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11 pages, 4086 KiB

Open AccessArticle

Lifetime Comparisons of Organic Light-Emitting Diodes Fabricated by Solution and Evaporation Processes

by Kuo-Chun Huang, Jeng-Yue Chen, Yen-Hua Lin, Fuh-Shyang Juang and Yu-Sheng Tsai

Micromachines 2023, 14(2), 278; https://doi.org/10.3390/mi14020278 - 21 Jan 2023

Viewed by 1273

In this paper, a blue fluorescent organic light-emitting diode (OLED) with a 1 cm² emitting area was fabricated by a solution process. The ITO/spin MADN:13% UBD-07/TPBi/Al was used as the basic structure in which to add a hole-injection layer PEDOT:PSS and an [...] Read more.

In this paper, a blue fluorescent organic light-emitting diode (OLED) with a 1 cm² emitting area was fabricated by a solution process. The ITO/spin MADN:13% UBD-07/TPBi/Al was used as the basic structure in which to add a hole-injection layer PEDOT:PSS and an electron-injection layer LiF, respectively. The device structure was optimized to obtain a longer lifetime. Firstly, the TPBi, which is an electron transport layer and a hole-blocking layer, was added to the structure to increase the electron transport rate. When the TPBi thickness was increased to 20 nm, the luminance was 221 cd/m², and the efficiency was 0.52 cd/A at a voltage of 8 V. Since the addition of the hole-injection layer (HIL) increased the hole current but did not increase the electron current, the electron transport layer (ETL) Alq₃ with the lowest unoccupied molecular orbital (LUMO) was added as stepped ETL to help the TPBi transport more electron current into the emitting layer. When the thickness of the TPBi/Alq₃ was 10 nm/15 nm, the luminance reached 862 cd/m², the efficiency was 1.29 cd/A, and the lifetime increased to 252 min. Subsequently, a hole-injection layer PEDOT:PSS with a thickness of 55 nm was added to make the ITO surface flatter and to reduce the probability of a short circuit caused by the spike effect. At this time, the luminance of 311 cd/m², the efficiency of 0.64 cd/A, and the lifetime of 121 min were obtained. Following this, the thickness of the emitting layer was doubled to increase the recombination probability of the electrons and the holes. When the thickness of the emitting layer was 90 nm, and the thermal evaporation method was used, the efficiency was 3.23 cd/A at a voltage of 8V, and the lifetime was improved to 482 min. Furthermore, when the thickness of the hole-injection layer PEDOT:PSS was increased to 220 nm, the efficiency increased to 3.86 cd/A, and the lifetime was increased to 529 min. An infrared thermal image camera was employed to detect the temperature variation of the blue OLEDs. After the current was gradually increased, it was found that the heat accumulation of the device became more and more significant. When the driving current reached 50 mA, the device burnt out. It was found that the maximum temperature that the OLED device could withstand was approximately 58.83 degrees C at a current of 36.36 mA. Full article

(This article belongs to the Special Issue Organic Light Emitting Diodes (OLEDs))

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13 pages, 5125 KiB

Open AccessArticle

Effects of Thermal Treatment on DC Voltage-Driven Color Conversion in Organic Light-Emitting Diode

by Tae Jun Ahn, Bum Ho Choi, Jae-Woong Yu, Young Baek Kim and Yun Seop Yu

Micromachines 2023, 14(1), 30; https://doi.org/10.3390/mi14010030 - 23 Dec 2022

Cited by 1 | Viewed by 1367

Abstract

A DC voltage-dependent color-tunable organic light-emitting diode (CTOLED) was proposed for lighting applications. The CTOLED consists of six consecutive organic layers: the hole injection layer, the hole transport layer (HTL), two emission layers (EMLs), a hole blocking layer (HBL), and an electron transport [...] Read more.

