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Inkjet-Printed Small-Molecule Organic Light-Emitting Diodes: Halogen-Free Inks, Printing Optimization, and Large-Area Patterning Lu Zhou, Lei Yang, MengJie Yu, Yi Jiang, Cheng-Fang Liu, Wen-Yong Lai, and Wei Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13355 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 29, 2017
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ACS Applied Materials & Interfaces
Inkjet-Printed Small-Molecule Organic Light-Emitting Diodes: Halogen-Free Inks, Printing Optimization, and Large-Area Patterning Lu Zhou,†,§ Lei Yang,†,§ Mengjie Yu,† Yi Jiang,† Cheng-Fang Liu,† Wen-Yong Lai,*,† Wei Huang*,†,‡ †
Key Laboratory for Organic Electronics and Information Displays (KLOEID) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China.
‡
Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an 710072, Shaanxi, China.
§
These authors contributed equally to this work.
*E-mail:
[email protected];
[email protected] 1 ACS Paragon Plus Environment
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ABSTRACT:
Manufacturing small-molecule organic light-emitting diodes (OLEDs) via inkjet printing is rather attractive for realizing high-efficiency and long-life-span devices yet challenging. In this paper, we present our efforts on systematical investigation and optimization of the ink properties and the printing process to enable facile inkjet printing of conjugated light-emitting small molecules. Various factors on influencing the inkjet printed film quality during the droplet generation, the ink spreading on the substrates and its solidification processes have been systematically investigated and optimized. Consequently, halogen-free inks have been developed and large-area patterning inkjet printed on flexible substrates with efficient blue emission have been successfully demonstrated. Moreover, OLEDs manufactured by inkjet printing the light-emitting small molecules manifested superior performance as compared with their corresponding spin-cast counterparts. KEYWORDS: inkjet printing, small molecules, large-area patterning, organic light-emitting diodes (OLEDs), halogen-free inks
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1. Introduction Organic light-emitting diodes (OLEDs) have attracted much academic and industrial interest due to their promising applications in solid-state lighting sources,1,2 full-color flat-panel displays,3-6 solid-state lasing diodes,7,8 wearable and flexible electronic devices, etc.9-12 Currently, thermal-evaporated small-molecule OLEDs have already entered industrialization due to their significant progress in device physics and materials chemistry. However, thermal evaporation methods require complicated shadow masks and vacuum processes, involving limited device scalability, inefficient material utilization, complicated fabrication process, and consequently high cost.13 Alternatively, solution-processed OLEDs are more attractive for their facile mass production towards cost-effective, large-area, flexible, and even wearable devices via simple and scalable solution manufacturing.14-16 Various techniques have been developed to fabricate solution-processed OLEDs, including spin coating,17 gravure printing,18 screen printing,19 inkjet printing, and so on.20 Among these, inkjet printing shows the advantages of a non-contact technique that offers facile patterning of large-area films with controllable thicknesses and material conservation, which is ideally suited to pattern the organic films for large-area solution-processed OLEDs.21-23 To our knowledge, there are specific requirements for inks during the inkjet printing process, such as surface tension,24,25 viscosity,26 solvent density,27 and solvent evaporation rate,28,29 etc. Although polymers with high molecular weights and amorphous characteristics have been regarded as the suitable choice of materials for inkjet printing,30-32 the drawbacks regarding the tedious purification and the charge unbalance problem have largely limited the device efficiency and lifetime. In contrast, small molecules are advantageous in achieving high performance required for practical applications because of their well-defined structures, high chemical purity, and controllable reproducibility with no 3 ACS Paragon Plus Environment
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batch-to-batch variations.33-35 Therefore, manufacturing small-molecule OLEDs via inkjet printing is rather attractive and highly desired. However, inkjet printed small-molecule OLEDs is challenging. On one hand, commercial OLED small molecules generally show limited solubility and poor film morphologies via solution processing.36,37 On the other hand, re-crystallization, phase separation and coffee-drop-effects of small molecules pose large obstacles via inkjet printing to form uniform films, which are essential for the layer structured OLEDs to achieve optimal device efficiency and long life span.38 Microstructures of substrates39,40 and binary solvents system of inks41,42 have been reported to improve the quality of inkjet printed small-molecule films. Nevertheless, the performance of the inkjet printed OLEDs are still far from optima and generally lower than the spin-cast devices. Moreover, halogenated solvents such as chlorobenzene (CB) and chloroform are commonly used as the processing solvents,39-42 which are toxic and volatile with harmful environmental impact. In this contribution, we present our efforts on systematical investigation and optimization of the ink properties and the printing process with the aim to enable facile inkjet printing of conjugated light-emitting small molecules. Various factors on influencing the inkjet printed film quality43-50 during the droplet generation, the ink spreading on the substrates and its solidification processes have been systematically investigated and optimized. First, the impact of ink viscosity, density and surface tension on droplet formation were studied. A binary solvent system was successfully developed to make up printable halogen-free inks for inkjet printing and its effects on the printed film morphology were then elucidated. Subsequently, the interactions between the ink spreading and surface properties of the substrates were established. Finally, the effects of annealing temperature on the morphological evolution and device performance were discussed. As a result, uniform films were achieved by inkjet printing the small molecules and large-area light-emitting 4 ACS Paragon Plus Environment
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flexible patterns with bright blue emission have been successfully constructed. Moreover, OLEDs manufactured by inkjet printing the light-emitting small molecules manifested superior performance as compared with their corresponding spin-cast counterparts. 2. Results and discussion 1,1'-(9,9-Bis(4-(hexyloxy)phenyl)-9H-fluorene-2,7-diyl)dipyrene (PFP-3) was selected as an example to study the ink properties and the film forming properties of small molecules by inkjet printing because of its simple chemical structure and bright blue emission as revealed in our previous studies.51 Figure 1 depicts the chemical structure of PFP-3. Various solvents such as dimethylbenzene
(DB),
cyclohexnone
(CHN),
CB,
o-dichlorobenzene
(DCB),
N-methyl-2-pyrrolidone (NMP), cyclohexylbenzene (CHB), and 3,4-dichlorotoluene (DCT) were used to make up the inks. The ink properties were studied with using single solvent or a component solvent mixed with the main solvent and the assistant solvent by weight. All the solvents were purchased from Aldrich without further purification. The concentration of emissive materials in all inks was 5 mg mL-1.
Figure 1. The chemical structure of PFP-3 used in this study.
2.1. Effect of characteristic number Z on droplet formation Prior to printing the films, it is essential to form a stable ink jetting from the nozzle without long-lived filaments or satellite drops.52 Various parameters affect the stable ink droplet formation, 5 ACS Paragon Plus Environment
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such as surface tension, viscosity and density.53,54 The characteristic number Z is adopted to predict the stable droplet formation as determined as follows:
(1) Where d is the diameter of jetting nozzle for inkjet printing, ρ, γ and η is the density, surface tension and viscosity of ink, respectively. The Z value for stable jetting is expected between 1 and 10.38 For Z value lower than 1, it is hard for ink to be ejected from the nozzle because of its high viscosity. In the case of Z values above the upper bound, the primary drop will be inevitably accompanied by many satellite droplets. First of all, the printing effect of DB, CHN, CB and DCB as the main solvent for PFP-3 was studied. The basic properties of the solvents and their corresponding Z number are summarized in Table 1. Unfortunately, none of them are within the scope of 1
