Tandem Organic Light-emitting Diodes Research Articles

Achieving efficient and stable deep-blue tandem organic light-emitting diodes with high color purity via novel charge generator layer

Achieving efficient and stable deep-blue tandem organic light-emitting diodes with high color purity via novel charge generator layer

Enhanced Emitting Dipole Orientation Based on Asymmetric Iridium(III) Complexes for Efficient Saturated-Blue Phosphorescent OLEDs.

Three novel asymmetric Ir(III) complexes have been rationally designed to optimize their emitting dipole orientations (EDO) and enhance light outcoupling in blue phosphorescent organic light-emitting diodes (OLEDs), thereby boosting their external quantum efficiency (EQE). Bulky electron-donating groups (EDGs), namely: carbazole (Cz), di-tert-butyl carbazole (tBuCz), and phenoxazine (Pxz) are incorporated into the tridentate dicarbene pincer chelate to induce high degree of packing anisotropy, simultaneously enhancing their photophysical properties. Angle-dependent photoluminescence (ADPL) measurements indicate increased horizontal transition dipole ratios of 0.89 and 0.90 for the Ir(III) complexes Cz-dfppy-CN and tBuCz-dfppy-CN, respectively. Analysis of the single crystal structure and density functional theory (DFT) calculation results revealed an inherent correlation between molecular aspect ratio and EDO. Utilizing the newly obtained emitters, the blue OLED devices demonstrated exceptional performance, achieving a maximum EQE of 30.7% at a Commission International de l'Eclairage (CIE) coordinate of (0.140, 0.148). Optical transfer matrix-based simulations confirmed a maximum outcoupling efficiency of 35% due to improved EDO. Finally, the tandem OLED and hyper-OLED devices exhibited a maximumEQE of 44.2% and 31.6%, respectively, together with good device stability. This rational molecular design provides straightforward guidelines to reach highly efficient and stable saturated blue emission.

Charge generation layer with octylamine assistant layer for efficient tandem organic light-emitting diodes

Charge generation layer with octylamine assistant layer for efficient tandem organic light-emitting diodes

High-efficiency tandem red organic light-emitting diodes adopting exciplex hosts

Abstract Deep-red organic light-emitting diodes (OLEDs) exhibit significant potential for applications in infrared medical treatment and infrared imaging. However, the OLED performance drops significantly with increasing emission wavelength due to the energy gap law. To overcome this challenge, an efficient interfacial exciplex host, NPB/TPBi, has been developed. Simulation results indicate that the interfacial exciplex host demonstrates superior performance, compared with the bulk exciplex host and single host. To further boost the performance of red OLEDs, the interfacial exciplex host was incorporated into tandem devices, achieving an external quantum efficiency of up to 27.3%. This work offers important insights for the fabrication of efficient red OLEDs.

Purely Organic Room‐Temperature Phosphorescence Molecule for High‐Performance Non‐Doped Organic Light‐Emitting Diodes

AbstractPurely organic molecules with room‐temperature phosphorescence (RTP) are potential luminescent materials with high exciton utilization for organic light‐emitting diodes (OLEDs), but those exhibiting superb electroluminescence (EL) performances are rarely explored, mainly due to their long phosphorescence lifetimes. Herein, a robust purely organic RTP molecule, 3,6‐bis(5‐phenylindolo[3,2‐a]carbazol‐12(5H)‐yl)‐xanthen‐9‐one (3,2‐PIC‐XT), is developed. The neat film of 3,2‐PIC‐XT shows strong green RTP with a very short lifetime (2.9 μs) and a high photoluminescence quantum yield (72 %), and behaviors balanced bipolar charge transport. The RTP nature of 3,2‐PIC‐XT is validated by steady‐state and transient absorption and emission spectroscopies, and the working mechanism is deciphered by theoretical simulation. Non‐doped multilayer OLEDs using thin neat films of 3,2‐PIC‐XT furnish an outstanding external quantum efficiency (EQE) of 24.91 % with an extremely low roll‐off (1.6 %) at 1000 cd m−2. High‐performance non‐doped top‐emitting and tandem OLEDs are also achieved, providing remarkable EQEs of 24.53 % and 42.50 %, respectively. Delightfully, non‐doped simplified OLEDs employing thick neat films of 3,2‐PIC‐XT are also realized, furnishing an excellent EQE of 17.79 % and greatly enhanced operational lifetime. The temperature‐dependent and transient EL spectroscopies demonstrate the electrophosphorescence attribute of 3,2‐PIC‐XT. These non‐doped OLEDs are the best devices based on purely organic RTP materials reported so far.

Purely Organic Room-Temperature Phosphorescence Molecule for High-Performance Non-Doped Organic Light-Emitting Diodes.

