Chemical Modification-Regulated Excited-State Dynamics and Charge Separation in TPA-C60 Donor-Acceptor Systems: A...

Understanding and regulating photoinduced charge-transfer dynamics in donor-acceptor (D-A) systems are crucial for the development of high-performance organic solar cells (OSCs). In this work, the excited-state properties and nonadiabatic dynamics of four chemically modified TPA-C60 D-A systems, namely DA-C60, DDA-C60, TPA-CH2-C60, and TPA-NH-C60, were systematically investigated using linear-response time-dependent density functional theory (LR-TDDFT) and nonadiabatic molecular dynamics (NAMD) simulations. The effects of donor conjugation length and linker-group modification on excited-state electronic structures, optical absorption, relaxation dynamics, and charge-transfer behaviors were comprehensively analyzed. The results show that DA-C60 and DDA-C60 mainly exhibit visible-light absorption, while TPA-CH2-C60 and TPA-NH-C60 display broader absorption regions extending into the ultraviolet region. Electron-hole density and fragment-based exciton analyses reveal that DA-C60 and DDA-C60 are dominated by localized excitons on the C60 fragment after photoexcitation, whereas TPA-CH2-C60 and TPA-NH-C60 exhibit pronounced donor-to-acceptor charge-transfer characteristics. Nonadiabatic dynamics simulations demonstrate that DA-C60 and DDA-C60 undergo ultrafast excited-state relaxation and rapidly evolve into localized |C60*> excitons. In contrast, TPA-CH2-C60 and TPA-NH-C60 exhibit significantly slower relaxation dynamics and efficiently generate long-lived charge-transfer excitons dominated by |TPA-CH2+C60-> and |TPA-NH+C60->, respectively. Moreover, TPA-NH-C60 exhibits a larger electron-hole separation distance than TPA-CH2-C60, indicating that the -NH- linker is more favorable for enhancing charge separation than the -CH2- linker. These results demonstrate that donor conjugation length and linker-group engineering can effectively regulate excited-state relaxation pathways and charge-transfer behaviors in TPA-C60 systems.