Doping is a widely-implemented strategy for enhancing the inherent electrical properties of metal oxide charge transport layers in photovoltaic devices because higher conductivity of electron transport layer (ETL) can increment the photocurrent by reducing the series resistance. To improve the conductivity of ETL, in this study we doped the ZnO layer with aluminum (Al), then investigated the influence of AZO on the performance of inverted bulk heterojunction (BHJ) polymer solar cells based on poly [[4,8-bis [(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b'] dithiophene-2,6-diyl]-[3-fluoro-2[(2-ethylhexyl)- carbonyl] thieno-[3,4-b]thiophenediyl]] (PTB7):[6, 6]- phenyl C71 butyric acid methyl ester (PC71BM). The measured conductivity of AZO was similar to 10(-3) S/cm, which was two orders of magnitude higher than that of intrinsic ZnO (similar to 10(-5) S/cm). By decreasing the series resistance (R-s) in a device with an AZO layer, the short circuit current (J(sc)) increased significantly from 15.663 mA/cm(2) to 17.040 mA/cm(2). As a result, the device with AZO exhibited an enhanced power conversion efficiency (PCE) of 8.984%.