Mixed Metals Slow Down Nonradiative Recombination in Saddle-Shaped Porphyrin Nanorings: A Time-Domain Atomistic Simulation

Saddle-shaped zinc porphyrin nanorings are utilized as light-harvesting materials. To achieve high performance, both fast charge transfer and slow charge recombination are required. Fast transfer favors efficient separation of exciton into free carriers, enhancing photocurrent. Slow recombination reduces charge and energy losses. We simulated both processes using time-dependent self-consistent-charge density functional tight binding theory combined with nonadiabatic (NA) molecular dynamics. The obtained picosecond charge recombination times agree well with experiment. The simulations demonstrate that the carrier lifetime depends strongly on the metals present in the porphyrin nanoring. When the porphyrin units are composed of Zn centers only, the simulated lifetime is 55 ps. If nanorings contain both Zn and Cd, the nonradiative recombination is suppressed to 200 ps, nearly 4 times. Incorporation of Cd partially localizes the photogenerated charges, weakens the NA coupling, and accelerates phonon-induced loss of electronic coherence. The heavier and slower Cd also decreases the NA coupling. The nonradiative recombination is driven by low-frequency phonons, with a moderate contribution from the C-C stretch. Our study demonstrates a straightforward pathway to reducing charge losses in the porphyrin nanorings by partial exchange of Zn atoms with Cd and provides a valuable guideline for improvement of the material efficiency for solar energy applications.