Achieving Highly Sensitive Near-Infrared Organic Photodetectors using Asymmetric Non-Fullerene Acceptor

Organic photodetectors (OPDs) based on non-fullerene acceptors (NFAs) have received considerable attention because of their potential for use in various commercial applications as near-infrared (NIR) light sensing platforms. However, recent OPDs suffer from low NIR photoresponse and large dark/noise currents with narrow bandgap organic photoactive materials. Herein, a pi-bridge molecular engineering strategy replacing alkoxythienyl with benzothiadiazole for ultra-narrow bandgap (ultra-NBG) NFAs is designed to achieve simultaneously high photoresponse at NIR region and low noise current density, thereby leading to excellent NIR (approximate to 1050 nm) detectivity (D*). The newly synthesized ultra-NBG NFAs, namely COB and CBT with optical bandgaps below 1.14 eV, present high responsivity (R) with 0.369 and 0.080 A W-1, respectively, at a wavelength of 1050 nm. Especially, with effectively suppressed noise current density, COB-based OPD exhibits a high NIR (approximate to 1050 nm) D* value of 2.18 x 10(11) cm Hz(1/2) W-1 at -0.5 V bias. The obtained R and D* values for these NFAs exceed or are comparable to those of a commercial Si photodetector at 1050 nm. This work provides important insight into the pi-bridge molecular engineering strategy for ultra-NBG NFAs, which facilitate achieving highly sensitive NIR OPDs with high NIR photoresponse and low dark/noise current.