Summary

基于溶液沉积的高效OLED开发

Published: November 04, 2022
doi:

Summary

这里介绍的是一种协议,用于制造高效、简单、溶液沉积的具有低滚降的有机发光二极管。

Abstract

基于热激活延迟荧光(TADF)概念的高效有机发射器的使用很有趣,因为它们具有100%的内部量子效率。这里介绍一种基于TADF发射器的溶液沉积方法,该方法基于简单器件结构中的TADF发射器制造高效有机发光二极管(OLED)。这种快速、低成本和高效的工艺可用于遵循主机-来宾概念的所有OLED发光层。描述了基本步骤以及进一步繁殖的必要信息。目标是建立一个通用协议,可以很容易地适应目前正在研究和开发的主要有机排放者。

Introduction

日常生活中使用的有机电子产品的增加已成为无与伦比的现实。在几种有机电子应用中,OLED可能是最具吸引力的。其图像质量、分辨率和色彩纯度使OLED成为显示器的首选。此外,在极薄、柔韧、轻便且易于调色的OLED中实现大面积发射的可能性在照明方面也有应用。然而,与大面积发射器制造工艺相关的一些技术问题推迟了进一步的应用。

随着第一个OLED在低施加电压下工作1,已经设计了固态照明的新范式,尽管外部量子效率(EQE)较低。OLED EQE是通过发射光子(光)与注入的载流子(电流)的比率获得的。最大预期 EQE 的简单理论估计等于 η x ηint 2内部效率(ηint)可以用ηint=γ  x x Equation 1 ΦPL近似,其中γ对应于电荷平衡因子,ΦPL是光致发光量子产率(PLQY), Equation 1 是发射激子(电子空穴对)产生的效率。最后,η 外耦合效率2。如果不考虑外耦合,则注意力集中在三个主题上:(1)材料在产生辐射复合激子方面的效率如何,(2)发射层的效率如何,以及(3)器件结构在促进良好平衡的电气系统方面的效率如何3。

纯荧光有机发射器只有25%的内部量子效率(IQE)。根据自旋规则,禁止从三重态到单重态(T→S)的辐射跃迁4。因此,75%的激发电载流子对光子5的发射没有贡献。这个问题首先在有机发射器磷光OLED678910中使用过渡金属克服,据报道IQE接近100%111213141516.这是由于有机化合物和重过渡金属之间的自旋轨道耦合。这种发射器的缺点是成本高,稳定性差。最近,Adachi1718关于激发三重态和单重态(∆EST)之间低能量分离的纯有机化合物的化学合成报告产生了一个新的框架。虽然不是新的19,但在OLED中成功应用TADF工艺使得在不使用过渡金属配合物的情况下获得高效率成为可能。

在这种无金属有机发射器中,处于三重态的激发载流子很有可能填充到单重态;因此,IQE可以达到100%520,2122的理论极限。这些TADF材料提供了可以辐射重组的激子。然而,这些发射器需要在矩阵主体中分散,以避免在主客体概念中淬灭320212324此外,其效率取决于主机(有机基质)如何适应客体(TADF)材料25。此外,有必要将器件结构(即薄层、材料和厚度)理想化,以实现电平衡器件(空穴和电子之间的平衡以避免损耗)26。为电平衡设备实现最佳的主机-来宾系统是提高EQE的基础。在基于TADF的系统中,这并不简单,因为EML中电载流子迁移率的变化不容易调整。

使用TADF发射器,大于20%的EQE值很容易获得26272829然而,器件结构通常由三到五个有机层(空穴传输/阻塞和电子传输/阻塞层,分别为HTL / HBL和ETL / EBL)组成。此外,它采用热蒸发工艺制造,成本高,技术复杂,几乎仅用于显示应用。根据HOMO(最高占据分子轨道)和LUM(最低未占据分子轨道)水平,载流子的电迁移率和厚度,每一层都可以注入,运输和阻断载流子,并保证发射层(EML)中的复合。

降低器件复杂性(例如,简单的两层结构)通常会导致EQE明显降低,有时降至5%以下。这是由于EML中的电子和空穴迁移率不同而发生的,并且器件变得电不平衡。因此,EML中的发射效率变得很低,而不是激子产生的高效率。此外,由于在高施加电压下激子浓度高,激发寿命长,EQE会随着亮度的大幅降低而发生明显的滚降243031。克服这些问题需要强大的能力来操纵发射层的电特性。对于使用溶液沉积方法的简单OLED架构,EML的电性能可以通过溶液制备和沉积参数32来调节。

以前已使用有机基器件的溶液沉积方法31。与热蒸发工艺相比,OLED制造因其结构简化,成本低和大面积生产而备受关注。随着过渡金属配合物OLED的高度成功,主要目标是增加发射面积,但保持器件结构尽可能简单33。卷对卷(R2R)3435,36,喷墨打印373839和槽模40等方法已成功应用于OLED的多层制造这是一种可能的工业方法。

尽管有机层的溶液沉积方法是简化器件架构的不错选择,但并非所有所需的材料都可以轻松沉积。使用两种类型的材料:小分子和聚合物。在溶液沉积方法中,小分子有一些缺点,例如薄膜均匀性差,结晶和稳定性差。因此,聚合物主要用于,因为它能够在大型柔性基材上形成具有低表面粗糙度的均匀薄膜。此外,材料在适当的溶剂(主要是氯仿、氯苯、二氯苯等有机溶剂)、水或醇衍生物中应具有良好的溶解性。