Purely organic molecules with room-temperature phosphorescence (RTP) are potential luminescent materials with high exciton utilization for organic light-emitting diodes (OLEDs), but those exhibiting superb electroluminescence (EL) performances are rarely explored, mainly due to their long phosphorescence lifetimes. Herein, a robust purely organic RTP molecule, 3,6-bis(5-phenylindolo[3,2-a]carbazol-12(5H)-yl)-xanthen-9-one (3,2-PIC-XT), is developed. The neat film of 3,2-PIC-XT shows strong green RTP with a very short lifetime (2.9 μs) and a high photoluminescence quantum yield (72 %), and behaviors balanced bipolar charge transport. The RTP nature of 3,2-PIC-XT is validated by steady-state and transient absorption and emission spectroscopies, and the working mechanism is deciphered by theoretical simulation. Non-doped multilayer OLEDs using thin neat films of 3,2-PIC-XT furnish an outstanding external quantum efficiency (EQE) of 24.91 % with an extremely low roll-off (1.6 %) at 1000 cd m-2. High-performance non-doped top-emitting and tandem OLEDs are also achieved, providing remarkable EQEs of 24.53 % and 42.50 %, respectively. Delightfully, non-doped simplified OLEDs employing thick neat films of 3,2-PIC-XT are also realized, furnishing an excellent EQE of 17.79 % and greatly enhanced operational lifetime. The temperature-dependent and transient EL spectroscopies demonstrate the electrophosphorescence attribute of 3,2-PIC-XT. These non-doped OLEDs are the best devices based on purely organic RTP materials reported so far.

Deterioration of Li-doped phenanthroline-based charge generation layer for tandem organic light-emitting diodes

Understanding the deterioration of the charge generation layer (CGL) is an uprising issue for efficient and stable tandem organic light-emitting diodes. Here, we comprehensively investigated the change in the electrical characteristics of Li-doped CGLs according to different stress conditions and compared the stability of Li diffusion in two phenanthroline-based electron transport materials. Through current density–voltage and capacitance-frequency measurements, it was revealed that electrical stress and thermal stress affect CGL’s electrical properties differently. Moreover, the exponential trap distribution model analysis informed that the Li diffusion is expedited toward narrowing depletion width in the CGL. Diffusion of Li and thermal effect to the diffusion was experimentally observed by X-ray photoelectron spectroscopy (XPS) depth profiling. We also evaluated variations in the operating voltage of tandem devices that differed only in nCGL. Overall, Li diffusion occurs more favorably in sparse Bphen film with weak binding energy rather than closely packed BPPB film with strong binding energy, which highlights that atomic geometry and morphological characteristics are crucial for the stability of tandem devices.

Exploring charge generation and separation in tandem organic light-emitting diodes based on magneto-electroluminescence

5,6,11,12-tetraphenylnaphthacene (rubrene) exhibits resonant energy properties (E S1,rub ≈ 2E T1,rub), resulting in rubrene-based organic light-emitting diode (OLED) devices that undergo the singlet fission (STT) process at room temperature. This unique process gives rise to a distinct magneto-electroluminescence (MEL) profile, differing significantly from the typical intersystem crossing (ISC) process. Therefore, in this paper, we investigate charge generation and separation in the interconnector, and the mechanism of charge transport in tandem OLEDs at room temperature using MEL tools. We fabricate tandem OLEDs comprising green (Alq3) and yellow (Alq3:rubrene) electroluminescence (EL) units using different interconnectors. The results demonstrate that all devices exhibited significant rubrene emission. However, the MEL did not exhibit an STT process with an increasing magnetic field, but rather a triplet–triplet annihilation (TTA) process. This occurrence is attributed to direct carrier trapping within doped EL units, which hinders the transport of rubrene trapped charges, consequently prolonging the lifetime of triplet excitons (T1,rub). Thus, the increased T1,rub concentration causes TTA to occur at room temperature, causing the rapid decrease of MEL in all devices under high magnetic fields. In devices where only the TTA process occurs, the TTA increases with the increasing current. Consequently, the high magnetic field of devices A–C is only related to TTA. Notably, there exists a high magnetic field TTA of device D in the Alq3/1,4,5,8,9,11-Hexaazatriphenylene-hexacarbonitrile interconnector regardless of the current. This occurs because both EL units in the device emit simultaneously, resulting in the triplet-charge annihilation process of Alq3 in the high magnetic field of the MEL. Moreover, the rapid increase in MEL at low magnetic field across all devices is attributed to the ISC between Alq3 polaron pairs. This entire process involves Förster and Dexter energy transfer. This article not only provides novel insights into charge generation and separation in the interconnector but also enhances our understanding of the microscopic mechanisms in tandem OLED devices.