A DC voltage-dependent color-tunable organic light-emitting diode (CTOLED) was proposed for lighting applications. The CTOLED consists of six consecutive organic layers: the hole injection layer, the hole transport layer (HTL), two emission layers (EMLs), a hole blocking layer (HBL), and an electron transport layer (ETL). Only one metal-free phthalocyanine (H₂Pc) layer with a thickness of 5 nm was employed as the EML in the CTOLED on a green organic light-emitting diode (OLED) structure using tris (8-hydroxyquinoline) aluminum (III) (Alq₃). The current density-voltage-luminance characteristics of the CTOLEDs before and after thermal treatment were characterized and analyzed. Several Gaussian peaks were also extracted by multipeak fitting analysis of the electroluminescent spectra. In the CTOLED before thermal treatment, green emission was dominant in the entire voltage range from low to high voltages, and blue and infrared were emitted simultaneously and at relatively low intensities at low and high voltages, respectively. In the CTOLED after thermal treatment, the dominant color conversion from blue to green was observed as the applied voltage increased, and the infrared emission was relatively low over the entire voltage range. By simulating the CTOLED with and without traps at the H₂Pc interface using a technology computer-aided design simulator, we observed the following: 1. After thermal treatment, the CTOLED emitted blue light by exciton generation at the H₂Pc–HBL interface because of the small electron transport through the H₂Pc thin film due to the dramatic reduction of traps in the low-voltage regime. 2. In the high-voltage regime, electrons reaching the HBL were transferred to Alq₃ by resonant tunneling in two quantum wells; thus, green light was emitted by exciton generation at the HTL–Alq₃ interface. Full article

(This article belongs to the Special Issue Organic Light Emitting Diodes (OLEDs))

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9 pages, 2427 KiB

Open AccessArticle

Optoelectronic Properties of MAPbBr₃ Perovskite Light-Emitting Diodes Using Anti-Solvent and PEDOT:PSS/PVK Double-Layer Hole Transport Layers

by Kai Zhang, Shisong Yu, Peng Tu, Xiangcheng Cai, Yuanming Zhou and Fei Mei

Micromachines 2022, 13(12), 2122; https://doi.org/10.3390/mi13122122 - 30 Nov 2022

Cited by 1 | Viewed by 1305

Abstract

Perovskite light-emitting diodes (PeLEDs) have attracted extensive attention due to their advantages such as low-temperature solution processing, high photoluminescence quantum efficiency, high color purity, tunable wavelength, and excellent carrier mobility. The hole transport layer plays an important role in the device’s performance. In [...] Read more.

Perovskite light-emitting diodes (PeLEDs) have attracted extensive attention due to their advantages such as low-temperature solution processing, high photoluminescence quantum efficiency, high color purity, tunable wavelength, and excellent carrier mobility. The hole transport layer plays an important role in the device’s performance. In this paper, the effect of anti-solvent (ethyl acetate) on the performance of PeLEDs was studied in order to determine the optimal anti-solvent condition. The effect of PEDOT:PSS/PVK double-layer hole transport layers on the optoelectronic properties of MAPbBr₃ PeLEDs was investigated. The device with 8 mg/mL PVK produced the best results, with a maximum luminance of 5139 cd/m² and a maximum current efficiency of 2.77 cd/A. Compared with the control device with PEDOT:PSS HTL, the maximum luminance of the device with 8 mg/mL PVK is increased by 2.02 times, and the maximum current efficiency is increased by 188%. The experimental results show that the addition of PVK helps to reduce the size of perovskite particles, contributing to the spatial confinement of excitons, and suppress the quenching of luminescence occurring at the interface between PEDOT:PSS and MAPbBr₃, thereby enhancing the optoelectronic performance of PeLEDs. The results of this paper can provide a basis for the improvement and industrialization of PeLEDs. Full article

(This article belongs to the Special Issue Organic Light Emitting Diodes (OLEDs))

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6 pages, 2038 KiB

Open AccessArticle

Ultra-Smooth and Efficient NiO/Ag/NiO Transparent Electrodes for Flexible Organic Light-Emitting Devices

by Yu Bai, Yahui Chuai, Yingzhi Wang and Yang Wang

Micromachines 2022, 13(9), 1511; https://doi.org/10.3390/mi13091511 - 12 Sep 2022

Viewed by 1111

Abstract

We demonstrate highly flexible and efficient organic light-emitting devices (OLEDs) using an ultra-smooth NAN anode. A template-stripping process was employed to create an ultra-smooth NAN anode on a photopolymer substrate. The flexible OLEDs obtained by this method maintained good electroluminescent properties and mechanical [...] Read more.

