Compared with the popular conjugated polyelectrolytes as interfacial materials in polymer solar cells (PSCs), small molecule conjugated electrolytes (SMCEs) interlayers remains limited and the relationship between structures of small molecule electrolytes and device performance is still unclear. Herein, a series of alcohol-soluble small molecule conjugated electrolytes, combined with different conjugated skeletons and identical ammonium salt polar side chains, namely, 2T-(FN-Br)(2), 3T-(FN-Br)(2) and 4T-(FN-Br)(2) were designed and synthesized as interfacial materials for the inverted polymer solar cells (I-PSCs), to explore the relevance between the pi-conjugated skeleton and the device performance. As the conjugated bridge length and terminal group increased from 2T-(FN-Br)(2), 3T-(FN-Br)(2) to 4T-(FN-Br)(2), the photovoltaic performances were gradually improved. The improved device performance caused by the enlarged pi-conjugated skeleton mainly resulted from the optimized interfacial contact, effective modulation on the morphology of the active layer and the improvement of charge mobility. Encouragingly, devices with 4T-(FN-Br)(2) as interfacial material achieved a high power conversion efficiency of 8.40%, based on thieno [3,4-b]thiophene/benzodithiophene (PTB7): [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as light-harvesting layer. This work provided a new avenue to exploit novel promising interfacial modification materials by simply and slightly modulating the pi-conjugated skeleton without complex molecular design. (C) Elsevier B.V. All rights reserved.