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Solution processed organic light-emitting devices: structure, device physics and fabrication process
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摘要:
有机电致发光器件(OLED)在照明领域的产业化发展仍然受到高成本的制约，而湿法OLED可显著降低制造成本。但是，相比于热蒸镀OLED，湿法OLED在构建多层薄膜体系上面临更大的挑战。鉴于已有相关综述从材料工程角度对湿法OLED进行了总结，本文将主要从器件物理和制备工艺方面对湿法OLED进行概述。首先从器件载流子动力学和光物理角度分析各功能层的必要性。接着介绍常用的湿法薄膜制备工艺，并讨论制备多层湿法薄膜所涉及的问题。最后对湿法OLED的发展进行了展望。
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关键词:
- 湿法有机电致发光器件 /
- 多层结构 /
- 湿法制备工艺 /
- 电流平衡性 /
- 金属-电介质界面表面等离子体共振吸收
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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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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图 1 湿法OLED的器件结构示意图。
Figure 1. Schematic diagrams of device structures of solution-processed OLEDs.
图 2 空穴主导型OLED器件的电流平衡性示意图。
Figure 2. Schematic diagrams of current balance in hole-dominated OLEDs.
图 3 (a) 电介质-金属界面的表面等离子体激元示意图； (b) OLED中偶极子辐射的功率耗散谱， TMv、TMh及TEh分别对应于纵向偶极子的TM模，水平偶极子的TM模和TE模[14]；(c) 不同ETL厚度OLED的光功率模态分析。Air、Sub、WV、SPP和Loss分别对应于空气模、衬底模、波导模、SPP模和金属吸收模[56]
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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