New Advances in Wireless Power Transmission Technology for Wearables

Abstract
Due to their inherent flexibility, solution-processable conjugated polymers are increasingly being considered for the cost-effective production of thin-film semiconductor devices used in Internet of Everything (IoE) applications. With considerable improvements in charge carrier mobilities, the final challenge impeding the commercialization of conjugated polymers may be improving their environmental and electrical stabilities. Recent studies have improved the stability of computing devices (i.e., transistors) by eliminating interface traps and water molecules within conjugated polymers. However, the stability issue of Schottky diodes, which play a crucial role in configuring thin-film IoE devices used in wireless communication and energy harvesting, has been largely overlooked. This study reveals that aluminum, which is commonly used as a cathode metal in polymer Schottky diodes, creates a nonstoichiometric effect when deposited on conjugated polymers, thereby leading to the formation of charge traps over time, which reduces the rectification ratio of the Schottky diodes and induces a significant bias stress effect during operation. To address this issue, we introduce a zinc-oxide sacrificial interlayer between the conjugated polymer and cathode. This interlayer effectively eliminates the penetrated Al metal or ionized Al-induced nonstoichiometric effect without reducing the charge injection efficiency, achieving exceptional environmental and operational stability. The printed polymer Schottky diodes demonstrate consistent rectifying operation at 13.56 MHz for several months with negligible changes in electrical characteristics.

A research team, affiliated with UNIST has unveiled a novel technology, capable of wirelessly power wearable devices, hence promising to enhance their longevity and efficiency.

Professor Jimin Kwon and Dr. Yongwoo Lee in the Department of Electrical and Electronic Engineering at UNIST, in collaboration with Professor Sungjune Jung from POSTECH, have developed a method to enhance the stability of polymer Schottky diodes over prolonged periods. This advancement addresses the significant issue of performance degradation in diodes caused by exposure to oxygen and moisture.

By resolving the challenges associated with the interaction between the polymer and metal components, the research team successfully created a thin yet stable Schottky diode. This diode allows for unidirectional current flow, enabling the use of high-performance wearable devices with low power consumption.

Notably, the Schottky diode maintains electrical flow efficiency by introducing an oxide layer between the metal and the polymer. This innovation is expected to significantly advance the development of rapid wireless communication and energy harvesting technologies, as it can reliably process 13.56 MHz signals even on flexible substrates.

The research team conducted extensive experiments to identify the underlying causes of diode performance degradation. They discovered that the primary issue lies in the interaction between the semiconductor layer and the cathode metal layer. These interactions were systematically analyzed using X-ray photoelectron spectroscopy and secondary ion mass spectrometry, in conjunction with electrical analysis.

“While considerable progress has been made in research on wearable devices, stability issues in wireless power transmission remain unresolved,” stated lead author Dr. Lee. “The technology developed in this study offers a flexible wireless power transmission solution that maintains performance for several months, which will be crucial for advancing next-generation wearable devices,” he added.

The findings of this research were published online on July 18 in npj Flexible Electronics, a prestigious international academic journal in the field of electronic engineering. The study received support from the National Research Foundation of Korea (NRF), the Ministry of Science and ICT (MSIT), the Institute of Information and Communication Planning and Evaluation (IITP), and the Korea Innovation Foundation (INNOPOLIS).

Journal Reference
Yongwoo Lee, Boseok Kang, Sungjune Jung, Jimin Kwon, “Stabilizing Schottky junction in conjugated polymer diodes enables long-term reliable radio-frequency energy harvesting on plastic,” npj Flex. Electron., (2024).