Alcohol‐Soluble n‐Type Polythiophenes as Cathode Interlayer in Organic Photodetectors for Hole Blocking

Abstract For organic photodetectors (OPDs), cathode interlayer (CIL) with low‐lying highest occupied molecular orbital (E_HOMO) energy level is required to reduce dark current density (J_d). In this study, toward application as CIL in OPDs, an alcohol‐processable n‐type conjugated polymer (n‐PT8) based on imide/cyano‐substituted polythiophene backbone and branched oligo(ethylene glycol) side chain is developed. Owing to the electron‐accepting imide/cyano groups on the backbone, n‐PT8 exhibits deep‐lying E_HOMO of −5.80 eV and a high conductivity of 5.12 × 10⁻⁶ S cm⁻¹. OPD with n‐PT8 as CIL exhibits dark current density one order of magnitude lower than that with the state‐of‐the‐art CIL material because of the large energy barrier for reverse hole injection. Moreover, the device performance is insensitive to the n‐PT8 layer thickness, which is very desirable for the mass production of OPDs. The near‐infrared (NIR) OPDs based on n‐PT8 exhibit an ultra‐low dark current density of 1.76 × 10⁻⁹ A cm⁻² and a specific detectivity of 2.88 × 10¹² cm Hz^1/2 W⁻¹ at −1 V. To the best knowledge, this is the first report on n‐type polythiophene derivatives as CIL in organic opto‐electronic devices.