We demonstrate highly flexible and efficient organic light-emitting devices (OLEDs) using an ultra-smooth NAN anode. A template-stripping process was employed to create an ultra-smooth NAN anode on a photopolymer substrate. The flexible OLEDs obtained by this method maintained good electroluminescent properties and mechanical stability after bending. The efficiency of the flexible OLEDs was improved by 30.6% compared with conventional OLEDs deposited on PET/ITO substrate due to the enhanced hole injection from the ultra-smooth anode. Full article

(This article belongs to the Special Issue Organic Light Emitting Diodes (OLEDs))

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9 pages, 3338 KiB

Open AccessArticle

Realization of Efficient Phosphorescent Organic Light-Emitting Devices Using Exciplex-Type Co-Host

by Lishuang Wu, Huiwen Xu and Huishan Yang

Micromachines 2022, 13(1), 51; https://doi.org/10.3390/mi13010051 - 29 Dec 2021

Viewed by 1311

Abstract

High-performance phosphorescent organic light-emitting devices with an exciplex-type co-host were fabricated. The co-host is constituted by 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene, and 4,4,4-tris (N-carbazolyl) triphenylamine, and has obvious virtues in constructing efficient devices because of the thermally activated delayed fluorescence (TADF) resulting from a reverse intersystem [...] Read more.

High-performance phosphorescent organic light-emitting devices with an exciplex-type co-host were fabricated. The co-host is constituted by 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene, and 4,4,4-tris (N-carbazolyl) triphenylamine, and has obvious virtues in constructing efficient devices because of the thermally activated delayed fluorescence (TADF) resulting from a reverse intersystem crossing (RISC) process. The highest external quantum efficiency and luminance are 14.60% and 100,900 cd/m² for the optimal co-host device. For comparison, 9.22% and 25,450 cd/m² are obtained for a device employing 4,4,4-tris (N-carbazolyl) triphenylamine as a single-host. Moreover, the efficiency roll-off is notably alleviated for the co-host device, indicated by much higher critical current density of 327.8 mA/cm², compared to 120.8 mA/cm² for the single-host device. The alleviation of excitons quenching resulting from the captured holes and electrons, together with highly sufficient energy transfer between the co-host and phosphorescent dopant account for the obvious boost in device performances. Full article

(This article belongs to the Special Issue Organic Light Emitting Diodes (OLEDs))

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10 pages, 970 KiB

Open AccessCommunication

Fabrication of a Solution-Processed White Light Emitting Diode Containing a Single Dimeric Copper(I) Emitter Featuring Combined TADF and Phosphorescence

by Gang Cheng, Dongling Zhou, Uwe Monkowius and Hartmut Yersin

Micromachines 2021, 12(12), 1500; https://doi.org/10.3390/mi12121500 - 30 Nov 2021

Cited by 10 | Viewed by 2349

Abstract

Luminescent copper(I) complexes showing thermally activated delayed fluorescence (TADF) have developed to attractive emitter materials for organic light emitting diodes (OLEDs). Here, we study the brightly luminescent dimer Cu₂Cl₂(P∩N)₂ (P∩N = diphenylphosphanyl-6-methyl-pyridine), which shows both TADF and phosphorescence [...] Read more.

Luminescent copper(I) complexes showing thermally activated delayed fluorescence (TADF) have developed to attractive emitter materials for organic light emitting diodes (OLEDs). Here, we study the brightly luminescent dimer Cu₂Cl₂(P∩N)₂ (P∩N = diphenylphosphanyl-6-methyl-pyridine), which shows both TADF and phosphorescence at ambient temperature. A solution-processed OLED with a device structure ITO/PEDOT:PSS/PYD2: Cu₂Cl₂(P∩N)₂/DPEPO (10 nm)/TPBi (40 nm)/LiF (1.2 nm)/Al (100 nm) shows warm white emission with moderate external quantum efficiency (EQE). Methods for EQE increase strategies are discussed. Full article

(This article belongs to the Special Issue Organic Light Emitting Diodes (OLEDs))

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12 pages, 3422 KiB

Open AccessArticle

Inkjet-Printed Highly Conductive Poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonate) Electrode for Organic Light-Emitting Diodes

by Yadong Liu, Juxuan Xie, Lihui Liu, Kai Fan, Zixuan Zhang, Shiyan Chen and Shufen Chen

Micromachines 2021, 12(8), 889; https://doi.org/10.3390/mi12080889 - 28 Jul 2021

Cited by 10 | Viewed by 2743

Abstract

Recently, inkjet printing technology has attracted much attention due to the advantages of drop-on-demand deposition, low-cost and large-area production for organic light-emitting diode (OLED) displays. However, there are still some problems in industrial production and practical application, such as the complexity of ink [...] Read more.

