KEYWORDS: Thin films, Microscopes, Metrology, Thin film solar cells, Thin film devices, Atomic force microscopy, Transmission electron microscopy, Phase measurement, Laser interferometry, Transistors
A transmission-mode high sensitivity quantitative phase microscope (QPM) is developed for profiling transparent thin film structures. The QPM system is implemented with a common-path interferometry design and a high well-depth camera, which has allowed us to achieve an optical path length difference sensitivity of around 50 picometers. A frame averaging method can be used to further improve the sensitivity. To account for multiple interference within thin films, a transmission matrix model is developed to achieve accurate height profile reconstruction. With the correction model, the profiling accuracy can be improved from 20.6% to 4.0% for a MoS2 thin film with a thickness of around 25 nm.
Near-infrared high-sensitivity photodetectors are the key component of wearable optical systems for noninvasive physiological monitoring, such as photoplethysmography (PPG) and near-infrared spectroscopy. Compared to the high-voltage driven avalanche photodiodes and photomultipliers, organic phototransistors based on a bulk heterojunction (BHJ) structure have a set of unique advantages including self-amplification (via a photoconductive gain mechanism), low operation voltage, lightweight, flexible, printable and CMOS compatible. By employing a bilayer dielectric design and an ultrathin encapsulation structure, we have realized a flexible/epidermal low-voltage (< 3V) driven BHJ phototransistor with ultra-high responsivity (3.5 ×10^5 AW^-1) and low noise equivalent power (1.2 × 10^−15 W Hz^−1/2). We combined the phototransistor with a high-efficiency III-V LED to realize a hybrid PPG sensor and demonstrated low-power and high stability continuous tracking of heart rate variability and pulse pressure. To reveal the fundamental correlations between the heterojunction morphology and the device’s figures-of-merits, such as responsivity, operation bandwidth, and noise, we carried out a systematical investigation combining morphological characterizations with photo-physics and charge transport studies. The results highlight the importance of optimizing interface charge separation and bulk charge transport through morphology control. This study not only reveals the physical mechanisms that govern the operation of organic phototransistors but also provides know-hows to realize highly flexible and stable photodetection systems.
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