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Impact of Interface Mixing on the Performance of Solution Processed Organic Light Emitting Diodes -Impedance and Ultraviolet Photoelectron Spectroscopy Study Dong A Ahn, Seungjun Lee, JaeGwan Chung, Yongsup Park, and Min Chul Suh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03557 • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017
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ACS Applied Materials & Interfaces
Impact of Interface Mixing on the Performance of Solution Processed Organic Light Emitting Diodes -Impedance and Ultraviolet Photoelectron Spectroscopy Study Dong A Ahn†, Seungjun Lee‡, Jaegwan Chung§, Yongsup Park‡,*, and Min Chul Suh†,* †
Organic Electronic Materials Laboratory, Dept. of Information Display, Kyung Hee University, Seoul 02447, Republic of Korea ‡
Dept. of Physics and Institute of Basic Sciences, Kyung Hee University, Seoul 02447, Republic of Korea §
Platform Technology Lab, Samsung Advanced Institute of Technology, 130 Samsung-ro,
Yeongtong-gu, Suwon-si, Gyeonggi-do 16678,
ABSTRACT
We investigated interfacial mixing of solution-processed organic light-emitting devices (OLEDs) using impedance spectroscopy (IS) and ultraviolet photoelectron spectroscopy (UPS) and its impact on device performance. We focused on interfacial mixing between a solutionprocessed cross-linkable hole transport layer (XM) and an emitting layer (EML), formed either by solution processing or vacuum evaporation. The results of IS and UPS clearly indicated that extensive interfacial mixing was unavoidable, even after the XM was cross-linked to make it insoluble and rinsed to remove residual soluble species, if the subsequent EML was solution processed. In addition, we also demonstrated that interfacial mixing indeed increased hole current density in corresponding hole only device (HOD). In fact, the hole injection efficiency could be an order of magnitude better when the EML was solution processed rather than vacuum evaporated. We investigated such behavior to find the desirable process condition of solution processed OLEDs.
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Keywords: Solution-processed OLEDs, Impedance spectroscopy, Ultraviolet photoelectron spectroscopy, Ar clustering sputtering, interfacial mixing, †
Corresponding authors. E-mail:
[email protected] (Prof. M. C. Suh) ‡ Corresponding authors. E-mail:
[email protected] (Prof. Y. Park) INTRODUCTION Significant progress has been made in organic materials and device architecture over the last decades for highly efficient organic light-emitting diodes (OLEDs), and mass-produced OLED TV sets and mobile displays are now readily available. Furthermore, OLEDs are considered one of the most promising future lighting technologies owing to their many desirable properties such as surface emission, conformability to curved structures, and transparency. The fabrication of a multilayer structure in an OLED has been achieved mostly through vacuum thermal evaporation, which enables highly efficient and stable device characteristics.1–6 Unfortunately, there are some drawbacks to this technique: an expensive vacuum system is required, the material utilization rate is low, and it is difficult to uniformly deposit materials over a large area while maintaining high pixel resolution. Solution processing technology for OLEDs have recently attracted much attention as a way to overcome problems associated with vacuum thermal deposition. However, despite the efforts to develop better materials and device fabrication processes, the performance of solutionprocessed OLEDs requires improvement to compete with its vacuum-evaporated counterparts.7–11 Problems with solution processing are mainly associated with the difficulties of forming well-defined multilayer structures because the sequential deposition of more than one layer can lead to the partial dissolution of the underlying layer. This may result in interfacial mixing, which could be detrimental to device performance. For example, the
