In this study, a low-temperature solution-processed zinc tin oxide (ZTO) films are successfully utilized as the cathode buffer layer in the inverted organic P3HT: PCBM bulk heterojunction solar cells. ZTO film cathode buffer layer with an appropriate Sn-doping concentration outperforms the zinc oxide (ZnO) film with an improved power conversion efficiency (1.96% (ZTO film) vs. 1.56% (ZnO film)). Furthermore, ZTO nanoparticles (NPs) are also synthesized via low-temperature solution route and the device with ZTO NPs buffer layer exhibits a significant improvement in device performance to reach a PCE of 2.60%. The crystallinity of the cathode buffer layer plays an influential factor in the performance. From impedance spectroscopy analysis, a correlation between short circuit current (J(sc)), carrier life time (tau(avg)) and, thus, PCE is observed. The interplay between composition and crystallinity of the cathode buffer layers is discussed to find their influences on the solar cell performance. (C) 2016 Elsevier Ltd. All rights reserved.