On the Physical Origins of Charge Separation at Donor–Acceptor Interfaces in Organic Solar Cells: Energy Bending versus Energy Disorder. Issue 4 (21st January 2020) - de Sousa, Leonardo Evaristo
Abstract: Charge separation (CS) is a central process in the working of organic solar cells (OSC). Despite the strong electron–hole (e–h) Coulombic attraction at the donor–acceptor (D–A) interface, the bound e–h pairs do separate into free charges following an ultrafast process. To explain these results, several models have been proposed. By means of kinetic Monte Carlo simulations, the energy bending (EB) at the D–A interface is considered as the driving force for CS against the impact of energy disorder. The results suggest that, while entropy and energy disorder alone allow for the bound e–h pairs to escape charger recombination, the efficiency of CS increases by several times and its timescale decreases significantly in the presence of EB, approaching experimental findings. The impact of external electric fields on CS efficiency is found to stem from insufficient amount of EB. Importantly, the results indicate that, in the absence of EB, simply improving charge carrier mobility in bulk has no effect on the performances of OSC, thus orienting the new materials design strategy toward preferably targeting interfacial properties. Guidelines regarding how to introduce EB at the D–A interface are also discussed. Abstract : In organic solar cells, the energy bending (EB) at the donor–acceptor interface constitutes a key driving force for efficient charge separation (CS). Kinetic Monte Carlo simulations based on the impact of EB allow explaining i) the generation of large current densities, ii) the electric field dependence of CS, and iii) the sub‐nanosecond CS time.