron quenching that results in molecular break down and device degradation.

High performance blue and white phosphorescent organic light-emitting diodes obtained by sensitizing both light-emitting and electron transport layers

In this work, novel blue and white organic light-emitting diodes (OLEDs) were designed by utilizing iridium(Ⅲ)bis(4-(tert-butyl)-2,6-diuoro-2,3-bipyridinne)acetylacetonate (FK306) as sensitizer in both light-emitting and electron transport layers. Analysis of device configuration demonstrated that FK306 molecules work as electron trappers in view of their low-lying energy levels and triplet energy, which broaden the width of recombination zone, thus enhancing carriers' recombination probability and suppressing the annihilation of excitons. Consequently, blue and white devices with low turn-on voltages were reported with current efficiencies of 71.95 and 72.23 cd A−1, external quantum efficiencies of 32.62% and 31.30%, power efficiencies of 74.85 and 70.80 lm W−1 and brightness of 43171 and 37848 cd m−2, respectively. Within electron transport layer, the optimal doping concentration of FK306 is 2 wt%, which can be easily controlled in device manufacture processes. In addition, it is much more flexible in terms of sensitizer choice for doped electron transport layer compared with light-emitting layer.

High-Throughput Virtual Screening of Host Materials and Rational Device Engineering for Highly Efficient Solution-Processed Organic Light-Emitting Diodes.

The appropriate choice of host and electron-transporting material (ETM) plays a very crucial role in the generation and collection of radiative excitons in the desired recombination zone of organic light-emitting diodes (OLEDs). Due to the sustainable development of material organic chemistry, there is a big library of functional materials that leads to uncountable combinations of device structures, which might achieve a desirable high device performance. However, there is no appropriate methodology available for the fast virtual screening of organic materials and designing a suitable device structure. Here, we have used the electrical software package SETFOS 4.5 for high-throughput virtual screening of host materials and developed a highly efficient multistack OLED device structure. To further enhance the device performance, a co-host approach has been used, and the final device structure has also been optimized with two different ETMs. The best-optimized Ir(ppy)3-based solution-processed green OLED device exhibited a maximum power efficiency (PE) of 83.20 lm/W and brightness of 61,362 cd/m2 with a driving voltage of 2.1 V without using any light extraction outcoupling techniques, which is the best among the OLEDs in its own category. The developed device structure has also been utilized to fabricate highly efficient blue hazard-free low-color temperature OLEDs for a physiologically friendly light at night. The resultant 2083 K OLED device displayed a maximum PE of 51.4 lm/W and luminance of 44,548 cd/m2 with a turn-on voltage of 2.1 V that is also 42 and 104 times safer in terms of retinal protection and ∼4 and ∼11 times safer in terms of melatonin generation when compared with those of a real candle and incandescent bulb, respectively. The observed excellent device performance may be attributed to the balanced charge carrier in the recombination zone, broader emissive layer due to a mixed-host system, less accumulation of charges at the injecting surfaces, well-aligned triplet energy and molecular orbital energy level of the host and guest, and high electron mobility and enhanced hole blocking ability of the employed ETM in the designed OLED device structure.

Highly efficient blue phosphorescent organic light-emitting diodes with low operation voltage

In this work, we report an efficient blue organic electroluminescent device based on phosphorescent emitter iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6), which displayed excellent thermal stability. To balance carriers’ distribution and reduce operation voltage, double light-emitting layers device structure was designed by selecting 4,4′,4′-tris(N-carbazolyl)triphenylamine (TcTa) and 2,6-bis(3-(9Hcarbazol-9-yl)phenyl)phyridine (26DCzPPy) as host materials because of their stepwise energy levels. By optimizing the doping concentration of FIr6 to be 26 wt%, well balanced carriers’ distribution on emitter molecules and broadening recombination zone were realized. Finally, high performance blue device with slow efficiency roll-off and high current efficiency up to 56.83 cd/A (obtained at 700 cd/m2) was realized. At the certain brightness of 1000 (3.9 V) and 5000 cd/m2 (4.8 V), the same device retained efficiencies of 56.33 and 51.45 cd/A, respectively.

Spectrum-stable tetra-chromatic white organic light-emitting diodes with red emitter outside the exciton recombination zone

Achieving the stable spectrum in complementary-color white organic light-emitting diodes (WOLEDs) with multiple emitters is a challenge. Here, on the basis of ultrathin emitters and spacers, we were successful in achieving a four-chromatic (Blue-Green-Red-Orange) WOLED with superior spectral stability. By placing the red emitter outside the exciton recombination zone, but within the energy transfer radius, the carrier trapping effect from the red dye which is extremely detrimental to spectral stability, is minimized. By adjusting the orange emitter and the unipolar spacer layer adjacent to the red emitter, the red emission from the energy transfer mechanism which is insensitive to voltage is further increased, resulting in further improvement of the spectral stability. When the brightness changed in the range of 1000 cdm-2 to 10000 cdm-2, the color coordinate change of the device with the most stable spectrum is only (0.004,0.006). Simultaneously, the color rendering index (CRI) of this device is up to 86.

