Researchers fabricated green-absorbing transparent organic photodetectors that are highly sensitive and compatible with CMOS fabrication methods. They combined one of the green organic photodetectors with a red- and blue-sensitive silicon photodiode to create an organic-silicon hybrid RGB imaging sensor. [Image: S. Park, Ajou University]
Inorganic photodetectors are a key component of today’s CMOS-based imaging sensors. Incorporating their organic counterparts could boost the sensitivity of these devices, but fabricating them at a large enough scale for commercial viability has been a challenge.
Now researchers at two laboratories in the Republic of Korea have designed transparent organic photodetectors that absorb green light and are compatible with CMOS manufacturing processes (Optica, doi: 10.1364/OPTICA.449557). These highly light-sensitive devices have low dark current and easily endure high temperatures. Eventually, they could find their way into many medical and security devices.
Easier manufacturing, better temperature resistance
Scientists in South Korea and elsewhere have been vigorously working on developing transparent organic photodetectors for multiple applications. However, it’s been difficult to find materials that withstand high post-processing temperatures and exhibit high external quantum efficiency.
The team that produced the current work, led by Sungjun Park of Ajou University and Kyung-Bae Park of the Samsung Advanced Institute of Technology (SAIT), found that the key ingredient to a better organic photodetector was a combination bathocuproine and C60 mixed-buffer layer as the electron-transporting layer (ETL) material.
The researchers built up the organic photodetector by sequentially depositing layers of organic bulk heterojunction materials and the ETLs on a glass substrate coated with indium tin oxide. The team measured the device’s active pixel size as 0.04 cm2.
After analyzing parameters such as signal-to-noise ratio and quantum efficiency, the group stacked the green-sensitive organic photodetector material on top of blue- and red-sensitive silicon photodetectors to create a hybrid image sensor, then measured color rendering indices and average visible transmittances. The organic ETL minimized the optical loss of the device, gave it a smooth shape and improved its thermal stability. The organic photodetector continued working after sitting in a 150 °C environment for two hours and after being exposed to 85 °C heat for 50 days.
“Narrow and selective green-light absorption”
According to the study’s authors, the most promising feature of the organic photodetector is “its narrow and selective green-light absorption.” Its spectral full width at half maximum is only 100 nm, which allows for good transmission of red and blue wavelengths.
Potential applications for the photodetector include heart-rhythm and blood-oxygen monitoring, fingerprint reading, optical switches and imaging.