A Highly Sensitive Photoplethysmography Sensor Enabled by a Highly Transparent Micromesh OLED Fabricated via an In Situ Mask Evaporation Process

Photoplethysmography (PPG) is a noninvasive technique for monitoring physiological signals, but conventional reflective configurations suffer from long and scattered reflection paths, leading to degraded signal quality. In this study, we propose a vertical reflective PPG structure using a transparent organic light-emitting diode (TOLED) with a mesh electrode, enabling reflected light to directly reach the photodiode (PD) through the mesh openings. The TOLED was fabricated by forming a Mg:Ag mesh cathode using a plus-shaped shadow mask, and at a pitch size of 1140 μm, the mesh electrode exhibited high optical transmittance (T₅₅₀ > 80%) and low sheet resistance (≤12 Ω/□), while the overall device maintained good transparency (T₅₅₀ = 71.5%). PPG measurements confirmed that the vertical structure enhanced signal collection efficiency, achieving the highest signal-to-noise ratio (SNR) of 35.18 dB at a pitch of 1140 μm. These results demonstrate the potential of mesh-based TOLEDs as a promising platform for next-generation wearable healthcare and transparent display applications.