In this work we studied the influence of the annealing treatments of a sol-gel derived ZnO electron transport layer deposited on ITO substrate, on the performances of inverted bulk heterojunction polymer solar cells using a blend of poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b']dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiopene)-2,6-diyl] and [6,6]-phenyl C-71 butyric acid methyl ester. Since the annealing treatments needed to complete the formation of the solution-processed ZnO film can modify the underlying ITO electrode, we analyzed the performance of the fabricated cells in terms of the properties of ITO and ZnO films. We found a linear relationship between the sheet resistance of the ITO layer and the series resistance of the corresponding device, which strongly influences the fill factor. The best power conversion efficiency (7%) under simulated AM 1.5G illumination of 100 mW/cm(2) was achieved for the polymer solar cell fabricated using a ZnO film annealed at 150 degrees C for only 5 mm. Higher annealing temperatures and times increase the sheet resistance of the ITO worsening the device performances. (C) 2015 Elsevier B.V. All rights reserved.