Effect of photonic flash annealing with subsequent compression rolling on the topography, microstructure and electrical performance of carbon-based inks

Screen-printed carbon-based pastes and inks are extensively used in a wide range of printed electronics applications including resistive heater panels [1‐3], electrochemical sensors [4, 5] pressure sensors [6], printed batteries [7] and supercapacitors [8] as well as photovoltaic applications including perovskite and dye-sensitised solar cells [9‐12]. Graphite, carbon black and graphite nanoplatelets (GNPs) are the primary forms of carbon used in a large proportion of these devices in industry. This is due to their ease of use and economic advantage over other commercial conductive materials, such as silver and more recent derivatives of carbon such as graphene and carbon nanotubes (CNTs).

Consequently, research has been conducted into identifying the effects of modifying the ink formulations of graphite and carbon black-based inks. These include investigating the effect of the solvents used in the ink on the drying rate of the prints as well as its subsequent effects on film conductivity and process consistency [13]. Studies have also been conducted to assess the effects of the number of printed layers [1] on the electrical performance of the carbon electrodes, as well as the effects of different particle loadings [14] and ratios of the different carbon forms [13, 15]. These are improvements to the inks which have been quite well established industrially, such as in the blood glucose sensor industry. Despite these improvements, significant gains in conductivity might not be possible through formulation alone. In the case of altering the ratios of different carbon forms, it was found that the small carbon black particles were able to fill the gaps between the large graphite flakes [13]. Therefore, identifying methods which could be used to reduce the physical size of these gaps using postprocessing methods rather than alterations to the print formulation might also help drive further improvements to the print performance. This could enable print topography and conductivity to be enhanced without changing the ink formulation, which could otherwise compromise the quality of dispersion, shelf life and print performance for these common forms of carbon-based inks. From an electrochemical perspective, the binders in inks can block the active surfaces and thus result in slower reaction kinetics [16], making methods capable of reducing the binder in the print without compromising performance desirable. Postprocessing methods which have shown promise in the literature include photonic flash annealing followed by subsequent compression rolling.

Photonic methods, in the form of intensive pulsed light (IPL) sintering and annealing, have been used to provide a rapid burst of high energy to heat materials such as metal nanoparticles and copper oxide. This allows them to sinter and become conductive, and crucially, it can heat the print to high temperatures without damaging low-cost substrates such as polymer films or paper [17, 18], providing opportunities for low-cost plastic electronics. IPL sintering and annealing have been used in a range of printed electronics such as photovoltaic applications [19‐21], where photonic annealing has been identified as a lower-cost alternative to high-energy, thermal processes. This enables a higher throughput along with improvements in the surface roughness, edge resolution and conductivity of the prints. It has also been used to enhance the electrical performance of RFID (radio frequency identification) tags [22, 23] and other forms of printed electronics [24, 25]. Arapov et al. [26] used photonic annealing to reduce the resistance of printed binder-based graphene inks from 200 Ω □⁻¹ after thermal drying at 100 °C for 5 min (based on 6 µm dry film thickness) to 40 Ω □⁻¹ after photonic annealing.

Despite improvements in electrical performance, it was noted that photonic annealing also resulted in a reduction in layer adhesion and cohesion. The degradation of the binder caused a decrease in the number of contact points between the graphene sheets and therefore a loss of structural integrity [26]. To counteract this disruption, Arapov et al. used compression rolling to improve the structural integrity of the prints, as well as provide further improvements to resistivity, down to 20 Ω □⁻¹, along with a reduction in film thickness and surface roughness from 6 µm and 0.945 µm, respectively, after thermal drying to 1.7 µm and 0.065 µm, respectively, after photonic annealing followed by compression rolling [26]. Compression rolling alone has also been investigated as a possible postprocessing method for a range of applications including radio frequency identification (RFID) antenna and other graphene-based prints and coatings, which in these cases were binder-free, where it produced improvements in the electrical and thermal conductivity of the devices due to better flake alignment [26‐29]. There are also reports of compression rolling leading to improvements in the electrochemical performance of screen-printed lithium-ion batteries with electrodes consisting of graphite [30‐32] and amorphous carbon [33]. In these cases, the compression rolling process has been found to reduce the roughness of the electrode by compressing the graphite flakes to become orientated with the basal plane, parallel to the current connector [32]. This leads to increases in the electrode’s density and a reduction in porosity which enables improvements in the ionic and electrical conductive pathways [30, 31].

Although these analyses assess the effect of photonic annealing and compression rolling on single morphologies of carbon, these processes could also provide an effective method for improving the electrical performance of inks containing a range of carbon morphologies without having to change the ink formulations. Therefore, this paper investigated the effect of photonic annealing and compression rolling on inks containing a range of common, low-cost forms of carbon. Inks were made with the following carbon morphologies dispersed in solvent and polymeric binder: (1) graphite only, (2) graphite nanoplatelets (GNPs) only and (3) graphite with carbon black. The inks were manufactured, screen-printed, thermally dried and then posttreated. The effects of posttreatment were established using topography and resistance measurements of both printed solid areas and printed lines, as well as scanning electron microscopy (SEM) imaging after each of the stages. Thermogravimetric analysis (TGA) tests were also conducted to establish the equivalent temperatures and durations required to remove the binder using traditional thermal drying methods.

