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

Energy loss (E _loss) consisting of radiative recombination loss (ΔE ₁ and ΔE ₂) and nonradiative recombination loss (ΔE ₃) 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 E _loss with sulfur (S) as a comparison. N7IT-based OSCs achieve not only a higher PCE (13.8%), but also a much lower E _loss (0.57 eV) than those of the analogue (a-IT)-based OSCs (PCE of 11.5% and E _loss of 0.72 eV), which are mainly attributed to N7IT's significantly enhanced charge carrier density (promoting J _SC) and largely suppressed nonradiative E _loss by over 0.1 eV (enhancing V _OC). 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 E _loss, which could inspire the community to design and synthesize more N-functionalized SMAs.