Extensive research on organic photovoltaics has granted impressive power conversion efficiencies, nowadays exceeding 13% for state-of-the-art photoactive material combinations. Nevertheless, different strategies can be adopted to further enhance the efficiency and the competitiveness with alternative photovoltaic technologies. Conjugated polyelectrolytes have been applied as anode or cathode interlayers to optimize ohmic contacts and lower the contact resistance, thereby improving the ultimate device efficiency. Here, we present an interlayer material belonging to the emerging class of narrow bandgap conjugated polyelectrolytes, based on an imidazolium functionalized 4H-cyclopenta[2,1-b: 3,4-b'] dithiophene (CPDT) as the electron-rich polymer building block and 4H-thieno[3,4-c] pyrrole-4,6(5H)-dione (TPD) as the electron-deficient subunit. The ionic polymer is applied as cathode interlayer for PBDTTPD:[70]PCBM (poly[bis(2'-ethylhexyloxy) benzo[1,2-b: 4,5-b'] dithiophene-alt-N-octylthieno[3,4-c] pyrrole-4,6-dione]:[6,6]-phenyl-C-71-butyric acid methyl ester) bulk heterojunction polymer solar cells, improving the overall device performance from 6.9 to 7.8%. (C) 2018 Elsevier Ltd. All rights reserved.