Dibenzothiophene‑S,S‑dioxide and Bispyridinium-Based Cationic Polyfluorene Derivative as an Efficient Cathode Modifier for Polymer Solar Cells

A novel n-type conjugated polymer containing dibenzothiophene-S,S-dioxide (FSO), bispyridinium, and fluorene scaffolds in the backbone (PFSOPyCl) was synthesized and used in the cathode interfacial layers (CILs) of conventional polymer solar cells (PSCs). The high electron affinities and large planar structures of the FSO and bispyridinium units endowed this polymer with good energy level alignments with [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM) and metal cathode, and excellent electron transport and extraction properties. Polymer solar cells (PSCs) based on the poly­[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):PC₇₁BM system with PFSOPyCl CIL exhibited simultaneous enhancement in open-circuit voltage (V_oc), short-circuit current density (J_sc), and fill factor (FF), while the power conversion efficiency increased from 5.47% to 6.79%, relative to the bare Al device. Besides, PSC based on the poly­[4,8-bis­(2-ethylhexyloxyl)­benzo­[1,2-b:4,5-b′]­dithio-phene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno [3,4-b]­thiophene-2-carboxylate-4,6-diyl] (PTB7):PC₇₁BM system achieved a PCE of 8.43% when using PFSOPyCl as CIL. Hence, PFSOPyCl is a promising candidate CIL for PSCs.