Low‐Cost Copper Electrode for High‐Performance Panchromatic Multiplication‐Type Organic Photodetectors with Optical Microcavity Effect

Low‐cost and flexible panchromatic organic photodetectors (OPDs) are one of the most promising alternatives in next‐generation wearable electronics, but they still face the formidable challenges of replacing brittle indium tin oxide electrode and suffer from low near‐infrared (NIR) photo‐response. Herein, the low‐cost metal copper (Cu) is used as a semi‐transparent anode to fabricate high‐performance panchromatic multiplication‐type OPDs. Because of the advantages of smooth surface, lower sheet resistance, and good transmittance, high‐quality optical resonant cavity forming between Cu anode and aluminum cathode greatly enhanced the weak sub‐bandgap response of intermolecular charge transfer states in NIR region, while also retaining a good response in the UV–vis region. Due to the good hole‐collecting ability, the Cu electrode is suitable for realizing the photo‐multiplication effect. Accordingly, the resulting OPDs achieve a panchromatic response ranging from the UV (300 nm) to NIR (900 nm) region. The maximum external quantum efficiency (EQE) reaches 117 040% at 350 nm, and the relatively high EQE of 25 468% is realized even at 765 nm. Furthermore, a flexible OPD is successfully fabricated by using polyethylene terephthalate substrate with Cu anode to achieve the real‐time detection of human blood oxygen pulse signals. A microcavity structure based on a low‐cost copper semi‐transparent electrode is adopted to manipulate the optical field distribution in organic photodetectors and enhance the absorption of intermolecular charge transfer states in the near‐infrared region to broaden the device response range. The introduction of the multiplication mechanism greatly improves the external quantum efficiency of the device, and finally yields high‐performance panchromatic organic photodetectors.