High-Performance Wearable Organic Photodetectors by Molecular Design and Green Solvent Processing for Pulse Oximetry and Photoplethysmography.

White-light detection from the visible to the near-infrared region is central to many applications such as high-speed cameras, autonomous vehicles, and wearable electronics. While organic photodetectors (OPDs) are being developed for such applications, several challenges must be overcome to produce scalable high-detectivity OPDs. This includes issues associated with low responsivity, narrow absorption range, and environmentally friendly device fabrication. Here, an OPD system processed from 2-methyltetrahydrofuran (2-MeTHF) sets a record in light detectivity, which is also comparable with commercially available silicon-based photodiodes is reported. The newly designed OPD is employed in wearable devices to monitor heart rate and blood oxygen saturation using a flexible OPD-based finger pulse oximeter. In achieving this, a framework for a detailed understanding of the structure-processing-property relationship in these OPDs is also developed. The bulk heterojunction (BHJ) thin films processed from 2-MeTHF are characterized at different length scales with advanced techniques. The BHJ morphology exhibits optimal intermixing and phase separation of donor and acceptor moieties, which facilitates the charge generation and collection process. Benefitting from high charge carrier mobilities and a low shunt leakage current, the newly developed OPD exhibits a specific detectivity of above 10  Jones over 400-900 nm, which is higher than those of reference devices processed from chlorobenzene and ortho-xylene.