Transparent electrode: What else after ITO? State of the art and future prospects. Paris, 21 November 2011
Transparent electrodes are an essential element of numerous ubiquitous devices – a for ever increasing multi billion dollars market - used by billions of people such as displays (LCDs and OLEDs for PCs or cellular phones; e-paper;…), photovoltaic and lighting.
Today, tin-doped indium oxide (ITO) is by far the dominant material (3 B$ in 2010 with a 20% growth rate through 2013). This situation is about to change because ITO suffers of several drawbacks: (i) indium is becoming a scarce and expensive resource ;(ii) it exhibits serious technical issues (deposition techniques are expensive, quite inefficient and slow; ITO films are fragile, sensitive to corrosion and have a relatively high index of refraction). These issues have stimulated numerous developments aiming at finding out the next generation of transparent electrode materials.
It gathered internationally recognized experts - from industry or academics - and working on new materials for transparent electrodes. The aim of this workshop was to present the state of the art of ITO alternative materials for transparent electrodes:
- Indium reduced or indium free oxides,
- Conducting (or hybrids) polymers,
- Carbon nanotubes,
- Graphene and other bidimensionnal materials,
- Nanowires or nanostructured films, nanoparticles…
The core of the workshop presented and compared the features of the various materials in competition and their deposition processes. Presentations detailed their performances according to targeted applications as well as their technological maturity (time to market). Barriers to overcome before industrialization were also discussed.
The introduction detailed economic relevance of transparent electrodes and the problematic attached to ITO. As a conclusion, integration of these emerging materials from a system point of view was presented.
Transparent electrode: What else after ITO? State of the art and future prospects
10h00 “Economic importance of ITO as a transparent electrode: applications, markets, and barriers to its replacement”
Didier JOUSSE, Director of products strategy, Saint Gobain Solar, France
Transparent electrodes are key materials of large area devices such as displays, touchscreens and solar panels. The annual coated area is estimated at 300 Mm² and represent a global value of 2 b€. Thin films of ITO which are deposited by magnetron sputtering under vacuum account for 82% of the total area, the remaining being split between SnO2:F deposited by atmospheric pressure CVD (11%) and ZnO deposited by magnetron sputtering or low pressure CVD (7%). Fortunately, we are talking of very thin layers between 100 and 300 nm and there is enough Indium on earth at this moment to adequately satisfy the demand. However, the cost of ITO targets remains very high because Indium is either extracted as a by-product of Zn mining at low concentration or through a recycling process. The price pressure will not decrease with the arrival of new applications like OLED lighting and CIGS (stands for Copper-Indium-Gallium-Selenide) solar panels which are expected to develop at large scale in the next 3 years. Many alternatives to ITO have been proposed with a limited success so far, except for the case of solar cells. By looking at each specific application, we show that alternative proposals very often have achieved the required levels of conductivity and transparency but they have been lacking the chemical properties that are needed for them in order to fit the existing industrial workflow without adding extra process steps or new pieces of equipment. Those properties to be looked at include etchability and compatibility with photolithographic processes in the case of LCD or plasma displays, as well as durability under high temperatures and/or humid atmospheres for all applications. It is worth noting that some applications like OLED and solar would greatly benefit from electrical and optical performances beyond the classical transmission/conductivity space achievable with ITO and alternative oxides. Examples of promising new approaches based on metal coatings will be presented.
10h40 "Transparent Electrodes: an Industrial Perspective"
Dr. Darwin ENICKS, Manager Thin Films Research Science and Technology Division, Corning Incorporated
Indium Tin Oxide (ITO) is a typical study in the difficulty of dethroning an incumbent. However, new applications in the major markets; display, PV, and lighting offer significant opportunities for new transparent electrode materials and structures. This presentation will discuss, from an industrial point of view, the application requirements, and hurdles these new systems must overcome to gain wider acceptance over ITO. Attention will be given to the innovation pipeline and barriers to scale-up.
11h10 Coffee Break
ITO ALTERNATIVES MATERIALS : STATE OF THE ART & FUTURES PROSPECTS
11h40 “Transparent Conductive Oxides alternatives through vacuum deposition”
Jean Christian BERNEDE, Moltech Anjou, Université de Nantes, France
Depending on the resistivity and transmittance, transparent conductive oxides (TCO) are widely used in thin film optoelectronic devices. In2O3:Sn (ITO) is the main TCO used in OPV. Since indium is scarce and ITO is limited in flexibility, due to its ceramic structure, works have been devoted to others TCO (FTO, AZO) and dielectric / metal / dielectric structures, as alternative to ITO. After a short reminder on the last developments concerning the TCO which could replace the ITO, through surface treatment, I present the state of the art concerning the structures dielectric / metal / dielectric. These structures can be deposited onto substrate at room temperature by simple evaporation under vacuum. Moreover it is possible to manage the anode work function through the choice of the dielectric, which can allow using them as cathode or anode and as intermediate electrode in tandem solar cells
12h10 “Transparent mesostructured metaloxide thin films prepared by liquid deposition techniques”
Dr. Cedric BOISSIERE, Laboratoire Chimie de la Matière Condensée de Paris - Collège de France, France
This talk will present an overview of the different methods of preparation of dense and mesostructured metaloxide thin films of optical quality obtained by liquid deposition techniques. Advantages and drawbacks of their processing, considered being cheap and highly flexible if compared to physical deposition techniques, will be exposed. More specifically, the presentation will highlight how the fine tuning of metaloxide films properties (such as refractive index or the mechanical properties) can be achieved via the controlled insertion of mesoporosity by sol-gel chemistry and the controlled crystallization of the deposited layer. The versatility of this approach for the controlled preparation of semiconductor and conductor transparent thin films will be presented via several examples.
