Cheap, green printed electronics

Inkjet printing in electronics production has attracted considerable attention for a wide range of applications because it is an environmentally friendly, low-cost technique.¹ Low-voltage electronics materials with excellent charge transport must meet other key requirements, such as high chemical stability, low-temperature processability, and low hysteresis, to be suitable for inkjet printing. Until recently, such materials were not available. But we have now demonstrated low-cost green manufacturing through precisely controlled inkjet printing of single-walled carbon nanotube (SWCNT) films.

Printable technology has the potential to drastically reduce ecological impact, manufacturing energy consumption, and material wastage by controlling the quantity and location of ink deposition. Compared to other solution-phase methods, inkjet printing offers targeted film deposition and is suitable for industrial-scale production. Moreover, the material utilization efficiency of this method is ~100%, which makes it appropriate for expensive novel materials.

Until now, research in this area has focused largely on organic semiconductors¹ because they offer the high carrier mobilities needed to create printable electronics (≤1cm²V⁻¹s⁻¹, comparable to amorphous silicon). Although highly crystalline organic thin-film transistors (TFTs) fabricated by spin-coating processes also offer sufficient charge mobility, inkjet-printed organic TFTs do not (≤0.1cm²V⁻¹s⁻¹). These lower mobilities are mainly attributed to the lower levels of crystallinity in inkjet films. The drying process of inkjet microdroplets differs significantly from that of other droplets, and highly crystalline films are almost unattainable. Among all inkjet-printed TFTs, polycrystalline silicon films using a silane-based liquid precursor have shown the best performance.² Yet, despite the high carrier mobilities exhibited by these films (6.5cm²V⁻¹s⁻¹), the processing temperature is too high (>500°C) to be cost-effective for commercial inkjet manufacturing and to apply flexible plastic substrates.

SWCNTs are among the most promising materials for inkjet-printable technology. Their electronic properties include high room-temperature charge mobility of semiconducting SWCNTs that is more than an order of magnitude larger than that of crystalline silicon.³ Recently, TFTs consisting of random networks or aligned arrays of SWCNTs have been attracting increasing attention for their possible use in transparent, flexible, high-speed, high-current, and high-frequency electronics. The use of network geometry in flexible electronics is of interest because it can be easily achieved from solution suspensions. Indeed, we have reported high performance (>1cm²V⁻¹s⁻¹) for spaghetti-like solution-processed random networks of SWCNTs, which may be attainable in an inkjet process.^4–6

Despite its critical importance, as far as we know, there has been only one previous report on inkjet-printed SWCNT TFTs, and those were fabricated on inflexible silicon substrates.⁸ Moreover, high performance has remained out of reach for such a TFT because of the presence of both semiconducting and metallic tubes inherent to all SWCNT synthesis methods. The only carrier mobility and on/off current ratio reported thus far are 0.07cm²V⁻¹s⁻¹ and 10², respectively.⁸Metallic SWCNTs may be eliminated after fabrication by being burned out, but this additional processing step is neither well controlled nor scalable. In particular, a high on/off current ratio (>10⁴) is indispensable for practical application. As a first step, we have succeeded in fabricating high on/off ratio (~10⁴) SWCNT flexible TFTs by inkjet printing a water-based dispersion without any additional processing steps to eliminate the effect of metallic SWCNTs (see Figure 1).⁹

Figure 1. An inkjet-printed single-walled carbon nanotube (SWCNT) flexible thin-film transistor (TFT).

In networks with high SWCNT coverage, especially in the form of small bundles, the metallic tubes (normally coexisting with semiconducting tubes in a 1:2 ratio) form a percolating network that behaves like a conducting film.³ With moderate SWCNT coverage, only the semiconducting tubes create a percolating network, and the film shows semiconducting properties. In the past, SWCNT TFTs have been inkjet-printed without precise density control, resulting in low transistor performance (≤0.4cm²V⁻¹s⁻¹).^8,9

We inkjet-printed SWCNT networks on a silicon dioxide/silicon (SiO₂/Si) wafer where the SiO₂ layer was 500nm thick. This created both source/drain electrodes and semiconducting active layers (see Figure 2). In the electrode regions, the SWCNT dispersion was deposited 100 times at each position. For the active semiconducting layers, we printed four types of networks by depositing 40, 20, 10, or two droplets per position. An optical micrograph of printed SWCNT transistors shows a clear contrast between networks, strongly suggesting successful density control (see Figure 2).

Figure 2. Schematic of materials, device layouts, and fabrication procedures.⁷V_DS: Voltage between drain and source electrodes. V_GS: Voltage between gate and source electrodes. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. (Reproduced with permission.)

Inkjet-printed SWCNT TFTs can potentially be used in flexible electronics due to their high performance and low temperature processability. However, various properties need to be improved before application to achieve low hysteresis, low-voltage operation, and full printability with printable-dielectric materials. To this end, we changed the dielectric layer from solid-state SiO₂ to an ionic liquid known as DEME-TFSI—see Figure 3(a) for the composition—as a high-capacitance gate dielectric compatible with TFTs and solution processing. We fabricated high-performance SWCNT TFTs by the inkjet method and then printed the SWCNT gate electrode: see Figure 3(b) for a schematic illustration of device layouts and fabrication procedures for exclusively inkjet-printed SWCNT TFTs. In the final fabrication step, we inkjet-printed an ionic liquid as a gate dielectric material. We compared the characteristics of back-gate-operated solid-state SiO₂ and ionic-liquid gating (see Figure 4). With ionic-liquid gating, the operating voltage was significantly reduced, and the hysteretic response against gate voltages was improved, possibly due to efficient screening of charged impurities by the ionic liquid.

Figure 3. Fabrication of exclusively inkjet-printed SWCNT TFTs. (a) Schematic representation of DEME-TFSI. (b) Schematic representation of fabrication procedures for exclusively inkjet-printed SWCNT-TFTs and atomic force microscopy image.⁷ (Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

Figure 4. Transfer characteristics of silicon dioxide (SiO₂) back-gated (blue) and ionic-liquid-gated (red) SWCNT TFTs.⁷ (Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

In summary, we prepared very dilute SWCNT dispersions and controlled both the densities and the electrical properties of the networks by optimizing the inkjet-printing process.¹⁰ This transistor exceeded the performance of conventional organic transistors (a mobility of 1.6–4.2cm²V⁻¹s⁻¹ and an on/off ratio of 10³–10⁴) and was fabricated at moderate temperatures (80°C).⁷ We also printed ionic-liquid gate dielectrics. This is the first demonstration of TFTs produced exclusively by inkjet printing. We are working toward producing high-performance SWCNT-printed electronics on inexpensive plastic films in the near future.