Realizing high-performance color-tunable WOLED by adjusting the recombination zone and energy distribution in the emitting layer

Color-tunable white organic light-emitting diodes (CT-WOLEDs) having daylight chromaticity and a wide correlated color temperature (CCT) span can mimic our circadian cycle and realize application for lighting or decoration. The effects of the recombination zone and energy distribution on the electro-optical properties and color span are investigated in this paper. We find that it is beneficial to expand the CCT span by increasing the distance between the red ultrathin phosphorescent emissive layer and the center of the recombination zone. By increasing the concentration (y) of mCP in mixed host material in the emitting layer and the thickness (z nm) of the red ultrathin phosphorescent emissive layer, the CCT spans can be expanded towards high CCT and low CCT, respectively. The widest CCT span reaches 4032 K (2391–6423 K) in a simple all-phosphorescent CT-WOLED with a maximum luminance and power efficiency of 9249 cd m−2 and 15.35 lm W−1, respectively.

Effect of Carrier-Transporting Layer on Blue Phosphorescent Organic Light-Emitting Diodes

This study presented the effects of carrier-transporting layer (CTL) on electroluminescence (EL) performance of a blue phosphorescent organic light-emitting diodes (PHOLEDs) with electron transporting host based on three kinds of electron-transporting layers (ETLs) including 3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), diphenyl-bis[4-(pyridin-3-yl)phenyl]silane (DPPS) and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) and two kinds of hole-transporting layers (HTLs) such as 4,4′-bis[N-1-naphthyl-N-phenyl-amino]biphenyl (NPB), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC). The carrier recombination and exciton formation zones in blue PHOLEDs strongly depend on the carrier mobility of CTLs and the layer thickness, especially the carrier mobility. Between ETLs and HTLs, the high electron mobility of ETL results in a lower driving voltage in blue PHOLEDs than the high hole mobility of HTL did. In addition, layer thickness modulation is an effective approach to precisely control carriers and restrict carriers within the EML and avoid a leakage emission of CTL. For CTL pairs in OLEDs using the electron transporting host system, ETLs with low mobility and also HTLs with high hole mobility are key points to confine the charge in EML for efficient photon emission. These findings show that appropriate CTL pairs and good layer thickness are essential for efficient OLEDs.

Optical Outcoupling Efficiency of Organic Light‐Emitting Diodes with a Broad Recombination Profile

AbstractAlthough a broadened recombination zone is beneficial for the lifetime of organic light‐emitting diodes (OLEDs), its effect on the optical outcoupling efficiency is unknown. Here, a numerical model to simulate the fraction of photons coupled to air for devices with a broad recombination zone is presented. It is demonstrated that the total outcoupling can be calculated as the integral of the outcoupling efficiency over different positions of emitting dipoles, weighted by the sum‐normalized recombination profile, as obtained from electrical modeling. Using single‐layer OLEDs based on the thermally delayed activated fluorescence emitter 9,10‐bis(4‐(9H‐carbazol‐9‐yl)‐2,6‐dimethylphenyl)‐9,10‐diboraanthracene (CzDBA) as a model system, it is demonstrated that an outcoupling efficiency to the air mode as high as ≈26% can be obtained. These simulations are validated with experiments on CzDBA OLEDs of different thickness. The results show that single‐layer devices with a broadened emission zone can achieve similar outcoupling efficiency to multilayer OLEDs with an optimized confined emission zone, establishing a route to efficient, stable, and simplified OLEDs.

Simple-Structured OLEDs Incorporating Undoped Phosphorescent Emitters Within Non-Exciplex Forming Interfaces: Towards Ultraslow Efficiency Roll-Off and Low Driving Voltage for Indoor R/G/B Illumination.

To meet the requirement of indoor R/G/B monochrome illumination a simplified OLEDs structure and fabrication process must occur. Herein, a design philosophy of low efficiency roll-off and simple-structure OLEDs incorporating R/G/B phosphorescent ultrathin non-doped emissive layers (EMLs) within non-exciplex forming interfaces a luminescent system by a direct charge trapping mechanism has been reported, which uses bis(2-methyldibenzo[f,h]-quinoxaline)(acetylacetonate)iridium(III) (MDQ)2Ir(acac), bis(3-phenylpyridin-e)iridium(III) (Ir(ppy)3), and bis(3,5-difluoro-2 -(2-pyridyl)phenyl-(2-carboxypyridyl) iridiumII) (Firpic) as R/G/B luminescent dyes, respectively. Although the recombination zone is narrow in the designed OLEDs, the efficiency roll-off of the designed OLEDs are unexpectedly slow, due to stable charge trapping of the emitters and are refrained from concentration quenching in relatively low current density, but the luminance meets the requirement of indoor lighting. With a low threshold voltage of 2.9/2.9/3.5 V, the designed R/G/B phosphorescent OLEDs show an efficiency roll-off as low as 7.6/3.2/4.3% for indoor luminance from 10 cd/m2 to 1,000 cd/m2, respectively. The perspective of R/G/B luminescent dyes on luminous efficiency, chromaticity coordinate drifts, efficiency roll-off, and direct charge trapping has been thoroughly studied. Therefore, our research may help to further develop ideal indoor lighting using a simplified undoped R/G/B OLEDs structure with simultaneous ultraslow efficiency roll-off, low threshold voltage, simplified fabrication process, low reagent consumption, and cost.