Vitality surveillance at distance using thin-film tandem-like narrowband near-infrared photodiodes with light-enhanced responsivity

Abstract

Remote measurement of vital sign parameters like heartbeat and respiration rate represents a compelling challenge in monitoring an individual's health in a noninvasive way. This could be achieved by large field-of-view, easy-to-integrate unobtrusive sensors, such as large-area thin-film photodiodes. At long distances, however, discriminating weak light signals from background disturbance demands superior near-infrared (NIR) sensitivity and optical noise tolerance. Here, we report an inherently narrowband solution-processed, thin-film photodiode with ultrahigh and controllable NIR responsivity based on a tandem-like perovskite-organic architecture. The device has low dark currents (<10^-6 mA cm^-2), linear dynamic range >150 dB, and operational stability over time (>8 hours). With a narrowband quantum efficiency that can exceed 200% at 850 nm and intrinsic filtering of other wavelengths to limit optical noise, the device exhibits higher tolerance to background light than optically filtered silicon-based sensors. We demonstrate its potential in remote monitoring by measuring the heart rate and respiration rate from distances up to 130 cm in reflection.

Fig. 1.. Tandem-like device architecture, working principle, and materials.

(A) Schematics of PD architecture. (B) Illustration of optical field distributions for visible and NIR wavelengths in the two optically active layers based on FAMAPbI₃ visible absorber and PM6:Y6 NIR absorber. For visible λ up to 650 nm, photons are predominantly absorbed by the perovskite film, while, for NIR λ (850 nm), photon absorption occurs within the organic BHJ layer and is subjected to cavity effects. (C) Schematics of the visible wavelengths filtering the tandem-like PD. HTL and ETL stand for hole and electron transport layer, respectively. The collection of photocarriers generated in the perovskite film is blocked by the perovskite/PFN-Br interface. (D) EQE as a function of wavelength (colored circle) of tandem-like devices using different perovskite/BHJ combinations, as indicated in the legend. EQE spectrum is compared with that of single PPD (black line) and single OPD (red line) and with the difference of their EQE shapes (dotted black line with colored area).

Fig. 2.. Performance of narrowband PDs based on tandem-like architecture with FAMAPbI₃ and PM6:Y6 active layers.

(A) J-V characteristics in the dark and under low light intensity illumination (0.5 mW cm⁻²) for different wavelengths; in the dark, solid circles are current density values derived from constant voltage measurements over time at discrete biases (fig. S2). (B) Linearity plot measured at 0 V showing J_ph as a function of NIR light intensities (850 nm). (C) Continuous tracking over 8 hours of normalized transient photocurrent response of the device upon 850-nm light pulses of 1 Hz, corresponding to 60 bpm, a typical resting heartbeat.

Fig. 3.. Enhancement of NIR sensitivity by green light illumination.

(A) EQE as function of wavelength measured with and without additional green (540 nm) light illumination, showing an enhancement in the NIR region. Green light intensities are indicated in the legend. (B) Corresponding EQE max (∼830 nm) obtained with and without green illumination plotted as a function of applied bias voltage. For comparison, EQE_max under the additional 60 mW cm⁻² 540-nm light is considered. The inset shows the relative enhancement of EQE due to the additional green light as a function of applied bias expressed as the ratio of EQE_max with and without green light. Data are extracted from EQE spectra (fig. S6). (C) Spectral responsivity (SR) of tandem-like PD (under 60 mW cm⁻² 540-nm illumination) compared to that of commercial Si PD. Colored circles indicate the SR at 850 nm for our device, Si and ideal PDs having 100% EQE. (D) Detectivity at different wavelengths measured at 0 V for tandem-like PD under additional green light (blue solid line) and for commercial Si reference PD (i_n = 1.2 × 10⁻¹⁴ A Hz^−1/2) (gray dashed line). Graph in semilogarithmic scale is provided in fig. S10. (E) Comparison of noise current–based specific detectivity (D^*) of our device with state-of-the-art NIR narrowband solution–processed photodetectors (both PD and photomultiplication types, indicated in the legend as PD-type and PM-type, respectively) and with main commercial inorganic PDs, namely, Si (Thorlabs, FDS100-CAL), Ge (Thorlabs, FDG03-CAL), and InGaAs (Thorlabs, FGA21-CAL). The comparison focuses on reported devices with design wavelength lying in the NIR region between 700 and 1200 nm. Further details are provided in fig. S10.

Fig. 4.. Detecting weak PPG signals using the tandem-like PD with enhanced NIR sensitivity.

(A) Schematic overview of the experimental conditions under which a PPG signal has been measured by the tandem-like PD with enhanced NIR sensitivity by green light illumination. As NIR light source, we used a LED emitting at 940 nm, i.e., at the edge of device SR, located at 1 or 2 cm above the finger. The PD was place instead ∼1.5 cm below the finger, while the green LED (540 nm) was placed within this gap. In addition, an optical long-pass filter (λ > 590 nm) was placed underneath the finger to prevent unwanted PPG signal generation from the direct interaction of green light with the latter. (B) PPG signal measured in transmission from the finger as described in (A) without and with enhanced NIR sensitivity enabled by green light illumination. For the experiment, the green LED was driven at 1.5 V. (C) Enlargement of the PPG waveform indicated by the gray outline in (B), which shows the typical PPG waveform features, such as the systolic and diastolic peaks (and phases), dicrotic notch, and pulse width. a.u., arbitrary units.

Fig. 5.. Demonstration of vital sign measurements at distance for constant, wireless monitoring.

(A) Schematic illustration of the application of the tandem-like PD in a hospital bed for heart and respiration rate monitoring at distance of a patient during sleep/resting time. The optical noninvasive wireless setup that can be integrated in the hospital’s equipment and maximize the comfort of the patient. (B) Normalized PPG signal measured at different PD-finger distances using NIR light (850 nm). (C) Normalized respiration rate signal measured through clothing at different PD-chest distances using NIR light (850 nm). (D) Comparison of PPG signals measured at distance (d = 50 cm) under the same experimental conditions using our tandem-like device and a broadband commercial Si PD without and with 780- and 830-nm optical filters to suppress ambient optical noise.