Effects of functionalization on ultrafast nonadiabatic dynamics of photoexcited electrons in PC₆₁BM molecule

Polymer functionalized fullerenes, such as phenyl-C₆₁ butyric acid methyl ester (PC₆₁BM), are potential electron acceptors in fullerene-based photovoltaic devices [1,2]. In general, down the relaxation path, the photoexcited electron can intermittently relocate among the fullerene, ester and phenyl sub-structure regions of the molecule. This may weaken the non-adiabatic coupling between excited orbitals to significantly delay the relaxation time by reducing dynamical inter-orbital transition rates. As a result, the electron will stay “hot” for a longer time increasing chances of dissociation benefitting the carrier collection efficiency. In this research, we use electron-phonon coupled nonadiabatic molecular dynamics [3,4] to simulate the relaxation of a photoexcited PC₆₁BM molecule. The methodology relies on a combination of the single-particle description in density functional theory (DFT) and fewest-switches surface hopping approach, implemented using the Libra software [5]. Comparisons with the results of pristine C₆₀ will capture effects of functionalization on the electron dynamics.

[1] G.J. Hedley et al, Nat. Commun. 4, 2867 (2013).

[2] C.-Z. Li et al, J. Mater. Chem. 22, 4161, (2012).

[3] M.E. Madjet, E. Ali, M. Carignano, O. Vendrell, and H.S. Chakraborty Phys. Rev. Lett. 126, 183002, (2021).

[4] E. Ali, M.E. Madjet, R. De, M.B. Wholey, T. Frauenheim, and H.S. Chakraborty, J. Phys. Chem. A (in review).

[5] M. Shakiba et al, Software Impacts 14 100445 (2022).