Isomerization of Asymmetric Ladder-Type Heteroheptacene-Based Small-Molecule Acceptors Improving Molecular Packing: Efficient Nonfullerene Organic Solar Cells with Excellent Fill Factors

Abstract

Moving one subunit of the central core to the other side in a small-molecule acceptor (SMA) is an effective approach to achieve the goal of isomerization, which has proven viable to enhance photovoltaic performance. Herein, two isomeric SMAs of ThPy5 and ThPy6 are developed by changing the position of the thiophene unit of the dithieno[3,2-b:2 ',3 '-d]pyrrol in IDTP-4F. The effect of altering the thiophene position on morphological characteristics, macroscopic factors, and device performance is thoroughly investigated among these three isomeric SMAs. Compared to ThPy5, IDTP-4F and ThPy6 show planar molecular structures and better molecular orientation. Moreover, tighter pi-pi stacking as well as enhanced electron mobility is observed in ThPy6 relative to IDTP-4F. PM6:ThPy6-based organic solar cells (OSCs) achieve the maximum efficiency of 16.11%, along with an excellent fill factor (FF) of 0.789, which are among the best results for A-D-A-type SMA-based OSCs. The high FF ascribes to the improved molecular packing and charge collection/extraction efficacy and the reduced charge recombination. The structure-morphology-performance relationship drawn from this work can offer better guidance for designing the molecular structure, especially the central cores of SMAs.