Effects of DIO on the charge recombination behaviors of PTB7:PC71BM photovoltaics

The influence of the solvent additive 1, 8-diiodooctane (DIO) on the charge carrier recombination behaviours of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b0] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediy1]] (PTB7): phenyl-[6, 6]-C71-butyric acid methyl ester(PC71BM) organic photovoltaics (OPVs) was investigated. The measurement of transient photocurrent (TPC) implies that the devices processed with 3% DIO have less charge carrier recombination loss than the devices without DIO. The photo-induced charge extraction by linearly increasing voltage (photo-CELIV) technique shows that the device with DIO has less charge carrier trap population and thus the bimolecular recombination can be reduced. The modified Onsager–Braun model with bimolecular recombination rate γbi, calculated from the photo-CELIV, indicated that by adding DIO, the binding energy of the charge-transfer exciton at the interface of PTB7 and PC71BM reduced, thus the geminate recombination loss decreased. Atomic Force Microscope (AFM) images show that DIO can optimize the morphology of the active layer, which is beneficial to the exciton dissociation and the reduction of the charge carriers trapping. The reduction of the charge carrier recombination including geminate recombination and non-geminate recombination lead to the large increase of current density (JSC), fill factor (FF), and power conversion efficiency (PCE) of PTB7:PC71BM OPVs.