A simple method is demonstrated to improve the film-forming properties and air stability of a conjugated polyelectrolyte (CPE) without complicated synthesis of new chemical structures. An anionic surfactant, sodium dodecybenzenesulfonate (SDS), is mixed with cationic CPEs. The electrostatic attraction between these two oppositely-charged materials provides the driving force to form a stable CPE-surfactant complex. Compared with a pure CPE, this electrostatic complex is not only compatible with highly hydrophobic bulk-heterojunction (BHJ) films, e.g. poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM), but also works well with other low bandgap polymer-based BHJ films. Using this complex as a cathode interface layer, a high power conversion efficiency of 4% can be obtained in P3HT:PCBM solar cells together with improved stability in air. Moreover, 20% performance enhancement can also be achieved when the complex is used as an interlayer to replace calcium metal for low bandgap polymer-based BHJ systems.