Nanostructured and Hybrid Materials Enabled Photodetection Technologies for Biomedical Applications

Optical sensors for biomedical applications typically have to operate under very weak light intensity due to strong scattering of photons by tissues of biological objects. To optimize the sensing performance, the photodetector units of the optical sensors should possess high detectivity and mechanical flexibility, thus allowing for efficient collection of photons over curved skin/tissue surfaces and with high signal-to-noise ratio. In this talk, I will introduce our recent efforts in developing photodetector structures with self-amplification mechanisms, which are particularly suitable for weak light biomedical applications. I will first talk about organic bulk heterojunction (BHJ) phototransistors with a tri-layer gate dielectric design to realize high fabrication reproducibility and excellent light bias stability. Such devices enabled us to demonstrate, for the first time, portable functional near-infrared spectroscopy (fNIRS) for brain monitoring. In the second part, I will extend the discussion to the comparison between polymer/fullerene and polymer/non-fullerene BHJ systems and analyze how the recent material development in the organic photovoltaic community can be leveraged to enhance the performance of organic photodetectors. Finally, I will focus on the commonly seen gain – bandwidth trade-off problem of photodetectors and introduce a monolithically integrated photovoltaic transistor (PVT) design to solve this dilemma. The PVT exploiting a lead halide perovskite as the photoactive layer achieved a record high gain – bandwidth product of ~ 10¹¹. The unique advantage of such devices for fNIRS and other weak light detection applications will be discussed.