Isomerization Directed Molecular Packing Toward Suppressing Energy Loss in Nonfullerene Ternary Solar Cells

Organic semiconductors based upon conjugated frameworksare oftenisomeric and display distinct optoelectronic properties within minorstructural variation. In this work, two isomeric nonfullerene acceptors(NFAs) ThMeCl-1 and ThMeCl-2, having the methyl and chlorine atomsattached on different positions of the electron-withdrawing end group,are synthesized and incorporated as the third component in ternarysolar cells. Although these NFA isomers exhibit a similar bandgap,energy levels, and energy loss in their PM6 based binary devices,the efficiency enhancements in ternary devices differ significantly.Compared to ThMeCl-1, the incorporation of ThMeCl-2 in PM6:C5-16 solarcells enables less energy loss, leading to an extra 0.03 eV open-circuitvoltage gain and a maximum efficiency increase from 17.8 to 18.9%.Grazing-incidence X-ray diffraction and molecular dynamics simulationsreveal that this is attributed to the versatile intermolecular pi-pi stacking forms between ThMeCl-2 and the host NFA, which result inimproved charge transport and suppressed recombination. This workprovides a rational guidance for controlling the molecular packingin ternary systems to prepare high performance organic photovoltaics.