Morphology-driven photocurrent enhancement in PTB7/PC71BM bulk heterojunction solar cells via the use of ternary solvent processing blends

Abstract

PTB7/PC71BM bulk heterojunction solar cell devices with high photocurrents were fabricated through processing their active layers from a ternary solvent system. The active layers were deposited from solutions where chlorobenzene (CB) was used as the main solvent, DIO (3%) as the solvent additive and cyclohexanone (CHN), a solvent in which both the polymer and fullerene are poorly soluble. The morphology of the different active layers was characterized by atomic force microscopy (AFM) and grazing incidence X-ray scattering (GIXRS) while charge extraction (CE) and transient photovoltage (TPV) techniques were used to measure charge carrier recombination kinetics. It was found that this ternary solvent system affects the J-V characteristics to a significant extent, mainly affecting the JSC and FF, in a trend where the JSC is seen to increase with increasing ratio of CHN/CB, while the FF decreases concomitantly. This increase in JSC has been correlated with an increase in crystallinity of the active layer and more specifically with an increase of the crystalline volume of the fullerene domains and the increase in the polymer crystallites size, while the decrease in FF was linked to non-optimal vertical segregation. Despite the drop in FF, the addition of CHN in the blend still leads to an overall increase in power conversion efficiency (PCE) with respect to the devices processed from pristine CB (6.86% vs. 7.31%).