Tuning charge transfer and recombination in exTTF/CNT nanohybrids by choice of chalcogen: A time-domain density functional analysis

Abstract

Supramolecular nanohybrids composed of carbon nanotubes (CNTs) and organic molecules are appealing candidates for many applications. We investigate charge separation and recombination dynamics in extended tetrathiafulvalene (exTTF), a well-known sulfur (S)-rich electron donor, immobilized on a CNT surface, and study the role of the chalcogen atom by comparing with the selenium (Se)-rich tetraselenafulvalene (exTSeF) analog. Using real-time time-dependent tight-binding density-functional theory combined with nonadiabatic molecular dynamics, we show that photo-excitation of exTTF results in electron transfer (ET) into the CNT conduction band, while CNT excitation leads to hole transfer (HT) to exTTF. The ET is sub-picosecond in both systems, while the HT transfer time depends strongly on the chalcogen. The simulated ET times agree with available experiments. HT from the excited CNT is accelerated by two orders of magnitude more in exTSeF/CNT than exTTF/CNT, because of smaller energy gap, larger nonadiabatic charge–phonon coupling, and longer coherence time. In comparison, nonradiative decay of the charge-separated state takes place on nanosecond time scales. Electrons and holes recombine more slowly by an order of magnitude in the exTTF/CNT hybrid because of weaker nonadiabatic coupling and shorter coherence time. The coupling is weaker since high frequency phonons are less active. The coherence is shorter due to participation of a broader spectrum of low-frequency modes. The state-of-the-art atomistic quantum dynamics simulation demonstrates the strong influence of the chalcogen atom on the separation and recombination dynamics of photo-generated carriers in the molecule/CNT hybrids. The insights provide valuable guidelines for optimization of photovoltaic efficiency in modern nanoscale materials.