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Final Report Summary - IRENA (Indium replacement by single-walled carbon nanotube thin films)

Executive Summary:
Indium, one of critical metals, is essential for manufacturing indium tin oxide (ITO), transparent conductive thin films which are commonly used in displays, touch sensors, and for manufacturing of emerging indium gallium zinc oxide (IGZO) based thin film transistors (TFTs), which offer better performance and energy-saving operation of displays over traditional amorphous silicon technology.
Single-walled carbon nanotube (CNT) films are a strong candidate for the replacement of the critical metal-based transparent conductive thin films and TFTs in displays. They also offer a possibility of realization of future flexible displays. The main goals of this project are to develop high performance SWCNT thin film materials: metallic and semiconducting films to completely eliminate the use of these critical metals in display industry. However, the transmittance-conductivity performance of SWCNT-based transparent conductive films should be still improved to meet the industrial requirements for wide range of display applications. The most important issues of SWCNT-based TFTs are the uniformity of device property and the current density sufficient for high-speed driving of display devices such as OLEDs.
The development of SWCNT materials is achieved by rational approaches where nanoscale characterization and modelling are used to guide the discovery of new configurations and optimization of material technology for device level applications and experimental demonstrations, with focus on the interplay between theory and large-scale computational screening and experimental methods. In-situ TEM analysis is also introduced to understand growth mechanism. Post-growth separation technique is also studied to obtain highly-pure semiconducting SWCNTs for TFTs. The achievements of this project are proved by demonstrating high-performance transparent conductive films, touch sensors, and high-performance TFT array with uniform property on plastic film.

Project Context and Objectives:
This project aims to develop high performance materials, i.e. both metallic and semiconducting single-walled carbon nanotube (SWCNT) thin films to completely eliminate the use of the critical metals in electron devices:
i) Indium in transparent conducting films (TCF, indium oxide doped by tin, ITO) and ii) Indium and Gallium as semiconductor In–Ga–Zn–O (a-IGZO) in thin film field effect transistors (TFTs). The target values for fully flexible transparent electrodes based on SWCNT thin films are 10 ohms/sq at 90% transparency, i.e. comparable to ITO-on-glass, with the midterm milestone of 40 ohms/sq at 90% transparency. The applicability of the developed SWCNT films will be further demonstrated in the high-performance TFTs on flexible and transparent polymer (Ion >1mA/mm, Ion/Ioff >10^5) and in 48 zone capacitive SWCNT touch sensor. Because of the rich resource of carbon element, recycling is not needed and the film material supports the friendly environment approach. During the growth of SWCNTs common transition metal nanoparticles will be used as catalyst. Based on the fundamental understandings, the performance and reliability of SWCNT transparent conductors and TFTs will be improved in this project, so that they can be used in highly performing products in the long term, such
as AMOLEDs and future flexible electron devices with very large commercial potential in future consumer electronics. The project contributes to reduce the European and Japanese electronics industry dependence
on the indium resources as well as the cost of manufacturing. Industrial partners from both Japan and EU will be invited to join the dissemination meetings to learn about the project results. Thus the project contributes
to increase the competitiveness of the industry, especially to the SME's developing novel flexible electronics products. Project involves 3 world class teams from both Europe and Japan, having complementary expertise in nanotube synthesis, thin film manufacturing and flexible device manufacturing, in addition to detailed modeling of nanotube growth and thin film charge transport processes. Active exchange of researchers (minimum of 12 person months from EU to Japan and vice versa) will deepen the EU-Japan collaboration. IRENA project European partners are Aalto University (Finland), AALTO, The Coordinator, DTU (Denmark) and CNRS (France).
Project Results:
Project main results
Significant results were obtained in this project as listed below,
• Deep understandings of SWCNT growth mechanism, which are quite valuable for mass production of SWCNT material and devices as well as for fundamental nano science, have been obtained.
• A novel technology for large scale, rapid extraction of semiconducting SWCNT was developed.
• State-of-the-art SWCNT transparent conductors with a sheet resistance of 29 ohm/sq. at transmittance of 87% have been realized. Highly-transparent film with 69 ohm/sq. at 97%, which is unachievable by conventional ITO, has also been developed.
• 12x12-zone touch sensor, fabricated by one-step transfer printing method, was demonstrated.
• SWCNT TFTs with high on-current of >1 mA/mm have been developed by using purified semiconductors.
• High-yield fabrication of both p- and n-TFTs (>99.2% for >1,000 devices) on plastic film has been achieved, implying a possibility of large-scale integration of SWCNT TFTs.
• Novel high efficiency perovskite solar cells employing SWCNT film as hole transport layer and electrode with power conversion efficiency of >17% has been achieved.