Charge generation layer with Yb assistant interlayer for tandem organic light-emitting diodes

Tandem organic light-emitting diode (OLED) devices require an efficient charge generation layer (CGL) between two stacked OLED units. In this study, a CGL with an Yb assistant interlayer was fabricated and investigated. The optical transmittance and charge generation performances of the CGLs were analyzed with respect to the Yb thickness. The best result was obtained at a Yb thickness of 3 nm, at which both the electrical and optical properties exhibited optimal performances. The optical transmittance was greater than 90 %; this was superior to the performances using Ag and Al interlayers. The insertion of a 3-nm Yb layer reduced the threshold voltage from 18 to 6 V. Capacitance measurements revealed that the insertion of the Yb layer enhanced charge generation in the CGLs, reducing the threshold voltage and enhancing charge transport. The tandem OLED device fabricated using Bphen:Liq/Yb/MoO3/TAPC as a CGL exhibited a 1.92 × enhancement in the current efficiency, which is nearly twice the ideal value, compared to a single-unit device. Thus, Yb is a promising candidate for use as an assistant interlayer in CGLs for high-performance tandem OLED devices.

Highly efficient tandem organic light-emitting diodes using p-type metal halide copper iodide (CuI)

Efficient charge generation is essential for tandem organic light-emitting diode (OLED) devices. In this study, we introduce a p-type copper iodide (CuI) layer within the charge generation layer (CGL) that exhibits high charge generation efficiency in tandem OLED devices. Briefly, CuI is deposited as a thin layer via thermal evaporation technique. X-ray diffraction (XRD) analysis reveals the formation of γ-CuI, and the ultraviolet-visible (UV-Vis) spectral analysis demonstrates outstanding optical transparency (∼95%) of the deposited CuI films in the visible range. The current-voltage (I-V) and capacitance-voltage (C-V) analyses show that the p-type CuI layer is a vital and excellent charge generator and enhances the current density significantly. The tandem OLED device fabricated using Bphen:Liq/CuI/HAT-CN as the CGL results in a substantial 1.9-fold increase in current efficiency compared to the single-unit OLED device, approaching the theoretical two-fold increase. These findings highlight the potential of CuI as a promising p-type material for CGL design in tandem OLED devices.

Largely improving the lifetime by inserting a Yb metal layer into the charge generation layer in tandem organic light-emitting diodes

Largely improving the lifetime by inserting a Yb metal layer into the charge generation layer in tandem organic light-emitting diodes

Blue Electrophosphorescence from Iridium(III) Phosphors Bearing Asymmetric Di-N-aryl 6-(trifluoromethyl)-2H-imidazo[4,5-b]pyridin-2-ylidene Chelates.

Efficient blue phosphors remain a formidable challenge for organic light-emitting diodes (OLEDs). To circumvent this obstacle, a series of Ir(III)-based carbene complexes bearing asymmetric di-N-aryl 6-(trifluoromethyl)-2H-imidazo[4,5-b]pyridin-2-ylidene chelates, namely, f-ct6a‒c, are synthesized, and their structures and photophysical properties are comprehensively investigated. Moreover, these emitters can undergo interconversion in refluxing 1,2,4-trichlorobenzene, catalyzed by a mixture of sodium acetate (NaOAc) and p-toluenesulfonic acid monohydrate (TsOH·H2O) without decomposition. All Ir(III) complexes present good photoluminescence quantum yield (ΦPL = 83-88%) with peak maximum (max.) at 443-452nm and narrowed full width at half maximum (FWHM = 66-73nm). Among all the fabricated OLED devices, f-ct6b delivers a max. external quantum efficiency (EQE) of 23.4% and Commission Internationale de L'Eclairage CIEx , y coordinates of (0.14, 0.12), whereas the hyper-OLED device based on f-ct6a and 5H,9H,11H,15H-[1,4] benzazaborino [2,3,4-kl][1,4]benzazaborino[4',3',2':4,5][1,4]benzazaborino[3,2-b]phenazaborine-7,13-diamine, N7,N7,N13,N13,5,9,11,15-octaphenyl (ν-DABNA) exhibits max. EQE of 26.2% and CIEx , y of (0.12, 0.13). Finally, the corresponding tandem OLED with f-ct6b as dopant gives a max. luminance of over 10000 cd m-2 and max. EQE of 42.1%, confirming their candidacies for making true-blue OLEDs.

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.

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.

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.

