Dithieno[3,2-b:2ʹ,3ʹ-d]pyrrol-Fused Asymmetrical Electron Acceptors: A Study into the Effects of Nitrogen-Functionalization on Reducing Nonradiative Recombination Loss and Dipole Moment on Morphology

Abstract

Energy loss (Eloss) consisting of radiative recombination loss (ΔE1 and ΔE2) and nonradiative recombination loss (ΔE3) is considered as an important factor for organic solar cells (OSCs). Herein, two N-functionalized asymmetrical small molecule acceptors (SMAs), namely N7IT and N8IT are designed and synthesized, to explore the effect of N on reducing Eloss with sulfur (S) as a comparison. N7IT-based OSCs achieve not only a higher PCE (13.8%), but also a much lower Eloss (0.57 eV) than those of the analogue (a-IT)-based OSCs (PCE of 11.5% and Eloss of 0.72 eV), which are mainly attributed to N7IT's significantly enhanced charge carrier density (promoting JSC) and largely suppressed nonradiative Eloss by over 0.1 eV (enhancing VOC). In comparison, N8IT, with an extended π-conjugated length, shows relatively lower photovoltaic performance than N7IT (but higher than a-IT) due to the less favorable morphology caused by the excessively large dipole moment of the asymmetrical molecule. Finally, this work sheds light on the structure–property relationship of the N-functionalization, particularly on its effects on reducing the Eloss, which could inspire the community to design and synthesize more N-functionalized SMAs.