Researchers from multiple institutions in South Korea developed and demonstrated green-light-absorbing transparent organic photodetectors that are highly sensitive and compatible with CMOS fabrication methods. Incorporating these photodetectors into organic-silicon hybrid image sensors could be useful for applications such as light-based heart-rate monitoring and fingerprint recognition, and for devices that detect the presence of nearby objects. Whether used in smartphones or scientific cameras, most modern image sensors are based on CMOS technology and inorganic photodetectors that convert light signals into electronic signals. Although photodetectors made from organic materials are attracting attention because they can help boost sensitivity, for example, high-performance organic photodetectors have proven difficult to fabricate. A conceptual image of an organic-silicon hybrid RGB imaging sensor based on a green-light-absorbing transparent organic photodetector. The newly developed photodetectors are compatible with CMOS fabrication methods. The researchers said their design could allow high-performance organic photodiodes to become the main component for the imaging modules and optoelectronic sensors used in mobile and wearable sensors, as well as for other applications. Courtesy of Sungjun Park, Ajou University. "For organic photodetectors to be incorporated into mass-produced CMOS image sensors requires organic light absorbers that are easy to fabricate on large scales and can accomplish vivid image recognition and produce distinct images in the dark with a high frame rate," said Sungjun Park, who co-led the research team. "We developed transparent green-sensitive organic photodiodes that can meet these requirements." The researchers also created a hybrid RGB imaging sensor by superposing the transparent green-absorbing organic photodetector onto a silicon photodiode with red and blue filters. “The green-selective light-absorbing organic layer used in these image sensors considerably reduced crosstalk between the different colored pixels thanks to the introduction of a mixed organic buffer layer,” said research team co-leader Kyung-Bae Park from the Samsung Advanced Institute of Technology (SAIT). “This new design could allow high-performance organic photodiodes to become the main component for imaging modules and optoelectronic sensors used in a variety of applications.” Most organic materials are not suitable for mass production because they cannot withstand the high temperatures used for post-processing, or they become unstable during long-time use at moderate temperatures. To overcome this challenge the researchers focused on modifying the photodetector's buffer layer to improve stability, efficiency, and detectivity — a measure of how well a sensor detects weak signals. “We introduced a bathocuproine (BCP):C60 mixed buffer layer as an electron transporting layer,” Park said. “This gave the organic photodetectors exceptional characteristics, including higher efficiency and an extremely low dark current, which reduces noise.” Additionally, the photodetector can be placed on a silicon photodiode with red and blue filters to create a hybrid image sensor. The team's photodetectors exhibited a detectivity comparable to that of conventional silicon photodiodes. The detectors operated stably under temperatures above 150 °C for two hours and showed long-term operational stability at 85 °C for 30 days. The photodetectors also exhibited good color expression. The team plans to tailor the photodetectors and the hybrid image sensor for use in various applications such as mobile and wearable sensors, including CMOS image sensors, as well as proximity sensors and fingerprint-on-display devices. The research was published in Optica (www.doi.org/10.1364/OPTICA.449557).