In the present work, a systematic study has been carried out to understand the effect of In doping on the various properties of the ZnO nanocrystalline thin films. In-doped ZnO nanocrystalline thin films with different indium concentrations (1.98%, 4.03%, 6.74%, 8.62% and 10.48% In) have been synthesized by sol-gel method. The grain size and surface roughness of the In-doped ZnO thin films are observed to be smaller than those of the ZnO thin films. 6.74% In-doped ZnO films with a low resistivity of 1.95 x 10(-3) Omega cm and a high mobility of 2.19 cm(2) V-1 S-1 have been prepared under optimal deposition conditions. Inverted organic solar cells containing In-doped ZnO as an electron extraction layer with the structure indium tin oxide (ITO)/In-doped ZnO/poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT): [6,6]-phenyl C-71-butyric acid methyl ester (PC71BM)/MoO3/Al have been fabricated. The inverted organic solar cell with 6.74% In-doped ZnO exhibited a power conversion efficiency of 5.58%, which is the best efficiency reported so far for these type of solar cells. The device performance has been optimized by varying the indium doping concentration. The results clearly demonstrate that significant improvement in power conversion efficiency can be obtained by incorporating In into the ZnO films. (C) 2014 Elsevier Ltd. All rights reserved.