Recently, inkjet printing technology has attracted much attention due to the advantages of drop-on-demand deposition, low-cost and large-area production for organic light-emitting diode (OLED) displays. However, there are still some problems in industrial production and practical application, such as the complexity of ink modulation, high-quality films with homogeneous morphology, and the re-dissolution phenomenon at interfaces. In this work, a printable poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) ink is developed and obtains an adjustable viscosity. Finally, a patterned PEDOT:PSS electrode is fabricated by inkjet printing, and achieves a high conductivity of 1213 S/cm, a transparency of 86.8% and a uniform morphology without coffee-ring effect. Furthermore, the vacuum-evaporated and solution-processed OLEDs are fabricated based on this electrode and demonstrate a current efficiency of 61 cd/A, which is comparable to that of the indium tin oxide counterpart. This work confirms the feasibility of inkjet printing technology to prepare patterned electrodes and expects that it can be used to fabricate highly efficient optoelectronic devices. Full article

(This article belongs to the Special Issue Organic Light Emitting Diodes (OLEDs))

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Review

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21 pages, 8451 KiB

Open AccessFeature PaperEditor’s ChoiceReview

Recent Progress in Blue Thermally Activated Delayed Fluorescence Emitters and Their Applications in OLEDs: Beyond Pure Organic Molecules with Twist D-π-A Structures

by Yiting Gao, Siping Wu, Guogang Shan and Gang Cheng

Micromachines 2022, 13(12), 2150; https://doi.org/10.3390/mi13122150 - 05 Dec 2022

Cited by 5 | Viewed by 4309

Abstract

Thermally activated delayed fluorescence (TADF) materials, which can harvest all excitons and emit light without the use of noble metals, are an appealing class of functional materials emerging as next-generation organic electroluminescent materials. Triplet excitons can be upconverted to the singlet state with [...] Read more.

Thermally activated delayed fluorescence (TADF) materials, which can harvest all excitons and emit light without the use of noble metals, are an appealing class of functional materials emerging as next-generation organic electroluminescent materials. Triplet excitons can be upconverted to the singlet state with the aid of ambient thermal energy under the reverse inter-system crossing owing to the small singlet–triplet splitting energy (ΔE_ST). This results from a specific molecular design consisting of minimal overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, due to the spatial separation of the electron-donating and electron-releasing part. When a well-designed device structure is applied, high-performance blue-emitting TADF organic light-emitting diodes can be realized with an appropriate molecular design. Unlike the previous literature that has reviewed general blue-emitting TADF materials, in this paper, we focus on materials other than pure organic molecules with twist D-π-A structures, including multi-resonance TADF, through-space charge transfer TADF, and metal-TADF materials. Cutting-edge molecules with extremely small and even negative ΔE_ST values are also introduced as candidates for next-generation TADF materials. In addition, OLED structures used to exploit the merits of the abovementioned TADF emitters are also described in this review. Full article

(This article belongs to the Special Issue Organic Light Emitting Diodes (OLEDs))

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22 pages, 5829 KiB

Open AccessReview

Recent Advances of Interface Exciplex in Organic Light-Emitting Diodes

by Jianhua Shao, Cong Chen, Wencheng Zhao, Erdong Zhang, Wenjie Ma, Yuanping Sun, Ping Chen and Ren Sheng

Micromachines 2022, 13(2), 298; https://doi.org/10.3390/mi13020298 - 14 Feb 2022

Cited by 8 | Viewed by 3264

Abstract

The interface exciplex system is a promising technology for reaching organic light-emitting diodes (OLEDs) with low turn-on voltages, high efficiencies and long lifetimes due to its unique virtue of barrier-free charge transport, well-confined recombination region, and thermally activated delayed fluorescence characteristics. In this [...] Read more.

The interface exciplex system is a promising technology for reaching organic light-emitting diodes (OLEDs) with low turn-on voltages, high efficiencies and long lifetimes due to its unique virtue of barrier-free charge transport, well-confined recombination region, and thermally activated delayed fluorescence characteristics. In this review, we firstly illustrate the mechanism frameworks and superiorities of the interface exciplex system. We then summarize the primary applications of interface exciplex systems fabricated by doping and doping-free technologies. The operation mechanisms of these OLEDs are emphasized briefly. In addition, various novel strategies for further improving the performances of interface exciplex-based devices are demonstrated. We believe this review will give a promising perspective and attract researchers to further develop this technology in the future. Full article

(This article belongs to the Special Issue Organic Light Emitting Diodes (OLEDs))

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