Conjugated polyelectrolyte-assisted vacuum-free transfer-printing of silver nanowire network for top electrode of polymer light-emitting diodes
Graphical abstract
Conjugated polyelectrolyte (CPE)-assisted vacuum-free transfer-printing of silver nanowire (AgNW) electrode for efficient polymer light-emitting diode (PLED) was demonstrated. The CPE layer provides a good adhesion between organic and metal layers as well as an enhancement of electron injection.
Introduction
Organic optoelectronics have received much attention in recent years due to their device flexibility and low-cost fabrication via solution process. There have been tremendous research efforts in the development of high-throughput and large-area manufacturing of organic electronics [1], [2], [3]. Particularly, a roll-to-roll process along with its compatible technologies, such as inkjet printing [4], [5], [6], was extensively explored as it allows continuous deposition of organic active layers in ambient conditions with no limit to the size of substrate. Previously, much work has focused on the use of organic materials simply due to their good solubility in common organic solvents and film formation ability. The detailed investigations were performed on the organic thin films fabricated by different processing techniques to probe the nature of molecular alignment and orientation within the layer and to understand their impact on the device performance [7], [8].
However, it is still difficult to effectively deposit top electrode layer in the devices, in which low work-function metal is necessary, without assistance of high-temperature vacuum environment. One of the most widely studied methods is a pressure-induced direct transferring or lamination of conductive thin films [9], which allows easy and fast formation of top electrode layer on the desired area of device using pre-prepared conductive films from various source materials, i.e. carbon nanotubes (CNTs) [10], [11], graphenes [12], [13], conducting polymers [14] and metal nanowires [15], [16], [17]. Among those various source materials, silver nanowire (AgNW) network is known as a versatile nanostructured material in a wide range of device applications in that it can be simply prepared in aqueous solution, and shows not only good electrical conductivity and optical transparency but also mechanical robustness in thin films [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. There have been many reports on successful demonstration of organic material-based light-emitting diodes such as organic light emitting diodes (OLEDs) and polymer light emitting diodes (PLEDs) with AgNW bottom electrode [28], [29], [30], [31]. However, there have been few investigations on OLEDs/PLEDs based on AgNW top electrode, due to difficulty in deposition of top electrode without deterioration in a device performance. Note that the OLEDs/PLEDs are subject to damage, because thickness of an active layer is typically tens of nm for OLEDs/PLEDs. Therefore, in the OLEDs/PLEDs, the attempt to deposit the AgNWs top electrode has been limited in mild solution-based deposition method (drop-cast) [3] to ensure integrity of structured layers underneath during the deposition of AgNWs. Another major concern with the lamination of AgNW networks as a top electrode is that the contact at the interface between the organic and electrode layer is not homogeneous due to poor adhesion of the materials. For this reason, the devices with laminated top electrode layer show lower device performance compared to its reference devices with vacuum-deposited metal electrode layer [17]. In addition, the interface issue becomes more significant when AgNWs are used because either unfavorable adhesion or penetration of the nanowires to the underlying organic layer is likely to take place during lamination. Therefore it is required to establish a strategy to have a full transferring of AgNWs to arbitrary surfaces and/or substrates with no interference at the interface.
In this context, we demonstrate conjugated polyelectrolyte (CPE)-assisted direct transfer-printing of AgNW network as a top electrode of standard green-emitting PLEDs under ambient conditions. To the best of our knowledge, no experimental study has been reported on PLEDs with an AgNW top electrode laminated by pressure-induced direct transfer-printing technique, probably due to the significant deterioration in a device performance caused by penetration of AgNWs during lamination. In our earlier report, poly[(9,9-bis(8′-(3″-methyl-1″-imidazolium)octyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] dibromide (F8imBT-Br) CPE was utilized as an efficient electron injection layer (EIL) on top of F8BT active layer for achieving solution-processable high performance PLEDs [32]. In principle, the same approach was used in this paper. However, a CPE thin film was applied on top of AgNW and F8BT layer prepared on separate substrate, respectively, prior to the lamination. (See experimental section for details) We found that the CPE treatment offers successful transferring of AgNW network from a donor substrate to the surface of F8BT layer through a poly(dimethylsiloxane) (PDMS) mold stamp. On the contrary, without CPE overcoating, AgNW layer was not delivered uniformly on top of F8BT layer and a substantial amount of the nanowires were observed on the PDMS side even after lamination. Our results suggest that the surface treatment using amphiphilic CPE at the interface between organic and metal contact greatly improves adhesion property of AgNW network film to the hydrophobic conjugated polymer thin film. F8BT-based PLEDs with laminated AgNW cathode layer were investigated by varying the CPE film thickness.
Section snippets
Synthesis of AgNWs
AgNWs were synthesized according to the method in the previous literature with modification [33]. Ethylene glycol (EG, anhydrous, 99.8%), lead(II) chloride (99.999%), silver nitrate (99.9999%) and polyvinylpyrrolidone (PVP, molecular weight of ∼55,000) were purchased from Sigma Aldrich. Lead(II) chloride (0.6 μmol) was added into a three-neck round-bottom flask containing 10 ml of EG, and heated at 160 °C for 2 h. PVP (4.5 mmol) dissolved in 5 ml of EG was then added into the flask. Afterwards,
Results and discussions
Fig. 1a describes the schematic of transfer-printing process of AgNW network film and the corresponding sample photo images. The detailed experimental procedures including the synthesis of AgNWs are presented in the experimental section. Briefly, an aqueous dispersion of AgNWs was prepared and filtered through an anodized aluminum oxide (AAO) membrane. A PDMS mold stamp was used to transfer the AgNW layer towards a desired substrate. AgNWs were spontaneously transferred from the AAO membrane
Conclusion
We have successfully demonstrated vacuum-free lamination of AgNW network film as a top electrode of standard PLEDs using a CPE interfacial layer. AgNW films were uniformly delivered from donor substrates to conjugated polymer thin films through a transfer printing technique without compromising the nanowire interconnection and electrical conductivity. F8BT-based PLEDs with laminated AgNW cathode layer were examined in terms of the CPE film thickness in order to gain better understanding on the
Acknowledgements
The authors thank to Prof. Chong Ook Park and Inkun Lee at the Department of Materials Science and Engineering at KAIST for providing the press machine. This research was supported by Korea Science and Engineering Foundation WCU (World Class University) program grant (R32-2011-10051-0) funded by the Korean Ministry of Education, Science and Technology, and was performed within the program of the AMOREPACIFIC Open Research ‘ORS18-01-R16C403003’ supported by a grant from AMOREPACIFIC. This work
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- 1
These authors contributed equally to this work.
- 2
Present address: Department of Physics and Centre for Plastic Electronics (CPE), Imperial College London, Prince Consort Rd, London SW7 2AZ, United Kingdom.
- 3
Present address: Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States.
- 4
Present address: Center for Theragnosis, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea.