Spray-coated nanoscale conductive patterns based on in situ sintered silver nanoparticle inks
© Zheng et al.; licensee Springer. 2014
Received: 31 December 2013
Accepted: 19 January 2014
Published: 25 March 2014
Nanoscale patterns with high conductivity based on silver nanoparticle inks were fabricated using spray coating method. Through optimizing the solution content and spray operation, accurate nanoscale patterns consisting of silver nanoparticles with a square resistance lower than 1 Ω /cm2 were obtained. By incorporating in situ sintering to substitute the general post sintering process, the time consumption could be significantly reduced to one sixth, qualifying it for large-scale and cost-effective fabrication of printed electronics. To testify the application of spray-coated silver nanoparticle inks, an inverted polymer solar cell was also fabricated, which exhibited a power conversion efficiency of 2.76%.
Printed electronics constitute an emerging class of materials with potential application in flexible devices including organic light-emitting diodes[1, 2], organic thin film transistors[3–5], flexible and conformal antenna arrays, photovoltaic devices[7–10], radio-frequency identification[11, 12], electronic circuits fabricated in clothing, and biomedical devices. Recently, the exploration of silver nanoparticle inks has yielded a promising potential for the design of nanoscale conductive patterns for integration on plastic, textile, and paper substrates, which is compatible with the high-throughput and cost-effective fabrication of printed electronics.
Among the conventional pattern technologies of printed electronics based on silver nanoparticle inks, inkjet printing is the most widely applied due to its great potential for a variety of substrates as well as high-throughput and cost-effective system. Silver nanoparticle inks were directly ejected from the nozzle to the substrate and then sintered at about 140°C ~ 250°C for 5 min to form final conductive patterns[15–17]. Silver nanoparticle inks based on inkjet printing are still hampered from practical application due to the reasons below. Firstly, solution properties including ink viscosity, surface tension, and solubility have a significant influence on the preparation of printed patterns. The complex film preparation and drying process for different types of silver nanoparticle inks restrict their potential for reproducible conductive patterns[19, 20]. Secondly, the coalescence of subsequently ejected ink droplets would cause edges in a type of wave rather than a straight line. Although this phenomenon could be modified by adjusting the component of the solvent, the wave-like edge is hard to avoid which would be even worse accompanied with the patterns at the nanometer scale, leading to conduction between the adjacent lines detrimental to the device. Besides, both the low printing speed of inkjet printing and general time-consuming post sintering process hinder the potential of silver nanoparticle inks for the cost-effective fabrication of printed electronics.
Alternatively, emerged as a promising method, spray coating has been successfully applied in printing electronics[23, 24]. Compared to inkjet printing, spray coating exhibits higher printing speed and easier control of the deposited film morphology. However, there are only a few reports about spray-coated conductive patterns based on silver nanoparticle inks until now[22, 26]. Therefore, in this work, the influence of spray coating silver nanoparticle inks on the properties of silver nanoscale conductive patterns was studied, and the morphology of the conductive patterns was characterized and analyzed by scanning electron microscopy (SEM) and electronic dispersive spectrometry (EDS) in detail. Also, based on the obtained silver nanoscale conductive patterns, polymer solar cells were fabricated using spray coating method, and the performance of the solar cells was also investigated.
Throughout the whole PSC spray coating process, the airbrush was powered by N2 gas at a high pressure of approximately 60 psi to ensure a fine nebulization of solution.
The morphology of the nanoscale conductive pattern was characterized by SEM (JSM-6610LV) and metallurgical microscopy (Olympus BX41, Shinjuku-ku, Japan). The component of the pattern was analyzed by EDS (Oxford Instruments, Abingdon, UK). Current density-voltage (J-V) curves under illumination were measured with a Keithley 4200 programmable voltage–current source (Cleveland, OH, USA). A xenon lamp (CHF-XM35, Beijing Trusttech, Beijing, China) with an illumination power of 100 mW/cm2 was used as an illumination source. The thicknesses of the film obtained from the solution process were measured with a stylus profiler (Dektak 150 stylus profiler, New York, USA). All the measurements were carried out in air at ambient circumstance without device encapsulation.
Results and discussion
where η is the viscosity of the film, γ the surface tension, x the volume fraction of the low surface tension solvent, Al and Ah the evaporation velocity, and αl and αh the activity coefficient of the low and high surface tension solvent, respectively. Through optimizing the content of silver nanoparticle inks, it was found that 45 vol. % ethanol could not only reduce the contact angle of the mixture but also balance the Marangoni flow and convective flows in a drying droplet, resulting in the relatively homogeneous solid film.
Compared to inkjet printing, spray coating has an obvious advantage on fabricating accurate patterns. Figure2a shows the wave-like edge of inkjet-printed patterns, which is mainly attributed to the drop-to-drop distance and component of the solvent. As depicted in Figure2b, the 10-μm inkjet-printed line is along the 1.5 ~ 3-μm scalloped edge. If the adjacent conductive lines were set closer than 3 μm, the wave-like edge would result in the crosstalk of electrical signal or even worse. Figure2c reveals a spray-coated silver line with a width of 20 μm, while the edge of the silver line is only 1 μm. It also shows that the edge of the spray-coated line is composed of a mass of silver dots, resulting from the inevitable diffraction of the spraying process. The enlarged view exhibits that the majority of divergent dots are isolated with each other. This indicates that the edge of spray-coated patterns is not conductive, which guarantees the potential of spray-coated silver nanoparticle inks for fabricating accurate patterns in the scale of nanometer.
R sq of spray-coated Ag patterns based on various sintering operations
(in situ sintering)
(in situ sintering)
In order to facilitate the pattern fabrication process to be compatible with the cost-effective fabrication process of printed electronics, an in situ sintering process was employed to substitute the general post sintering process. The silver nanoparticle inks were sprayed directly towards the substrate at high temperature (140°C ~ 200°C), in which the drying process of wet droplets and the sintering process of silver nanoparticles took place at the same time. It was shown that a highly conductive pattern with Rsq of 6 Ω/cm2 could be obtained at a low sintering temperature of 140°C, compared to 20 Ω/cm2 of the post sintering-processed pattern at the same temperature. More importantly, the time consumption of the in situ sintering process to obtain highly conductive patterns at 140°C was significantly reduced to 20 s, which was about one sixth of that of the post sintering process, as listed in Table1. Meanwhile, the advantages of the in situ sintering process on pattern conductivity and time consumption were not further existent when the sintering temperature was higher than 170°C, as shown in Figure3 and Table1.
Furthermore, SEM was employed to understand the change in the morphology of spray-coated silver nanoparticle inks. Figure4d,e shows the morphology of spray-coated post sintered and in situ sintered conductive patterns, respectively. In Figure4d, it is obvious that there are a large number of nanoscale dark bulges on the surface of post sintered patterns, and the surface roughness is about 40 nm. However, in situ sintered patterns significantly exhibit a lower density of dark bulges. Additionally, in situ sintered patterns exhibit a smoother surface with a roughness of 23 nm. Characterized by EDS, a detailed elemental analysis of the sample has been performed. The dark bulges were corresponding to the C element peaking at 0.3 keV. The flat surface was related to the binding energies of Ag Lα and Ag Lβ at the peaks of 3.0 and 3.2 keV, respectively. The main reason for dense dark bulges in the post sintered pattern was that there was a large space for the stabilizer polymer to transfer to the surface and aggregate to become bulges during sintering at high temperature. In comparison, the relatively sparse dark bulges of the in situ sintered pattern can be attributed to the simultaneous evaporation of the stabilizer polymer and sintering of silver inks. Dried droplet limited the mobility of the stabilizer polymer, which was not affected by the latish wet droplet inks. Hence, there were a few dark bulges detected on the surface, but many of them were distributed into the whole pattern vertically. This was also consistent with the lower conductivity of in situ sintered conductive patterns at high sintering temperature.
Device characteristics of spray-coated PSCs
In situ sintering
In conclusion, spray coating method was successfully applied for the fabrication of accurate nanoscale conductive patterns consisting of silver nanoparticle inks. Homogeneous and highly conductive patterns with low Rsq less than 1 Ω/cm2 were obtained by optimizing the spray coating parameters. Meanwhile, in situ sintering was incorporated to facilitate the sintering process, leading to less time consumption and lower energy cost compared to the general post sintering process. Finally, the potential of silver nanoparticle inks for printed electronics was also testified by fabricating an inverted PSC based on the spray-coated silver electrode, which exhibited a high PCE of 2.76%. This approach would be significantly beneficial to widen the application of silver nanoparticle inks and facilitate it to match the cost-effective and large-scale fabrication process of printed electronics.
This work was supported by the National Science Foundation of China (NSFC) (grant no. 61177032), the Foundation for Innovative Research Groups of the NSFC (grant no. 61021061), the Fundamental Research Funds for the Central Universities (grant no. ZYGX2010Z004), and SRF for ROCS, SEM (grant no. GGRYJJ08-05).
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