Effect of an External Electric Field on the Ordered Structures of Blended Donor Polymers in Solar Cells

Abstract

This research investigated the effect of a high external electric field (EEF) on the ordered structure of a blended film composed of poly­(3-hexylthiophene) (P3HT) and 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]­thiophenediyl]] (PTB7) through atomic force microscopy, high-resolution transmission electron microscopy, micro-Raman spectroscopy, ultraviolet–visible absorption spectroscopy, photoluminescence spectroscopy, and space-charge-limited current measurements. With the application of an optimal electric field, the orderness of the PTB7/P3HT blended film was improved, and its hole mobility was evidently enhanced from 2.72 × 10–3 to 6.76 × 10–3 cm2 V–1 s–1. PTB7 and P3HT formed an interpenetrating structure in the precursor solution. P3HT oriented along the electric field direction and drove PTB7 molecular chains to move in a manner that enabled the PTB7 molecular chain segment that was originally unaffected by the EEF to overcome steric hindrance and twist to achieve tight π–π stacking, forming more ordered structures in the PTB7/P3HT blended film. The ordered structure of PTB7 in the blended film was described by an order parameter R = A 0–0/A 0–1 in the UV–vis absorption spectra. This parameter R increased from 1.01 to 1.12 after the addition of 30% P3HT and further increased to 1.14 under the effect of the EEF, indicating that the order degree of PTB7 molecular chains continued to increase in the PTB7/P3HT blended film. This research contributes to the understanding of the effect of the EEF on the ordered structures of PTB7/P3HT blended films. It reveals the formation mechanism and regularity of the ordered structures of two donor polymers, enabling the fabrication of ternary solar cells with high efficiency on the basis of polymer physics.