The significant advance of organic solar cells performance has been the result of a combination of materials and devices architecture developments. In particular, electron-extraction layers, such as phenantrolines, have been very effective at improving devices efficiency. During our investigations of the role of BPhen and BCP phenantroline derivatives we found that thin layers thermally deposited on top of either F8T2:PCBM or PTB7:PCBM films form columnar grain structures, with columns heights largely exceeding the “average” thickness. Additionally, when LiF/Al low work function electrodes were used, significant efficiency improvements were observed, reaching 4.33% for the F8T2-based devices and 8.84% in the case of the PTB7-based devices. No improvements were observed for cells without LiF.
XPS investigations are consistent with the existence of BPhen doping by LiF, which explains that, despite the tall pillars of BPhen formed, no detrimental effects are found in the electron transport.