Researchers have developed and demonstrated new green light-absorbing transparent organic photodetectors that are highly sensitive and compatible with CMOS fabrication methods. Incorporating these new photodetectors into hybrid organic-silicon image sensors could be useful for many applications. These applications include light-based heart rate monitoring, fingerprint recognition, and devices that detect the presence of nearby objects.
Whether used in scientific cameras or smartphones, most of today’s imaging sensors are based on CMOS technology and inorganic photodetectors that convert light signals into electrical signals. Although photodetectors made of organic materials are attracting attention because they can help improve sensitivity, as so far, it has proven difficult to make high-performance organic photodetectors.
Sungjun Park of Asia University in Korea, who co-led the research team, said, “Incorporating organic photodetectors into mass-produced CMOS image sensors requires organic light absorbers that are easy to mass-produce and can accomplish vivid image recognition to produce clear images at high frame rates in the dark. We have developed transparent green-sensitive organic photodiodes that can meet these requirements.”
The researchers described the new organic photodetector in Optica on Aug. 25 local time. In addition, they created a hybrid RGB imaging sensor by superimposing a 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 greatly reduces crosstalk between pixels of different colors due to the introduction of a hybrid organic buffer layer,” said Kyung-Bae Park, co-leader of the research team from Samsung Advanced Institute of Technology (SAIT) in South Korea. “This new design allows high-performance organic photodiodes to become a major component of imaging modules and photodetectors used in a variety of applications.”
A more practical organic photodetector
Most organic materials are not suitable for mass production because they are temperature sensitive. They either cannot withstand the high temperatures used for post-processing or become unstable when used at moderate temperatures for long periods of time. To overcome this challenge, scientists have focused on modifying the buffer layer of photodetectors to improve stability, efficiency and detectability. Detectability is a measure of how well a sensor can detect weak signals.
“We introduced a bath copper spirit (BCP):C60 hybrid buffer layer as the electron transport layer,” said Sungjun Park, “which gives the organic photodetector special properties, including higher efficiency and very low dark current, which reduces noise. ” This photodetector can be placed on a silicon photodiode with red and blue filters to create a hybrid image sensor.
The researchers demonstrated that the new photodetector exhibited detection rates comparable to those of conventional silicon photodiodes. The detectors were stable for 2 hours at temperatures above 150 °C and showed long-term operational stability of 30 days at 85 °C. These photodetectors also exhibit good color performance.
As a next step, they plan to customize new photodetectors and hybrid image sensors for various applications such as mobile and wearable sensors, proximity sensors and fingerprint devices on displays.
