The study of flexible electronics has taken many steps forward recently, with large market growth predicted for the future. Ellis Davies investigates the latest in the field.
The continued escalation in demand for compact and lightweight electronic devices is the fuel for the growth of the flexible electronics market. This demand has sparked a range of innovations in the field, ranging from materials consisting of nanosized wire, to self-healing electronic materials.
A recent report by Grand View Research predicted an estimated US$87, 210 million market for flexible electronics by 2024, with increased demand for smart glasses, e-books, smartphones, smart watches, and e-papers expected to provide a boost over the next eight years. ‘There are significant research efforts being made both within the UK and worldwide that are supporting the flexible electronics industry’, says Dr Stuart Higgins, Research Associate in the Optoelectronics Group of the Cavendish Laboratory, University of Cambridge, UK. Can this research keep up with the demand and growth predicted?
Nanowires for touchscreens
The smartphone market is a saturated area, estimated to reach 1.5 billion units shipped in 2016. Perhaps unsurprisingly, the market is facing a supply shortfall in the traditional touchscreen material that relies on electrodes made from indium tin oxide (ITO). Alongside this shortfall, companies such as Samsung, Apple, and Google are investing heavily in flexible electronics for wearable devices.
Potentially offering a solution to both issues, a team from the University of Surrey, UK, in collaboration with Thomas Swan and Oxford based touch-sensor manufacturer M-SOLV Ltd have looked to alternative materials to replace ITO as the main component in multi-touch sensors. Alan Dalton, Professor of Experimental Physics at the University of Sussex, led the research. Speaking to Materials World on the importance of collective work, he said, ‘The collaboration with Thomas Swan (and M-SOLV) came about after an Innovate UK call for proposals under their “Realising the graphene revolution” programme, which aims to fund feasibility studies relating to real-world applications of graphene. With M-SOLV, we had previously developed a technology to create graphene-metal nanowire electrodes that were inexpensive and, in many respects, out-performed ITO in certain applications. Obviously, getting an industrial manufacturer of high-quality graphene on board meant that we could concentrate on optimising the electrode fabrication process in a scalable manner.’
Partnership between academic institutes and manufacturers can often be troublesome, but ‘these types of collaboration are of the upmost importance’. According to Dalton, ‘overcoming the apparent disconnect between academic laboratories and industry is a major bottleneck to commercialising many technologies, including flexible electronics. Getting early input from industry is key. We have worked with M-SOLV for several years on alternative materials for flexible electrodes. The collaboration has been of major benefit, in terms of project and student funding, and having their technologists’ input at a relatively early stage of material or process development has been key to giving our research team focus in our research efforts.’
The study, Predicting the optoelectronic properties of nanowire films based on control of length polydispersity, published in Scientific Reports, showed that silver nanowire not only matched the transmittances and conductivities of ITO, but exceeded them.
The team also discovered that the incorporation of silver nanowires into a multi-touch sensor actually reduced the production cost and energy usage. Dalton elaborated, ‘M-SOLV sell commercial machines to pattern ITO and other transparent electrode materials using laser ablation. For instance, this allows well-defined control of electrode pixilation on touch sensors for smartphones or tablets. Interestingly, using this same process, one can pattern the silver nanowires at much lower laser fluency, thus reducing the energy requirements. Moreover, the cost of the nanowire electrodes is significantly less than that of ITO. This was a conscious goal for our work but only because we had input from M-SOLV at an early stage, highlighting the relative financial issues.’
Other materials were explored as alternatives to ITO. ‘Silver and silver/graphene hybrids are probably the most viable alternatives to existing technologies. Others, as well as ourselves, have studied several alternative materials. The main issue is that the majority of other materials do not effectively compete with ITO or they are too costly to produce, at least at the moment.’ The team is now looking to commercially scale the process.
One of the major barriers facing the advance of flexible electronics is the ability of the existing materials to function following breaking and healing. Materials capable of restoring one function after breaking have been created before. However, the ability to heal a range of functions is crucial for creating effective flexible or wearable electronics. Self-healing materials are able to repair themselves with little or no external influences following physical deformation.
Through a combination of boron nitride nanosheets and plastic polymer, scientists at Pennsylvania State University, USA, have created a material that restores properties needed for continued use as a dielectric in flexible electronics. These include mechanical strength, breakdown strength to protect against surges, electrical resistivity, thermal conductivity and dielectric or insulating properties. The resulting material is tough in comparison to the usual soft self-healing materials. Similar to graphene, boron nitride nanosheets are two-dimensional, although they differ because of their resistance to electricity, while graphene conducts it. Instead of conducting electricity like most self-healing materials, the boron nitrite insulates, giving protection to the microelectronics.
The hydrogen bonds that connect the boron nitride nanosheets give the material its self-healing ability. When two pieces are placed in close proximity, they are drawn closer together by a naturally occurring electrostatic attraction, healing the hydrogen bond and the material. The nanosheets are also impervious to moisture, unlike other hydrogen bonded healable materials. This gives an advantage for implementation into commercial technology, allowing use in environments of high humidity.
Qing Wang, Professor of Materials Science and Engineering at Penn State, and lead researcher on the project, said, ‘Flexible electronics are emerging as a disruptive technology in numerous applications such as wearable devices and implantable sensors. The electronic materials used in flexible devices are susceptible to mechanical deformation-induced damage, resulting in loss of functionality.’
Highlighting the focus of his work, Wang told Materials World, ‘This work reports electronic materials that can be healed efficiently and repeatedly to fully recover multiple functionalities, including mechanical strength, breakdown strength, thermal conductivity and dielectric properties, after mechanical damages. It is also found that, different from conventional self-healing materials, the nanocomposites reported herein can be operative and healed under the condition of high humidity.’
In another innovation, research scientists at Leibniz-Institute for New Materials (INM), Germany, have combined the benefits of organic and inorganic electronic materials to create hybrid inks to be used to print electronic materials. The inks also allow electronic circuits to be applied to paper with a pen.
To create the inks, the researchers coated nanoparticles made of metals with organic conductive polymers, suspending them in mixtures of water and alcohol. These suspensions can then be applied directly to paper using a pen, to dry with no further processing required to form electrical circuits. The combination of metal and nanoparticles combines mechanical flexibility with robustness, increasing electrical conductivity. The organic compounds act as hinges, maintaining conductivity when bent – the combination of materials means that when bent, the electrical conductivity is greater than either would possess independently.
Dalton touched on the viability of flexible electronics for the future, stating ‘The field of flexible electronics encompasses a wide range of technologies that are at different points on the “technology readiness” scale. However, there is a range of disruptor technologies that are already economically and technologically viable compared to existing technologies. For instance, technology using percolating networks of metal nanowires, graphene or conducting polymers as an alternative to traditional transparent electrode materials such as ITO is happening now or for certain applications is just round the corner. These new materials have major benefits in terms of cost and material performance.’
Higgins commented that one of the biggest challenges for flexible electronics is system integration. ‘How do you bring together all of the technical developments of flexible electronics into a complete system? This needs to be done in a way that takes advantage of the benefits of flexible electronics, namely large-area conformable form factors, and desirable manufacturing techniques. Bringing together different processes and materials poses a unique set of challenges that need to be overcome in order to facilitate the uptake of flexible electronics.’ Looking at the greater scope for application of flexible electronics, Higgins mentioned that ‘similar materials and processes can be applied to areas such as smart packaging, medical diagnostics, and sensing systems. These are areas where both the form factor and the possibility of high-throughput manufacturing techniques, such as printing, are well served by flexible electronics.