Fully Printed ZnO Photosensors for Next Generation User Interfaces

Printing techniques have improved the conformability and flexibility of electronic devices. With on demand fabrication, the cost of printed electronics is lower than that of conventional electronics, and their impact to the environment is improved, with less material wastage and lower temperature processes.[1] These properties allow for electronic functionality to be incorporated into everyday objects, thus bridging the gap between the physical and the digital domain without compromising the users’ experience.<br/>One of the applications enabled by printed electronics is that of augmented paper. We have created a sustainable and manufacturable ecosystem, the Magic Bookmark, for linking physical paper books, with digital media and the web.[2] The system comprises of photosensors embedded on an electronic bookmark, which detect a different light pattern for each open page. For a seamless users’ experience, we propose replacing the off-the-self sensors with fully printed photodetectors. This will make the system more flexible, better integrated with the book, and allow for a more streamlined fabrication process.<br/>In this work, we are exploring the use of Al doped ZnO for creating photodetectors that will be used for user interfaces, such as the Magic Bookmark. ZnO has been selected for its wide band gap and photosensitivity. A nanoparticle based printable solution has been used, with a work function of 3.6±0.1eV. The proposed structure is a planar metal-semiconductor-metal interface, which is easily scaled and printed with very few manufacturing steps (2 layers of materials). Interdigitated electrodes have been used to increase the conduction of the final device, since the photogenerated charge will be the dominant source of current in this architecture, due to Schottky barriers in both metal-semiconductor interfaces.<br/>The proposed device was first tested with evaporated Au electrodes. The current was low (approximately 9x10^-11A under 4.5V bias and UV illumination), and the photosensitive effect undetected. This was due to the high work function of Au, at approximately 5.1eV,[3] which created large Schottky barriers that the photo generated charge was not sufficient to overcome. Simultaneously Au is not a suitable metal considering the final goal for scalability and printed manufacturing on-demand. Sputtered Al electrodes (work function of 4.28eV)[3] showed promising results, with currents between 25-40nA under 4.5V bias, and an on/off ratio of around 46% between dark conditions, and environmental light of 250lux. The structure was also tested with Ag (work function of 4.26eV)[3], which can be printed for scalability. Under a 10V bias the dark current was approximately 0.85nA, and the current under a 250lux light was 50nA.<br/>In conclusion, we have indicated that such a simple architecture combined with printing techniques can be optimised for scalable, power efficient solutions to a versatile user interface system that requires light detection. Due to the nature of the printing process, devices are non-uniform. However, the use of digital codes in the example of the Magic Bookmark allows for these to be used. The use of a printed intermediate layer, such as graphene, between the metal and the semiconductor, enables higher currents, improving conduction. In that case, the ZnO acts as a modulating photosensitive gate. The use of Ag allows for a simpler fully printed sensor. Finally, we are demonstrating a version of the Magic Bookmark with the developed sensors.<br/>[1] Y. Khan et al., “Flexible Hybrid Electronics: Direct Interfacing of Soft and Hard Electronics for Wearable Health Monitoring,” Adv. Funct. Mater., vol. 26, no. 47, pp. 8764–8775, Dec. 2016.<br/>[2] G. Bairaktaris et al., "Magic Bookmark: A Nonintrusive Electronic System for Functionalizing Physical Books", Adv. Int. Sys., p. 2100138, 2021.<br/>[3] H. Michaelson, "The work function of the elements and its periodicity", Journal of Applied Physics, vol. 48, no. 11, pp. 4729-4733, 1977.