Enhanced performance and morphological evolution of PTB7:PC71BM polymer solar cells by using solvent mixtures with different additives

Organic photovoltaics (OPVs) were fabricated with blended active layers of 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]thiophene-diyl]]:[6,6]-phenylC71-butyric acid methyl ester (PTB7:PC71BM). The active layers were prepared in chlorobenzene (CB) with different additives of 1,8-diiodooctane (DIO) and 1,8-dibromooctane and different concentrations using a wet process with spin coating. The effects of different solvent additives were studied with respect to photovoltaic parameters such as fill factor, short circuit current density, and power conversion efficiency. The absorption and surface morphology of the active layers were investigated by UV-visible spectroscopy, atomic force microscopy (AFM) and time-of-flight secondary ion mass spectrometry (TOF-SIMS), respectively. The results indicated that structural and morphological changes were induced by the solvent additives. The polymer solar cells (PSCs) of PTB7/PC71BM prepared by a spin coating method using DIO and 1,8-dibromooctane showed more improved PCE of 6.76%. The enhancement of performance of PSCs could be mainly attributed to the absorption enhancement and the improved charge carrier transportation.