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 10 13 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.