Semifluoroalkyl n-Type Conjugated Polymers: Novel Cathode Interlayers for Air-Stable Organic Photodetectors

Typical organic cathode interlayer (CIL) materials for solution-processed organic optoelectronic devices usually have polar side chains or polar/ionic groups, making them hydrophilic and thus limiting device stability. This manuscript reports a novel CIL material based on an n-type polythiophene backbone with semifluoroalkyl side chains (n-PT11) for developing high-performance organic photodetectors (OPDs) with excellent device stability. The n-type polythiophene backbone of n-PT11 has a low-lying LUMO/HOMO energy level of −4.27 eV/−6.21 eV, suppressing reversed hole injection. The semifluoroalkyl side chains enable n-PT11 processing with 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) for fabricating multilayer OPDs and exhibit low trap density for suppressing dark current density (J_d). As a result, n-PT11-based near-infrared (NIR) OPDs exhibit an ultra-low J_d of 1.00 × 10⁻⁹ A cm⁻² and a high specific detectivity (D^*) of 3.06 × 10¹² cm Hz^1/2 W⁻¹ at −1 V, which is among the highest D^* values reported so far for NIR OPDs. Most importantly, the hydrophobicity of n-PT11 allows the OPD unprecedented air stability, with unencapsulated OPDs retaining 94% of their initial responsiveness after 2000 h in ambient conditions. To the best knowledge, this is the first report of conjugated polymers with semifluoroalkyl side chains as CILs in OPDs.