除了溶解度问题外,还必须保证一层中使用的溶剂不应充当前一层的溶剂。这允许通过湿法工艺沉积多层结构;但是,存在限制41。最典型的器件结构使用一些溶液沉积层(即发射层)和一个热蒸发层(ETL)。此外,薄膜均匀性和形态在很大程度上取决于沉积方法和参数。通过这些层的电荷传输完全受这种形态的支配。然而,应该明智地在所需的最终器件和制造过程的兼容性之间进行权衡。调整沉积参数是成功的关键,尽管这是一项耗时的工作。例如,旋涂不是一个简单的技术。虽然看起来很简单,但从纺丝基板顶部的溶液形成薄膜有几个方面需要注意。

除了薄膜厚度优化、纺丝速度和时间的操纵(厚度是两个参数的指数衰减)外,还必须调整实验者的行动以获得良好的结果。正确的参数还取决于溶液粘度、沉积面积和溶液在基材上的润湿性/接触角。没有唯一的参数集。只有对溶液/底物进行特定调整的基本假设才能产生预期的结果。此外,可以按照此处描述的方案优化依赖于层分子构象和形态的电性能以获得所需的结果。一旦完成,该过程就简单可行。

然而,降低器件结构复杂性会导致最大EQE降低;虽然,在效率与亮度方面可以实现折衷。由于这种折衷方案允许实际应用,简单、大面积兼容和低成本工艺的盈余可以成为现实。本文介绍这些要求以及如何开发方法来处理所需问题。

该协议侧重于绿色TADF发射器2PXZ-OXD [2,5-双(4-(10H-吩噁嗪-10-基)苯基)-1,3,4-恶二唑]42 作为由PVK [聚(N-乙烯基咔唑)]和OXD-7 [1,3-双[2-(4-叔丁基苯基)-1,3,4-恶二氮-5-基]苯组成的宿主基质中的客体,对应于EML。使用TmPyPb[1,3,5-三(间吡啶-3-基苯基)苯]的电子传输层(ETL)。阳极和阴极的功函数都得到了优化。阳极由ITO(氧化铟锡)和高导电聚合物PEDOT:PSS[聚(3,4-乙烯二氧噻吩)-聚(苯乙烯磺酸酯)]组成,阴极由双层铝和LiF(氟化锂)组成。

最后,PEDOT:PSS和EML(PVK:OXD-7:2PXZ-OXD)都通过旋涂沉积,而TmPyPb,LiF和Al则热蒸发。考虑到PEDOT:PSS的导电金属性质,该器件是典型的“两个有机层”,结构尽可能简单。在EML中,TADF客体(10%重量)分散在由PVK0.6+OXD-70.4组成的宿主(90%重量)中。

Protocol

注意:以下步骤涉及使用不同的溶剂和有机材料,因此在处理时必须格外小心。使用通风柜和防护设备,如实验室眼镜、口罩、手套和实验室外套。材料的称重应使用高精度秤机精确完成。为了确保基材的清洁度、薄膜的溶液沉积和蒸发,建议所有程序在受控环境或手套箱中进行。在使用旋涂机、微量移液器、热蒸发器、有机材料和溶剂之前,必须查阅所有安全数据表。 1. 主?…

Representative Results

图5 显示了该器件的主要结果。导通电压极低(~3 V),这对于两层有机层器件来说是一个有趣的结果。最大亮度约为 8,000 cd/m2 ,不使用积分球。ηc、ηp和EQE的最大值分别约为16 cd/A、10 lm/W和8%。虽然结果不是该TADF发射器的最佳品质因数,但它们是使用该发射器通过求解工艺方法在如此简单的器件结构中找到的最佳品质因数。 据…

Discussion

这里用于在简单的器件结构中制造高效OLED的协议相对简单。电迁移率不仅受器件层的材料成分的调节,而且严重取决于薄膜形态。溶液的制备以及溶剂和浓度的适当选择很重要。不会发生材料聚集,这意味着在纳米尺度上完全溶解。观察溶液的粘度也很重要。高粘度导致溶液在基材上的高接触角,反之亦然。在这两种情况下,都可以通过旋涂形成非均匀的薄膜。此外,应避免在滴下溶液之前开始…

Declarações

The authors have nothing to disclose.

Acknowledgements

作者要感谢欧盟地平线2020研究和创新计划根据Marie Sklodowska-Curie资助协议编号674990的“EXCILIGHT”项目。这项工作也是在i3N,UIDB / 50025 / 2020和UIDP / 50025 / 2020项目范围内开发的,由国家基金通过FCT / MEC资助。

Materials

2PXZ-OXD (2,5-bis(4-(10H-phenoxazin-10-yl)phenyl)-1,3,4-oxadiazole) Lumtec ltd 1447998-13-1
Aluminum (99.999%) Alfa Aesar 7429-90-5
Acetone (99.9%) Sigma Aldrich 67-64-1
Hellmanex Ossila 7778-53-2
Isopropyl alcohol Sigma Aldrich 67-63-0
ITO patterned substrates Ossila 65997-17-3
Lithium Fluoride (99.99%) Sigma Aldrich 7789-24-4
OXD-7 (1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene) Ossila 138372-67-5
PEDOT: PSS (Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) Ossila 155090-83-8
PVK (Polyvinlycarbazole) (average Mn 25,000-50,000) Sigma Aldrich 25067-59-8
TmPyPb (1,3,5-Tri(m-pyridin-3-ylphenyl)benzene) Ossila 138372-67-5

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Kumar, M., Pereira, L. Development of Efficient OLEDs from Solution Deposition. J. Vis. Exp. (189), e61071, doi:10.3791/61071 (2022).

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