High‐Quality Photoplethysmography Signal Enabled by Narrowband Red Phosphor Converted Light‐Emitting Diodes for Accurate Blood‐Pressure Monitoring

ABSTRACT Photoplethysmography (PPG) is widely used for noninvasive cardiovascular monitoring, yet raw‐signal quality is often limited by the broadband emission and thermal spectral drift of conventional red LEDs, increasing reliance on complex downstream processing. Here, we develop a red light source by integrating an Rb 2 LiGaF 6 :Mn 4+ (RLGF:Mn 4+ ) microcrystal phosphor with a blue InGaN chip to form a narrowband phosphor‐converted LED (pc‐LED, FWHM = 13.4 nm) as a replacement for a commercial red module. The crystallized RLGF:Mn 4+ microcrystals deliver a high external quantum efficiency (EQE = 52%) and enable clearer structure–emission correlation via experimentally resolved dual‐site Mn 4+ emission. The resulting pc‐LED exhibits high electro‐optical efficiency (η = 32.03%) and minimal thermal spectral drift (peak shift ≤ 0.3 nm from 20°C–100°C). Using this narrowband, thermally stable light source in a wearable PPG prototype, we quantitatively demonstrate improved signal quality and more stable VPG/APG feature localization compared with a commercial red module. Finally, we provide a proof‐of‐concept demonstration of blood‐pressure estimation by lightly training a basic convolutional neural network, achieving mean absolute errors of 2.86 mmHg for SBP and 2.40 mmHg for DBP on an independent test set. This work establishes a materials‐to‐device pathway for improving PPG quality through optical spectral engineering.