Fluorinated polyethylene glycol as cathode interlayer with enhanced dipole strength for efficient organic solar cells

Interlayers play important roles in elevating the efficiency of the organic solar cells (OSCs). It is generally believed that organic cathode interlayer (CIL) can form dipole at the interface to reduce the work function of the metal electrode and minimize the contact resistance between the active layer and electrode. However, in most cases, the interlayer materials are deposited onto an active layer by spin coating and the polar groups thus randomly distribute on the surface of the active layer. In this work, we adopted fluorinated polyethylene glycol (FSO-100, a commercialized surfactant) as cathode interlayer in OSCs and achieved remarkable performance enhancement. Scanning Kelvin probe microscopy (SKPM) and ultraviolet photoelectron spectrometer (UPS) results prove that FSO-100 effectively decrease the work function of the metal electrode. Angle resolved XPS (ARXPS) shows that fluorinated alkyl chains on FSO-100 prefer to spontaneously aggregate on one side and thus result in highly ordered ethylene glycol chains, which enhance the dipole strength as well as improve the device performance, even much better than device with polyethylene glycol (PEG) as CIL. These results demonstrate that introducing a fluorinated alkyl chain on CIL is an effective strategy to promote the molecular orientation, as well as improve the electrode modification ability of CIL.