Elucidating the Detectivity Limits in Shortwave Infrared Organic Photodiodes

While only few organic photodiodes have photoresponse past 1 mu m, novel shortwave infrared (SWIR) polymers are emerging, and a better understanding of the limiting factors in narrow bandgap devices is critically needed to predict and advance performance. Based on state-of-the-art SWIR bulk heterojunction photodiodes, this work demonstrates a model that accounts for the increasing electric-field dependence of photocurrent in narrow bandgap materials. This physical model offers an expedient method to pinpoint the origins of efficiency losses, by decoupling the exciton dissociation efficiency and charge collection efficiency in photocurrent-voltage measurements. These results from transient photoconductivity measurements indicate that the main loss is due to poor exciton dissociation, particularly significant in photodiodes with low-energy charge-transfer states. Direct measurements of the noise components are analyzed to caution against using assumptions that could lead to an overestimation of detectivity. The devices show a peak detectivity of 5 x 10(10) Jones with a spectral range up to 1.55 mu m. The photodiodes are demonstrated to quantify the ethanol-water content in a mixture within 1% accuracy, conveying the potential of organics to enable economical, scalable detectors for SWIR spectroscopy.