Solution-Processed Indium Doped Zinc Oxide as Electron Transport Layer for Inverted Polymer Solar Cells

In this work, we report the synthesis and the application of a low-temperature sol-gel In-doped zinc oxide (IZO) thin film with various indium content as an electron transport layer (ETL) for high efficiency inverted polymer solar cells (PSCs). The IZO precursor was prepared by dissolving zinc acetate and ethanolamine in the 2-methoxyethanol in the presence of InCl3 at different concentration. Doped ZnO thin films were then deposited on indium tin oxide (ITO)/glass substrates by spin coating the above solution and annealed at 150°C for 5' in air. Inverted polymer solar cells with the configuration ITO/IZO/photoactive layer/MoO3/Ag were realized in order to investigate the performance of the IZO thin film. The photoactive layer was a blend of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) and [6,6]-phenyl C71 butyric acid methyl ester ([70]PCBM). We made a comparative study of the photovoltaic behavior of PSCs realized employing IZO films with various indium contents. The best efficiency of 9.27% was reached using a 1 at% IZO film. The improved performance of such device is due to the better charge collection efficiency of this ETL.