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appropriated energy barriers between charge transport layer [e.g. hole transport layer (HTL), electron transport layer (ETL), etc.] and emitting layer (EML) or that between EML and charge blocking layer [e.g. hole blocking layer (HBL), electron blocking layer (EBL), etc.] cannot be maintained although they are very important to control the charge balance. In addition, unwanted interfacial mixing can also lead to undesirable charge scattering or charge trapping behavior inside EML, which may result in adverse charge recombination behavior. These factors can be very important for device efficiency and lifetime. The most common approach to avoid the interfacial mixing is the crosslinking of the underlying materials layer before the overlying layer is deposited.12,13 Another approach is employing materials with very different solubilities (e.g., materials with orthogonal solubilities) such that the underlying layer is not dissolved by the solvent of the overlying layer.14–17 The crosslinking approach is more common because the orthogonal solubility is not always possible for a given set of materials. However, a perfect cross-linking of a given material is also not possible, and therefore, some degree of interfacial mixing is unavoidable in solution-processed OLEDs. In this study, we investigated the interfacial mixing between the solution-processed crosslinkable hole transport layer (XM) and an EML as well as its influence on device performances. Using both impedance spectroscopy (IS) and ultraviolet photoelectron spectroscopy (UPS) we show that extensive interfacial mixing was unavoidable even after the XM was cross-linked to make it insoluble and rinsed to remove the residual soluble species, if the subsequent EML was solution processed. In addition,we also demonstrated that interfacial mixing indeed increased hole current density in corresponding hole only device (HOD). In fact, the hole injection efficiency could be an order of magnitude better when the EML was solution processed rather than vacuum evaporated. We investigated such behavior to find the desirable process condition of solution processed OLEDs.
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EXPERIMENTS Materials. We used PEDOT:PSS (Heraeus Clevios P VP CH 8000) as a hole injection layer(HIL) and HL-X026 (Merck) as a cross-linkable hole transport layer (XM.)18–20 We used 11-(4,6-diphenyl-[1,3,5]triazin-2-yl)-12-phenyl-11,12-dihydro-11,12-diaza-indeno[2,1a]fluorine
(DIC-TRZ)
and
Iridium(III)bis(2-(3,5-dimethylphenyl)
quinolinato-
N,C2’)tetramethyl-heptadionate [Ir(mphq)2tmd] (red dopant, RD) (Lumtec) as host and dopant materials for EML, respectively. We also used 2,2′,2″-(1,3,5-phenylene)tris(1-phenyl-1Hbenzimida-zole) (TPBi) (Lumtec) as an ETL, lithium fluoride (LiF) (Sigma-Aldrich) an electron injection layer (EIL), molybdenum oxide (MoO3) (Sigma-Aldrich) as a material for the electron blocking layer in HODs, and aluminum (Al) as the cathode. These materials were used as received, without further purification. Device Fabrication. For the fabrication of OLEDs, glass substrates with patterned indium−tin oxide (ITO) (thickness: 150 nm) and a bank layer were utilized. We adopted an aperture area of 9 mm2 and 4 mm2 to prepare full devices and HODs, respectively. Those ITO substrates were first cleaned by sonication in acetone and isopropyl alcohol. Then, they were successively rinsed in deionized water, and treated using UV−ozone equipment to eliminate all possible organic impurities from the surface. For the fabrication of full devices, PEDOT:PSS was spin-coated on the ITO substrates in ambient conditions, and annealed at 150 °C for 20 min in nitrogen atmosphere. The XM solution prepared in chlorobenzene (CB) (0.4 wt %) was poured onto the ITO substrate and spin-coated to yield a pristine XM film. It was then cross-linked at 200 °C for 1 h in a nitrogen environment 20. After cross-linking, the XM film was rinsed by CB during spinning for 30 s. We called this process “post-rinsing.” Next, we dried the resultant samples again at 200 °C for 1 h in nitrogen. For the deposition of EML materials, we poured the red EML solution prepared
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with DIC-TRZ and RD onto the ITO substrates, and spin-casted them to yield a 30-nm-thick EML (s-EML: EML formed by solution process) film, which was dried at 100 °C for 15 min. For the vacuum deposited EML (v-EML: EML formed with vacuum deposition process), the DIC-TRZ and RD were thermally deposited at the rates of 0.5 Å /s and 0.03 Å /s, respectively. After all aforementioned procedures, we deposited the TPBi by thermal evaporation at a rate of 0.5 Å /s, under the typical vacuum condition (
