We report on significant efficiency enhancement of bulk
heterojunction organic photovoltaic cells based on blends of
poly(9,9-dioctylfluorene-alt-bithiophene), F8T2, or 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, with [6,6]-phenyl-C61-butyric acid methyl ester, PC61BM, upon thermally depositing a layer of a phenanthroline derivative, 4,7-diphenyl-1,10-phenanthroline, BPhen, or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, BCP, and LiF. We found that both phenanthroline derivatives form nanostructured layers and that the introduction of the LiF layer is crucial for the cells performance improvement. The interface between BPhen and LiF was investigated by X-ray photoelectron spectroscopy (XPS). This analysis suggested that charge-transfer doping of BPhen by fluoride is at the origin of improved electron transport throughout the BPhen layer. The best performing cells display power conversion efficiencies of 2.89% and 5.89%, for the blend with F8T2 and PTB7, respectively