Highly sensitive water pollution monitoring using colloid-processed organic photodetectors

Water stands at the heart of the Sustainable Development Goals, and risk management for clean water often requires in situ monitoring of contaminants, where a portable photodetector with high detectivity is highly desired. The emerging portable organic photodetectors (OPDs) with intrinsic flexibility and light weight, which are promising candidates for in situ water pollution monitoring, often exhibit unsatisfactory specific detectivity (D^*) lower than 10¹³ Jones, failing to meet monitoring requirements set below safety limits (~0.01–0.1 mg l⁻¹) for pollutant discharge. Here we present OPDs with D^* as high as 4.1 × 10¹³ Jones made from dense and ordered polymer films via colloid processing, attributed to their substantially lower energy disorder and fewer traps than those deposited via traditional solution processing. Detectable light intensities as weak as approaching 0.1 pW cm⁻² of colloid-processed OPDs (CP-OPDs) at visible to near-infra-red light are achieved, on par with the premium commercial Si photodiode and overwhelming some widely used models. Highly sensitive and accurate monitoring of trace dye pollutants/chlorophyllin A analogue in water with a detection limit of 0.02/0.05 mg l⁻¹ (below the discharge/eutrophication standard) is successfully accomplished through the developed CP-OPDs, surpassing the commercial Si photodiode that cannot effectively respond to the concentration below the standard. Colloid processing endows OPDs with the highest detectivity among reported portable photodetectors and makes CP-OPDs superior in in situ monitoring trace contaminants in water relative to traditional methods.