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Chung-Ang University Researchers Unveil the Mechanism of Noise and Signal Detection, Enhancing Organic Photodetector Performance for Advanced Image Sensors

Researchers interpret the dominant mechanism for unexpected charge injection in organic photodetectors

The performance of organic photodetectors (OPDs) is degraded by noise. Although charge injection is the major source of dark current in the photodetector, few studies have been conducted to determine the noise origin in OPDs. To this end, researchers from Chung-Ang University, Korea, have experimentally interpreted the dominant mechanism of dark current generated in OPDs with theoretical calculation, paving the way for improved photoconductors.

Title: Researchers determine the dominant mechanism for dark current generation in organic photodetectors

Caption: In a new study, Chung-Ang University researchers experimentally interpreted the mechanism of noise and signal detection in organic photodetectors with theoretical calculation. Their findings will facilitate development of better devices with faster response as well as excellent detection.

Credit: Copyright (2023) Advanced Functional Materials.

License type: Wiley Online Library (The current cover photo is unpublished. It is scheduled for January. The paper was published online.) (https://onlinelibrary.wiley.com/doi/10.1002/adfm.202209615)

Organic photodetectors (OPDs) are the "eyes" of the image sensor and can be applied to various diode systems for next-generation advanced devices. The distinct advantages of OPDs over conventional inorganic photodetectors are their extreme flexibility and low power requirement. However, noise in OPDs impedes performance. Therefore, noise reduction is important for enhancing the performance of OPDs.

Several mechanisms potentially induce the origin of noise. Among them, thermionic emission (the flow of charged particles from a charged metal surface, owing to the thermal energy overcoming the electrostatic binding forces) and direct electron tunnelling (a quantum-mechanical process by which an electron can penetrate a potential barrier even when the energy of the particle is lower than the barrier height), are possible models at low applied bias. However, the dominant noise-causing mechanism remained hitherto unknown.

To this end, a research collaboration led by Professor Dong Hwan Wang and his post-doctoral researcher Dr. Woongsik Jang from the School of Integrative Engineering in Chung-Ang University, Korea investigated dark current in OPDs by experimentally modeling mechanisms related with the thermionic emission and tunneling to ascertain the dominant cause of noise generated in an OPD. The researchers presented their findings in a recent article made available online on November 4, 2022, in Advanced Functional Materials. In addition, the scientific achievements of this study were introduced through the domestic magazine.

“Despite many efforts to reduce noise in OPDs, little work has been done to identify the mechanism of the origin of noise.”, explains Prof. Wang. The team experimentally demonstrated that the formation of a barrier energy for charge carriers formed as the “Schottky barrier”, a metal–semiconductor junction at the electrode–photosensitive layer interface, bias and thermal energy together enable charge injection from one electrode to the other. This, in turn, becomes dark current.

The researchers analyzed correlations between current and voltage to demystify the origin of noise. They confirmed the relationship between current and barrier energy by controlling three acceptors with different energy levels by adjusting the barrier energies. The researchers unearthed a linear relationship between the natural logarithm of the dark current density under reverse bias and the square root of the Schottky barrier height. This finding strongly points to direct tunneling as the dominant mechanism for dark current injection.

Armed with the mechanism of the origin of noise, the team developed an OPD with an efficient injection barrier that enhances detection speed owing to noise suppression. The OPDs developed in this research can be expanded to facilitate a curved design allowing a miniature form factor without color filter. “We could achieve these incredible features owing to interpretation of noise generated in OPDs. The curved design OPDs developed are capable of 100% omnidirectional sensing without the constraints of lens design and can detect singular wavelengths”, notes Prof. Wang.

Taken together, this development experimentally demonstrates the primary, dominant mechanism of noise in OPDs with theorical calculation, which leads to fast response as well as excellent detection. This enables high integration density of hundreds of millions of pixels owing to the miniaturization of the image sensor by introducing the OPDs. Further research on the dominant noise-causing mechanism contributes to suppress dark current for improved OPD performance in advanced applications.

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Chung-Ang University

About Chung-Ang University

Chung-Ang University is a private comprehensive research university located in Seoul, South Korea. It was started as a kindergarten in 1916 and attained university status in 1953. It is fully accredited by the Ministry of Education of Korea. Chung-Ang University conducts research activities under the slogan of “Justice and Truth.” Its new vision for completing 100 years is “The Global Creative Leader.” Chung-Ang University offers undergraduate, postgraduate, and doctoral programs, which encompass a law school, management program, and medical school; it has 16 undergraduate and graduate schools each. Chung-Ang University’s culture and arts programs are considered the best in Korea.

Website: https://neweng.cau.ac.kr/index.do

About Professor Dong Hwan Wang and Dr. Woongsik Jang

Prof. Dong Hwan Wang obtained his PhD degree in Chemical and Biomolecular Engineering from KAIST in 2012 and joined the group of Professor Alan J. Heeger (Novel Prize Winner in Chemistry, 2000) at the University of California, Santa Barbara in 2012–2014 as a postdoctoral research fellow. He is currently a professor at the School of Integrative Engineering in Chung-Ang University since 2014. He is full professor from 2020 with over 130 published SCI papers. His research interests include solution processed organic electronics such as organic and perovskite solar cells, organic photodetectors fabricated by newly designed nanomaterials, and nanostructure with specific optoelectronic properties.

Dr. Woongsik Jang is currently post-doctoral researcher of Prof. Dong Hwan Wang’s group at the school of integrative engineering in Chung-Ang university. He received his Ph.D. degree in 2022, and his research interests include organic/perovskite solar cells and organic photodetectors. He is an excellent student in CAU with currently published 50 important SCI papers.