Flexible PTB7:PC70BM Bulk-Heterojunction Solar Cells with LiF Cathode Buffer Layer

Bulk heterojunction solar cells were fabricated using [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) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) after a layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on a flexible indium tin oxide (ITO)-coated polyethylene terephthalate substrate. The fabricated structures were Al/LiF/PTB7:PC71BM/PEDOT:PSS/ITO with or without a lithium fluoride (LiF) buffer layer, and the influence of the LiF buffer layer on the performance of the solar cells was investigated. The LiF layer significantly enhanced the open-circuit voltages and fill factors of the solar cells presumably because of the work function shift of the aluminum cathode. As a result, the conversion efficiency increased from 2.31 % to 4.02 % owing to the presence of the LiF layer