Organic photodetector
performance for
enhancing the sensing abilities of an organic photoplethysmography
sensor was investigated. The optimized organic photodetector with
an anode interlayer and a cathode interlayer showed a reverse dark
current density of 22 nA cm–2 at −2 V and
an external quantum efficiency of 53.3% at 0 V. This organic photodetector
was fabricated monolithically with an organic light-emitting diode
on a glass substrate to achieve a reflectance-mode photoplethysmography
sensor, demonstrating the impact of organic photodetector device performance
on the measured photoplethysmography signal for sensing applications.
Furthermore, we estimated the optimal sensor design for circular geometry
in terms of device area and distance between the organic light-emitting
diode and organic photodetector to maximize the signal-to-noise ratio
and lower the power consumption of organic photoplethysmography sensor
devices. For the most favorable photoplethysmography sensor design,
a signal strength of 130 mV with 600 μW power consumption was
measured.
The electrical performance of the back-channel etched Indium–Gallium–Zinc–Oxide (IGZO) thin-film transistors (TFTs) with copper (Cu) source and drain (S/D) which are patterned by a selective etchant was investigated. The Cu S/D were fabricated on a molybdenum (Mo) layer to prevent the Cu diffusion to the active layer (IGZO). We deposited the Cu layer using thermal evaporation and performed the selective wet etching of Cu using a non-acidic special etchant without damaging the IGZO active layer. We fabricated the IGZO TFTs and compared the performance in terms of linear and saturation region mobility, threshold voltage and ON current (ION). The IGZO TFTs with Mo/Cu S/D exhibit good electrical properties, as the linear region mobility is 12.3 cm2/V-s, saturation region mobility is 11 cm2/V-s, threshold voltage is 1.2 V and ION is 3.16 × 10−6 A. We patterned all the layers by a photolithography process. Finally, we introduced a SiO2-ESL layer to protect the device from external influence. The results show that the prevention of Cu and the introduced ESL layer enhances the electrical properties of IGZO TFTs.
The pulse oximetry device has been used for decades to monitor human pulse rate and oxygen saturation. There are two types of pulse oximetry which are transmission and reflection based. However, most devices are unsuitable for daily health monitoring due to the bulkiness and inconvenience of long-term monitoring while continuously doing everyday activities. Therefore, developing a wearable device such as a patch would benefit the users. Several factors can be considered for such a system. One of them is the distance between the source and detector since both are the major components of this system. However, there is still a lack of information in this regard. This study used the ray-tracing Monte Carlo method to simulate transmittance and reflectance-based oximetry principles with a 663 nm wavelength as the light source. The results show the ray tracing behavior from the light source to the photodetector in the biological tissue under two different structures mentioned previously. The separation between the light source and the detector should be less than 3 mm for the reflection type. A significant difference was observed for a distance greater than 3 mm compared with the transmission-based, which has a higher photocurrent even at a 7 mm distance. However, this transmission-based device is limited to the placement of the device on the body part. It is due to the thickness, which varies depending on the body parts themselves. Therefore, wearable pulse oximetry devices with the reflectance-based principle are better due to higher signal acquisition than the transmittance-based, especially for the daily health monitoring system. Furthermore, it also can be used throughout any body part. This reflection-based device can fully utilize microfabrication to integrate the light source and photodetector.Keywords: PPG sensor, Monte Carlo, tissue optics, pulse oximetry, photoplethysmography.
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