Interfacial charge transfer study of hexacarbonitrile-based intermediate connecter in blue tandem organic light-emitting diodes

The charge generation process of hexaazatriphenylene hexacarbonitrile-based intermediate connectors (ICs) in blue tandem organic light-emitting diodes (OLEDs) is investigated via photoelectron spectroscopic and device studies. Though 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) is well-known to be a p-type hole-injection layer, our photoelectron spectroscopic results indicate that HATCN becomes the n-type when it is sandwiched between Li-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) and N,N″-diphenyl-N,N″-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) layers, and the charge generation and separation at the Li:BPhen/HATCN/NPB interfaces become energetically favorable, as depicted by their energy level alignments. This explains the superior device performance when Li:BPhen/HATCN/NPB is used as a connecting unit in tandem devices.

Lateral current suppression in tandem organic light-emitting diodes by adopting a buffer layer

For the tandem organic light emitting diodes (TOLEDs) with the charge generation unit (CGU) of LiF/Al/MoO 3 , there is a significant current lateral spreading causing light emission over an extremely large area outside the OLEDs pixel, due to the conductive interfacial layer caused by the oxidation-reduction reaction between Al and MoO 3 layers. To crack this nut, a buffer layer of 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) is inserted between Al and MoO 3 layers. The result shows the HAT-CN buffer layer eliminates the spread light emission in the TOLEDs. What's more, the device characteristics show the new CGU of LiF/Al/HAT-CN/MoO 3 has stronger charge generation and injection capabilities. The white TOLEDs with the new CGU exhibit a high external quantum efficiency (EQE) of 28.4%. Compared with the single OLEDs and the TOLEDs with the CGU of LiF/Al/MoO 3 , the efficiency is increased by 143% and 44%, respectively. • The efficient charge generation unit (CGU) of LiF/Al/HAT-CN/MoO 3 was developed. • The current lateral spreading was completely suppressed by adopting the CGU of LiF/Al/HAT-CN/MoO 3 . • The white tandem OLEDs exhibited an external quantum efficiency of 28.4%.

Corrigendum to “blue-hazard free candlelight-style tandem organic light-emitting diode”

Long exposure to intensive blue light may damage retina and suppress, at night, melatonin secretion. To devise a blue hazard-free light source with a long lifetime, we demonstrated here a tandem organic light-emitting diode (OLED) with candlelight-style emission, especially via the use of all commercially-available materials. The resultant device shows a color temperature of 1,500 K, significantly lower than the 1,800 K of oil lamps, 1,900 K of candles and 2,500 K of incandescent bulbs. It permits, at 100 lx, a retinal exposure-limit of 12 h, while merely 5 min for the 6,000 K cold-white LED or CFL. Notably, it is 15 times more human eye-friendly and 30% more melatonin generation-friendly than the candle. It shows a lifetime of 40,000 h at 1,000 candelas per meter square, which is about 20,000 times that of a candle and 40 times that of an incandescent bulb. It is 65 times more energy-saving as comparing with candles. Commercially viable human-friendly lighting can be expected in the near future to safeguard human health. • We demonstrated a blue-hazard free tandem OLED with candlelight-style emission via all commercially-available materials. • The device shows a color temperature of 1500 K. • The device shows a lifetime of 40,000 h at 1000 nits and 65 times more energy-saving as comparing with candles. • It permits, at 100 lx, a retinal exposure-limit of 12 h and 30% more melatonin generation-friendly than the candle. • Commercially viable human-friendly lighting can be expected in the near future to safeguard human health.

Blue-hazard free candlelight-style tandem organic light-emitting diode

Long exposure to intensive blue light may damage retina and suppress, at night, melatonin secretion. To devise a blue hazard-free light source with a long lifetime, we demonstrated here a tandem organic light-emitting diode (OLED) with candlelight-style emission, especially via the use of all commercially-available materials. The resultant device shows a color temperature of 1500 K, significantly lower than the 1800 K of oil lamps, 1900 K of candles and 2500 K of incandescent bulbs. It permits, at 100 lx, a retinal exposure-limit of 12 h, while merely 5 min for the 6000 K cold-white LED or CFL. Notably, it is 15 times more human eye-friendly and 30 % more melatonin generation-friendly than the candle. It shows a lifetime of 40,000 h at 1000 cd per meter square, which is about 20,000 times that of a candle and 40 times that of an incandescent bulb. It is 65 times more energy-saving as comparing with candles. Commercially viable human-friendly lighting can be expected in the near future to safeguard human health. • We demonstrated a blue-hazard free tandem OLED with candlelight-style emission via all commercially-available materials. • The device shows a color temperature of 1500 K. • The device shows a lifetime of 40,000 h at 1000 nits and 65 times more energy-saving as comparing with candles. • It permits, at 100 lx, a retinal exposure-limit of 12 h and 30 % more melatonin generation-friendly than the candle. • Commercially viable human-friendly lighting can be expected in the near future to safeguard human health.