Charge Separation in Donor–C₆₀ Complexes with Real-Time Green Functions: The Importance of Nonlocal Correlations

Abstract

We use the Nonequilibrium Green's Function (NEGF) method to perform real-time simulations of the ultrafast electron dynamics of photoexcited donor-C60 complexes modeled by a Pariser-ParrPople Hamiltonian. The NEGF results are compared to mean-field Hartree-Fock (HF) calculations to disentangle the role of correlations. Initial benchmarking against numerically highly accurate time-dependent Density Matrix Renormalization Group calculations verifies the accuracy of NEGF. We then find that charge-transfer (CT) excitons partially decay into charge separated (CS) states if dynamical non-local correlation corrections are included. This CS process occurs in ∼ 10 fs after photoexcitation. In contrast, the probability of exciton recombination is almost 100% in HF simulations. These results are largely unaffected by nuclear vibrations; the latter become however essential whenever level misalignment hinders the CT process. The robust nature of our findings indicate that ultrafast CS driven by correlation-induced decoherence may occur in many organic nanoscale systems, but it will only be correctly predicted by theoretical treatments that include time-nonlocal correlations.

The generalized Kadanoff-Baym ansatz with initial correlations

“…One of the reasons is the reinvention of the Generalized Kadanoff-Baym Ansatz (GKBA) 4 for the solution of the NEGF equations, which has made it possible to perform ab-initio simulations of atoms, molecules, and bulk systems thanks to a drastic reduction of the computational effort. The NEGF-GKBA has been used to study, e.g., atoms 5 , biologically relevant molecules 6 , organic compounds 7,8 as well as a large class of extended systems 9,10 including several twodimensional layered materials 11,12 . Recently, the scheme has also been used to study model Hamiltonians with Hubbard or extended Hubbard interactions [13][14][15][16][17] .…”

CHEERS: a tool for correlated hole-electron evolution from real-time simulations

“…We mention that the 2B approximation has been successfully applied to equilibrium spectral properties [49] and total energies [50] of molecular systems. Comparisons against numerically accurate real-time simulations of 1D systems [18,51] and weakly correlated model nanostructures of different geometries [29,30,39,[52][53][54][55][56] indicate that the 2B approximation remains accurate even out of equilibrium. The last term I Auger can be written as in Eq.…”

Initial correlated states for the generalized Kadanoff–Baym Ansatz without adiabatic switching-on of interactions in closed systems

“…If the second and higher orders of the expansion are negligible, the approximate Green's function will be the same as in Eq. (29). The advantage of this equation is that it has the Dyson structure and gives a numerically less delicate evaluation of ρ.…”