Electronic and electrical study of transition metal oxide - conjugated polymer interfaces as hole injection systems for organic electronics

Polymer based organic electronics research is motivated by the promise that ultra-cheap, printed, flexible electronic devices will be able to disrupt many consumer and industrial applications. The prediction is that the organic electronics market will be worth more than the traditional inorganic microelectronics sector in less than 20 years, and have value of more than $200 billion. Information displays lead this expansion with a fledgling industry already being sustained. Lighting, flexible displays and disposable electronics are following with organic photovoltaics a significant future hope. Although progressing rapidly, the ability of organic materials to fulfil their promise requires significant research effort since many fundamental properties of these materials and devices are still not well understood. This proposal addresses hole injection in organic electronic devices by investigating the electronic properties of novel organic - inorganic interfaces comprising transition metal oxides such as vanadium, molybdenum, chromium, nickel, tungsten, copper etc. Motivation arises from several sources; firstly as a replacement for the current state-of-the-art anode material, indium tin oxide, since indium is a costly and increasingly rare material; secondly as materials with which to integrate organic optoelectronic components with other existing advanced electronic technology such as CMOS to make devices such as microdisplays, sensors or lab-on-chip; thirdly as very high workfunction electrodes to enable the use of intrinsically more stable organic materials, since stability and lifetime still pose a barrier to widespread adoption; and finally with the possibility of utilising the rich ferromagnetic and electrochemical functionality of transition metals in organic-inorganic hybrid spintronic and electrochromic devices. This work will provide the first well-founded scientific study of the operation of transition metal oxides in organic electronic devices by investigating a wide range of materials using photoelectron and impedance spectroscopy. The findings of this work will enable improvements in the performance, processability and cost of devices such as light emitting diodes (OLED), field effect transistors (OFET) and organic photovoltaics (OPV) as well as providing an opportunity to realise exciting new hybrid technology.

Organic electronics is currently undergoing an acceleration of investment and development around the world as commercial products start to enter the mainstream marketplace. By 2012 it is expected that the organic electronics economy will be worth $20 billion of which contributions from information displays and lighting will dominate. Over the next 20 years this market is expected to expand to >$100 billion as technologies such as disposable printed electronics and organic photovoltaics gain market penetration. Our TMO hole injection system has intrinsic benefits in terms of workfunction, processability and integration and has the potential to be an enabling part of this expansion by improving current technology and being the disruptive catalyst for new ones. There is direct relevance of this work for the integration of polymer and molecular OLED structures with CMOS display backplanes to form microdisplays. OLED-on-CMOS researchers and manufacturers include MicroOLED (Grenoble, France), eMagin (Rochester, US) and the Frauenhofer IPMS (Dresden, Germany). The wider field of OLED displays and lighting will benefit our new range of electrode materials and UK based Cambridge Display Technology (CDT) and has expressed a significant interest in this work (detailed in their letter of support). We will talk to the CDT technical team to discuss experimental results. CDT is particularly interested in the application of impedance spectroscopy as a diagnostic of OLED devices and of our concept for a solution processed TMO hole injection layer. One important direct economic impact of our proposed work is as a replacement material for ITO in OLED displays since the abundance and cost of Indium metal will in time prohibit the further growth of the displays industry. Mo metal on the other hand is 100 times more abundant and 20 times cheaper than Indium. Our project will generate information about how to achieve hole injection from different metal electrodes. This will help enable new applications where electrical connection is required from metal interconnect tracks to OLED, OFET and OPV structures, for example, in the integration of an OLED light source with a lab-on-chip bio-sensor. The Scottish microelectronics centre in Edinburgh is a leader in these new integrated devices and we will communicate directly with centre director Ian Underwood to establish potential routes for exploitation. In the longer term the ability of TMO to have tunable high workfunction will enable intrinsically more stable OLED devices by increasing the HOMO and LUMO levels of the organic semiconductors. In particular the use of much more stable cathode materials can be realised if the LUMO level can be increased to greater than 3.5eV. We will discuss the possibilities of these deep HOMO-LUMO materials with both industrial collaborators at CDT and Sumitomo Chemical and in house with Ahmed Iraqi in the Department of Chemistry at the University of Sheffield. Along with the traditional academic methods of dissemination we will specifically pursue two routes that we believe can raise the profile of our research in both the academic and industrial communities. We will become actively involved in the UK Displays and Lighting Network; a rapidly growing body of industrial and academic researchers that includes a significant organic device based contingent. The second opportunity lies with the current lack of a clear reference dataset of workfunction values for OLED electrode materials. We will start a web-based archive of workfunction with UPS measurement data from our TMO systems. This is likely to become a well used resource, with a wide audience, and enable us to communicate our other research very effectively.