Synergistically Enhancing Light Harvesting and Mechanical Flexibility for Ultra-Flexible Organic Biosensors.

Ultra-flexible organic optoelectronic biosensors, as key components of next-generation wearable electronics, require devices that sufficiently harvest light, tolerate oblique illumination, and maintain mechanical compliance under bending or stretching. Herein, we report a solvent-vapor spin-coating (SVS) strategy combining chloroform:methanol mixed solutions, which simultaneously induces a shallow island-like surface modulation and optimizes molecular packing. The former improves light harvesting and reduces angular sensitivity, whereas the latter contributes to improved optoelectronic performance; together, they are also associated with enhanced mechanical flexibility. Consequently, organic photovoltaics (OPVs) based on this strategy achieve a power conversion efficiency (PCE) of 20.28% on rigid substrates; more importantly, ultra-flexible devices exhibit a record PCE of 19.03%, accompanied by improved mechanical robustness and reduced angular sensitivity. For organic photodetectors (OPDs), the enhanced light-harvesting translates to a high specific detectivity (D*) exceeding 1013 Jones across the 320-920 nm range, and a response time of < 10 µs. Finally, for the first time, we demonstrate a top-illumination/top-emission ultra-flexible photoplethysmography (PPG) sensor by integrating an OPV module, an organic light-emitting diode (OLED), and an OPD, which accurately records on-skin pulse signals. This study provides a promising route to co-optimize the power output, detection sensitivity, and mechanical ductility of organic optoelectronics for practical applications.