14h00 “Transparent Electrodes based on Conducting Polymers”
Dr. Wilfried LOEVENICH, Head of R&D, Heraeus Clevios GmbH, Germany
A number of basic properties distinguish conductive polymers from the most widely used material for transparent electrodes, indium tin oxide (ITO). Polymers are inherently flexible and they come as solutions or dispersions that allow cost efficiently coating. On the other side polymer as organic materials are suspected to suffer from long term degradation. We will show that this issue can easily be overcome with suitable stabilizers against aging. Formulation and coating of PEDOT is known technology which has been practiced for almost 10 years due to its application as antistatic layer e.g. in flat screens. The development of PEDOT grades exhibiting conductivity close to ITO motivated to extend the use of PEDOT for transparent electrodes. A new patterning process which allows electrical structures without any visible pattern currently propels the application of such PEDOT-electrodes e.g. in touch panels
14h30 "Transparent & flexible nano-structured carbon thin films"
Dr. Matt COLE, Electrical Engineering Division, Dept. of Engineering, Cambridge University, UK
Nanotubes have been suggested as a candidate replacement material for ITO. Metallic only SWCNTs have been reported to show more than 90% transmission at 550 nm optical with sheet resistances as low as 200 ?/sq. However the successful exploitation of nanotubes lies in applications that make use of their high structural anisotropy in order to complement, and replace ITO only in certain specific applications.
This talk will report on the pros and cons of state-of-the-art nanotube thin film fabrication and processing, with regards to ITO. Viable application examples will be presented throughout where the porosity and one-dimensionality of the nanotubes networks can, and has, been used to complement more transparent and conductive materials. Finally, we will attempt to address the question “why haven’t nanotubes superseded ITO yet?” and “how do nanotubes realistically fit into the scheme of things?”
15h00 “Graphene for transparent conducting film: Comparison to carbon nanotubes”
Prof. Young Hee LEE, Chairman of Department of Energy Science, Sungkyunkwan University, Korea
Nanocarbons such as fullerenes, carbon nanotubes, carbon nanofibers, graphite oxides, and graphenes have been the key materials in 21th century and have led nanoscience and nanotechnology. In particular, carbon nanotubes and graphene have been recently intensively investigated for electronic applications. Both materials have strong potentials for transparent conducting electrodes. While their electronic properties are similar to each other, their applications are still limited by the available technologies in many cases. I will describe the recent progresses of graphene in transparent conducting film and furthermore advantages and drawbacks compared to carbon nanotubes.
15h30 Coffee Break
16h00 “Metallic Nanowires: Emerging Building Blocks for TCO Alternative”
Dr. Jean Pierre SIMONATO, CEA - LITEN, France
Recent research has focused on finding cost-efficient alternatives for the fabrication of flexible transparent electrodes. Some emerging technologies are based on nanomaterials. Among them, metallic nanowires appear very promising. Either patterned grid or random networks based on metallic nanostructures can lead to large scale electrodes having excellent properties.
16h30 Novel electrodes for organic devices
Prof. Dr. Karl LEO, Fhg IPMS, Dresde, Germany
For organic light-emitting diodes (OLED), and organic solar cells, transparent electrodes with high conductivity are needed. The standard approach – Indium tin oxide – has many disadvantages, like brittleness and the scarcity of Indium. The challenges for alternative electrodes are the need for very smooth materials which allow ohmic contacts.
I will report about our recent studies using alternative inorganic and organic ITO replacements, both on OLED and organic solar cells. We have shown that Al-doped ZnO is well suited and gives in combination with organic devices with doped organic layers good contacts for both OLED /1/ and solar cells /2/, with IV-curves and efficiencies comparable to ITO. Lifetime tests indicate also similar values. Also, we have successfully tested various organic materials /3,4,5/ and combinations of nanowires with organics. The main challenges are the limited conductivity which requires additionally metal grids, and the large roughness of many of the materials, which leads in the organic thin film devices often to shortcuts.
17.00 END OF THE WORKSHOP