Potential Impact:
Rare metals like indium (In) and gallium (Ga) have high socio-economical and technological importance, while being prone to supply-demand fluctuations. Indium is currently used as ITO (indium-tin oxide) to provide transparent conducting films (TCF) for a wide variety of consumer electronics devices, such as displays as well as touch screens of mobile phones and ipad-style portable computers. The global annual market revenue for touch panels is about 25 B€, with annual growth rate of about 10 %. The market for flexible devices is now beginning to take off, with the forecast for the flexible display revenue to reach 20 B€ during 2020. In the flexible devices ITO cannot be used.
In order to replace indium, IRENA project aims to develop flexible single-walled carbon nanotube (SWNT) thin films. The final goal with respect to film conductivity and transmittance was to challenge ITO-on-glass i.e. 10 omhs/sq at 90 % transmittance. Now we have already the world record performance below 30 ohms/sq at 90 % transmittance with patterned SWNT film and 65 omhs/sq with non-patterned film, using direct dry nanotube deposition method from the floating catalyst CVD reactor for the film manufacturing. These results were achieved using a small scale laboratory FC-CVD reactor. Earlier experience has shown that when using large scale industrial reactor, typically factor 2 to 3 more conductive films can be produced. Accordingly, when scaling the results we have reached here to the industrial scale manufacturing, the overall project goal of 10 ohms/sq at 90 % transmittance can be reached.
The films we have produced surpass the properties of ITO on polymer (typically PET) thus being ready for the market adaption. More than 50 % of the TCF used in touch panels currently use ITO-on-PET. Therefore IRENA project results can already now contribute to really large global markets. In the field of touch sensors, IRENA project results have potential to significantly impact the global market for ITO replacement and especially in the developing new markets of flexible touch panel products. The direct dry deposition SWNT thin film manufacturing method developed in this project is continuous, environmentally friendly yet economic one, being currently adapted by companies for flexible touch sensor development.
Polycrystalline silicon is the traditional semiconductor material used in the thin film field effect transistors (TFT-FET) of the high quality display back planes. However, silicon TFTs are not suited for the flexible devices. In–Ga–Zn–O (a-IGZO) TFT-FETs have been introduced to increase the flexibility, however the charge carrier mobility if IGZO is lower than that of polycrystalline silicon. We have demonstrated that SWNT network TFT-FETs with mobility comparable to that of IGZO devices and with high enough on-current, demonstrating the potential of nanotube transistors for flexible display applications. IRENA project offers new solutions for the globally very large markets of backplanes in future flexible displays.

To sum up the dissemination efforts of European IRENA team, total of 49 refereed journal papers were published and total of 163 conference talks were given.

Added Value from International collaborative work
Japanese groups studied the growth mechanism and its control in the supported-catalyst CNT growth, purification of CNT materials, and thin-film device integration. EU groups studied the growth and control in the floating-catalyst CNT growth and in-situ TEM observation to understand the growth mechanism. By engaging both teams, the performance and reliability of CNT thin film devices have been drastically improved on the basis of full understandings of the growth and physical properties.

List of Websites:
Project website: www.irena.aalto.fi

Factual information
The IRENA project is a fundamental research project coordinated by Esko I. Kauppinen in EU and by Shigeo Maruyama in Japan. It associated Christophe Bichara, French National Centre for Scientific Research, Jakob B. Wagner, Technical University of Denmark, as well as Hisanori Shinohara and Yutaka Ohno, Nagoya University. The project started on September 2nd, 2013 and lasted March 31, 2017. EU grant amounted to €1,799,648 and JST grant amounted to ¥199,953,000

Reported by

AALTO-KORKEAKOULUSAATIO
Finland
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