Polarizer-free polarimetric organic photodetectors for enhanced signal monitoring

Polarimetric photodetectors can discern the polarization state of incident linearly polarized light and have applications in bioelectronics, optical security, and spectral polarization imaging. However, their sensitivity is affected by the intensity attenuation of the polarized light passing through the polarizer. In this study, polarizer-free polarimetric organic photodetectors (P-OPDs) were constructed from stretched polymer semiconductor thin films as electron donors. When the poly[4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b; 4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2,6-diyl] (PTB7-Th) thin films were stretched to 150%, the P-OPDs exhibited the highest responsivity (R ≥ 160 mA W⁻¹) and specific detectivity (D^∗ ≥ 10¹² Jones) at 700 nm among the tested samples. The responsivity and specific detectivity values of the P-OPDs were polarization sensitive, and their highest responsivity ratio (R_0°/R_90°) approached 3.0. When integrated into a wearable circuit, the P-OPDs achieved motion-tolerant photoplethysmography (PPG) monitoring in the absence of polarizers, with a perfusion index (PI) ≥11.5 and enhanced sensitivity. These P-OPD-based wearable PPG sensors promise precise heart rate monitoring without motion artifacts.