Common Defects Accelerate Charge Separation and Reduce Recombination in CNT/Molecule Composites: Atomistic Quantum Dynamics.

Carbon nanotubes (CNTs) are appealing candidates for solar and optoelectronic applications. Traditionally used as electron sinks, CNTs can also perform as electron donors, as exemplified by coupling with perylenediimide (PDI). To achieve high efficiencies, electron transfer (ET) should be fast, while subsequent charge recombination should be slow. Typically, defects are considered detrimental to material performance because they accelerate charge and energy losses. We demonstrate that, surprisingly, common CNT defects improve rather than deteriorate the performance. CNTs and other low dimensional materials accommodate moderate defects without creating deep traps. At the same time, charge redistribution caused by CNT defects creates an additional electrostatic potential that increases the CNT work function and lowers CNT energy levels relative to those of the acceptor species. Hence, the energy gap for the ET is decreased, while the gap for the charge recombination is increased. The effect is particularly important because charge acceptors tend to bind near defects due to enhanced chemical interactions. The time-domain simulation of the excited-state dynamics provides an atomistic picture of the observed phenomenon and characterizes in detail the electronic states, vibrational motions, inelastic and elastic electron-phonon interactions, and time scales of the charge separation and recombination processes. The findings should apply generally to low-dimensional materials, because they dissipate defect strain better than bulk semiconductors. Our calculations reveal that CNT performance is robust to common defects and that moderate defects are essential rather than detrimental for CNT application in energy, electronics, and related fields.