The excellent adhesion property of polydopamine (PDA) provides a new way to fixing nanoparticles. This study investigated the effects of PDA/ZnO nanocomposite as an electron transport layer (ETL) material on the performance of polythieno[3,4-b]-thiophene/benzodithiophene:[6,6]-phenyl-C70-butyric acid methyl ester bulk heterojunction solar cell. On one hand, the strong adhesive property of PDA to ZnO nanoparticles is beneficial to the formation of a uniform ZnO thin film, which is favorable for electron transfer. On the other hand, an energy level mismatch exists between PDA and ZnO, which hinders electron transfer from ZnO to ITO. Upon optimization, filling factor and photoelectric conversion efficiency are improved by 23.2% and 13.8%, respectively, when the single ZnO layer is replaced by PDA/ZnO nanocomposite as the ETL. Open circuit voltage resulted in 9.8% enhancement from 0.61V to 0.67V. However, the short circuit current density is reduced.