Significant Dark Current Suppression in Organic Photodetectors Using Side Chain Fluorination of Conjugated Polymer

Dark current density (J(dark)) is the most important factors determining signal-to-noise ratio, linear dynamic range and detectivity (D*) of organic photodetectors (OPDs). However, the solution-processed OPDs generally suffer from high J(dark) under bias because the origin of J(dark) still remains unclear and related to complicate factors. In this work, the effect of fluorinated alkyl side chain (FAC) of conjugated polymers (CPs) on the OPD performance is systematically investigated according to the number of fluorine atom. The OPDs, comprising the CPs with a FAC (CP-FAC), exhibit at least two orders lower J(dark) (approximate to 3.73 x 10(-10) A cm(-2) at -2 V) compared to that without FAC while significantly maintaining high external quantum efficiency (approximate to 78%). This is because the side chain fluorination leads to the formation of a more ordered molecular structure of the CPs, which reduces the trap density and energetic disorder, thus resulting in the effective suppression of J(dark) and in the enhancement of charge carrier mobility. The CP-FAC based OPD exhibits an exceptionally high D* (approximate to 3.39 x 10(13) cm Hz(1/2) W-1 at -2 V) and a reasonable response speed (f(-3db) = approximate to 56.31 kHz). Furthermore, a photoplethysmography sensor comprising the CP-FAC based OPD in a transmission mode is demonstrated.