The effect of compression rolling on inks containing binder in comparison with those which have had their binder removed has also not been fully assessed. To establish whether the removal of binder is required to enable optimisation of print conductivity, the graphite only ink was used to assess the effect of compression rolling both with and without prior photonic annealing. These methods could provide a feasible technique for enhancing the print consistency and conductivity of low-cost carbon inks, which could lead to improvements in printed carbon electrodes for a host of devices, without increasing the cost or practicality of the ink formulation.

In the case of the graphite-based ink, reductions in surface roughness, film thickness and resistivity were apparent for samples which were compression rolled, both with and without prior photonic annealing. However, less significant improvements were seen for the samples which were compression rolled after drying alone, with a 68% reduction in surface roughness (2.80–0.90 µm) and a 61% reduction in resistivity (1.91–0.74 Ω cm). For samples which were photonically annealed prior to compression rolling, there were greater reductions of 56% in surface roughness (2.80–1.22 µm) and 88% in resistivity (1.91–0.23 Ω cm). TGA suggests that photonic annealing was able to reduce the binder in the print over a significantly shorter duration than would be required for traditional drying methods, where prolonged exposure to high temperatures would lead to warping of the polymer substrate [35]. This also complements the results by Secor et al. which found that photonic annealing was able to reduce the resistance of printed graphene inks by exceeding the threshold temperature required for the degradation of the polymer binder without damaging the substrate, as it was conducted on a short enough timescale. Their results found a reduction from 165 Ω □⁻¹ when thermally dried to 135 Ω □⁻¹ when photonically annealed [24]. This removal of insulating barriers between the graphite flakes could enable more direct contact points between the conductive flakes, and provide more space for the flakes to be compacted into during compression. On the other hand, for electrochemical applications, such as sensors, the rougher surface produced by photonic annealing only will provide a greater number of exposed edges and potentially increase the electron transfer rate [36, 37]. However, this must be weighed against durability and stability issues, as during photonic annealing the layered structure is interrupted and mechanical stability is reduced, resulting in poorer adhesion between the print and substrate. Subsequent compression rolling was used to consolidate the layer, and produce significantly smoother prints with the graphite flakes aligned to the horizonal plane, providing more substantial improvements in conductivity.

Similar improvements were shown in surface roughness and resistivity for the GNP-based inks, where there were reductions of 40% in surface roughness (0.82–0.49 µm) and 86% in resistivity (2.37–0.32 Ω cm) after both photonic annealing and compression rolling. These resemblances are likely due to the similarities in the morphologies of the materials, i.e. layered planar structures which are initially randomly orientated. The random orientation of the GNPs provided large gaps between nanoplatelets from which binder could easily escape during photonic annealing and allow the GNPs to be compacted into during compression rolling. These changes in the print microstructure and improvements in the print conductivity, seen for the graphite- and GNP-based inks, are in line with the trends found for the graphene-based inks assessed by Arapov et al. that found a 80% reduction in sheet resistance after photonic annealing [26].

The graphite and carbon black ink was substantially more conductive than the GNP and graphite-based inks alone and therefore had lower potential for improvement in the conductive matrix. The carbon black particles acted as conductive bridges between the graphite flakes with the graphite-to-carbon black ratio optimised for conductivity by Phillips et al. [13]. This meant that there was less space for the graphite flakes to be flattened into during photonic annealing and subsequent compression rolling, when compared with the plain graphite and GNP inks. However, there were still improvements seen in the print topography which may have potential benefits; for example, Lee et al. suggest that polishing of an electrode surface will extend its working life and potentially make single-use sensors reusable [16]. There were reductions of 59% in surface roughness (1.7 µm to 0.7 µm) for the graphite and carbon black-based prints with reductions of 49% in resistivity (0.037–0.019 Ω cm) after photonic annealing and subsequent compression rolling. However, the SEM images (Fig. 5) appear to show that some carbon black has been removed from the graphite flakes as a result of processing. So, although there is an overall improvement in print topography and electrical performance, due to an improvement in the area of carbon flakes and particles in contact with one another, it is done so whilst potentially removing the some of the uppermost carbon black particles. The high surface area and small size of these particles make them likely to be entrained in the vapour when the binder is degraded.

Overall, these postprocessing methods could provide a quick and simple technique to enhance conductivity with low-cost, easy-to-process materials. However, further improvements in conductivity could be made by using thicker printed layers, as found by Secor et al. for printed graphene inks, where the sheet resistance was found to reduce from around 135 Ω □⁻¹ to around 20 Ω □⁻¹, when increasing the number of printed layers from one to eight for samples which were photonically annealed [24]. Similar improvements were found for thermally dried samples, with a reduction from 165 Ω □⁻¹ for one layer to around 30 Ω □⁻¹ for eight layers, although the sheet resistance was consistently higher for all printed layers when compared with the photonically annealed counterparts. However, due to the increasing print thickness with number of passes, the pulse length and energy required for best results during photonic annealing would alter with deposit thickness and ink consistency [24].

This study has demonstrated that the photonic annealing and compression rolling can provide a suitable postprocessing method for improving the topography and electrical performance of carbon-based inks of a range of carbon morphologies. These processes both can be integrated into a roll to roll production of printed electronics and may be beneficial for a range of applications as well as potentially widening the range of uses for low-cost carbon inks, so that they can displace some of the applications using earth metal inks. Future work could be conducted to assess the effect of photonic annealing and compression rolling on the electrochemical performance of a range of redox couples for and electrochemical sensors, as well as battery electrodes, to assess whether these postprocessing methods can lead to advantages other than improvements in electrical conductivity.