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Liu S H, Zhang L T, Xie W F. Solution processed organic light-emitting devices: structure, device physics and fabrication process[J]. Opto-Electron Eng, 2022, 49(5): 210407. doi: 10.12086/oee.2022.210407
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Solution processed organic light-emitting devices: structure, device physics and fabrication process
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Abstract
The industrialization of organic light-emitting devices (OLEDs) in the field of lighting still faces the challenge of high costs, while solution-processed OLEDs can dramatically reduce their manufacturing cost. However, compared with thermally evaporated OLEDs, solution-processed OLEDs encounter difficulty in building multilayer systems. As the relevant reviews on solution-processed OLED from the perspective of material engineering have been proposed, this paper will mainly summarize multilayer structures of solution-processed OLED from the aspects of device physics and preparation processes. First, we will analyze the necessity of each functional layer based on the carrier/exciton dynamics and optical physics. Next, we will introduce the commonly used solution-processing techniques and discuss the problems involved in the preparation of multilayer films. Finally, we will present the future prospects of solution-processed OLEDs. -
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References
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Overview
In this paper, we reviewed multilayer structures of the solution-processed organic light-emitting devices (OLEDs), including normal structure, inverted structure, and tandem structure. From a physical point of view, we discussed and summarized that those multilayer structures are important to improve current balance. The improved current balance can reduce internal accumulations of carriers or current leakages, both of which are detrimental to the device's efficiency and lifetime. We also discussed that multilayer structures are necessary to avoid coupling between dipole sources and surface plasmon polariton (SPP) at the metal-dielectric interface. Thus, we concluded that multilayer structures actually play a key role in achieving solution-processed OLEDs with high performances.
Next, we summarized four solution-processing technologies for OLEDs: spin coating process, spray coating process, blade coating process, and inkjet printing process, as well as their problems involved in preparing multilayer structures. Due to self-leveling under centrifugal force, spin-coated films have a huge advantage in film uniformity. With the aid of the orthogonal solvent system and cross-linked material strategy, the spin coating process can prepare multilayer structures. But the two strategies may cause the problems such as carrier trap and accumulation, which would degrade the device's performances. Besides, due to the capillary flow and Marangoni flow, spray coating process, blade coating process, and inkjet printing process require the add of a surfactant or dual solvent systems to prepare uniform films. In addition, the amount of liquid film and its drying rate can be precisely controlled by the blade coating process and inkjet printing process. As a result, solution-processed multilayer structures can be achieved by the two fabrication processes without using the orthogonal solvent system and crosslinking material strategy. But they are still limited by equipment precision and solvent characteristics.
In conclusion, multilayer structures are necessary to achieve high-performance for solution-processed OLEDs. It is demonstrated that the existing solution-processing technologies can prepare multilayer structures for OLEDs. However, due to similar polarities of organic molecules, the existing solution-processing technologies still have problems with the universality of multilayer structures. The advantage of the existing solution-processing technologies with low costs is that it does not rely on high vacuum, and their problems are mainly caused by the use of solvent. In the field of perovskite, Professor Han Liyuan's team developed a new solvent-free and non-vacuum deposition process for the preparation of methyl ammonium halide lead perovskite film, namely, soft pressure processing. This process can not only retain the low-cost advantages of the existing solution-processing technologies, but also avoid the problems caused by the introduction of solvents. Thus, this process could have great potential in the preparation of low-cost multi-layer OLEDs.
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Liu S H, Zhang L T, Xie W F. Solution processed organic light-emitting devices: structure, device physics and fabrication process[J]. Opto-Electron Eng, 2022, 49(5): 210407. doi: 10.12086/oee.2022.210407
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Figure 1.
Schematic diagrams of device structures of solution-processed OLEDs.
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Figure 2.
Schematic diagrams of current balance in hole-dominated OLEDs.
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Figure 3.
(a) Schematic diagram of SPP at metal-dielectric surface; (b) Power dissipation spectra of dipole radiation. TMv, TMh and TEh respectively represent TM mode from vertical dipole, TM mode and TE mode from horizontal dipole[14]; (c) Optical power modal analysis vs ETL thickness of a conventional OLED. The power is distributed into air (Air), substrate (Sub), waveguide (WV), surface plasmon polariton (SPP) and lossy metal (Loss) modes [56]
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Figure 4.
Schematic diagram of solution-processing technologies.
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