This Small Business Innovation Research (SBIR) Phase I project aims to develop a flexible transparent conductor with better performance and lower cost than indium-doped tin oxide (ITO). The approach is to produce smooth films composed of sparse meshes of silver nanowires, which are highly conductive and flexible, yet transmit light with high efficiency. Films prepared on a laboratory scale have demonstrated higher conductivity for a given transparency, and higher total conductivity than the most conductive ITO available. This project will focus on process development for the production and patterning of large-area conductive films at high throughputs required by industrial applications.
The broader/commercial impact of this project will be the potential to provide transparent conductive films with better performance than ITO at significantly lower cost and better mechanical flexibility. Transparent conductors are crucial parts of optoelectronic devices, including liquid crystal displays (LCDs), light emitting diodes (LEDs), capacitive touch screens, solar cells, electronic ink readers, and others. ITO is the primary transparent conductor currently in use that meets the performance required by these devices. However, the price of ITO has been increasing steadily as the supply of indium dwindles and demand for optoelectronic devices grows. Furthermore, ITO is a brittle material and cannot be used in flexible devices. Silver nanowire films do not suffer from these limitations, and thus are expected to be a low-cost alternative to ITO and an enabling technology that allows the flexible electronics market to emerge.
Transparent conductive films are used as electrodes in many types of optoelectronic devices, including touch panels, displays, thin-film solar cells, and large-area solid-state lighting. Roll-to-roll deposition of such films directly from solution has long promised to lower device costs, but all of the solution-processed alternatives have fallen short in terms of performance requirements, namely low sheet resistance, high transparency, and low surface roughness. Meanwhile, the incumbent material, tin-doped indium oxide (ITO), though having good performance on glass, has poor performance on plastic substrates due to the high-temperatures required to form highly conductive ITO films. Sinovia Technologies has created a new composite transparent electrode structure that has electrical and optical performance superior to ITO, on both glass and plastic substrates. To date, it is the first known drop-in replacement for ITO that has been shown to function effectively in thin-film optoelectronic devices, and it is able to be fabricated using potentially scalable, solution-processed deposition techniques. Sinovia has been able to fabricate samples on a small scale, but needed to investigate whether or not the same films can be fabricated on a large scale using commercial roll-to-roll equipment. In addition, an investigation of the degradation properties of these films had never been performed. Over the course of this SBIR Phase I study, Sinovia successfully demonstrated, as a proof-of-concept, that every fabrication step of our solution-processed transparent electrode can be executed using large-scale, commercial coating equipment. This was carried out on a 9-inch wide plastic web, using various types of scalable roll-to-roll printing processes. While there is a need for further optimization, manufacturing speeds from 3 m/min to 5 m/min were demonstrated, showing great potential for a complete, single-pass, wide-web, high-speed, roll-to-roll manufacturing scheme for Sinovia electrodes. We have also investigated the effects of exposure to oxygen and moisture on our electrodes during an 80-day ambient storage experiment. We found that when our electrodes were protected from exposure, no changes to the electrical or optical performance of our material occurred. For the samples exposed to oxygen and moisture, over 80 days, no change in optical performance was observed. Electrical performance declined slowly over time. These results inform us that Sinovia transparent electrodes will be stable in standard device structures, which are encapsulated with moisture and oxygen barriers, and that packaging our films in industry-standard Mylar bags with desiccant will prevent degradation of the films during shipping and storage.
